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Bathgate Hills - an excursion

2016-01-26T10:14:33Z

JenniferFindlay1:

[[File:Lothian Geology cover.jpg|thumb|100px|right|[http://www.edinburghgeolsoc.org/p_sales.html#lothiangeology Buy the book]]]
'''By D. Stephenson and S.K. Monro. From: Lothian geology an excursion guide. Edited by A.D. McAdam and E.N.K. Clarkson. 1996'''

O.S. 1:50000 Sheet 65 Falkirk and West Lothian

B.G.S. 1:63360 Sheet 31 Airdrie

B.G.S. 1:50000 Sheet 32W Livingston

=== Introduction ===
[[File:BathgateHillsExcursionMap.jpg|400px|thumbnail|Bathgate Hills - excursion map]]
The Bathgate Hills owe their general elevation to the presence of a considerable thickness (up to 500 m) of hard, resistant volcanic rocks of basaltic composition. These replace much of the normal sequence of softer, less resistant, sedimentary rocks between the top of the Upper Oil-Shale Group and the middle of the Upper Limestone Group. Individual hills, often craggy, are formed by east-west trending dykes and inclined sills of the late-Carboniferous quartz-dolerite suite. Intercalated sediments within the volcanic pile include several limestones of both freshwater and marine origin. It is difficult to correlate these limestones with the established successions in sedimentary basins outside the Bathgate Hills. The thickest and most persistent limestone-bearing strata the Petershill Formation formed around an elevated volcanic land area lying to the north of Bathgate (Jameson 1980). Lateral changes in the lithology and fauna of these strata can be traced from the seaward side northwards towards the inferred land area.

The East Kirkton Limestone, in contrast to the Petershill Limestone, is of freshwater origin but demonstrates an unusual lithology which is thought to be associated with hot springs. These beds have yielded an important and distinctive flora and fauna. The limestones of the Bathgate Hills have been extensively worked in the past for agricultural use both by quarrying and mining. An example of the latter will be seen at Hillhouse.

The aims of the excursion are: (i) to contrast the sedimentology and palaeontology of the freshwater East Kirkton Limestone with those of the marine Petershill and Hillhouse limestones; (ii) to examine the intercalated basaltic tuffs and lava flows; (iii) to examine the late-Carboniferous quartz-dolerite intrusions; and (iv) to see the remains of 17th, 18th and 19th century silver-nickel-lead workings at Hilderston Mine. In Map 27 showing the route the geology is based upon recent mapping, boreholes and air-photo interpretation. The outcrops are all easily accessible from public roads, making visits by vehicle convenient. but they may be traversed on foot, in the order described, involving a 9 km walk from the first to last outcrop or 12 km between public transport centres in Bathgate and Linlithgow, both of which are on Eastern Scottish service bus routes.

==== 1. East Kirkton Quarry: East Kirkton Limestone ====

Park at the gate to the quarry (NS 990 689).

[[File:P002882.jpg|thumb|300px|left|East Kirkton Quarry, near Bathgate, West Lothian. The East Kirkton Limestone in the upper part of the West Lothian Oil Shale Formation is a laminated freshwater limestone. A unique and varied terrestrial assemblage of fossils has been collected from this site, including Westlothiana lizziae, a vertebrate fossil intermediate between amphibians and reptiles. The rocks are dipping at 20 to 45 degrees to the west-south-west and are between 9 and 19 m. thick. P002882]]The East Kirkton Limestone is a laterally impersistent sedimentary sequence within a thick succession of basaltic lavas and tuffs. The lithological succession consists of limestone overlain by black mudstones (Muir and Walton 1957) but is laterally variable in detail. Within the limestone occur bands with contorted laminae, tuff horizons where spherulitic structures are common, and beds of impure limestone with chert nodules. These lithologies have been interpreted as having been laid down in shallow pools associated with hot springs.

The limestones have recently yielded an unusual flora and fauna including amphibians, Millipedes, eurypterids, scorpions, the earliest known harvestman (opilionid) and much plant material (Milner 1985; Wood el al. 1985). The presence of these forms, together with a striking absence of fishes, suggests a palaeoecology where amphibians were an integral part of the terrestrial fauna. The black mudstones, lying stratigraphically above the limestone, are also associated with laterally impersistent bands of tuff and have yielded a fauna containing both ostracods and fish remains.

==== 2. Petershill Reservoir: Seaward Development of the Petershill Limestone ====

Park in rough lay-by at the bend in the road (NS 985 694). On the east bank of the reservoir, the Petershill Limestone has a high carbonate content and is only slightly argillaceous. It was originally horizontally-bedded and contains an abundant fauna of solitary corals and low mounds of ''Lithostrotion junceum'' (a colonial coral giving rise to the popular name 'spaghetti rock'). The fauna also has delicate elements, epifaunal spinose productids and echinoids, which are well preserved with little disturbance, indicating quiet sedimentation. The trace fossil ''Thalassinoides'' is characteristic of this facies.

The southern wall of the reservoir is the central portion of a bioherm, built-up of successive beds composed mostly of skeletal remains. A three-part ecological succession shows a basal, small productid-fenestellid association with few species (low-diversity), passing up into a high-diversity productid-sponge-rostroconch association, replaced towards the top by an echinoderm-dominated association. The succession can be interpreted in terms of regional shallowing with regression and eventual exposure. About midway up the southern wall of the reservoir a thin (1-2 cm) impersistent band of clay can be seen. This is a bentonite horizon, consisting largely of montmorillonite formed from a weathered local ash fall.

==== 3. Craigs: Quartz-dolerite Sill ====

Park 500 m west of Wester Drumcross (NS 995 700); roadside parking may be difficult. A quartz-dolerite sill of the late-Carboniferous suite trends NNW-SSE across the hills. Here it dips moderately steeply ENE, contrary to the regional dip of the sediments and lavas, and forms a prominent west-facing scarp with brown, spheroidal-weathering outcrops. From the north of the road the scarp can be seen trending north-west towards Raven Craig. It is cut, and in some places displaced, by E-W trending valleys, interpreted as fault-lines scoured out by glacial action. A track follows the foot of the scarp northwards and eastwards for 300 m to the ruins of Craigs and a small quarry. Here, above the sill, 4 m of ill-sorted tuffs rest on 3·5 m of pale grey, well-bedded limestone with shale partings and some tuffaceous bands. The rocks show signs of baking and disturbance by the sill, the top contact of which may be traced in outcrops NNW of the ruins. These are some of the few natural outcrops of the tuffs near the base of the Bathgate Hills volcanic sequence, which are known from boreholes to reach considerable thicknesses. Other outcrops occur at Whitelaw (NS 994 692), in a stream (NS 998 695) and in a roadside quarry (NS 996 700). All are close to the top of the sill and probably crop out owing to an increased resistance to erosion produced by contact metamorphism. Dips, where seen, tend to conform to the top of the sill, rather than to the regional dip.

==== 4. The Rifle Range Quarries: Landward Development of the Petershill Limestone ====

Park in the roadside quarry at The Knock (NS 991 711). In the Rifle Range Quarries, which extend south-west for a kilometre, the Petershill Limestone consists of heterogeneous packstones, medium-grained bioclastic limestones, located north and palaeogeographically shoreward of the laterally equivalent limestones exposed at the Petershill Reservoir (locality 2). In the lower 3-4 m of the quarry-face, cross-stratified crinoidal packstones, large cerioid coral colonies, bands of ''Gigantoproductus'' and large ''Chaetetes'' colonies can be seen. Although most of the fauna is in situ, signs of extensive hydrodynamic influence on the original life-surfaces are present throughout the sequence. Overturned corals with the open ends (calices) of the corallites downwards and coral debris, as well as separated and stacked brachiopod shells all suggest that currents were at times a major destructive force. The heterogeneous packstone facies is interpreted as having accumulated in a relatively turbulent, nearshore zone.

At the northern end of the quarries, a blue-grey basalt lava flow rests directly upon the limestone. The quarries terminate at a vertical E-W dyke of quartz-dolerite which exhibits good columnar jointing perpendicular to its margins.

==== 5. Triangulation Station (NS 987 712): Basaltic Lavas of Dalmeny Type ====

To the north-west of the north end of the limestone quarries (locality 4), several low scarp features trend NNE across the hillside as far as the triangulation point. Outcrops on these features are of a fresh, fine-grained, blue-grey basalt-the hard, central parts of lava flows. The basalts appear non-porphyritic, but in thin-section abundant 1 mm microphenocrysts of fresh olivine show them to be of Dalmeny type (MacGregor 1928). The presence of abundant interstitial analcime suggests gradation from olivine-basalt to basanite. Between the scarp features, topographic hollows with no exposure are due mainly to rubbly, altered flow tops or may in some cases indicate thin intercalations of sediment or pyroclastic horizons.

==== 6. The Knock: Quartz-dolerite Sill, Viewpoint ====

[[File:P001208.jpg|thumb|300px|left|400 m. north of The Knock. West Lothian. Looking south. Near Cairnpapple. Quartz-dolerite intruded into the Bathgate Hill Volcanic Formation. The quartz-dolerite is a steeply-inclined, transgressive part of the Midland Valley Sill, here orientated approximately north-south. The quartz-dolerite is the flat-topped ridge running from the centre of the photograph obliquely left to the skyline. P001208]]Cross the road to The Knock, where the quartz-dolerite sill seen at locality 3, trends NNW and dips steeply ENE at 60°, probably marking a step-like transgression between stratigraphic levels. The sill forms the rocky knoll of The Knock and its top surface forms the steep slab on the east side. In the back wall of the quarry on the south-west side, the lower chilled contact is well exposed where it cuts blue-grey basalt lava dipping north-west at about 20°.

From the top of The Knock the regional geology of the Bathgate Hills and that of the Midland Valley as a whole may be appreciated. The underlying bedrock of the Midland Valley shows itself in the landforms, with the harder igneous rocks forming the hills and the softer sediments the lowland. Far to the north are the metamorphic rocks of the Grampian Highlands, with the andesitic lavas of the Ochil Hills in the middle distance. To the south-east, the Old Red Sandstone sediments and lavas of the Pentland Hills can be seen with the Lower Palaeozoic rocks of the Southern Uplands due south. Eastwards most of the prominent topographic features such as Arthur's Seat, the Bass Rock and North Berwick Law, all correspond to areas of igneous rock. Also to be seen are the flat-topped, red bings which are the remains of West Lothian's oil-shale industry and which pick out the outcrop of the Oil-Shale Groups. The oil-shale was mined and burned in large retorts to extract the hydrocarbons; the red bings are made up of the burnt oil-shale. Looking to the west, black conical bings can be seen. These are the spoil from coal mining and their distribution picks out the of the Coal Measures. Note the marked cast-west valleys due to glacial scouring along fault-lines. and crag-and-tail features such as The Knock itself, which indicate west to east ice movement.

==== 7. Hilderston Mine: Silver-Lead-Zinc Mineralisation ====

Park in the lay-by for Cairnpapple at the top of the hill (NS 989 718). Immediately west of the road, the quartz-dolerite sill of localities 3 and 6 crops out forming a good N-S feature some 40 m wide. The sill dips steeply to the cast as at The Knock, and is really a dyke-like step between levels of sill. A diversion may be made at this point to the bronze-age burial mound of Cairnpapple Hill. 250 m to the south-west, which is also a good viewpoint.

Follow the sill/dyke southwards into the valley to the ruins of Windywa's which are on the site of the original 17th century Hilderston silver mine (Cadell 1925. 359-378; Stephenson 1983). The mine was in operation initially from 1606 to 1614 but made little or no profit after the first two years (i.e. after it had been 'nationalised' by King James VI). The silver occurred in a vein as filaments of native silver in a gangue of baryte and niccolite. This vein was located on the margin of a thin E-W dolerite dyke which cut sandstones and siltstones above the Petershill Limestone. The economic vein extended for only 80 m to the cast of the N-S dyke/sill and for 18 m below the surface. In the 18th century the mine was reopened and worked for lead and zinc which occurred within a baryte and calcite gangue at deeper levels, where the vein cut the Petershill Limestone. A second, much longer vein, some 60m to the north, was also worked at this time, but this phase of working ceased in 1772. The original workings were re-excavated during the period 1865 to 1873 using money from the sale of the niccolite and again from 1896 to 1898. However, no further economic deposits of silver or lead were discovered.

The full list of known minerals from the mine is: baryte, calcite. Dolomite, quartz, galena, sphalerite, niccolite, erythrite (nickel bloom), annabergite (cobalt bloom), bravoite, pyrite, chalcopyrite, albertite (solid hydrocarbon) and native silver. Most of these, with the exception of the nickel. cobalt and silver minerals, have been obtained recently from the waste heaps.

Three depressions close to the road at Windywa's mark the site of the main group of 17th century shafts. A large mound in the field east of the road marks the main 18th century shaft on the northern lead vein. The 1873 shaft is probably marked by a pile of debris, crescentic in plan, south of the burn and 35 m east of the road. Two adits in the west wall of the flooded Silvermines quarry mark two branches of the northern lead vein, probably excavated in the 19th century. If the water level is low, these adits can be examined. Do not attempt to enter them. Thin calcite veins with galena can be seen cutting the sandstone roof of the northernmost one.

The Petershill Limestone is no longer visible in the quarries, but the overlying clastic sediments can be examined in the cut to the south. Here an upward-coarsening sequence of clastic sediments can be seen with mudstone at the base of the quarry, gradually passing upwards to sandstone at the top. Fallen sandstone blocks at the foot of the quarry show good examples of trace fossils, both burrows and feeding trails. This type of upward-coarsening succession is typical of that associated with deltaic sedimentation. A thin N-S tholeiite dyke in the west wall of the southern quarry is altered to 'white trap' at its northern end.

==== 8. North Mine Quarry: Petershill Limestone, Dyke ====

Park by the roadside (NS 995 722). In the quarry, only massive sandstone is now seen, with a capping of basalt lava forming knolls in the plantation above the north face. An E-W dyke of quartz-dolerite, 30 m wide, forms a feature which is followed by the road. Just over the fence on the south side of the road, immediately east of the plantation. the southern contact of the dyke with the Petershill Limestone is exposed. The limestone contains colonies of ''Lithostrotian'' and is intensely baked. Both limestone and dolerite are impregnated with pink baryte. 'Copper ore' has also been recorded from this locality.

==== 9. Riccarton Hills: Basalt Lavas ====

From North Mine drive east, past Tartraven (small, overgrown limestone quarries). At South Mains, take the road left (north) towards Beecraigs. From this road views are seen of good trap features in basalt lavas of the Riccarton Hills to the east.

==== 10. Beecraigs Wood: Quartz-dolerite Dyke ====

Park at Beecraigs Country Park (NT 007 743) and walk south to the quarry on the west side of the road, cleared as a climbing wall. The face shows a section across an E-W quartz-dolerite dyke. The dolerite is very fresh and exhibits horizontal columnar jointing. The fine-grained edge of the dyke can be seen beside the ladder.

==== 11. Hillhouse Quarry and Mine: Hillhouse Limestone ====

Park at Beecraigs Visitors' Centre (NT 006 746), walk northwards and turn left at the T-junction. Quarries trend N-S on both sides of the road where the Hillhouse Limestone (possibly a lateral equivalent of the Petershill Limestone) has been worked by both surface and underground methods. The strata dip west at 30-40 degrees and consequently the stoop and room workings north of the road, which are still open, slope very steeply underground. Do not enter these workings. There have been roof collapses in recent years, some of which have caused subsidence in the overlying road.

The limestone is 9 to 12 m thick, the most complete section being exposed north of the road. Here the limestone is mostly massive with fossiliferous bands rich in crinoids, solitary and colonial corals and productid brachiopods. South of the road only 2 m of limestone is seen, overlain by over 2·5 m of cross-bedded sandstones and siltstones. The upper part of the section consists of a sill of very fresh basalt having well-developed. sub-vertical columnar joints. This sill is the type locality for the Hillhouse type of olivine-basalt (MacGregor 1928). The characteristic abundant microphenocrysts of olivine and augite are clearly visible on the weathered surfaces of the basalt.

{{EGwalks}}

[[category:5. Midland Valley of Scotland]]

File:P001208.jpg

2016-01-26T09:39:21Z

JenniferFindlay1: 400 m. north of The Knock. West Lothian. Looking south. Near Cairnpapple. Quartz-dolerite intruded into the Bathgate Hill Volcanic Formation. The quartz-dolerite is a steeply-inclined, transgressive part of the Midland Valley Sill, here orientated appr...

== Summary ==
400 m. north of The Knock. West Lothian. Looking south. Near Cairnpapple. Quartz-dolerite intruded into the Bathgate Hill Volcanic Formation. The quartz-dolerite is a steeply-inclined, transgressive part of the Midland Valley Sill, here orientated approximately north-south. The quartz-dolerite is the flat-topped ridge running from the centre of the photograph obliquely left to the skyline.
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Bathgate Hills - an excursion

2015-12-15T12:14:58Z

JenniferFindlay1:

[[File:Lothian Geology cover.jpg|thumb|100px|right|[http://www.edinburghgeolsoc.org/p_sales.html#lothiangeology Buy the book]]]
'''By D. Stephenson and S.K. Monro. From: Lothian geology an excursion guide. Edited by A.D. McAdam and E.N.K. Clarkson. 1996'''

O.S. 1:50000 Sheet 65 Falkirk and West Lothian

B.G.S. 1:63360 Sheet 31 Airdrie

B.G.S. 1:50000 Sheet 32W Livingston

=== Introduction ===
[[File:BathgateHillsExcursionMap.jpg|400px|thumbnail|Bathgate Hills - excursion map]]
The Bathgate Hills owe their general elevation to the presence of a considerable thickness (up to 500 m) of hard, resistant volcanic rocks of basaltic composition. These replace much of the normal sequence of softer, less resistant, sedimentary rocks between the top of the Upper Oil-Shale Group and the middle of the Upper Limestone Group. Individual hills, often craggy, are formed by east-west trending dykes and inclined sills of the late-Carboniferous quartz-dolerite suite. Intercalated sediments within the volcanic pile include several limestones of both freshwater and marine origin. It is difficult to correlate these limestones with the established successions in sedimentary basins outside the Bathgate Hills. The thickest and most persistent limestone-bearing strata the Petershill Formation formed around an elevated volcanic land area lying to the north of Bathgate (Jameson 1980). Lateral changes in the lithology and fauna of these strata can be traced from the seaward side northwards towards the inferred land area.

The East Kirkton Limestone, in contrast to the Petershill Limestone, is of freshwater origin but demonstrates an unusual lithology which is thought to be associated with hot springs. These beds have yielded an important and distinctive flora and fauna. The limestones of the Bathgate Hills have been extensively worked in the past for agricultural use both by quarrying and mining. An example of the latter will be seen at Hillhouse.

The aims of the excursion are: (i) to contrast the sedimentology and palaeontology of the freshwater East Kirkton Limestone with those of the marine Petershill and Hillhouse limestones; (ii) to examine the intercalated basaltic tuffs and lava flows; (iii) to examine the late-Carboniferous quartz-dolerite intrusions; and (iv) to see the remains of 17th, 18th and 19th century silver-nickel-lead workings at Hilderston Mine. In Map 27 showing the route the geology is based upon recent mapping, boreholes and air-photo interpretation. The outcrops are all easily accessible from public roads, making visits by vehicle convenient. but they may be traversed on foot, in the order described, involving a 9 km walk from the first to last outcrop or 12 km between public transport centres in Bathgate and Linlithgow, both of which are on Eastern Scottish service bus routes.

==== 1. East Kirkton Quarry: East Kirkton Limestone ====

Park at the gate to the quarry (NS 990 689).

[[File:P002882.jpg|thumb|300px|left|East Kirkton Quarry, near Bathgate, West Lothian. The East Kirkton Limestone in the upper part of the West Lothian Oil Shale Formation is a laminated freshwater limestone. A unique and varied terrestrial assemblage of fossils has been collected from this site, including Westlothiana lizziae, a vertebrate fossil intermediate between amphibians and reptiles. The rocks are dipping at 20 to 45 degrees to the west-south-west and are between 9 and 19 m. thick. P002882]]The East Kirkton Limestone is a laterally impersistent sedimentary sequence within a thick succession of basaltic lavas and tuffs. The lithological succession consists of limestone overlain by black mudstones (Muir and Walton 1957) but is laterally variable in detail. Within the limestone occur bands with contorted laminae, tuff horizons where spherulitic structures are common, and beds of impure limestone with chert nodules. These lithologies have been interpreted as having been laid down in shallow pools associated with hot springs.

The limestones have recently yielded an unusual flora and fauna including amphibians, Millipedes, eurypterids, scorpions, the earliest known harvestman (opilionid) and much plant material (Milner 1985; Wood el al. 1985). The presence of these forms, together with a striking absence of fishes, suggests a palaeoecology where amphibians were an integral part of the terrestrial fauna. The black mudstones, lying stratigraphically above the limestone, are also associated with laterally impersistent bands of tuff and have yielded a fauna containing both ostracods and fish remains.

==== 2. Petershill Reservoir: Seaward Development of the Petershill Limestone ====

Park in rough lay-by at the bend in the road (NS 985 694). On the east bank of the reservoir, the Petershill Limestone has a high carbonate content and is only slightly argillaceous. It was originally horizontally-bedded and contains an abundant fauna of solitary corals and low mounds of ''Lithostrotion junceum'' (a colonial coral giving rise to the popular name 'spaghetti rock'). The fauna also has delicate elements, epifaunal spinose productids and echinoids, which are well preserved with little disturbance, indicating quiet sedimentation. The trace fossil ''Thalassinoides'' is characteristic of this facies.

The southern wall of the reservoir is the central portion of a bioherm, built-up of successive beds composed mostly of skeletal remains. A three-part ecological succession shows a basal, small productid-fenestellid association with few species (low-diversity), passing up into a high-diversity productid-sponge-rostroconch association, replaced towards the top by an echinoderm-dominated association. The succession can be interpreted in terms of regional shallowing with regression and eventual exposure. About midway up the southern wall of the reservoir a thin (1-2 cm) impersistent band of clay can be seen. This is a bentonite horizon, consisting largely of montmorillonite formed from a weathered local ash fall.

==== 3. Craigs: Quartz-dolerite Sill ====

Park 500 m west of Wester Drumcross (NS 995 700); roadside parking may be difficult. A quartz-dolerite sill of the late-Carboniferous suite trends NNW-SSE across the hills. Here it dips moderately steeply ENE, contrary to the regional dip of the sediments and lavas, and forms a prominent west-facing scarp with brown, spheroidal-weathering outcrops. From the north of the road the scarp can be seen trending north-west towards Raven Craig. It is cut, and in some places displaced, by E-W trending valleys, interpreted as fault-lines scoured out by glacial action. A track follows the foot of the scarp northwards and eastwards for 300 m to the ruins of Craigs and a small quarry. Here, above the sill, 4 m of ill-sorted tuffs rest on 3·5 m of pale grey, well-bedded limestone with shale partings and some tuffaceous bands. The rocks show signs of baking and disturbance by the sill, the top contact of which may be traced in outcrops NNW of the ruins. These are some of the few natural outcrops of the tuffs near the base of the Bathgate Hills volcanic sequence, which are known from boreholes to reach considerable thicknesses. Other outcrops occur at Whitelaw (NS 994 692), in a stream (NS 998 695) and in a roadside quarry (NS 996 700). All are close to the top of the sill and probably crop out owing to an increased resistance to erosion produced by contact metamorphism. Dips, where seen, tend to conform to the top of the sill, rather than to the regional dip.

==== 4. The Rifle Range Quarries: Landward Development of the Petershill Limestone ====

Park in the roadside quarry at The Knock (NS 991 711). In the Rifle Range Quarries, which extend south-west for a kilometre, the Petershill Limestone consists of heterogeneous packstones, medium-grained bioclastic limestones, located north and palaeogeographically shoreward of the laterally equivalent limestones exposed at the Petershill Reservoir (locality 2). In the lower 3-4 m of the quarry-face, cross-stratified crinoidal packstones, large cerioid coral colonies, bands of ''Gigantoproductus'' and large ''Chaetetes'' colonies can be seen. Although most of the fauna is in situ, signs of extensive hydrodynamic influence on the original life-surfaces are present throughout the sequence. Overturned corals with the open ends (calices) of the corallites downwards and coral debris, as well as separated and stacked brachiopod shells all suggest that currents were at times a major destructive force. The heterogeneous packstone facies is interpreted as having accumulated in a relatively turbulent, nearshore zone.

At the northern end of the quarries, a blue-grey basalt lava flow rests directly upon the limestone. The quarries terminate at a vertical E-W dyke of quartz-dolerite which exhibits good columnar jointing perpendicular to its margins.

==== 5. Triangulation Station (NS 987 712): Basaltic Lavas of Dalmeny Type ====

To the north-west of the north end of the limestone quarries (locality 4), several low scarp features trend NNE across the hillside as far as the triangulation point. Outcrops on these features are of a fresh, fine-grained, blue-grey basalt-the hard, central parts of lava flows. The basalts appear non-porphyritic, but in thin-section abundant 1 mm microphenocrysts of fresh olivine show them to be of Dalmeny type (MacGregor 1928). The presence of abundant interstitial analcime suggests gradation from olivine-basalt to basanite. Between the scarp features, topographic hollows with no exposure are due mainly to rubbly, altered flow tops or may in some cases indicate thin intercalations of sediment or pyroclastic horizons.

==== 6. The Knock: Quartz-dolerite Sill, Viewpoint ====

Cross the road to The Knock, where the quartz-dolerite sill seen at locality 3, trends NNW and dips steeply ENE at 60°, probably marking a step-like transgression between stratigraphic levels. The sill forms the rocky knoll of The Knock and its top surface forms the steep slab on the east side. In the back wall of the quarry on the south-west side, the lower chilled contact is well exposed where it cuts blue-grey basalt lava dipping north-west at about 20°.

From the top of The Knock the regional geology of the Bathgate Hills and that of the Midland Valley as a whole may be appreciated. The underlying bedrock of the Midland Valley shows itself in the landforms, with the harder igneous rocks forming the hills and the softer sediments the lowland. Far to the north are the metamorphic rocks of the Grampian Highlands, with the andesitic lavas of the Ochil Hills in the middle distance. To the south-east, the Old Red Sandstone sediments and lavas of the Pentland Hills can be seen with the Lower Palaeozoic rocks of the Southern Uplands due south. Eastwards most of the prominent topographic features such as Arthur's Seat, the Bass Rock and North Berwick Law, all correspond to areas of igneous rock. Also to be seen are the flat-topped, red bings which are the remains of West Lothian's oil-shale industry and which pick out the outcrop of the Oil-Shale Groups. The oil-shale was mined and burned in large retorts to extract the hydrocarbons; the red bings are made up of the burnt oil-shale. Looking to the west, black conical bings can be seen. These are the spoil from coal mining and their distribution picks out the of the Coal Measures. Note the marked cast-west valleys due to glacial scouring along fault-lines. and crag-and-tail features such as The Knock itself, which indicate west to east ice movement.

==== 7. Hilderston Mine: Silver-Lead-Zinc Mineralisation ====

Park in the lay-by for Cairnpapple at the top of the hill (NS 989 718). Immediately west of the road, the quartz-dolerite sill of localities 3 and 6 crops out forming a good N-S feature some 40 m wide. The sill dips steeply to the cast as at The Knock, and is really a dyke-like step between levels of sill. A diversion may be made at this point to the bronze-age burial mound of Cairnpapple Hill. 250 m to the south-west, which is also a good viewpoint.

Follow the sill/dyke southwards into the valley to the ruins of Windywa's which are on the site of the original 17th century Hilderston silver mine (Cadell 1925. 359-378; Stephenson 1983). The mine was in operation initially from 1606 to 1614 but made little or no profit after the first two years (i.e. after it had been 'nationalised' by King James VI). The silver occurred in a vein as filaments of native silver in a gangue of baryte and niccolite. This vein was located on the margin of a thin E-W dolerite dyke which cut sandstones and siltstones above the Petershill Limestone. The economic vein extended for only 80 m to the cast of the N-S dyke/sill and for 18 m below the surface. In the 18th century the mine was reopened and worked for lead and zinc which occurred within a baryte and calcite gangue at deeper levels, where the vein cut the Petershill Limestone. A second, much longer vein, some 60m to the north, was also worked at this time, but this phase of working ceased in 1772. The original workings were re-excavated during the period 1865 to 1873 using money from the sale of the niccolite and again from 1896 to 1898. However, no further economic deposits of silver or lead were discovered.

The full list of known minerals from the mine is: baryte, calcite. Dolomite, quartz, galena, sphalerite, niccolite, erythrite (nickel bloom), annabergite (cobalt bloom), bravoite, pyrite, chalcopyrite, albertite (solid hydrocarbon) and native silver. Most of these, with the exception of the nickel. cobalt and silver minerals, have been obtained recently from the waste heaps.

Three depressions close to the road at Windywa's mark the site of the main group of 17th century shafts. A large mound in the field east of the road marks the main 18th century shaft on the northern lead vein. The 1873 shaft is probably marked by a pile of debris, crescentic in plan, south of the burn and 35 m east of the road. Two adits in the west wall of the flooded Silvermines quarry mark two branches of the northern lead vein, probably excavated in the 19th century. If the water level is low, these adits can be examined. Do not attempt to enter them. Thin calcite veins with galena can be seen cutting the sandstone roof of the northernmost one.

The Petershill Limestone is no longer visible in the quarries, but the overlying clastic sediments can be examined in the cut to the south. Here an upward-coarsening sequence of clastic sediments can be seen with mudstone at the base of the quarry, gradually passing upwards to sandstone at the top. Fallen sandstone blocks at the foot of the quarry show good examples of trace fossils, both burrows and feeding trails. This type of upward-coarsening succession is typical of that associated with deltaic sedimentation. A thin N-S tholeiite dyke in the west wall of the southern quarry is altered to 'white trap' at its northern end.

==== 8. North Mine Quarry: Petershill Limestone, Dyke ====

Park by the roadside (NS 995 722). In the quarry, only massive sandstone is now seen, with a capping of basalt lava forming knolls in the plantation above the north face. An E-W dyke of quartz-dolerite, 30 m wide, forms a feature which is followed by the road. Just over the fence on the south side of the road, immediately east of the plantation. the southern contact of the dyke with the Petershill Limestone is exposed. The limestone contains colonies of ''Lithostrotian'' and is intensely baked. Both limestone and dolerite are impregnated with pink baryte. 'Copper ore' has also been recorded from this locality.

==== 9. Riccarton Hills: Basalt Lavas ====

From North Mine drive east, past Tartraven (small, overgrown limestone quarries). At South Mains, take the road left (north) towards Beecraigs. From this road views are seen of good trap features in basalt lavas of the Riccarton Hills to the east.

==== 10. Beecraigs Wood: Quartz-dolerite Dyke ====

Park at Beecraigs Country Park (NT 007 743) and walk south to the quarry on the west side of the road, cleared as a climbing wall. The face shows a section across an E-W quartz-dolerite dyke. The dolerite is very fresh and exhibits horizontal columnar jointing. The fine-grained edge of the dyke can be seen beside the ladder.

==== 11. Hillhouse Quarry and Mine: Hillhouse Limestone ====

Park at Beecraigs Visitors' Centre (NT 006 746), walk northwards and turn left at the T-junction. Quarries trend N-S on both sides of the road where the Hillhouse Limestone (possibly a lateral equivalent of the Petershill Limestone) has been worked by both surface and underground methods. The strata dip west at 30-40 degrees and consequently the stoop and room workings north of the road, which are still open, slope very steeply underground. Do not enter these workings. There have been roof collapses in recent years, some of which have caused subsidence in the overlying road.

The limestone is 9 to 12 m thick, the most complete section being exposed north of the road. Here the limestone is mostly massive with fossiliferous bands rich in crinoids, solitary and colonial corals and productid brachiopods. South of the road only 2 m of limestone is seen, overlain by over 2·5 m of cross-bedded sandstones and siltstones. The upper part of the section consists of a sill of very fresh basalt having well-developed. sub-vertical columnar joints. This sill is the type locality for the Hillhouse type of olivine-basalt (MacGregor 1928). The characteristic abundant microphenocrysts of olivine and augite are clearly visible on the weathered surfaces of the basalt.

{{EGwalks}}

[[category:5. Midland Valley of Scotland]]

File:P002882.jpg

2015-12-15T10:37:40Z

JenniferFindlay1: East Kirkton Quarry, near Bathgate, West Lothian. The East Kirkton Limestone in the upper part of the West Lothian Oil Shale Formation is a laminated freshwater limestone. A unique and varied terrestrial assemblage of fossils has been collected from th...

== Summary ==
East Kirkton Quarry, near Bathgate, West Lothian. The East Kirkton Limestone in the upper part of the West Lothian Oil Shale Formation is a laminated freshwater limestone. A unique and varied terrestrial assemblage of fossils has been collected from this site, including Westlothiana lizziae, a vertebrate fossil intermediate between amphibians and reptiles. The rocks are dipping at 20 to 45 degrees to the west-south-west and are between 9 and 19 m. thick.
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[[Category:License tags]]

Bathgate Hills - an excursion

2015-12-15T10:34:29Z

JenniferFindlay1:

[[File:Lothian Geology cover.jpg|thumb|100px|right|[http://www.edinburghgeolsoc.org/p_sales.html#lothiangeology Buy the book]]]
'''By D. Stephenson and S.K. Monro. From: Lothian geology an excursion guide. Edited by A.D. McAdam and E.N.K. Clarkson. 1996'''

O.S. 1:50000 Sheet 65 Falkirk and West Lothian

B.G.S. 1:63360 Sheet 31 Airdrie

B.G.S. 1:50000 Sheet 32W Livingston

=== Introduction ===
[[File:BathgateHillsExcursionMap.jpg|400px|thumbnail|Bathgate Hills - excursion map]]
The Bathgate Hills owe their general elevation to the presence of a considerable thickness (up to 500 m) of hard, resistant volcanic rocks of basaltic composition. These replace much of the normal sequence of softer, less resistant, sedimentary rocks between the top of the Upper Oil-Shale Group and the middle of the Upper Limestone Group. Individual hills, often craggy, are formed by east-west trending dykes and inclined sills of the late-Carboniferous quartz-dolerite suite. Intercalated sediments within the volcanic pile include several limestones of both freshwater and marine origin. It is difficult to correlate these limestones with the established successions in sedimentary basins outside the Bathgate Hills. The thickest and most persistent limestone-bearing strata the Petershill Formation formed around an elevated volcanic land area lying to the north of Bathgate (Jameson 1980). Lateral changes in the lithology and fauna of these strata can be traced from the seaward side northwards towards the inferred land area.

The East Kirkton Limestone, in contrast to the Petershill Limestone, is of freshwater origin but demonstrates an unusual lithology which is thought to be associated with hot springs. These beds have yielded an important and distinctive flora and fauna. The limestones of the Bathgate Hills have been extensively worked in the past for agricultural use both by quarrying and mining. An example of the latter will be seen at Hillhouse.

The aims of the excursion are: (i) to contrast the sedimentology and palaeontology of the freshwater East Kirkton Limestone with those of the marine Petershill and Hillhouse limestones; (ii) to examine the intercalated basaltic tuffs and lava flows; (iii) to examine the late-Carboniferous quartz-dolerite intrusions; and (iv) to see the remains of 17th, 18th and 19th century silver-nickel-lead workings at Hilderston Mine. In Map 27 showing the route the geology is based upon recent mapping, boreholes and air-photo interpretation. The outcrops are all easily accessible from public roads, making visits by vehicle convenient. but they may be traversed on foot, in the order described, involving a 9 km walk from the first to last outcrop or 12 km between public transport centres in Bathgate and Linlithgow, both of which are on Eastern Scottish service bus routes.

==== 1. East Kirkton Quarry: East Kirkton Limestone ====

Park at the gate to the quarry (NS 990 689).

The East Kirkton Limestone is a laterally impersistent sedimentary sequence within a thick succession of basaltic lavas and tuffs. The lithological succession consists of limestone overlain by black mudstones (Muir and Walton 1957) but is laterally variable in detail. Within the limestone occur bands with contorted laminae, tuff horizons where spherulitic structures are common, and beds of impure limestone with chert nodules. These lithologies have been interpreted as having been laid down in shallow pools associated with hot springs.

The limestones have recently yielded an unusual flora and fauna including amphibians, Millipedes, eurypterids, scorpions, the earliest known harvestman (opilionid) and much plant material (Milner 1985; Wood el al. 1985). The presence of these forms, together with a striking absence of fishes, suggests a palaeoecology where amphibians were an integral part of the terrestrial fauna. The black mudstones, lying stratigraphically above the limestone, are also associated with laterally impersistent bands of tuff and have yielded a fauna containing both ostracods and fish remains.

==== 2. Petershill Reservoir: Seaward Development of the Petershill Limestone ====

Park in rough lay-by at the bend in the road (NS 985 694). On the east bank of the reservoir, the Petershill Limestone has a high carbonate content and is only slightly argillaceous. It was originally horizontally-bedded and contains an abundant fauna of solitary corals and low mounds of ''Lithostrotion junceum'' (a colonial coral giving rise to the popular name 'spaghetti rock'). The fauna also has delicate elements, epifaunal spinose productids and echinoids, which are well preserved with little disturbance, indicating quiet sedimentation. The trace fossil ''Thalassinoides'' is characteristic of this facies.

The southern wall of the reservoir is the central portion of a bioherm, built-up of successive beds composed mostly of skeletal remains. A three-part ecological succession shows a basal, small productid-fenestellid association with few species (low-diversity), passing up into a high-diversity productid-sponge-rostroconch association, replaced towards the top by an echinoderm-dominated association. The succession can be interpreted in terms of regional shallowing with regression and eventual exposure. About midway up the southern wall of the reservoir a thin (1-2 cm) impersistent band of clay can be seen. This is a bentonite horizon, consisting largely of montmorillonite formed from a weathered local ash fall.

==== 3. Craigs: Quartz-dolerite Sill ====

Park 500 m west of Wester Drumcross (NS 995 700); roadside parking may be difficult. A quartz-dolerite sill of the late-Carboniferous suite trends NNW-SSE across the hills. Here it dips moderately steeply ENE, contrary to the regional dip of the sediments and lavas, and forms a prominent west-facing scarp with brown, spheroidal-weathering outcrops. From the north of the road the scarp can be seen trending north-west towards Raven Craig. It is cut, and in some places displaced, by E-W trending valleys, interpreted as fault-lines scoured out by glacial action. A track follows the foot of the scarp northwards and eastwards for 300 m to the ruins of Craigs and a small quarry. Here, above the sill, 4 m of ill-sorted tuffs rest on 3·5 m of pale grey, well-bedded limestone with shale partings and some tuffaceous bands. The rocks show signs of baking and disturbance by the sill, the top contact of which may be traced in outcrops NNW of the ruins. These are some of the few natural outcrops of the tuffs near the base of the Bathgate Hills volcanic sequence, which are known from boreholes to reach considerable thicknesses. Other outcrops occur at Whitelaw (NS 994 692), in a stream (NS 998 695) and in a roadside quarry (NS 996 700). All are close to the top of the sill and probably crop out owing to an increased resistance to erosion produced by contact metamorphism. Dips, where seen, tend to conform to the top of the sill, rather than to the regional dip.

==== 4. The Rifle Range Quarries: Landward Development of the Petershill Limestone ====

Park in the roadside quarry at The Knock (NS 991 711). In the Rifle Range Quarries, which extend south-west for a kilometre, the Petershill Limestone consists of heterogeneous packstones, medium-grained bioclastic limestones, located north and palaeogeographically shoreward of the laterally equivalent limestones exposed at the Petershill Reservoir (locality 2). In the lower 3-4 m of the quarry-face, cross-stratified crinoidal packstones, large cerioid coral colonies, bands of ''Gigantoproductus'' and large ''Chaetetes'' colonies can be seen. Although most of the fauna is in situ, signs of extensive hydrodynamic influence on the original life-surfaces are present throughout the sequence. Overturned corals with the open ends (calices) of the corallites downwards and coral debris, as well as separated and stacked brachiopod shells all suggest that currents were at times a major destructive force. The heterogeneous packstone facies is interpreted as having accumulated in a relatively turbulent, nearshore zone.

At the northern end of the quarries, a blue-grey basalt lava flow rests directly upon the limestone. The quarries terminate at a vertical E-W dyke of quartz-dolerite which exhibits good columnar jointing perpendicular to its margins.

==== 5. Triangulation Station (NS 987 712): Basaltic Lavas of Dalmeny Type ====

To the north-west of the north end of the limestone quarries (locality 4), several low scarp features trend NNE across the hillside as far as the triangulation point. Outcrops on these features are of a fresh, fine-grained, blue-grey basalt-the hard, central parts of lava flows. The basalts appear non-porphyritic, but in thin-section abundant 1 mm microphenocrysts of fresh olivine show them to be of Dalmeny type (MacGregor 1928). The presence of abundant interstitial analcime suggests gradation from olivine-basalt to basanite. Between the scarp features, topographic hollows with no exposure are due mainly to rubbly, altered flow tops or may in some cases indicate thin intercalations of sediment or pyroclastic horizons.

==== 6. The Knock: Quartz-dolerite Sill, Viewpoint ====

Cross the road to The Knock, where the quartz-dolerite sill seen at locality 3, trends NNW and dips steeply ENE at 60°, probably marking a step-like transgression between stratigraphic levels. The sill forms the rocky knoll of The Knock and its top surface forms the steep slab on the east side. In the back wall of the quarry on the south-west side, the lower chilled contact is well exposed where it cuts blue-grey basalt lava dipping north-west at about 20°.

From the top of The Knock the regional geology of the Bathgate Hills and that of the Midland Valley as a whole may be appreciated. The underlying bedrock of the Midland Valley shows itself in the landforms, with the harder igneous rocks forming the hills and the softer sediments the lowland. Far to the north are the metamorphic rocks of the Grampian Highlands, with the andesitic lavas of the Ochil Hills in the middle distance. To the south-east, the Old Red Sandstone sediments and lavas of the Pentland Hills can be seen with the Lower Palaeozoic rocks of the Southern Uplands due south. Eastwards most of the prominent topographic features such as Arthur's Seat, the Bass Rock and North Berwick Law, all correspond to areas of igneous rock. Also to be seen are the flat-topped, red bings which are the remains of West Lothian's oil-shale industry and which pick out the outcrop of the Oil-Shale Groups. The oil-shale was mined and burned in large retorts to extract the hydrocarbons; the red bings are made up of the burnt oil-shale. Looking to the west, black conical bings can be seen. These are the spoil from coal mining and their distribution picks out the of the Coal Measures. Note the marked cast-west valleys due to glacial scouring along fault-lines. and crag-and-tail features such as The Knock itself, which indicate west to east ice movement.

==== 7. Hilderston Mine: Silver-Lead-Zinc Mineralisation ====

Park in the lay-by for Cairnpapple at the top of the hill (NS 989 718). Immediately west of the road, the quartz-dolerite sill of localities 3 and 6 crops out forming a good N-S feature some 40 m wide. The sill dips steeply to the cast as at The Knock, and is really a dyke-like step between levels of sill. A diversion may be made at this point to the bronze-age burial mound of Cairnpapple Hill. 250 m to the south-west, which is also a good viewpoint.

Follow the sill/dyke southwards into the valley to the ruins of Windywa's which are on the site of the original 17th century Hilderston silver mine (Cadell 1925. 359-378; Stephenson 1983). The mine was in operation initially from 1606 to 1614 but made little or no profit after the first two years (i.e. after it had been 'nationalised' by King James VI). The silver occurred in a vein as filaments of native silver in a gangue of baryte and niccolite. This vein was located on the margin of a thin E-W dolerite dyke which cut sandstones and siltstones above the Petershill Limestone. The economic vein extended for only 80 m to the cast of the N-S dyke/sill and for 18 m below the surface. In the 18th century the mine was reopened and worked for lead and zinc which occurred within a baryte and calcite gangue at deeper levels, where the vein cut the Petershill Limestone. A second, much longer vein, some 60m to the north, was also worked at this time, but this phase of working ceased in 1772. The original workings were re-excavated during the period 1865 to 1873 using money from the sale of the niccolite and again from 1896 to 1898. However, no further economic deposits of silver or lead were discovered.

The full list of known minerals from the mine is: baryte, calcite. Dolomite, quartz, galena, sphalerite, niccolite, erythrite (nickel bloom), annabergite (cobalt bloom), bravoite, pyrite, chalcopyrite, albertite (solid hydrocarbon) and native silver. Most of these, with the exception of the nickel. cobalt and silver minerals, have been obtained recently from the waste heaps.

Three depressions close to the road at Windywa's mark the site of the main group of 17th century shafts. A large mound in the field east of the road marks the main 18th century shaft on the northern lead vein. The 1873 shaft is probably marked by a pile of debris, crescentic in plan, south of the burn and 35 m east of the road. Two adits in the west wall of the flooded Silvermines quarry mark two branches of the northern lead vein, probably excavated in the 19th century. If the water level is low, these adits can be examined. Do not attempt to enter them. Thin calcite veins with galena can be seen cutting the sandstone roof of the northernmost one.

The Petershill Limestone is no longer visible in the quarries, but the overlying clastic sediments can be examined in the cut to the south. Here an upward-coarsening sequence of clastic sediments can be seen with mudstone at the base of the quarry, gradually passing upwards to sandstone at the top. Fallen sandstone blocks at the foot of the quarry show good examples of trace fossils, both burrows and feeding trails. This type of upward-coarsening succession is typical of that associated with deltaic sedimentation. A thin N-S tholeiite dyke in the west wall of the southern quarry is altered to 'white trap' at its northern end.

==== 8. North Mine Quarry: Petershill Limestone, Dyke ====

Park by the roadside (NS 995 722). In the quarry, only massive sandstone is now seen, with a capping of basalt lava forming knolls in the plantation above the north face. An E-W dyke of quartz-dolerite, 30 m wide, forms a feature which is followed by the road. Just over the fence on the south side of the road, immediately east of the plantation. the southern contact of the dyke with the Petershill Limestone is exposed. The limestone contains colonies of ''Lithostrotian'' and is intensely baked. Both limestone and dolerite are impregnated with pink baryte. 'Copper ore' has also been recorded from this locality.

==== 9. Riccarton Hills: Basalt Lavas ====

From North Mine drive east, past Tartraven (small, overgrown limestone quarries). At South Mains, take the road left (north) towards Beecraigs. From this road views are seen of good trap features in basalt lavas of the Riccarton Hills to the east.

==== 10. Beecraigs Wood: Quartz-dolerite Dyke ====

Park at Beecraigs Country Park (NT 007 743) and walk south to the quarry on the west side of the road, cleared as a climbing wall. The face shows a section across an E-W quartz-dolerite dyke. The dolerite is very fresh and exhibits horizontal columnar jointing. The fine-grained edge of the dyke can be seen beside the ladder.

==== 11. Hillhouse Quarry and Mine: Hillhouse Limestone ====

Park at Beecraigs Visitors' Centre (NT 006 746), walk northwards and turn left at the T-junction. Quarries trend N-S on both sides of the road where the Hillhouse Limestone (possibly a lateral equivalent of the Petershill Limestone) has been worked by both surface and underground methods. The strata dip west at 30-40 degrees and consequently the stoop and room workings north of the road, which are still open, slope very steeply underground. Do not enter these workings. There have been roof collapses in recent years, some of which have caused subsidence in the overlying road.

The limestone is 9 to 12 m thick, the most complete section being exposed north of the road. Here the limestone is mostly massive with fossiliferous bands rich in crinoids, solitary and colonial corals and productid brachiopods. South of the road only 2 m of limestone is seen, overlain by over 2·5 m of cross-bedded sandstones and siltstones. The upper part of the section consists of a sill of very fresh basalt having well-developed. sub-vertical columnar joints. This sill is the type locality for the Hillhouse type of olivine-basalt (MacGregor 1928). The characteristic abundant microphenocrysts of olivine and augite are clearly visible on the weathered surfaces of the basalt.

{{EGwalks}}

[[category:5. Midland Valley of Scotland]]

Arthur's Seat, Calton Hill, Edinburgh - an excursion

2015-12-15T09:53:10Z

JenniferFindlay1:

== Arthur's Seat, Calton Hill, Edinburgh ==
[[File:Lothian Geology cover.jpg|thumb|100px|right|[http://www.edinburghgeolsoc.org/p_sales.html#lothiangeology Buy the book]]]
'''By G P Black. From: [[Lothian Geology: an excursion guide|Lothian geology: an excursion guide]]. Edited by A D McAdam and E N K Clarkson. 1996'''

[[Arthur’s Seat|Arthur's Seat, Edinburgh - introduction]]

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002357 BGS 1:50 000 Geological Survey of Scotland map. Bedrock]

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002355 BGS 1:50 000 Geological Survey of Scotland map. Bedrock and Superficial]

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1003916 BGS 1:25 000 series - Classical areas of British geology Edinburgh District]

The Calton Hill succession may be easily followed by ascending the steps at the east end of Waterloo Place (NT 261 741) and proceeding eastwards over the hill.

==== 32. Calton Hill: Lavas and Ashes ====

[[File:P001324.jpg|thumb|250px|right|Oblique aerial view of Calton Hill and the Arthur's Seat volcano, looking from the north-west. Calton Hill is a faulted fragment of the Arthur's Seat volcano lavas and tuffs. Glaciation from the west has left the crag-and-tail feature formed by the hard crag of Calton Hill Rock and the tail of Royal Terrace stretching eastwards. [http://geoscenic.bgs.ac.uk/asset-bank/action/viewAsset?id=2621 P001324]]]At the base of the succession alternations of ash and sediment occur. These represent the two ash beds of the Dry Dam, for the equivalents of Lavas 1 and 2 of the Whinny Hill are not found on the Calton Hill. Above the ashes lie two lavas of Craiglockhart basalt, the lower closely resembling Lava 3 of the Whinny Hill. Lavas 4,5,6 and 7 of the Whinny Hill are not found, their temporal equivalent being a thick bed of ash containing an unsorted assemblage of boulders, blocks and bombs of basalt. Above this there lies a group of three lavas of Markle basalt separated by thin ash-beds, the local equivalents of Lavas 8, 9 and 10 of the Whinny Hill. On the highest Markle flow lies another ash-bed which, in turn, is overlain by a group of three mugearite lavas, again separated by ash-beds; these flows are the approximate equivalents in time of Whinny Hill Lavas 11 and 12. The highest mugearite lava of the Calton Hill is directly covered by the Abbeyhill Shales.

{{EGwalks}}

[[category:5. Midland Valley of Scotland]]

Girvan and Ballantrae - an excursion

2015-12-09T13:51:11Z

JenniferFindlay1:

By P Stone. Excursion 8. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Girvan and Ballantrae: an obducted ophiolite ==

OS 1: 50 000 Sheet 76 Girvan

BGS 1: 50 000 Sheet 7 Girvan, 8W Carrick and 14W Ayr

BGS 1: 25 000 Sheet IVX08, 18 and 19 (in part)

=== Main points of interest ===
Early Ordovician Ballantrae ophiolite Complex (pillow lavas, gabbro, serpentinite, chert and melange), late Ordovician and early Silurian greywacke and conglomerate; late Ordovician reef limestone, Caledonian folding.

=== Logistics ===
There is ample parking for cars and space for a coach near most localities. Most of the exposures are on the coast and require much scrambling over steep and sometimes slippery rocks. Maximum walk at any one locality is about 2.5 km along beach and cliff paths. The outcrops are best seen at low tide and ideally the excursion should be planned to coincide with low water at about 1pm; this should allow adequate access from 9am to 5pm. Most of the localities described have been designated Sites of Special Scientific Interest (Appendix 2).

=== Introduction ===
[[File:GirvanBallantraeExcursionMap.jpg|400px|thumbnail|Locality map for the Girvan-Ballantrae excursion and outline geology of the Ballantrae Ophiolite Complex.]]
The ophiolitic Ballantrae Complex and its sedimentary cover are well exposed between Girvan and Ballantrae. The area has been a focus of considerable geological research (summarised by '''Stone and Smellie, 1988'''). The geological complexity of the area is such that several days are required to examine all of the varied features exposed. Thus, previous excursion guides have included a three-day itinerary ('''Robertson et al., 1990''') and the seven itineraries recently published by '''Bluck and Ingham (1992)'''. The field excursion described below is intended to bring out the main features of the complex and its unconformable sedimentary cover in a single day. Those with more time available are referred particularly to '''Bluck and Ingham (1992)'''.

The ophiolite complex consists of two main elements interleaved by faulting; serpentinised ultramafic rocks representing oceanic mantle and volcanic sequences representing the remains of island arc and ocean crust. The structure is dominated by NE–SW faults which divide the complex into discrete lithological zones such that northern and southern serpentinite belts separate three areas of mainly volcanic rock '''(Figure 30)'''. A late Tremadoc to early Arenig age has been established for the eruption of the volcanic components (Balcreuchan Group); the interbedded sedimentary strata contain graptolite faunas of that age ('''Stone and Rushton, 1983; Rushton et al., 1986)''' and Sm-Nd radiometric dating of the basalts has given ages of 501 ± 12 and 476 ± 14 Ma ('''Thirlwall and Bluck, 1984)'''. It is believed that the ophiolite was obducted on to the continental margin during the middle Arenig, since metamorphic rocks formed at this stage have been dated by the K-Ar method at 478 ± 8 Ma ('''Bluck et al., 1980'''). Some late Arenig sedimentary rocks, probably deposited during the final stages of obduction, are structurally included within the ophiolite ('''Smellie and Stone, 1992''') and the oldest strata within the unconformably overlying cover sequence (Barr Group) are of Llanvirn age. Sedimentation above the ophiolite was controlled by a series of faults, downthrowing to the south and sequentially stepping back northwards. Thus the basal conglomeratic facies becomes progressively younger northwards; in the south the basal conglomerate is Llanvirn, in the north Caradoc ('''Williams, 1959'''; '''Ince, 1984'''). Above the basal conglomerate, which is commonly associated with shallow-marine limestone, facies become progressively more deep water upwards so that turbiditic greywackes and shales form much of the exposed sequence. This is more or less continuous up into the Silurian, with only a slight stratigraphical break and small angular discordance in bedding at the Ordovician–Silurian boundary. The main tectonism occurred late in the Silurian and produced folding and north-directed thrusts.

The ophiolite complex is examined at Localities 1 to 4, and the sedimentary cover at Localities 5 to 7. Locality 7 should be regarded as an alternative stop in a single day excursion but could be included in a more leisurely 2-day schedule. Since many of the coastal exposures are tide-dependent (local details given below) the order in which localities are visited may have to be varied to suit conditions. '''Tide tables '''should be '''carefully checked '''and the excursion planned accordingly.

===Excursion===
==== 1 Balcreuchan Port: structural imbrication in Arenig lavas and lava breccias ====
[[File:GirvanBallentraeBalcreuchanPointExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for Balcreuchan Port (Locality 1) and the coastal section to the south-west.]]
A convenient place to begin the excursion is the large lay-by overlooking Balcreuchan Port on the seaward side of the A77 (Figure 31) (NX 100 876). There is a fine view over the Firth of Clyde towards Arran and to Ailsa Craig, a spectacular Tertiary micro-granite plug ('''Harrison et al., 1987''').

From the car park descend into Balcreuchan Port by the steep footpath on the east side of the cove. Take great care, the slopes are very steep so do not leave the footpath. However, note the cliffs to the south and east; pillows of basalt lava can be seen on both sides.

At beach level there is extensive intertidal rock outcrop. Prominent here is a Tertiary basalt dyke, up to about 50 cm across and trending generally north across the foreshore. The dyke has resisted erosion and now stands proud of its host rock. Abundant amygdales are concentrated into zones parallel to the dyke margin and are normally restricted to only one side of the dyke.

The Tertiary dyke is intruded into highly altered ultramafic rock which at this locality consists largely of a mass of secondary quartz and carbonate veins. Such alteration is fairly common at the margins of serpentinised ultramafic bodies and is generally regarded as a side effect of the serpentinisation process. A north–south fault marks the east margin of Balcreuchan Port, beyond which the steep sea cliffs are formed by basalt lavas, both massive and pillowed. The petrographical and geochemical characteristics of the volcanic rocks are typical of lavas erupted in oceanic island arcs above subduction zones (Thirlwall and Bluck, 1984; Smellie and Stone, 1988 and references therein). Most of the sequence is tholeiitic but it includes some boninitic lavas with exceptionally high contents of Cr and Ni. This lava variety is relatively rare and modern examples are found exclusively in oceanic island arcs ('''Smellie and Stone, 1992'''). Within the lava sequence a cave, controlled by minor faulting, is reputed to have been the home of Sawney Bean and his family, the notorious 16th century cannibals.

From the cave and the adjacent boninitic lavas cross SW towards the opposite side of the bay. About two-thirds of the way across '''(1a on Figure 31)''', serpentinised ultramafic lithologies are exposed below mid-tide level: these are dunite (almost entirely olivine) and harzburgite containing both olivine and orthopyroxene. The pyroxene can be seen as bronze-coloured flecks in the background of dark green serpentinised olivine. A gently inclined contact, possibly thrust, separates the ultramafic rocks from the overlying lavas and lava breccias forming the SW headland of Balcreuchan Port. The route continues SW over the headland, an easy scramble at mid to low tide but quite difficult at high tide, and across the next small bay to the cliffs on its far side. A prominent fault gully trends south (inland) from this point and its western side is formed by steeply dipping clastic sedimentary strata; note the marked swing in strike adjacent to the fault. Sandstones and melange-like breccia (probably formed by slumping) make up most of the sequence, but an important intercalation of laminated red and cream fine-grained sandstone '''(1b) '''can be seen at the west end of the rock platform (just above low water mark). This laminated bed is important on several accounts, not least for the contained graptolite fauna which establishes an early Arenig age for this part of the sequence ('''Stone and Rushton, 1983'''). The lithology is distinctive, making it a readily identifiable marker horizon, and its sedimentological features allow the younging direction to be established. Check the layering carefully; it is cut out westward by a coarse feldspathic sandstone which is in turn overlain by a repeat of the red and cream striped lithology, but this time in a jumbled, chaotic form. A likely interpretation is that a channel was eroded into the striped sandstone and partially filled by the coarse feldspathic sandstone; the channel walls then collapsed to give the chaotic deposit. The younging direction is clearly to the west.

[[File:P005992.jpg|thumb|300px|left|Between Balcreuchan Port and Port Vad, near Ballantrae. Basalt pillows. Large pillows of reddened, feldspar-phyric spilitic basalt. P005992]]Rejoin the footpath above these crags and continue west for a few metres to the next prominent outcrop. This is formed of reddened basaltic pillow lavas rich in feldspar phenocrysts and, at the east side of the outcrop, a conformable relationship can be established between the lava and fine-grained sandstone. Bearing in mind the younging direction established earlier, the lava almost certainly overlies the sandstone. These lavas, and those to be examined subsequently towards Port Vad and Bennane Head, have the geochemical characteristics of oceanic island or hot-spot lavas similar to the modern example of Hawaii. Such a major change in lava type emphasises the importance of the fault to the east of locality lb.

For the next few tens of metres westward the feldsparphyric pillow lavas are well exposed, particularly on the flat surfaces overlooking the sea. An approximate north–south strike of steeply inclined bedding can be readily established, and a continuation of the westward younging confirmed, from the shape of the pillows. These have smooth convex upper surfaces but more irregular lower surfaces which bulge and drape into underlying cavities. Note the red chert filling spaces between some of the pillows. The feldsparphyric pillow lavas are exposed on the next rocky headland (1c) but there they are cut by dykes of fine-grained basalt, best seen on the seaward end of the headland. The dykes are taken to be feeders for the next higher unit of pillow lavas because these have the same aphyric composition.

The contact between the two lava types is exposed in a fault gully about 15 m farther west (1d). A deep cleft is open to the sea but at the narrower, inland end the rock in the gully walls can he examined. '''Take great care; the sea is a long way down and the exposure is '''precarious. The eastern side of the gully is formed of feldsparphyric lava and the lowermost pillows in the western wall are also feldsparphyric. However, these are conformably overlain by aphyric pillow lavas, which then form the sequence continuing westward.

Continue west on the footpath, crossing faulted and brecciated aphyric lavas, towards the mouth of the Bennane Burn. Stratified, fine-grained elastic rocks overlying the brecciated lavas, are best examined beside a sea-water pool on the SW side of the burn ('''1e'''). Interbedded sandstone, chert and dark shale form a small cliff; the shale has yielded graptolites of middle Arenig age ('''Stone and Rushton, 1983)'''. Thus, the traverse has passed from the lower to the middle Arenig, confirming the sedimentological evidence for westward younging.

From the sea-water pool climb inland for a short distance up a cattle track into a shallow NW–SE gully with rock forming low crags along its SW side. These crags expose a familiar sequence: reddened feldsparphyric pillow lavas conformably overlie fine-grained elastic strata including a red and cream striped sandstone remarkably similar to the early Arenig example seen at Locality ('''1b'''). The feldsparphyric lavas are well exposed to the SW and provide abundant evidence of continued and consistent steep dip and SW-younging. Cheri and siliceous sandstone interbeds occur at intervals and from one of these an early Arenig graptolite fauna has been collected ('''Stone and Rushton, 1983'''). The comparison with the sequence traversed at Localities lb to Id is then further strengthened by the appearance of aphyric pillow lavas above and to the SW of the feldsparphyric pillows. The aphyric pillows are well exposed in landward-facing cliffs at the margin of a small embayment about 200 m SW from Bennane Burn ('''1f''') but from there the coastal cliffs become impassable. A major fault repeating the succession seems probable and the most likely site is the NW–SE gully followed by the cattle track at Locality le above Bennane Burn.

Return to the car park above Balcreuchan Port and proceed to Locality 2, where the southward continuation of the section can be seen.

==== 2 Bennane Lea: Balcreuchan Group (Arenig) conglomerate and chert ====
South from Balcreuchan Port the A77 has been re-routed inland for about 2 km to Bennane Lea. Some parking space is available on the seaward side of the road where it rejoins the coast and the extensive raised beach (NX 092 858). Vehicles should be left outside the cattle grid; access to the beach section is via a small sand pit. The old A77 road is now a private access route and should not be used. Further details and a full description of the sections are also provided by '''Bluck (1978)''' and in '''Bluck and Ingham (1992).'''

Exposures of Permian red sandstone beds, dipping gently south, can be examined at low tide. These lie at the edge of an extensive offshore Permian basin, the faulted eastern margin of which runs beneath the raised beach between Bennane Lea and Ballantrae. However, locally at Bennane Lea the Permian strata are unconformable on the Ballantrae Complex and the basal red sandstone contains clasts of spilitic lava ('''Stone, 1988''').

There is a marked topographical change at Bennane Lea: steep sea cliffs to the north contrast with the raised beach, backed by relic sea cliffs cut in glacial till, to the south. [[File:P219962.jpg|thumb|300px|right|Coast N. of Ballantrae, Bennane Lea. Tight, upright folds in bedded chert. The hinge of a synform is well exposed in the smaller stack on the right of the photograph. P219962]]The change takes place across a faulted junction between basaltic lava and breccia, to the north, and less-resistant ultramafic rock of the Southern Serpentinite Belt, to the south. The ultramafic rock exposed on the shore is altered and reddened, it also contains pods of gabbroic composition which are probably tectonic inclusions. The fault itself is exposed on the foreshore at Bennane Lea (subject to the vagaries of drifting sand) as a thin zone of silicification, north of which a massive tuff unit forms the first rocky outcrop. Traversing northwards a conformable contact between the tuff and underlying thinly bedded cherts can be seen. The cherts themselves contain altered radiolaria and are chaotically deformed into small-scale, disharmonic structures which seem most likely to be the result of soft sediment deformation through slumping. Slightly farther north, mass-flow conglomerates are interbedded with the cherts; the pebbles and cobbles present are mainly of spilitic lava and all can be related to lithologies exposed elsewhere in the Ballantrae Complex. Pale blocks, seemingly of limestone, at first appear out of place but contained chrome spinel grains suggest that they are likely to have originated as ultramafic rock. Alternations of bedded chert and conglomerate continue north for about 100 m, folded about several large, upright hinges which plunge steeply seawards. These are tectonic structures and can be correlated with the large anticlines and synclines clearly visible in the steep cliffs on the inland side of the old A77 road. The steep hinge plunges may bring down to the beach exposures the highest stratigraphical levels preserved, i.e. the mass flow conglomerates which are certainly not present in any of the inland outcrops.

Farther along, to the north of Bennane Cave, discontinuous layers of coarse feldspathic sandstone are interbedded with the chert. At one important exposure (NX 0909 8627) such sandstone locally forms about 30 per cent of the sequence; close by and slightly north, black siliceous mudstone, locally stained green by secondary copper minerals, is exposed between large boulders. There is a marked change of strike in this vicinity, and evidence for much minor faulting, but there is a consensus that the black mudstone stratigraphically underlies the chert and sandstone ('''Bluck and Ingham, 1992'''; '''Stone and Smellie, 1988''', table 5). Slightly farther north, and lower in the sequence, similar siliceous mudstone layers are interbedded with basalt lava and breccia. A graptolite fauna recovered from these mudstones gave a middle Arenig age ('''Stone and Rushton, 1983'''), similar to that obtained at the sea-water pool (Locality le) in the Balcreuchan Port traverse. Fracture planes within the black mudstone may be coated with green, secondary copper minerals such as malachite.

North towards Bennane Head the rock exposed at sea level for the first few hundred metres is predominantly basalt lava but the cliffs inland of the old A77 consist mainly of basalt breccia which extends seaward to form the steep cliffs of Bennane Head itself. The breccia is believed to overlie the middle Arenig sequence seen at Locality le ('''Stone and Smellie, 1988''', table 5). It can be most readily examined by continuing north for about 300 m to the southern flanks of Bennane Head (NX 091 865) where it includes beds of coarse-grained sandstone. However, the general lithology can be examined in the abundant large loose blocks which surround the black siliceous mudstone exposures. This completes the section from Balcreuchan Port to Bennane Lea, a traverse from island arc lavas into and through an oceanic island volcano-sedimentary assemblage. Return along the beach to the parking area.

==== 3 Carleton Fishery: ultramafic rock and altered dolerite ====
The next locality, Carleton Fishery, is about 5 km north along the A77. Ample parking is available in the well-signposted picnic site (123 894). Walk east for about 100 m to the old black boathouse and descend to the beach on the NE side of the rock outcrop.

The outcrop is within the Northern Serpentinite Belt and ultramafic rock, mainly dunite, is exposed between the loose boulders and shingle. Locally, layered relationships on a centimetre scale are developed between the dunite and coarser-grained, pyroxene-rich harzburgite. The main mass of the outcrop is composed of dolerite, which was probably originally intrusive into the dunite. It has suffered extensive calcium metasomatism and this has produced the fine-grained, flinty appearance of the marginal dolerite which is now composed of an assemblage of calcium-rich secondary minerals (e.g. prehnite, pectolite, hydrogrossular) known as rodingite. Large relict feldspar phenocrysts can be seen in some parts of the dolerite, and a large ultramafic enclave forms an eroded hollow on the top of the main rock mass. Another feature of interest is the network of thin (< 1.5 mm) chrysotile asbestos veins, seen in the ultramafic rock towards low water mark on the north side of the outcrop. The veins are mostly developed adjacent to the dolerite and parallel to its margins. Return to the parking area at the Carleton Fishery picnic site.

==== 4 Bonney's Dyke and Pinbain Beach: gabbro pegmatite, Balcreuchan Group melange and breccia ====
[[File:P556396.jpg|300px|thumbnail|left|Bonney's Dyke (pegmatitic gabbro sheet intruded into serpentinite) Ballantrae. Contact with serpentinite (darker rock at base). P556396.]]
About 2.5 km north from Carleton Fishery is a locality known as 'Bonney's Dyke'. Parking is available in a rough lay-by on the seaward side of the A77 (136 910).

Bonney's Dyke is a term used in the geological literature for a mass of pegmatitic gabbro within the Northern Serpentinite Belt. The name derives from Professor T G Bonney, the eminent Victorian mineralogist who recognised the igneous origins of much of the Ballantrae Complex ('''Bonney, 1878'''). When approached from the south the gabbro stands out as a paler, more resistant body within the ultramafic rock. It is exposed just above high water mark and in the intertidal zone. The outcrop is slightly arcuate owing to the cumulative effect of minor sinistral wrench faults. The pegmatitic texture is spectacular, with plagioclase and altered clinopyroxene crystals up to 3 cm across within zones of marked grain-size variation. There is no sign of chilling against either the surrounding ultramafic rock or the numerous serpentinite xenoliths. Three types of marginal contact relationship are seen:

* sharp gabbro-serpentinite contacts;

* less well-defined margins where the gabbro is in contact with coarse pyroxenite veins;

* sheared margins that are fine grained, flinty and particularly intensively Cametasomatised (rodingitised).

The south side of Bonney's Dyke shows a combination of types 1 and 2 whereas the north side is principally a sheared contact of type 3.

Pyroxenite veins occur intermittently throughout the ultramafic outcrop; they are pale green, coarse grained and up to about 50 cm across. The coarsest developments contain pyroxene crystals several centimetres across and can be seen slightly to the south of the gabbro towards low water mark.

From Bonney's Dyke walk north along the shore for about 350 m towards Pinbain Bridge. Several low intertidal outcrops expose fine-grained serpentinised harzburgite, although the movement of the beach sands may occasionally obscure them. Alternatively, drive up to a small lay-by on the seaward side of the road at Pinbain Bridge (NX 137 913). The Pinbain beach section, also described by '''Bluck (1978''') and in '''Bluck and Ingham (1992'''), exposes the contact between the Northern Serpentinite Belt and the Pinbain volcano-sedimentary sequence. A Tertiary dyke trending approximately east-west is intruded along the contact and has baked the adjacent serpentinite so that it now stands out as the more resistant lithology, a reversal of the normal situation. North of the dyke, a mélange deposit is well exposed in the intertidal zone; within its foliated muddy matrix are clasts of basalt and rarer amphibolite and schist. Prominent large, pale grey, carbonate blocks are not of organic or even sedimentary origin; residual grains of chrome spinel within the carbonate reveal that they are altered ultramafic rocks. The origin of the mélange deposit was probably by mass flow but much of the foliation through the matrix may have been imparted by subsequent tectonic shearing.

The highest point within this sequence of rocky outcrops consists of brecciated spilitic pillow lavas. Many of the pillows retain their shape despite pervasive cracks but others have completely disaggregated. There is no fine-grained matrix with this deposit and it is uncertain whether it forms a discrete unit interbedded with the mélange or whether it is part of a very large clast contained within the mélange. The latter lithology is also exposed to the north of the breccia but there the proportion of clasts is higher, and the foliation less marked, than in the exposure to the south.

Farther north a complex fault zone reintroduces tectonic slivers of serpentinite into the section. These are not exposed at beach level but form the cliff behind the isolated raised beach inland of the A77 where some dunite contains large pods of chrome spinel. The main mass of steep cliffs and rocky coastal outcrop to the north is composed of volcaniclastic sandstone, the faulted contact with the serpentinite slivers being intruded by Tertiary dykes. These are exposed at beach level immediately south of the sandstones. The latter are at the base of a thick sequence of lavas and elastic sedimentary rock which is exposed continuously for some distance north along the coast. Graptolites have been recovered at the base of the sequence from siltstones which are exposed on the inland roadside of the A77 behind the crash barriers. An early Arenig age was deduced by Rushton et al. (1986) but the specimens are generally fragmentary, very scarce, and only recovered with much patient effort.

==== 5 Kennedy's Pass: Caradocian Kilranny Conglomerate and Ardwell Formation ====
[[File:GirvanBallentraeStratigraphicalRelationships.jpg|300px|thumbnail|Schematic illustration of stratigraphical relationships above the Balantrae Complex south of Girvan. Northwards marine transgression was controlled by a sequence of faults (developed in the order 1 to 3 etc.) throwing down to the south.]]
Drive about 2.5 km north from Pinbain to Kennedy's Pass. Ample parking is available in a lay-by on the seaward side of the road (NX 149 932).

This locality allows examination of some of the late Ordovician strata which form an unconformable cover to the Ballantrae ophiolite. It is also described in an extensive field itinerary for the cover sequence given in '''Bluck and Ingham (1992)'''. The full cover sequence records north-westward marine transgression, from the late Llanvirn onwards, across the obducted ophiolite. Sedimentation was controlled by a series of faults throwing down to the south but becoming sequentially younger to the north. The overall geometry is summarised in Figure 32, based largely on the work of '''Williams (1959'''; cf. '''Ingham, 1978''' and '''Ince, 1984'''). The Barr Group rests unconformably on the ophiolitic rocks and continues up to the base of the Caradoc, whence the conformably succeeding Ardmillan Group ranges up to the high Ashgill.

[[File:P220329.jpg|thumb|300px|left|Kennedy's Pass, coast SW from Girvan. The Kilranny Conglomerate. Note inclined bedding defined by pebble imbrication. Late Ordovician. P220329]]At Kennedy's Pass the Kilranny Conglomerate is well exposed in the sea cliffs below the parking area. The Barr Group and the unconformable base of the sequence are here both faulted out and the Kilranny Conglomerate, low in the Ardmillan Group, is the lowest unit exposed. However, in lithology it is typical of the Barr Group conglomerates (e.g. Benan Conglomerate) which rest unconformably on the ophiolite elsewhere.

[[File:P556403.jpg|thumb|300px|right|Ardwell Farm Formation, Kennedy's Pass, Girvan. Folding (thought to be sedimentary - slump induced?) in flaggy sandstone/siltstone. P556403]]Overall the Kilranny Conglomerate is crudely stratified with both clast- and matrix-supported lithologies present. Some beds show clast imbrication and from this can be deduced a palaeocurrent flow from the north. Clasts range up to 1 m across and include red chert, basalt and gabbro (probably derived from the underlying ophiolite) and abundant felsitic and granitic rocks. Prominent amongst the latter are clasts of pink granite which have been dated by the Rb-Sr method at about 470 Ma (Longman et al., 1979). This suggests that intrusion occurred only a relatively short time before deposition (early Caradoc is approximately 455 Ma) and so rapid uplift and erosion of the source hinterland seem likely.

The conglomerate beds become younger northwards, and near the north end of the cliff exposure they are unconformably overlain by thinly bedded siltstones and greywackes of the Ardwell Formation. These are turbidites and appear to fill a channel eroded into the top of the underlying conglomerate. They are best examined about 100 m farther north where wave-polished surfaces reveal good examples of graded bedding and fine lamination. However, the most striking aspect of this section is the spectacular development of large and small chevron 'box' folds. Northwards from Kennedy's Pass the fold hinges trend approximately NE but plunge is variable; the southernmost examples plunge gently SW but, farther north, plunge passes through the horizontal and progressively steepens to about 45° NE. There is some controversy over the origin of these folds, which have been variously described as products of late Caledonian tectonism or as slump folds produced by the downslope movement of unconsolidated sediment: the former interpretation seems the most likely.

==== 6 Cow Rock, Horse Rock and Craigskelly: contrasting early Llandovery conglomerates ====
Drive north from Kennedy's Pass for about 5 km to a large car park beside the beach on the southern outskirts of Girvan (NX 182 964). There are toilet facilities here.

[[File:P005980.jpg|thumb|300px|left|Cow Rock, by ruins of the Haven Hotel, 1 km. S. of Girvan. Quartz-conglomerate (Silurian) channelled into slumped, laminate mudstones. Here seen disrupted and injected by the conglomerate. P005980]]From the car park walk south along the beach on the seaward side of the Ainslie Manor Nursing Home (formerly the Haven Hotel). This section also forms part of an extensive itinerary in '''Bluck and Ingham (1992)'''. Outcrops between the beach and the Nursing Home consist of a coarse, mainly matrix-supported conglomerate containing abundant quartz pebbles and siltstone clasts, many quite angular. A small proportion of metamorphic lithologies is also present. The bed is known informally as the `quartz conglomerate' and is stratigraphically a part of the Scart Grit. Southwards the number and size of siltstone clasts increases and at Cow Rock, NW of the Nursing Home, the base of the conglomerate can be seen (at low tide) to channel into an underlying siltstone and fine greywacke sequence. This is the Woodland Formation, which contains a sparse shelly fauna of Llandovery age ('''Cocks and Toghill, 1973'''). The topmost few metres of the Woodland Formation, beneath the quartz conglomerate, are much disturbed by slumping.

[[File:P005981.jpg|thumb|300px|right|Horse Rock, by ruins of the Haven Hotel, 1 km. S. of Girvan. Craigskelly conglomerate (Silurian) with interbedded channel-form turbiditic beds. P005981]]The Woodland Formation underlies the small sandy beach extending for about 30 m SW towards the next rock outcrop, the Horse Rock. This is also formed of conglomerate but of a very different character to that previously seen. At the Horse Rock, and also on Craigskelly which can be reached at low tide, the Craigskelly Conglomerate is well exposed as a polymict and clast-supported lithology. It contains rounded pebbles of acid and basic igneous rock, some metamorphic fragments, jasper and clastic turbidite strata. The beds are quite thick, reaching about 8 m on Craigskelly, but at the Horse Rock the conglomerate is interbedded with turbidite greywacke units up to about 50 cm thick. The base of the Craigskelly Conglomerate is exposed on the SW side of the Horse Rock and may be seen at low tide subject to the vagaries of the shifting beach sand. An unconformable but sharp planar contact occurs between the conglomerate and the underlying thin greywacke and shale beds of the Shalloch Formation. The latter is of mid-Ashgill (Ordovician) age and the Craigskelly Conglomerate is taken to mark the base of the Silurian.

Palaeocurrent evidence, deduced from clast imbrication and bottom structures, indicates that the Craigskelly and quartz conglomerates were both derived from the NW. The cause of the abrupt change in character of the source terrane during the early Silurian is a matter of speculation.

Localities 1 to 6 will provide a full day's excursion and cover many points of interest within the Ballantrae Complex and its sedimentary cover. However, many of the exposures require examination at low tide and an inland site, Locality 7, is suggested as a partial alternative if the tides are unfavourable.

==== 7 Craighead Quarry: Caradocian Reef Limestone ====
Craighead Quarry exposes Caradocian reef limestone overlying lavas of the Ballantrae Complex and is a Site of Special Scientific Interest for botanical as well as geological reasons. It is described as part of an extensive excursion itinerary for the Craighead Inlier in '''Bluck and Ingham (1992)'''.

From Girvan drive north on the A77 towards Ayr and then turn right on to the B741 and continue for about 5 km. At Low Craighead Farm turn left; there is parking space for 3 or 4 cars in the entrance of the track leading to a disused quarry about 200 m beyond the farm on the left of the road (NX 235 014). Take care not to obstruct the adjacent farm track. Access to the quarry is via the track and thence by a footpath on the right which leads down on to the quarry floor. The footpath is frequently overgrown and may not be obvious.

The quarry walls expose parts of a late Ordovician limestone reef assemblage (Craighead Limestone) stratigraphically equivalent to a level within the Ardwell Formation '''(Figure 32)'''. On the NW side of the quarry a dark mass of spilitic lava can be seen overlain by a limestone breccia containing much algal debris and algal-cemented basalt clasts. The lava is thought to be an inlier of the Ballantrae ophiolite seen at Localities 1-4. The stratigraphical relationships, with Caradocian reef limestone overlying Arenig ophiolite lava, continue the trend of north-westward transgression discussed earlier. The eastern walls of the quarry reveal a variety of reef-flank limestone types containing abundant, although mainly broken, fossils; corals and crinoids are the commonest groups. There is abundant evidence for slumping. The sequence within the quarry is much disrupted by faulting, probably related in the main to Carboniferous movement on the nearby Kerse Loch Fault.
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[[Category:6. The South of Scotland]]

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|description={{|1=Lower Silurian, Craigskelly Conglomerate Formation, as exposed on the coast of the Firth of Clyde at Horse Rock, south of Girvan. Ailsa Craig on the horizon (P005981). From: Plate 12 in Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland (Fourth edition). (Keyworth, Nottingham: British Geological Survey.)}}
|date=2015-07-26 17:59:02
|source=British Geological Survey
|author=British Geological Survey
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{{BGS}}

[[Category:Uploaded with UploadWizard]]

File:P005980.jpg

2015-12-09T13:29:37Z

JenniferFindlay1: Cow Rock, by ruins of the Haven Hotel, 1 km. S. of Girvan. Quartz-conglomerate (Silurian) channelled into slumped, laminate mudstones. Here seen disrupted and injected by the conglomerate.

== Summary ==
Cow Rock, by ruins of the Haven Hotel, 1 km. S. of Girvan. Quartz-conglomerate (Silurian) channelled into slumped, laminate mudstones. Here seen disrupted and injected by the conglomerate.
== Licencing ==
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File:P556403.jpg

2015-12-09T13:23:37Z

JenniferFindlay1: Ardwell Farm Formation, Kennedy's Pass, Girvan. Folding (thought to be sedimentary - slump induced?) in flaggy sandstone/siltstone.

== Summary ==
Ardwell Farm Formation, Kennedy's Pass, Girvan. Folding (thought to be sedimentary - slump induced?) in flaggy sandstone/siltstone.
== Licencing ==
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File:P220329.jpg

2015-12-09T13:20:48Z

JenniferFindlay1: Kennedy's Pass, coast SW from Girvan. The Kilranny Conglomerate. Note inclined bedding defined by pebble imbrication. Late Ordovician.

== Summary ==
Kennedy's Pass, coast SW from Girvan. The Kilranny Conglomerate. Note inclined bedding defined by pebble imbrication. Late Ordovician.
== Licencing ==
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The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
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====Warranty====
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[[Category:License tags]]

Gatehouse of Fleet - an excursion

2015-12-09T12:59:25Z

JenniferFindlay1:

By J A Weir. Excursion 12. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Gatehouse Of Fleet: structural complexities of the Hawick Group ==
OS 1:50 000 Sheet 83 Newton Stewart & Kirkcudbright

BGS 1:50 000 Sheet 4E Wigtown

=== Main points of interest ===
Sedimentology and structure in the late Llandovery Hawick Group, polyphase deformation, late Caledonian porphyry and felsite dykes.

=== Logistics ===
The area may be approached either from Gatehouse in the east or from Creetown in the west via the A75 road and the excursion may be started or curtailed at Ravenshall Point, Mossyard Farm or Low Auchenlarie Farm. Note that account has to be taken of the state of the tide at Auchenlarie, Mossyard and the Newton shore. The likelihood of congestion in the Mossyard Farm car park during the holiday season may also have to be considered. Localities visited are situated along the coast between the Ardwall shore (NX 585 545), on the Fleet estuary 2 km west of Gatehouse, and Ravenshall Point (NX 523 523) on the east side of Wigtown Bay. Exposures are in the wave-cut platform, intertidal skerries and cliffs. Nearby cuttings along the main A75 Carlisle–Dumfries–Stranraer road are informative but are not now recommended for examination due to the hazard of high-speed traffic. On a clear day the A75, which follows the coast a short distance inland, gives good views of the Cumbrian mountains and the Isle of Man.

=== Introduction ===
The eastern coastline of Wigtown Bay provides good sections through the Cairnharrow and Kirkmaiden formations of the late Llandovery Hawick Group. The succession consists of greywackes which are typically poorly graded and bedded in units mostly less than 1 m thick. Fine arenite forms up to 90 per cent thickness of each unit and grades upwards into pelite; Tabde (Figure 6) cycles predominate. Grading is expressed mainly in the progressive upward intensification of cleavage into the finer-grained intervals. A high quartz content commonly imparts a vitreous lustre to fresh surfaces. Calcareous nodule trails parallel to the bedding are dispersed through many of the thicker arenite intervals, reflecting a general abundance of secondary carbonate within the Hawick Group as a whole. Sole markings are well developed in the Cairnharrow Formation, but are scarce in the Kirkmaiden Formation. They are predominantly non-directional drag marks, with small directional flute casts, longitudinal ridge casts and prod marks. Directional sole markings denote palaeoflow towards the WSW, the prevalent trend within the area and subparallel to the regional fold plunge. Deposition in an intermediate to distal trench, or lateral lobe of a submarine fan, is suggested (Weir, 1974).

The strata have been subjected to a continuity of deformation in which two stages, designated `main' and late' (cf. Needham, 1993), are prominently developed. An `early' soft-sediment deformation is sparingly represented. The main-stage folds are of two contrasting styles: NW-facing homoclinal zones of steeply inclined bedding up to 1 km and more wide (`steep belts'), which alternate with comparably wide tracts of dominantly open, upright buckle folds; where asymmetrical these latter structures are SE-verging. Adjacent structurally contrasting belts are separated by strike faults downthrowing SE (Weir, 1968; 1979). Evidence from elsewhere in the outcrop of the Hawick Group suggests that the strike faults, though now steep, originated as thrusts over-riding towards the SE, and were subsequently steepened and back-rotated. Anticlines are commonly faulted along their axial surfaces, suggesting that the folds grew ahead of the developing thrusts, ultimately to be cut by them. The SE-downthrow of the strike faults, in combination with the NW-facing of the steep belts, has led to duplication of the succession, and there are no gross changes in stratigraphical horizon across the 10 km-wide outcrop of the Cairnharrow and Kirkmaiden formations (Craig and Walton, 1959). The main cleavage in the pelite intervals is steep, penetrative and slaty, with development of new sericite; an equally steep but irregular, non-penetrative and spaced cleavage occurs in the arenites. The cleavage is rotated by up to 15° clockwise relative to fold axial surfaces indicating continuous deformation under a sinistral transpressive regime. This also resulted in the reactivation of at least some of the rotated main strike faults with a sinistral strike-slip displacement (Stringer and Treagus, 1980).

Late-stage structures are more variable. Open folds with wavelengths of a few metres, asymmetrical with respect to dip though symmetrical in relation to limb lengths, verge NW and affect the steep belts. Minor NW-directed thrusts are associated with small recumbent folds which refold the main folds; an associated flat-lying crenulation cleavage displaces the main cleavage. This episode postdates the sinistral transpressive phase and is likely to relate to a period of back-thrusting (Stone et al., 1987). The late crenulation cleavage dips SSE at 45° or less. It is irregularly spaced, planar and weakly penetrative in pelites, and spaced, irregular, non-penetrative and strongly refracted to steeper inclinations in arenites.

A conjugate set of NNE sinistral and ESE dextral wrench faults displaces the main folds and succeeds the thrusting episode. Porphyry dykes of late Caledonian age are especially associated with axes of parasitic main-stage anticlines within the steep belts but also invade at least one late wrench fault.
=== Excursion ===
[[File:GatehouseOfFleetExcursionMap.jpg|500px|thumbnail|Locality map and outline geology for the Gatehouse of Fleet excursion.]]
==== 1 and 2 Ravenshall Point: Cairnharrow Formation ====
Follow the A75 west from Gatehouse for 10 km to a lay-by on the south side of the road (NX 525 524). This can accommodate more than one coach. Follow the path to Ravenshall Point, which breaches the wall near the west end of the lay-by and turns sharply west for 300 m.

The Cairnharrow Formation as developed at Ravenshall consists of turbidite units characterised by abundance and variety of sole markings both directional and non-directional. Exposures around high water mark (NX 522 522), across the small bay beyond the western headland (Locality 1), are reached through a natural arch which cuts the headland'''. '''Bed bases display a comprehensive range of sole markings.

A thick breccia horizon in the western headland may have been formed by slumping of a thick turbidite bed, probably during the late deformation. In such circumstances brecciation may be caused by bed-parallel fluid escape in response to seismic pumping following shear failure on a basement fault (Murphy, 1984). The locality is within a steep belt which has been refolded by late asymmetrical folds. These include a medium-sized monoform in which the upper limb has dips varying around 30° and the lower steep limb is vertical.

On the north (landward) side of an isolated stack (Locality 2) at mid-tide level 100 m east of the eastern headland (NX 524 522), one bedding surface displays current ripple marks denoting a SSE-directed palaeoflow. Another bedding plane exposed on the south side of the stack displays structures which mimic linguoid ripple marks with wavelengths comparable to those already examined. Close examination reveals that these are small monoformal folds cut by minute, evenly spaced thrusts dipping NNW and with top-to-SE displacements. These are late structures, the 'pseudo-ripples' of Craig and Walton (1959).

==== 3 and 4 Auchenlarie shore: Kirkmaiden Formation ====
Drive east on the A75 for 1.5 km to the caravan site at Low Auchenlarie Farm (NX 536 521). Transport may be parked by arrangement with the caravan site shop, which may charge a fee. Follow the metalled road on foot southwards to its eastward turn at the cliff top (250 m) and descend the steep gullied track for about 100 m to the shore '''(Locality 3). '''These exposures are also described in Excursion 16, Locality I.

Along the Gatehouse coastal section the greywackes of the Kirkmaiden Formation (Rust, 1965) contrast with those of the Cairnharrow Formation in displaying a scarcity of sole markings and internal structures, though grading is more convincingly developed, principally as Tae units. The turbidites of this formation are typified by a large thickness range. Packets of turbidite units reaching upwards of a metre thick, and including one succession of conspicuously thick beds, alternate with sequences of thinly bedded and pelite-dominated units a few metres thick at the most. Some thick packets constitute thinning- and fining-upward cycles, which may terminate in pelite-dominated sequences. The succession here crops out within a buckle fold belt 750 m wide.

Pelite partings, which are well exposed in fault gullies in the western headland of the small bay, carry a well-developed main cleavage which is penetrative with regular spacing and a near-vertical attitude. This cleavage carries crenulations on a scale of a few millimetres, related to a coarse, evenly spaced and weakly penetrative late cleavage with a shallow SSE dip. One conspicuously thick pelite interval near the mouth of the nearest gully clearly displays the intersecting relationship of the late and main cleavages. The crenulations are strongly asymmetrical, with long limbs facing upwards and short limbs facing downwards. This broadly reflects the style of the larger late asymmetrical folds which here have axial surfaces dipping at around 35° SSE.[[File:P008437.jpg|thumb|300px|left|Shore S. of Low Auchenlarrie (Boatdraught). Deformed sand volcanoes on top of sandstone bed. A bedding plane with numerous small sand volcanoes, many of which preserve a central hollow. The sand volcanoes have been distorted from their original circular shape into eliptical shapes aligned with a bedding cleavage intersection lineation parallel to the hammer shaft. See D3916 for detail. P008437]]Hinges in the larger folds are rounded with no thickening and the axial traces are nearly horizontal. Limbs are roughly equidimensional, around 5 m in length and with amplitudes of less than 20 per cent of wavelength. Arenites carry a coarse, irregular and irregularly spaced fracture cleavage dipping 45° SSE.

Quartz veins in arenite immediately south of the cliff path carry slickenfibres and the associated pelites a stretching lineation, both of which are aligned parallel to the regional fold hinge plunge. About 100 m east of the cliff path, the most thickly bedded turbidite sequence on the Gatehouse coast displays a series of late folds having uncommonly large wavelengths of around 25 m and amplitudes of up to 15 m. Axial separation of the folds is here controlled by bed thicknesses. A detached exposure to seaward shows an isolated example of back-thrusting, in which the steep limb of a small recumbent fold is displaced by a minor thrust with an indeterminate top-to NW displacement.

The wide and shallow gully (Locality 4) of the Boatdraught (NX 537 518) follows the boundary fault between the Auchenlarie buckle fold belt and a steep belt 1.3 km wide to the SE. This gully, though lacking exposures, is likely to have been eroded out along a disruption zone generated by the fault (Needham, 1993).

A bedding plane 50 m east of Boat-draught is covered with small sand volcanoes related to dewatering during the early, soft-sediment deformation. These structures are around 60 mm long and are elongated parallel to the main cleavage. This denotes axial extension during the main deformation and confirms the evidence of the slickenfibres and the stretching lineation at the previous locality.

==== 5 to 10 Mossyard shore: Kirkmaiden Formation and minor intrusions ====
Continue east along the A75 for a further 900 m to the junction with the Mossyard farm road (NX 544 525). Follow this road south for 1 km to the farm. The road beyond the farm is unmetalled and unsuitable for coaches; passengers should disembark at the farm and follow the road to the coast on foot, coaches returning to Auchenlarie to park. A parking charge is levied and to gain permission for access it is advisable to contact Mossyard Farm, Gatehouse of Fleet, Kirkcudbrightshire in advance.

The Kirkmaiden Formation here lies partly within the Auchenlarie—Mossyard steep belt (to the NW) and partly in a buckle fold belt characterised by upright, open to very tight main-stage folds with wavelengths of more than 100 m to less than 1 m. This belt continues past Mossyard at least as far as Gatehouse. From the car park, walk SW along the rocky shore for about 1 km; two parasitic main fold pairs within this steep belt can be examined at '''Locality 5 ('''NX 542 515). The more northerly fold pair has been coaxially refolded into a SE-verging recumbent attitude such that the main cleavage is also nearly horizontal. The main structure, which deforms a more thickly bedded sequence, originated as a tight fold pair with an axial separation of 3 m and a rounded axis with little or no thickening. The core of the anticline is occupied by a pale, pinkish grey porphyry sheet about 1 m thick. This has a dense in-situ growth along each margin consisting of alkali feldspar phenocrysts about 2 cm long with their long axes aligned roughly normal to the dyke margins. The intrusion belongs to the late Caledonian acidic suite and is unsheared; it thus postdates the main folding and was deformed along with the main fold pair during the late folding (Stringer and Treagus, 1981).

The second fold pair is situated 28 m farther south, and affects a pelite-dominated sequence. This fold is upright and very tight with a twofold axial thickening. It is nearly symmetrical with an axial separation of 1 m and a NE plunge of 5°. The main-stage cleavage is strong and rotated by some 15° clockwise relative to the anticlinal axis.

A third main-phase anticline at Locality 6 (NX 543 514) is tight and upright, and has a particularly thick and bright pink, felsitic dyke intruded into the core. This dyke is again unsheared, but also lacks marginal concentrations of aligned feldspar phenocrysts and may be entirely post-tectonic. There is little if any evidence of faulting along the axial surface of the anticline but it is displaced by a small ESE dextral strike-slip fault, one of only two observed within the area and now eroded to form a prominent gully parallel to the shoreline.

[[File:P259023.jpg|thumb|300px|right|Ringdoo Point. F1 folding. P259023]]The boundary between the Mossyard steep belt and the Mossyard—Gatehouse buckle fold belt is well exposed on Ringdoo Point at Locality 7 (NX 545 513) and takes the form of an axially faulted anticline. Each limb is nearly vertical and the fault plane is marked by a narrow, quartz-filled crush zone. Poorly developed grading and occasional bottom structures (mainly drag marks) indicate southerly facing seaward of the fault and northerly facing to landward. Strong reddening and ochreous discoloration of the strata in the vicinity of the fault are ascribed to weathering during development of the major Permo-Triassic depositional basin whose present margin lies a short distance offshore.

[[File:P259025.jpg|thumb|300px|left|Ringdoo Point. F1 syncline. P259025]]A low west-facing cliff at Locality 8 (NX 552 516) displays a symmetrical and upright main fold pair with a comprehensive range of minor structures. The syncline is wide and open and the anticline to the south narrow and very tight. There is considerable thickening of the anticline hinge in association with both small-scale disharmonic folding and flat-lying faults with quartz veining. The axial separation is about 25 m, and there is a gentle easterly plunge. Main slaty and late crenulation cleavages are especially well developed and replicate the features observed on the Auchenlarie shore. The relationship of the late cleavage to the crenulations is especially convincing, as is displacement of the main by the late cleavage. There is also a strong development of a steep main fracture cleavage in the arenite beds. The late cleavage in the pelites has a SE dip of 30°, strongly refracted in the arenites to dip 45°. The strata in the syncline constitute a thinning-and fining-upwards greywacke sequence which rests on pelites forming the anticline. This anticline is aligned with the Ringdoo Point anticline (Figure 39) but is unlikely to be an extension of it since the fold styles contrast markedly. There is likely to be a strike-slip fault through the sandy bay to the west of this locality.

Another main fold pair is well exposed in the three Garvellan Rocks (Locality 9). In the eastern rock (552 514) the anticline folds a fine-grained sequence, and the NW limb is slightly overturned. The main cleavage has a southerly dip of 40°-60° and is offset by small SE-facing monoformal folds with a 10 cm scale of wavelength. These folds are closely comparable in style to the late crenulations but are an order of magnitude larger and are associated with a weak crenulation cleavage. This has itself been coaxially refolded into a gentle anticline. A felsitic dyke which traverses the middle rock occupies a late fault trending ESE. Drag folds in the adjacent strata indicate a dextral strike-slip movement though there is no discernible displacement of the anticlinal axis. This is the second of the two dextral faults observed in the area.

Several folds within the Mossyard buckle fold belt crop out on intertidal skerries '''(Locality 10) '''between the Garvellan Rocks and the Cardoness shore (NX 570 538). The traces of these folds are assumed to terminate against faults trending between north and NNE, the prevailing trend of the late sinistral wrench set.

==== 11 Skyreburn Bay: tectonic structures in the Kirkmaiden Formation ====
Return to the main road and continue east for 3 km, parking in a lay-by on the seaward side (NX 576 545). Pass through the wicket gate near the eastern end of the lay-by and descend to the shore.

A series of small folds is exposed on the wave-cut platform below the east end of the lay-by. These folds have vertical axial surfaces with a north—south strike and are strongly asymmetrical, with east-facing limbs several centimetres long and west-facing limbs a metre or more in length. They are thus open, sharply angled chevron folds with interlimb angles of around 150° and near-vertical plunges; they represent a late brittle kink-band phase with dextral sense of displacement.

Several medium-sized main-stage folds are exposed above high water mark at this locality. They are symmetrical, have vertical axial surfaces striking ENE, are open with rounded hinges and show no axial thickening. Dips range up to 70° and hinge plunges, which are variable, are mostly steeper than 45° ENE. Wavelengths vary between 10 and 50 m. These moderately to steeply plunging folds occupy a tract 2.5 km long trending WSW between Skyreburn Bay and Cardoness, the plunge lessening progressively south-westwards. Apart from the steep plunges, the style and associated cleavages of these folds correspond with the other main buckle folds, including the clockwise rotation of the main cleavage relative to the axial surfaces. Late crenulations have plunges parallel to those of the main fold axes. Steepening of the plunges thus postdates the crenulation cleavage, and may relate to fold rotation by a major fault, as yet unproved, following the Fleet valley.

One arenite interval, just above high water mark close to the western limit of exposure displays sigmoidal tension gashes filled with quartz. These indicate a sinistral sense of movement in accordance with the late transpressive episode but contradicting the dextral kink bands which probably represent the latest Caledonian effect.
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

Gatehouse of Fleet - an excursion

2015-12-09T12:56:57Z

JenniferFindlay1:

By J A Weir. Excursion 12. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Gatehouse Of Fleet: structural complexities of the Hawick Group ==
OS 1:50 000 Sheet 83 Newton Stewart & Kirkcudbright

BGS 1:50 000 Sheet 4E Wigtown

=== Main points of interest ===
Sedimentology and structure in the late Llandovery Hawick Group, polyphase deformation, late Caledonian porphyry and felsite dykes.

=== Logistics ===
The area may be approached either from Gatehouse in the east or from Creetown in the west via the A75 road and the excursion may be started or curtailed at Ravenshall Point, Mossyard Farm or Low Auchenlarie Farm. Note that account has to be taken of the state of the tide at Auchenlarie, Mossyard and the Newton shore. The likelihood of congestion in the Mossyard Farm car park during the holiday season may also have to be considered. Localities visited are situated along the coast between the Ardwall shore (NX 585 545), on the Fleet estuary 2 km west of Gatehouse, and Ravenshall Point (NX 523 523) on the east side of Wigtown Bay. Exposures are in the wave-cut platform, intertidal skerries and cliffs. Nearby cuttings along the main A75 Carlisle–Dumfries–Stranraer road are informative but are not now recommended for examination due to the hazard of high-speed traffic. On a clear day the A75, which follows the coast a short distance inland, gives good views of the Cumbrian mountains and the Isle of Man.

=== Introduction ===
The eastern coastline of Wigtown Bay provides good sections through the Cairnharrow and Kirkmaiden formations of the late Llandovery Hawick Group. The succession consists of greywackes which are typically poorly graded and bedded in units mostly less than 1 m thick. Fine arenite forms up to 90 per cent thickness of each unit and grades upwards into pelite; Tabde (Figure 6) cycles predominate. Grading is expressed mainly in the progressive upward intensification of cleavage into the finer-grained intervals. A high quartz content commonly imparts a vitreous lustre to fresh surfaces. Calcareous nodule trails parallel to the bedding are dispersed through many of the thicker arenite intervals, reflecting a general abundance of secondary carbonate within the Hawick Group as a whole. Sole markings are well developed in the Cairnharrow Formation, but are scarce in the Kirkmaiden Formation. They are predominantly non-directional drag marks, with small directional flute casts, longitudinal ridge casts and prod marks. Directional sole markings denote palaeoflow towards the WSW, the prevalent trend within the area and subparallel to the regional fold plunge. Deposition in an intermediate to distal trench, or lateral lobe of a submarine fan, is suggested (Weir, 1974).

The strata have been subjected to a continuity of deformation in which two stages, designated `main' and late' (cf. Needham, 1993), are prominently developed. An `early' soft-sediment deformation is sparingly represented. The main-stage folds are of two contrasting styles: NW-facing homoclinal zones of steeply inclined bedding up to 1 km and more wide (`steep belts'), which alternate with comparably wide tracts of dominantly open, upright buckle folds; where asymmetrical these latter structures are SE-verging. Adjacent structurally contrasting belts are separated by strike faults downthrowing SE (Weir, 1968; 1979). Evidence from elsewhere in the outcrop of the Hawick Group suggests that the strike faults, though now steep, originated as thrusts over-riding towards the SE, and were subsequently steepened and back-rotated. Anticlines are commonly faulted along their axial surfaces, suggesting that the folds grew ahead of the developing thrusts, ultimately to be cut by them. The SE-downthrow of the strike faults, in combination with the NW-facing of the steep belts, has led to duplication of the succession, and there are no gross changes in stratigraphical horizon across the 10 km-wide outcrop of the Cairnharrow and Kirkmaiden formations (Craig and Walton, 1959). The main cleavage in the pelite intervals is steep, penetrative and slaty, with development of new sericite; an equally steep but irregular, non-penetrative and spaced cleavage occurs in the arenites. The cleavage is rotated by up to 15° clockwise relative to fold axial surfaces indicating continuous deformation under a sinistral transpressive regime. This also resulted in the reactivation of at least some of the rotated main strike faults with a sinistral strike-slip displacement (Stringer and Treagus, 1980).

Late-stage structures are more variable. Open folds with wavelengths of a few metres, asymmetrical with respect to dip though symmetrical in relation to limb lengths, verge NW and affect the steep belts. Minor NW-directed thrusts are associated with small recumbent folds which refold the main folds; an associated flat-lying crenulation cleavage displaces the main cleavage. This episode postdates the sinistral transpressive phase and is likely to relate to a period of back-thrusting (Stone et al., 1987). The late crenulation cleavage dips SSE at 45° or less. It is irregularly spaced, planar and weakly penetrative in pelites, and spaced, irregular, non-penetrative and strongly refracted to steeper inclinations in arenites.

A conjugate set of NNE sinistral and ESE dextral wrench faults displaces the main folds and succeeds the thrusting episode. Porphyry dykes of late Caledonian age are especially associated with axes of parasitic main-stage anticlines within the steep belts but also invade at least one late wrench fault.
=== Excursion ===
[[File:GatehouseOfFleetExcursionMap.jpg|500px|thumbnail|Locality map and outline geology for the Gatehouse of Fleet excursion.]]
==== 1 and 2 Ravenshall Point: Cairnharrow Formation ====
Follow the A75 west from Gatehouse for 10 km to a lay-by on the south side of the road (NX 525 524). This can accommodate more than one coach. Follow the path to Ravenshall Point, which breaches the wall near the west end of the lay-by and turns sharply west for 300 m.

The Cairnharrow Formation as developed at Ravenshall consists of turbidite units characterised by abundance and variety of sole markings both directional and non-directional. Exposures around high water mark (NX 522 522), across the small bay beyond the western headland (Locality 1), are reached through a natural arch which cuts the headland'''. '''Bed bases display a comprehensive range of sole markings.

A thick breccia horizon in the western headland may have been formed by slumping of a thick turbidite bed, probably during the late deformation. In such circumstances brecciation may be caused by bed-parallel fluid escape in response to seismic pumping following shear failure on a basement fault (Murphy, 1984). The locality is within a steep belt which has been refolded by late asymmetrical folds. These include a medium-sized monoform in which the upper limb has dips varying around 30° and the lower steep limb is vertical.

On the north (landward) side of an isolated stack (Locality 2) at mid-tide level 100 m east of the eastern headland (NX 524 522), one bedding surface displays current ripple marks denoting a SSE-directed palaeoflow. Another bedding plane exposed on the south side of the stack displays structures which mimic linguoid ripple marks with wavelengths comparable to those already examined. Close examination reveals that these are small monoformal folds cut by minute, evenly spaced thrusts dipping NNW and with top-to-SE displacements. These are late structures, the 'pseudo-ripples' of Craig and Walton (1959).

==== 3 and 4 Auchenlarie shore: Kirkmaiden Formation ====
Drive east on the A75 for 1.5 km to the caravan site at Low Auchenlarie Farm (NX 536 521). Transport may be parked by arrangement with the caravan site shop, which may charge a fee. Follow the metalled road on foot southwards to its eastward turn at the cliff top (250 m) and descend the steep gullied track for about 100 m to the shore '''(Locality 3). '''These exposures are also described in Excursion 16, Locality I.

Along the Gatehouse coastal section the greywackes of the Kirkmaiden Formation (Rust, 1965) contrast with those of the Cairnharrow Formation in displaying a scarcity of sole markings and internal structures, though grading is more convincingly developed, principally as Tae units. The turbidites of this formation are typified by a large thickness range. Packets of turbidite units reaching upwards of a metre thick, and including one succession of conspicuously thick beds, alternate with sequences of thinly bedded and pelite-dominated units a few metres thick at the most. Some thick packets constitute thinning- and fining-upward cycles, which may terminate in pelite-dominated sequences. The succession here crops out within a buckle fold belt 750 m wide.

Pelite partings, which are well exposed in fault gullies in the western headland of the small bay, carry a well-developed main cleavage which is penetrative with regular spacing and a near-vertical attitude. This cleavage carries crenulations on a scale of a few millimetres, related to a coarse, evenly spaced and weakly penetrative late cleavage with a shallow SSE dip. One conspicuously thick pelite interval near the mouth of the nearest gully clearly displays the intersecting relationship of the late and main cleavages. The crenulations are strongly asymmetrical, with long limbs facing upwards and short limbs facing downwards. This broadly reflects the style of the larger late asymmetrical folds which here have axial surfaces dipping at around 35° SSE.[[File:P008437.jpg|thumb|300px|left|Shore S. of Low Auchenlarrie (Boatdraught). Deformed sand volcanoes on top of sandstone bed. A bedding plane with numerous small sand volcanoes, many of which preserve a central hollow. The sand volcanoes have been distorted from their original circular shape into eliptical shapes aligned with a bedding cleavage intersection lineation parallel to the hammer shaft. See D3916 for detail. P008437]]Hinges in the larger folds are rounded with no thickening and the axial traces are nearly horizontal. Limbs are roughly equidimensional, around 5 m in length and with amplitudes of less than 20 per cent of wavelength. Arenites carry a coarse, irregular and irregularly spaced fracture cleavage dipping 45° SSE.

Quartz veins in arenite immediately south of the cliff path carry slickenfibres and the associated pelites a stretching lineation, both of which are aligned parallel to the regional fold hinge plunge. About 100 m east of the cliff path, the most thickly bedded turbidite sequence on the Gatehouse coast displays a series of late folds having uncommonly large wavelengths of around 25 m and amplitudes of up to 15 m. Axial separation of the folds is here controlled by bed thicknesses. A detached exposure to seaward shows an isolated example of back-thrusting, in which the steep limb of a small recumbent fold is displaced by a minor thrust with an indeterminate top-to NW displacement.

The wide and shallow gully (Locality 4) of the Boatdraught (NX 537 518) follows the boundary fault between the Auchenlarie buckle fold belt and a steep belt 1.3 km wide to the SE. This gully, though lacking exposures, is likely to have been eroded out along a disruption zone generated by the fault (Needham, 1993).

A bedding plane 50 m east of Boat-draught is covered with small sand volcanoes related to dewatering during the early, soft-sediment deformation. These structures are around 60 mm long and are elongated parallel to the main cleavage. This denotes axial extension during the main deformation and confirms the evidence of the slickenfibres and the stretching lineation at the previous locality.

==== 5 to 10 Mossyard shore: Kirkmaiden Formation and minor intrusions ====
Continue east along the A75 for a further 900 m to the junction with the Mossyard farm road (NX 544 525). Follow this road south for 1 km to the farm. The road beyond the farm is unmetalled and unsuitable for coaches; passengers should disembark at the farm and follow the road to the coast on foot, coaches returning to Auchenlarie to park. A parking charge is levied and to gain permission for access it is advisable to contact Mossyard Farm, Gatehouse of Fleet, Kirkcudbrightshire in advance.

The Kirkmaiden Formation here lies partly within the Auchenlarie—Mossyard steep belt (to the NW) and partly in a buckle fold belt characterised by upright, open to very tight main-stage folds with wavelengths of more than 100 m to less than 1 m. This belt continues past Mossyard at least as far as Gatehouse. From the car park, walk SW along the rocky shore for about 1 km; two parasitic main fold pairs within this steep belt can be examined at '''Locality 5 ('''NX 542 515). The more northerly fold pair has been coaxially refolded into a SE-verging recumbent attitude such that the main cleavage is also nearly horizontal. The main structure, which deforms a more thickly bedded sequence, originated as a tight fold pair with an axial separation of 3 m and a rounded axis with little or no thickening. The core of the anticline is occupied by a pale, pinkish grey porphyry sheet about 1 m thick. This has a dense in-situ growth along each margin consisting of alkali feldspar phenocrysts about 2 cm long with their long axes aligned roughly normal to the dyke margins. The intrusion belongs to the late Caledonian acidic suite and is unsheared; it thus postdates the main folding and was deformed along with the main fold pair during the late folding (Stringer and Treagus, 1981).

The second fold pair is situated 28 m farther south, and affects a pelite-dominated sequence. This fold is upright and very tight with a twofold axial thickening. It is nearly symmetrical with an axial separation of 1 m and a NE plunge of 5°. The main-stage cleavage is strong and rotated by some 15° clockwise relative to the anticlinal axis.

A third main-phase anticline at Locality 6 (NX 543 514) is tight and upright, and has a particularly thick and bright pink, felsitic dyke intruded into the core. This dyke is again unsheared, but also lacks marginal concentrations of aligned feldspar phenocrysts and may be entirely post-tectonic. There is little if any evidence of faulting along the axial surface of the anticline but it is displaced by a small ESE dextral strike-slip fault, one of only two observed within the area and now eroded to form a prominent gully parallel to the shoreline.

[[File:P008561.jpg|thumb|300px|right|S. of Back Bay, Monreith. Looking NE. Offsets in lamprophyre dyke facilitated by folding. One of the several sideways steps in a 1 m. thick dyke is facilitated by an open recumbent D2 fold, suggesting emplacement synchronous with folding. P008561]]The boundary between the Mossyard steep belt and the Mossyard—Gatehouse buckle fold belt is well exposed on Ringdoo Point at Locality 7 (NX 545 513) and takes the form of an axially faulted anticline. Each limb is nearly vertical and the fault plane is marked by a narrow, quartz-filled crush zone. Poorly developed grading and occasional bottom structures (mainly drag marks) indicate southerly facing seaward of the fault and northerly facing to landward. Strong reddening and ochreous discoloration of the strata in the vicinity of the fault are ascribed to weathering during development of the major Permo-Triassic depositional basin whose present margin lies a short distance offshore.

[[File:P259025.jpg|thumb|300px|left|Ringdoo Point. F1 syncline. P259025]]A low west-facing cliff at Locality 8 (NX 552 516) displays a symmetrical and upright main fold pair with a comprehensive range of minor structures. The syncline is wide and open and the anticline to the south narrow and very tight. There is considerable thickening of the anticline hinge in association with both small-scale disharmonic folding and flat-lying faults with quartz veining. The axial separation is about 25 m, and there is a gentle easterly plunge. Main slaty and late crenulation cleavages are especially well developed and replicate the features observed on the Auchenlarie shore. The relationship of the late cleavage to the crenulations is especially convincing, as is displacement of the main by the late cleavage. There is also a strong development of a steep main fracture cleavage in the arenite beds. The late cleavage in the pelites has a SE dip of 30°, strongly refracted in the arenites to dip 45°. The strata in the syncline constitute a thinning-and fining-upwards greywacke sequence which rests on pelites forming the anticline. This anticline is aligned with the Ringdoo Point anticline (Figure 39) but is unlikely to be an extension of it since the fold styles contrast markedly. There is likely to be a strike-slip fault through the sandy bay to the west of this locality.

Another main fold pair is well exposed in the three Garvellan Rocks (Locality 9). In the eastern rock (552 514) the anticline folds a fine-grained sequence, and the NW limb is slightly overturned. The main cleavage has a southerly dip of 40°-60° and is offset by small SE-facing monoformal folds with a 10 cm scale of wavelength. These folds are closely comparable in style to the late crenulations but are an order of magnitude larger and are associated with a weak crenulation cleavage. This has itself been coaxially refolded into a gentle anticline. A felsitic dyke which traverses the middle rock occupies a late fault trending ESE. Drag folds in the adjacent strata indicate a dextral strike-slip movement though there is no discernible displacement of the anticlinal axis. This is the second of the two dextral faults observed in the area.

Several folds within the Mossyard buckle fold belt crop out on intertidal skerries '''(Locality 10) '''between the Garvellan Rocks and the Cardoness shore (NX 570 538). The traces of these folds are assumed to terminate against faults trending between north and NNE, the prevailing trend of the late sinistral wrench set.

==== 11 Skyreburn Bay: tectonic structures in the Kirkmaiden Formation ====
Return to the main road and continue east for 3 km, parking in a lay-by on the seaward side (NX 576 545). Pass through the wicket gate near the eastern end of the lay-by and descend to the shore.

A series of small folds is exposed on the wave-cut platform below the east end of the lay-by. These folds have vertical axial surfaces with a north—south strike and are strongly asymmetrical, with east-facing limbs several centimetres long and west-facing limbs a metre or more in length. They are thus open, sharply angled chevron folds with interlimb angles of around 150° and near-vertical plunges; they represent a late brittle kink-band phase with dextral sense of displacement.

Several medium-sized main-stage folds are exposed above high water mark at this locality. They are symmetrical, have vertical axial surfaces striking ENE, are open with rounded hinges and show no axial thickening. Dips range up to 70° and hinge plunges, which are variable, are mostly steeper than 45° ENE. Wavelengths vary between 10 and 50 m. These moderately to steeply plunging folds occupy a tract 2.5 km long trending WSW between Skyreburn Bay and Cardoness, the plunge lessening progressively south-westwards. Apart from the steep plunges, the style and associated cleavages of these folds correspond with the other main buckle folds, including the clockwise rotation of the main cleavage relative to the axial surfaces. Late crenulations have plunges parallel to those of the main fold axes. Steepening of the plunges thus postdates the crenulation cleavage, and may relate to fold rotation by a major fault, as yet unproved, following the Fleet valley.

One arenite interval, just above high water mark close to the western limit of exposure displays sigmoidal tension gashes filled with quartz. These indicate a sinistral sense of movement in accordance with the late transpressive episode but contradicting the dextral kink bands which probably represent the latest Caledonian effect.
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P259025.jpg

2015-12-09T12:51:50Z

JenniferFindlay1: Ringdoo Point. F1 syncline. P259025

== Summary ==
Ringdoo Point. F1 syncline. P259025
== Licencing ==
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File:P259023.jpg

2015-12-09T12:48:43Z

JenniferFindlay1: Ringdoo Point. F1 folding. P259023

== Summary ==
Ringdoo Point. F1 folding. P259023
== Licencing ==
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| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

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File:P008437.jpg

2015-12-09T12:36:19Z

JenniferFindlay1: Shore S. of Low Auchenlarrie (Boatdraught). Deformed sand volcanoes on top of sandstone bed. A bedding plane with numerous small sand volcanoes, many of which preserve a central hollow. The sand volcanoes have been distorted from their original circula...

== Summary ==
Shore S. of Low Auchenlarrie (Boatdraught). Deformed sand volcanoes on top of sandstone bed. A bedding plane with numerous small sand volcanoes, many of which preserve a central hollow. The sand volcanoes have been distorted from their original circular shape into eliptical shapes aligned with a bedding cleavage intersection lineation parallel to the hammer shaft. See D3916 for detail. P008437
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Dundrennan Ranges, Gipsy Point - an excursion

2015-12-09T12:14:59Z

JenniferFindlay1:

By E N K Clarkson and C M Taylor. Excursion 11. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Dundrennan Ranges, Gipsy Point: Silurian submarine slump features ==

OS 1:50 000 Sheet 83 Newton Stewart & Kirkcudbright

BGS 1:50 000 Sheet 5W Kirkcudbright

=== Main points of interest ===
Major slump structure, sand volcanoes and sedimentology of the Raeberry Castle Formation, Wenlock graptolites.

=== Logistics ===
[[File:DundrennanExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Ministry of Defence Dundrennan Range.]]
The area lies within the Ministry of Defence Test and Evaluation Establishment near Dundrennan, a strip of coastline which has been '''used as a firing range for many years. '''Recently announced plans (1994) indicate that its future is uncertain. The MOD has no objection to people visiting the area, except '''when firing is in progress! '''At such times '''red flags and explanatory notices are displayed. '''Notification of firing times is held by the Harbour Master and Tourist Information Office in Kirkcudbright. Access on foot is feasible without special permission when the range is not in use, normally evenings, weekends and public holidays, although it is always best to inform the range authorities in advance. Large parties, or visitors wishing to take vehicles into the range area, should contact the Commanding Officer beforehand (Test and Evaluation Establishment, Dundrennan, Kirkcudbright DG6 4QZ). '''Do not touch '''any strange objects encountered during the excursion.

=== Introduction ===
The primary aim of this excursion is to examine a gigantic slump structure, lying within a channel in a sequence of Wenlock turbidites belonging to the Gipsy Point Unit of the Raeberry Castle Formation (Riccarton Group). A series of sand volcanoes, probably formed by de-watering of the slump, lies just above the disturbed beds. Other tectonostratigraphical units within the Raeberry Castle Formation, showing diverse facies associations, can be seen further east along the coast. These represent channels, lobes and fringes within a submarine fan complex that built out from the north; the dominant transport direction was from the NE. From the entrance to the MOD establishment on the A 711 Kirkcudbright–Dundrennan road (NX 718 474) proceed south towards the shore, along the road leading to Little Balmae (Figure 37). About 1 km from the shore there is a small car park (NX 692 449), to the left of which a track, with a sign marked 'Gipsy Point' leads down to the coast. A ten-minute walk brings the visitor to an area of numbered firing targets, and an old tank which forms a convenient landmark. From here proceed to the cliff top above Port Muddle. The localities of interest are indicated on Figure 38. Comprehensive details of the section are given by Kemp (1987b). For a longer excursion this itinerary could be extended to examine the stratigraphically adjacent Ross Formation at localities 3 and 4 of Excursion 5 (Kirkcudbright).

===Excursion===
==== 1 Port Muddle: Gipsy Point Unit ====
[[File:DundrennanGipsyPointExcursionMap.jpg|500px|thumbnail|Shore section between Gipsy Point and Mullock Bay showing localities and outline geology.]]
This bay is excavated at the seaward end of a fault trending NNE, which may be traced inland as a boggy depression. All the beds in this vicinity lie within the Raeberry Castle Formation, now known to be Wenlock in age. Four thin bands of graptolite-bearing hemipelagite, which crop out below the cliff on the east side of the bay, provide confirmation.

According to Kemp (1987a and b), there are three definable tectonostratigraphical units within the Raeberry Castle Formation: the Gipsy Point Unit, the Raeberry Unit, and the Mullock Bay Unit. It is the Gipsy Point Unit ''(riccartonensis ''to ''flexilis ''biozones) that occurs here, on both sides of Port Muddle. All the beds are vertical and show quite diverse facies associations. Kemp described an A Member, of rather monotonous thin- to medium-bedded turbidites with some hemipelagic muds, and a B Member, with channelised arenites, rudites and slumps, interbedded with thin-bedded sandstones, siltstones, and mudstones. Kemp interpreted these deposits as part of a meandering and laterally migrating channel-levee complex on a submarine fan, with the coarser deposits lying within the channels and the fine beds representing levees. All these sedimentary features are easily visible from the cliff top. It is possible to descend to the shore here, but the details are better seen at Gipsy Point.

==== 2 Gipsy Point: slump ====
A walk of some 300 m in a WSW direction, keeping the old tank to the left and crossing a ruined wall, brings the visitor to the cliff top at Gipsy Point (NX 685 436). A new military earthwork lies just to the west. The slump, which lies within the B Member of the Gipsy Point Unit, is well seen from the cliff top. It is some 30 m thick and lies within a channel-fill sequence consisting of chaotic and disordered thin-bedded sandstones. The sequence is vertical and youngs NW; above and below the slump the beds are undisturbed. One prominent horseshoe-shaped sedimentary fold within the slump sequence is particularly evident from this vantage point. It is clear that the sediment forming the slump must have slid down the process, but remaining cohesive.

It is possible, with care, to descend to the coast from here, to examine the slump more closely. The erosive base of the slump (SE side) can be seen cutting into the underlying beds, forming fossilized channels with flute casts; in some places rip-up clasts can be seen in the mudstone. The whole sequence has been folded by a large anticline–syncline pair which traverses part of the section and is clearly seen as a large box-fold on the vertical face near the point of descent from the cliff top.

==== 3 Gipsy Point: sand volcanoes ====
[[File:P258913.jpg|thumb|300px|left|Gipsy Point. Sand volcano. P258913]]Directly overlying the large slump is a single layer (now vertical) of spectacular sand volcanoes, originally described by Lovell (1974), who suggested that they formed as a result of dewatering of the underlying slump. Above lies a coarsening-upward sequence of thin sandstone–mudstone layers, interpreted as an interchannel or overbank sequence. The sharp junction between the top of the slump, with its attendant sand volcanoes, and the overlying beds is very well seen in the cliff face. This shows clearly that the original channel had been abandoned by the time renewed deposition began.

Lovell identified ten sand volcanoes, occurring ill three groups, over 100 m of strike section. Five of these are easily accessible, even at mid-tide. The largest and most obvious is over 3 m across, and is almost completely preserved. It is about 1.5 m deep and has a depressed central crater nearly 0.5 m across. Several of the other volcanoes are cut across, showing laminated sandstone with complex internal structure, as if the water had escaped in a series of pulses. One small volcano directly overlies another smaller one. A thin sandstone bed connects the sand volcanoes of the landward and central groups, but does not extend to the seaward set. In this latter group, however, is a sand volcano, cut so as to display the central pipe, which in this case is filled with coarse sandstone. Stringers of gritty material have developed in the underlying sandstone, and penetrated upwards into the sediment overlying the volcano during the dewatering process. This example is the only one to show a direct connection between the beds above and below the volcano.

[[File:P259111.jpg|thumb|300px|right|Gipsy Point. Sand volcano, (0986/29, rack 2). P259111]]The thin sandstone beds of the overbank sequence directly overlying the sand-volcano horizon show some interesting sedimentary structures. Firstly there are ripple marks with slightly different orientations, seen on successive vertical faces near the cliff edge. Secondly there are other structures, well worth examining, on the lower surfaces of the thin sandstone units down on the shore, a few metres above the sand volcanoes. Some of these faces are smooth but show convolute lamination in cross-section; other surfaces display very fine load casts. In some instances an otherwise smooth surface has only one or two isolated load casts, of which some are simple, whereas others are grouped together like bunches of grapes. Many faces, however, are entirely covered with load casts. One surface, about a metre above the sand volcanoes, displays strings of clearly orientated load casts, suggestive of modified ripples.

Small dolomitised concretions can be seen within the slump close to the base of the cliff, and several small fault-planes within the vicinity are likewise dolomitised. These may possibly have been synsedimentary faults penetrated by the diagenetic fluids associated with the concretion-forming process.

==== 4 Big Raeberry: Raeberry Unit ====
The section east from Gipsy Point to Mullock Bay shows many features of interest and should be visited if time permits. From the east side of Port Muddle, to the far side of Howwell Bay, underneath the prominent hill of Big Raeberry, the turbidites belong to the Raeberry Unit ''(rigidus ''to early ''flexilis ''biozones) of the Raeberry Castle Formation. Kemp (1987) who logged the whole section in detail, recognised three separate facies. The medium- to occasionally thick-bedded turbidites of the A Member were interpreted as deposits of the depositional lobe of a submarine km. The very regular, thin-bedded turbidites of the B Member were considered to be a fringe facies of the fan lobe, whereas the alternating medium- and thin-bedded base-absent turbidites of the C Member were taken to represent a channel-mouth facies.

These distinctions are evident in the cliff section east of Big Raeberry, where the beds young to the NW. Just west of the prominent feature known as the Haystack (NX 702 437), the alternating thick and thin beds of the C Member form spectacular Z-folds (Locality 4a in Figure 38). In the prominent north—south-trending cliff some 200 m to the east (Locality 4b) all three members are clearly seen. The seaward end is formed from the varied lithologies of the A Member, the top of which is marked by a thick bed forming a notch in the cliff top. Landward of this lie the strikingly regular, alternating sandstones and siltstones of the B Member, of which the top is marked by a prominent vertical band of structureless siltstone, known as the `homogenite wall'. This bed may be at the same horizon as the Gipsy Point slump, marking some large-scale event within the depositional basin. The ragged-looking C Member, with its projecting sandstone bands, is distinct and forms the north end of the cliff. The bases of many of the sandstones throughout this sequence are marked with flute casts showing variable transport directions.

==== 5 Mullock Bay: Mullock Bay Unit ====
About 1 km further east, excellent sections in the Mullock Bay Unit of the Raeberry Castle Formation are well exposed along the shore. The beds here yield graptolites of the ''ellesae ''and ''lundgreni ''biozones, and are distinguished by having more hemipelagite and less sandstone than any of the other units seen along this section. Kemp (1987) divided these into two members. Member A consists of thinly bedded fine sandstones, siltstones and mudstones, alternating with channelised arenites and rudites. They are quite like the B Member of the Gipsy Point Unit, and like them are interpreted as having formed in a channel-levee system on a submarine fan. The B Member consists of a complex of alternating thin- to medium-bedded sandstones and shales, with occasional coarser horizons considered to represent prograding depositional lobes.

Return to Port Muddle along the cliff top or the shore.

<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P259111.jpg

2015-12-09T12:04:38Z

JenniferFindlay1: Gipsy Point. Sand volcano, (0986/29, rack 2). P259111

== Summary ==
Gipsy Point. Sand volcano, (0986/29, rack 2). P259111
== Licencing ==
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File:P258913.jpg

2015-12-09T12:02:02Z

JenniferFindlay1: Gipsy Point. Sand volcano. P258913

== Summary ==
Gipsy Point. Sand volcano. P258913
== Licencing ==
__notoc__
{|class="wikitable" style="width:100% style="background:#f0f8ff; font-size:80%"
| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

====Non-commercial Use====

Use of the images downloaded from this site and reproduced digitally or otherwise may only be used for non-commercial purposes, which are:-

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When using the images please credit 'British Geological Survey' and include the catalogue reference ('P Number') of the item to allow others to access the original image or document.
Non-commercial users of the images from this site are restricted to downloading no more than 30 images, without seeking further permission from [mailto:enquiries@bgs.ac.uk enquiries@bgs.ac.uk]

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====Ordnance Survey topography====
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[[Category:License tags]]

Dob's Linn, Moffat - an excursion

2015-12-09T11:40:11Z

JenniferFindlay1:

'''By E.N.K. Clarkson and C.M. Taylor. From [[Scottish Borders geology: an excursion guide]] edited by A.D. McAdam, E.N.K. Clarkson, P. Stone. Edinburgh : Scottish Academic Press (for [http://www.edinburghgeolsoc.org/ Edinburgh Geological Society]), 1992.'''

O.S. 1:50000 Sheet 79 Hawick & Eskdale

B.G.S. 1.50000 Sheet 16W Moffat

=== Introduction ===
Some 19 km north-east of Moffat lies Dob's Linn, a spectacular gorge and waterfall deeply cut into the Ordovician and Silurian shales and greywackes of the Southern Uplands. [[File:P571542.jpg|thumb|300px|left|View towards Dob's Linn, Moffatdale, Main Branch round spur on right, Linn Branch on left in from of red scar. P571542]][[File:P772042.jpg|thumb|300px|right|Silurian (Llandovery) graptolites from southern Scotland. Locality is probabably Dob's Linn, Moffatdale. 1:50 k sheet 16E (Ettrick), NT 196 158. P772042]]The well-exposed, virtually continuous Caradoc to Llandovery sequence of graptolite-bearing strata, with its intercalated 'barren beds' was distinguished in the classic researches of Charles Lapworth (1878)<ref>Lapworth, C. 1878. The Moffat Series. Quart. J. Geol. Soc. Lond. 34, 240-346. Lapworth, C. and Wilson, J.1870. The Lower Silurian rocks in the neighbourhood of Galashiels. Trans. Edinb. Geol. Soc. 2, 46-48.</ref> as 'the only section of the Moffat Series which allows us to determine with certainty the sequence and palaeontological characteristics of its component beds, and at the same time exhibits the relationship of the group as a whole to the surrounding greywackes'. Dob's Linn was named after a Scottish Covenanter, Halbert Dobson, who hid from the English Dragoons in the small cave above the waterfall. It is an easily accessible section and abundantly fossiliferous, though at certain critical exposures the visitor is asked to refrain from hammering. Dob's Linn has been the subject of much research (Lapworth 1878<ref>Lapworth, C. 1878. The Moffat Series. Quart. J. Geol. Soc. Lond. 34, 240-346.</ref>, 1970<ref>Toghill, P. 1970. Highest Ordovician (Hartfell Shales) graptolite faunas from the Moffat area, south Scotland. Bull. Br. Mus. Nat. Hist. (Geol.) 19, 1-26.</ref>, Williams 1982-88)(see list in Bibliography below) and is now renowned as the global boundary stratotype for the Ordovician/Silurian boundary (Williams & Ingham 1989)<ref>Williams, S.H. and Ingham, J.K. 1989. The Ordovician-Silurian Boundary Stratotype at Dob's Linn, southern Scotland. In Holland, C.H. and Bassett, M.G. (Editors). A Global Standard for the Silurian System, Nat. Mus. Wales, Geol. Ser. 9, 27-35.</ref>. This lies within a black shale sequence (the Moffat Shale Group) extending from the peltifer graptolite zone of the early Caradoc(Ordovician) to the maximus subzone of the late Llandovery (Silurian). Interbedded volcanic ash horizons are common; Merriman and Roberts (1990)<ref>Merriman, R.J. and Roberts, B. 1990. Metabentonites in the Moffat Shale Group; Southern Uplands of Scotland; geochemical evidence of ensialic marginal basin volcanism. Geol. Mag. 127, 259-271.</ref> describe 135 metabentonite beds cumulating to about 6 m and representing up to 20% of the strata in some biozones (Figure 41). Following Lapworth the Moffat Shale Group is divided into four units with informal formation status: in ascending stratigraphic order these are the Glenkiln Shales, the Lower Hartfell Shales and Upper Hartfell Shales and the Birkhill Shales. The latter are abruptly but conformably overlain by greywackes of the Gala Group, locally assigned to the Queensbury Formation.
[[File:DobsLinnExcursionMap.jpg|thumbnail|Overhead view of Dob's Linn.]]

Dob’s Linn is conveniently adjacent to the A708 Mofat-Selkirk road. An appropriate starting point for a visit is the watershed some 5 km south-west from the Loch of the Lowes and St. Mary’s Loch. Here on the wall of Birkhill Cottage (NT 202 158) is a plcque erected by Scottish geologists in 1930 commemorating Lapworth’s residence therein at various times during 1872 and 1878, and his discovery of the uses of graptolites in unravelling the stratigraphy and structure of the Southeren Uplands. Some 700 m. west of the cottage are two parking spaces on the north side of the road and conveniently situated for leaving cars or coaches prior to the descent into the Linn. Follow the streamlet from the parking space down to the main burn. Crossing the burn is normally very easy though it can be hazardous after heavy rain.

The floor of the valley follows the line of a major fault (the Main Fault of Lapworth). It is still worth noting that to the east of this line the rocks (Birkhill shales) are intensely crushed and folded, and little of stratigraphical value can be determined. All the instructive exposures lie to the west of the fault, the total sequence there is summarized below in descending order. The common graptolites in each zone are indicated, some are illustrated.
[[File:DobsLinnGeologicalColumn.jpg|thumbnail|Geological column, timescale after Harland (1989).]]

=== Gala Group (Queensberry Formation) ===
Thick greywacke beds with sole marked bases, interbedded thin shale bands. Top not seen here.

=== Birkhill Shales (Llandovery -Ashgill) 43 m. ===
'''''Rastrites maximus'' zone (6.6 m) '''

Grey mudstones with thin, sparsely fossiliferous black shales.

''Rastrites maximus, R. linnaei, R hybridus ,R. fugax, Monograptus ''

''sedgwickii, M. halli, M. attenuatus, M. spiralis''.

'''''Monograptus sedgwickii'' zone (8.4 m) '''

Black and grey mudstones, highly fossiliferous.

''Monograptus sedgwickii, M. involutus, M. decipiens, Pristiograptus regularis, P. jaculum, Climacograptus scalaris, Petalograptus tenuis''.

'''''Monograptus convolutus'' zone (5.4 m) '''

Black and grey mudstones, highly fossiliferous.

''Monograptus convolutus, M.lobiferus,M. clingani,M.limatulus, ''

''M. communis, Monoclimacis crenularis, Rastrites hybridus, Cephalograptus cometa, C. tubulariformis, Glyptograptus tamariscus, Orthograptus bellulus''.

'''''Monograptus gregarius'' zone (8 m)''' Some workers divide this into three zones, termed in descending order the ''leptotheca'', ''magnus,''and ''triangulatus'' zones. Soft black shales, highly fossiliferous.

''Coronograptus gregarius, Monograptus triangulatus, M. fimbriatus, M. communis, M. revolutus, Pribylograptus leptotheca, Rastrites peregrinus, K longispinus, Diplograptus magnus, Petalograptus spp., Clyptograptus tamariscus, Rhaphidograptus toernquisti, Orthograptus cyperoides''.

'''''Monograptus cyphus'' zone (7.3 m) '''

Hard black shales, highly fossiliferous. The basal part is sometimes separated off as the acinaces zone.

''Coronograptus cyphus, Atavograptus atavus, Lagarograptus acinaces, Monograptus revolutus, Dimorphograptus confertus, D. erectus, Cystograptus vesiculosus, Glyptograptus tamariscus, Climacograptus spp., Pribylograptus sandersoni''.

'''''Cystograptus vesiculosus'' zone (1.3 m) '''

Hard black shales, highly fossiliferous.

''Cystograptus vesiculosus, Dimorphograptus elongatus, Atavograptus atavus, Climacograptus'' spp. (e.g. ''C. normalis, C. medius, C. rectangularis''), ''Rhaphidograptus extenuatus''.

'''''Parakidograptus acuminatus'' zone (5 m) '''

Black shales, fossiliferous.

''Parakidograptus acuminatus, Akidograptus ascensus, Diplograptus modestus, Climacograptus'' spp. (e.g. ''C. normalis, C. angustus, C. medius, C. trifilis''), ''Atavograptus ceryx''.

'''Base of Silurian '''

'''''Glyptograptus persculptus'' zone (0.6 m) '''

Black shales with grey-green mudstone at base.

''Glyptograptus persculptus, Climacograptus angustus''.

A thin band with Climacograptus extraordinarius occurs about 1m below the base of the ''persculptus'' zone. Below this ''extraordinarius'' band a thin horizon of grey calcareous siltstone contains the blind trilobite, ''Mucronaspis'' sp.

=== Upper Hartfell Shales (Ashgill) ===
'''''Dicellograptus anceps'' zone (3.5 m) '''

Grey-green 'barren mudstones', unfossiliferous, except near the top where there are 5 thin 'anceps bands' of black shale.

''Dicellograptus anceps, D. ornatus, D. minor, Climacograptus supernus, Amplexograptus latus, C. normalis, C. miserabilis, Orthograptus abbreviatus, 'O'. fastigatus, Pleurograptus lui'', inarticulate brachiopods.

'''''Dicellograptus complanatus'' zone (8 m) '''

Grey-green 'barren mudstones', unfossiliferous, except for two thin 'complanatus bands' in the lower part.

Dicellograptus complanatus, Orthograptus socialis, inarticulate brachiopods.

=== Lower Hartfell Shales (Upper Caradoc) ===
'''Pleurograptus linearis zone (5 m) '''

Black shales, highly fossiliferous.

Pleurograptus linearis, Orthograptus quadrimucronatus, O. quo spinigerus, O. pauperatus, O. calcaratus basilicus, Dicellograptus morrisi, D. carruthersi, D. elegans, D. pumilus, Leptograptus flaccidus macer, L. capillaris, Climacograptus mohawkensis, C. tubuliferus, C. miserabilis, Amphigraptus divergens.'' ''

'''''Dicranograptus clingani'' zone (c. 8 m) '''

Hard, black, thin-bedded shales, highly fossiliferous.

''Dicranograptus clingani, D. ramosus, D. nicholsoni, Dicellograptus caduceus, D. flexuosus, D. morrisi, D. moffatensis, Climacograptus spiniferus, C. dorotheus, Orthograptus calcaratus, O. cal. basilicus, O. pageanus, O. amplexicaulis, Leptograptus flaccidus, Corynoides calicularis, Neurograptus margaritatus, Lasiograptus harknessi, Glyptograptus daviesi. ''

'''''Climacograptus wilsoni'' zone (c. 7 m) '''

Black shales with pale grey cherty horizons, sparsely fossiliferous. Some beds yield graptolites very well preserved in relief.

''Climacograptus wilsoni, Climacograptus bicornis, C. bi. tridentatus, Orthograptus calcaratus vulgatus, Dicellograptus forchhammeri, D. moffatensis, Glossograptus hincksi, Dicranograptus nicholsoni, Pseudoclimacograptus scharenbergi, Corynoides curtus, C. calicularis. ''

=== Glenkiln Shales (Lower Caradoc) ===
'''''Climacograptus peltifer'' zone (5 m, base not seen). '''

Hard cherty black shales, sparsely fossiliferous.

''Climacograptus bicornis peltifer, C. antiquus, Dicranograptus ziczac, D. nicholsoni, Dicellograptus cyathiformis, Orthograptus whitfieldi, Hallograptus mucronatus, Pseudoclimacograptus scharenbergi, Didymograptus superstes, Cryptograptus tricornis, Amplexograptus perexcavatus.''

The first three recommended sites for investigation lie at the foot of the Main Cliff on the west side of the valley (Plates 2 and 3), the others are along the Long Burn and the Linn Branch.

==== 1. Glenkiln age shales ====
The oldest beds at Dob's Linn are dark grey, poorly fossiliferous shales of Glenkiln age. These beds are not well exposed but fragments showing the characteristic lithology can be picked up from the scree slope. This is the only locality where the ''peltifer'' zone is exposed at Dob's Linn.

==== 2. The Main Cliff ====
The Main Cliff is formed by shales ranging from the ''clingani'' zone upwards into the Birkhill Shales. Post-glacial surface creep has rotated the beds from the regional near-vertical attitude to the sub-horizontal. This cliff thus enables several zones to be studied in ascending sequence.

The ''clingani'' zone is now largely covered by scree, but the linearis zone is well represented in a projecting bluff. The black shales here yield abundant ''Orthograptus'' and the zone fossil ''Pleurograptus linearis''. PLEASE DO NOT HAMMER THIS FRAGILE EXPOSURE; excellent specimens can be conceted from the scree, though they may be mixed with higher Ordovician and Silurian material derived from further up the cliff. Above the ''linearis'' zone the Barren Mudstones with their characteristic grey-green colour, form a substantial cliff, and the highly fossiliferous ''anceps'' bands may be located at the top of this barren sequence by those prepared to undertake the somewhat hazardous ascent. Still higher in the section are the lower zones of the Birkhill Shale, hard black shales yielding abundant graptolites. At the base of the Birkhill Shales here is seen the thin ''persculptus'' zone, with the zonal fossil and associated climacograptids, and above this the base of the Silurian is marked by the ''acuminatus'' zone in which are the first small monograptids ''Atavograptus ceryx''. Before leaving this locality, turn round and examine the eastern cliff which is formed of highly contorted Birkhill Shales. In these the fossils are very poorly preserved. Sma11 recumbent isoclinal folds are clearly seen some two or three metres above the river bed; the ubiquitous presence of such folds at various scales in the Southern Uplands may have influenced Lapworth and his contemporaries in the conception that isoclinal folding was the dominant structural control. Notice also the presence of 'head' on the ground surface; a fossil scree consisting of shale fragments and consolidated by a ferruginous cement during the post-glacial period.

==== 3. Barren mudstones ====
A small crag here exposes Barren Mudstones which were deposited during the late Ordovician Ice-age. At this time ice may have extended to quite low latitudes, and the Barren Mudstones may have been deposited beneath an ice-covered sea. Within this sequence are two thin ''complanatus'' bands which presumably signify a local warming episode during an otherwise glacial event. This is another fragile exposure and IT IS REQUESTED THAT YOU DO NOT HAMMER IT.

==== 4. Birkhill shales ====
Ascend to the prominent shaly scree slope just north of the junction of the Linn Branch and the Long Burn. From here one may look southwards to see the disturbed Birkhill Shales on the eastern bank of the main stream. The vertical bedding in the lower slopes close to the burn has been rotated by near-surface hill creep in the higher part of the cliff. Westwards along the Linn Branch one is confronted by the magnificent waterfall of Dob's Linn which plunges down a near-vertical series of bedding planes to the Linn Branch gorge. A large curving tool-mark is evident on the lower facing surface of the main greywacke unit. This was cut into the underlying mud, possibly by a flat skating pebble, immediately before deposition of the overlying greywacke sediment. The beds on the North Cliff, as seen at locality 2, have been overturned and inverted. Despite faulting they preserve a sequence consistently younging towards the waterfall.

==== 5. Long Burn ====
For the moment, however, proceed along the western side of the Long Burn until a small tributary is reached. This IS usually choked with large blocks of shale from the ''clingani'' zone with exquisitely preserved, and often large, graptolites such as ''Dicranograptus ramosus'' and ''Orthograptus calcaratus'' on the fissile bedding surfaces.

==== 6. clingani zone ====
The source of this material is a bowl-shaped depression,

much higher in the slope, which may be reached by direct ascent and via a narrow defile through which the tributary emerges. The depression is floored by masses of loose shale and is probably the best collecting ground for the graptolites of the ''clingani'' zone. The somewhat folded shale is exposed in situ in the walls of the depression.

==== 7. Top ''clingani'' zone ====
It is possible to climb out, though with care, to this locality, a trench dug in the late 1970s by Henry Williams. He collected in meticulous detail through the top ''clingani'' zone and the full extent of the ''linearis'' zone to establish the order of succession of graptolite faunas as firmly as possible (Williams, 1982)<ref>Williams, S.H. 1982. Upper Ordovician graptolites from the top Lower Hartfell Shale Formation (D. clingani and P. linearis zones) near Moffat, southern Scotland. Trans. R. Soc. Edinb: Earth Sci. 72, 229-255.</ref>.
[[File:AncepsBandsInLinnBranch.jpg|400px|thumbnail|The anceps bands and Ordovician/Silurian boundary stratotype in Linn Branch tranch (modified after Williams 1983).]]

==== 8. ''anceps'' bands ====
Descending to the Long Burn once more one may see a further exposure of the ''anceps'' bands just east of the Main Fault in a second trench cut by Williams and Ingham. Here the succession is, remarkably, about twice the thickness seen in localities to the west of the Main Fault. This is one reason for believing that the Main Fault is a major tectonic thrust which has juxtaposed rock units which may have originally been deposited a long distance away from each other. The ''anceps'' zone faunas are here both abundant and well preserved.

==== 9. ''cyphus'' zone of the Birkhill Shales ====

An energetic scramble up the Long Burn eventually gains a small waterfall some 3 m high (locality 9). Here is exposed a greywacke horizon, the first known in the sequence, lying within the ''cyphus'' zone of the Birkhill Shales (Rushton & Stone 1991)<ref>Rushton, A.W.A. and Stone, P. 1992. Terrigenous input to the Moffat Shale sequence, Southern Uplands of Scotland. Scott. Geol. 27, 167-170.</ref>; it can actually be traced some distance along the burn southwards. Above it, just south of the waterfall, are exposed shales of the cyphus and gregarius zones; they are highly fossiliferous and this is one of the best locations for seeing strata from these lower zones of the Silurian in an undisturbed state. Note, too, the soft pale metabentonites characteristic of this part of the sequence.
[[File:MetabentoniteDobsLinn.jpg|300px|thumbnail|Biostratigraphical distribution of metabentonite horizons at Dob's Linn and amounts and proportion of metabentonite in graptolitic zones.]]

==== 10. Lower to Upper (Barren) Hartfell Shales ====
Return to the junction of the Long Burn and the Linn Branch and now proceed westwards via a rough footpath above the Linn Branch, towards the waterfall and locality 10. The succession in the north face of the Linn Branch is near vertical to slightly overturned, younging is constant towards the west and this traverse first passes from Lower to Upper (Barren) Hartfell Shales. Further west a conspicuous eroded gully marks the line of the West Fault which downthrows to the north. Downthrow on this fault causes the aneeps zone at the top of the Ordovician to crop out at the level of the footpath, and the next few metres expose the critical Ordovician/Silurian boundary stratotype section.

This section, which spans strata containing evidence of a major extinction episode, was carefully excavated and documented by Henry Williams; his paper of 1988 records the succession in great detail. The five ''anceps'' bands are clearly shown, highly inclined in the eastern part of the trench within the upper part of the Barren Mudstone sequence. The fauna in these bands is remarkably rich, up to 18 species, including ''Dicellograptus anceps'', having being recorded. The thin ''extraordinarius'' band lies a metre higher than the top ''anceps'' band in the succession (and just above a nodula rlimestone which contains rare blind dalmanitid trilobites). Only three diplograptid species occur in the ''extraordinarius'' band, a dramatic drop from the rich fauna of the ''anceps'' bands. After a further metre of barren beds, black shales resume with abundant graptolites of the uppermost Ordovician ''perseulptus'' zone, and the Ordovician/Silurian boundary is taken at the base of the succeeding ''aeuminatus'' zone.

==== 11. ''vesieulosus'' and ''gregarius'' (''triangulatus'') zones ====
On moving into the Corrie, strata from two further zones may be examined on the eastern wall, the bedding is now overturned by hill creep. These are the flaggy beds of the ''vesieulosus'' zone, and the hard blocky shales of the ''gregarius'' ''(triangulatus)'' zone in which abundant and finely preserved ''Rastrites peregrinus'' are found. The thin shale bands with numerous interbedded bentonites may be followed right up to the top of the Corrie. Bentonites are especially abundant in the Birkhill Shales and represent successive episodes of volcanic ash-fall which, at least in some instances, have been correlated with successive episodes of graptolite extinction (Batchelor & Weir 1988)<ref>Batchelor, R. and Weir, J.A. 1988. Metabentonite geochemistry: magmatic cycles and graptolite extinctions at Dob's Linn, southern Scotland. Trans. R. Soc. Edinb: Earth Sci. 79, 19-41.</ref>. Shales of the ''sedgwickii'' zone are exposed in the floor of the Corrie and form a flat plane along its western wall. This zone, however, is better exposed on the south face of the Linn Branch.

==== 12 maximus subzone ====
Towards the foot of the waterfall the shales and mudstones become increasingly grey, with only a few black shale bands containing graptolites indicative of the ''maximus'' subzone. Fossils are, however, extremely rare at this locality. The overlying greywacke beds display a variety of turbidite features such as graded bedding and various sale structures.

==== 13. sedgwickii zone ====
Having examined the Gala greywackes an ascent may be made up the south face of the Linn Branch to the shales of the ''sedgwickii'' zone. The beds curve sinuously up the cliff, and fine specimens of this fossil and attendant faunas may be collected from near the top. However, this ascent is hazardous and is not recommended for large parties. We stress that a direct ascent from locality 12 to locality 15 should NOT be attempted.

==== 14. The corrie ====
It is recommended that an ascent be made up the Corrie to the top, where the major features of the geology can be seen from the grassy meadow. From here Birkhill Cottage is clearly visible and also the valley excavated along the line of the great strike fault to the south-west.

==== 15. Hartfell and Birkhill Shales ====
[[File:NorthCliffDobsLinn.jpg|400px|thumbnail|Panoramic view of North Cliff from locality 15.]]
Then proceed west and south crossing the stream above the waterfall, and descending eastwards to the flat platform above the south face of the Linn Branch where the spectacular panorama of the north bank of the Linn Branch can be compared with the diagram. Note the overturned succession of the Hartfell and Birkhill Shales, Williams' two excavations, the West Fault, and several thrusts in the shales and the greywackes. From the corner just above the north end of the Main Cliff good specimens of ''Cephalograptus corneta'' may be found in the ''convolutus'' zone. Descend from this point to valley floor, either down the scree slope or along the top of the Main Cliff, and thence return to the road.

===References===

<References/>

===Bibliography===
Williams, S.H. 1982. The Late Ordovician graptolite fauna of the Anceps Bands at Dob's Linn, southern Scotland. Geologica Palaeont. 16, 29-56.

Williams, S.H. 1982. Upper Ordovician graptolites from the top Lower Hartfell Shale Formation (D. clingani and P. linearis zones) near Moffat, southern Scotland. Trans. R. Soc. Edinb: Earth Sci. 72, 229-255.

Williams, S.H. 1983. The Ordovician-Silurian boundary graptolite fauna of Dob's Linn, southern Scotland. Palaeontology 26, 605-639.

Williams, S.H. 1986. Top Ordovician and lowest Silurian of Dob's Linn. In Hughes, C.P. and Rickards, R.B. (Editors). Palaeoecology and Biostratigraphy of Grapolites. Geol. Soc. Spec. Pub. 20, 165-171.

Williams, S.H. 1987. Upper Ordovician graptolites from D. complanatus Zone of the Moffat and Girvan districts and their significance for correlation. Scott. J. Geol. 23, 65-92.
Williams, S.H.1988. Dob's Linn - the Ordovician Silurian Boundary Stratotype. Bull. Br. Mus. Nat. Flist.(Ceol.) 43,17-30.

{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P772042.jpg

2015-12-09T11:34:28Z

JenniferFindlay1: Silurian (Llandovery) graptolites from southern Scotland. Locality is probabably Dob's Linn, Moffatdale. 1:50 k sheet 16E (Ettrick), NT 196 158. P772042

== Summary ==
Silurian (Llandovery) graptolites from southern Scotland. Locality is probabably Dob's Linn, Moffatdale. 1:50 k sheet 16E (Ettrick), NT 196 158. P772042
== Licencing ==
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File:P571542.jpg

2015-12-09T10:22:20Z

JenniferFindlay1: View towards Dob's Linn, Moffatdale, Main Branch round spur on right, Linn Branch on left in from of red scar. P571542

== Summary ==
View towards Dob's Linn, Moffatdale, Main Branch round spur on right, Linn Branch on left in from of red scar. P571542
== Licencing ==
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| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

====Non-commercial Use====

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When using the images please credit 'British Geological Survey' and include the catalogue reference ('P Number') of the item to allow others to access the original image or document.
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[[Category:License tags]]

Girvan and Ballantrae - an excursion

2015-11-17T12:27:03Z

JenniferFindlay1:

By P Stone. Excursion 8. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Girvan and Ballantrae: an obducted ophiolite ==

OS 1: 50 000 Sheet 76 Girvan

BGS 1: 50 000 Sheet 7 Girvan, 8W Carrick and 14W Ayr

BGS 1: 25 000 Sheet IVX08, 18 and 19 (in part)

=== Main points of interest ===
Early Ordovician Ballantrae ophiolite Complex (pillow lavas, gabbro, serpentinite, chert and melange), late Ordovician and early Silurian greywacke and conglomerate; late Ordovician reef limestone, Caledonian folding.

=== Logistics ===
There is ample parking for cars and space for a coach near most localities. Most of the exposures are on the coast and require much scrambling over steep and sometimes slippery rocks. Maximum walk at any one locality is about 2.5 km along beach and cliff paths. The outcrops are best seen at low tide and ideally the excursion should be planned to coincide with low water at about 1pm; this should allow adequate access from 9am to 5pm. Most of the localities described have been designated Sites of Special Scientific Interest (Appendix 2).

=== Introduction ===
[[File:GirvanBallantraeExcursionMap.jpg|400px|thumbnail|Locality map for the Girvan-Ballantrae excursion and outline geology of the Ballantrae Ophiolite Complex.]]
The ophiolitic Ballantrae Complex and its sedimentary cover are well exposed between Girvan and Ballantrae. The area has been a focus of considerable geological research (summarised by '''Stone and Smellie, 1988'''). The geological complexity of the area is such that several days are required to examine all of the varied features exposed. Thus, previous excursion guides have included a three-day itinerary ('''Robertson et al., 1990''') and the seven itineraries recently published by '''Bluck and Ingham (1992)'''. The field excursion described below is intended to bring out the main features of the complex and its unconformable sedimentary cover in a single day. Those with more time available are referred particularly to '''Bluck and Ingham (1992)'''.

The ophiolite complex consists of two main elements interleaved by faulting; serpentinised ultramafic rocks representing oceanic mantle and volcanic sequences representing the remains of island arc and ocean crust. The structure is dominated by NE–SW faults which divide the complex into discrete lithological zones such that northern and southern serpentinite belts separate three areas of mainly volcanic rock '''(Figure 30)'''. A late Tremadoc to early Arenig age has been established for the eruption of the volcanic components (Balcreuchan Group); the interbedded sedimentary strata contain graptolite faunas of that age ('''Stone and Rushton, 1983; Rushton et al., 1986)''' and Sm-Nd radiometric dating of the basalts has given ages of 501 ± 12 and 476 ± 14 Ma ('''Thirlwall and Bluck, 1984)'''. It is believed that the ophiolite was obducted on to the continental margin during the middle Arenig, since metamorphic rocks formed at this stage have been dated by the K-Ar method at 478 ± 8 Ma ('''Bluck et al., 1980'''). Some late Arenig sedimentary rocks, probably deposited during the final stages of obduction, are structurally included within the ophiolite ('''Smellie and Stone, 1992''') and the oldest strata within the unconformably overlying cover sequence (Barr Group) are of Llanvirn age. Sedimentation above the ophiolite was controlled by a series of faults, downthrowing to the south and sequentially stepping back northwards. Thus the basal conglomeratic facies becomes progressively younger northwards; in the south the basal conglomerate is Llanvirn, in the north Caradoc ('''Williams, 1959'''; '''Ince, 1984'''). Above the basal conglomerate, which is commonly associated with shallow-marine limestone, facies become progressively more deep water upwards so that turbiditic greywackes and shales form much of the exposed sequence. This is more or less continuous up into the Silurian, with only a slight stratigraphical break and small angular discordance in bedding at the Ordovician–Silurian boundary. The main tectonism occurred late in the Silurian and produced folding and north-directed thrusts.

The ophiolite complex is examined at Localities 1 to 4, and the sedimentary cover at Localities 5 to 7. Locality 7 should be regarded as an alternative stop in a single day excursion but could be included in a more leisurely 2-day schedule. Since many of the coastal exposures are tide-dependent (local details given below) the order in which localities are visited may have to be varied to suit conditions. '''Tide tables '''should be '''carefully checked '''and the excursion planned accordingly.

===Excursion===
==== 1 Balcreuchan Port: structural imbrication in Arenig lavas and lava breccias ====
[[File:GirvanBallentraeBalcreuchanPointExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for Balcreuchan Port (Locality 1) and the coastal section to the south-west.]]
A convenient place to begin the excursion is the large lay-by overlooking Balcreuchan Port on the seaward side of the A77 (Figure 31) (NX 100 876). There is a fine view over the Firth of Clyde towards Arran and to Ailsa Craig, a spectacular Tertiary micro-granite plug ('''Harrison et al., 1987''').

From the car park descend into Balcreuchan Port by the steep footpath on the east side of the cove. Take great care, the slopes are very steep so do not leave the footpath. However, note the cliffs to the south and east; pillows of basalt lava can be seen on both sides.

At beach level there is extensive intertidal rock outcrop. Prominent here is a Tertiary basalt dyke, up to about 50 cm across and trending generally north across the foreshore. The dyke has resisted erosion and now stands proud of its host rock. Abundant amygdales are concentrated into zones parallel to the dyke margin and are normally restricted to only one side of the dyke.

The Tertiary dyke is intruded into highly altered ultramafic rock which at this locality consists largely of a mass of secondary quartz and carbonate veins. Such alteration is fairly common at the margins of serpentinised ultramafic bodies and is generally regarded as a side effect of the serpentinisation process. A north–south fault marks the east margin of Balcreuchan Port, beyond which the steep sea cliffs are formed by basalt lavas, both massive and pillowed. The petrographical and geochemical characteristics of the volcanic rocks are typical of lavas erupted in oceanic island arcs above subduction zones (Thirlwall and Bluck, 1984; Smellie and Stone, 1988 and references therein). Most of the sequence is tholeiitic but it includes some boninitic lavas with exceptionally high contents of Cr and Ni. This lava variety is relatively rare and modern examples are found exclusively in oceanic island arcs ('''Smellie and Stone, 1992'''). Within the lava sequence a cave, controlled by minor faulting, is reputed to have been the home of Sawney Bean and his family, the notorious 16th century cannibals.

From the cave and the adjacent boninitic lavas cross SW towards the opposite side of the bay. About two-thirds of the way across '''(1a on Figure 31)''', serpentinised ultramafic lithologies are exposed below mid-tide level: these are dunite (almost entirely olivine) and harzburgite containing both olivine and orthopyroxene. The pyroxene can be seen as bronze-coloured flecks in the background of dark green serpentinised olivine. A gently inclined contact, possibly thrust, separates the ultramafic rocks from the overlying lavas and lava breccias forming the SW headland of Balcreuchan Port. The route continues SW over the headland, an easy scramble at mid to low tide but quite difficult at high tide, and across the next small bay to the cliffs on its far side. A prominent fault gully trends south (inland) from this point and its western side is formed by steeply dipping clastic sedimentary strata; note the marked swing in strike adjacent to the fault. Sandstones and melange-like breccia (probably formed by slumping) make up most of the sequence, but an important intercalation of laminated red and cream fine-grained sandstone '''(1b) '''can be seen at the west end of the rock platform (just above low water mark). This laminated bed is important on several accounts, not least for the contained graptolite fauna which establishes an early Arenig age for this part of the sequence ('''Stone and Rushton, 1983'''). The lithology is distinctive, making it a readily identifiable marker horizon, and its sedimentological features allow the younging direction to be established. Check the layering carefully; it is cut out westward by a coarse feldspathic sandstone which is in turn overlain by a repeat of the red and cream striped lithology, but this time in a jumbled, chaotic form. A likely interpretation is that a channel was eroded into the striped sandstone and partially filled by the coarse feldspathic sandstone; the channel walls then collapsed to give the chaotic deposit. The younging direction is clearly to the west.

Rejoin the footpath above these crags and continue west for a few metres to the next prominent outcrop. This is formed of reddened basaltic pillow lavas rich in feldspar phenocrysts and, at the east side of the outcrop, a conformable relationship can be established between the lava and fine-grained sandstone. Bearing in mind the younging direction established earlier, the lava almost certainly overlies the sandstone. These lavas, and those to be examined subsequently towards Port Vad and Bennane Head, have the geochemical characteristics of oceanic island or hot-spot lavas similar to the modern example of Hawaii. Such a major change in lava type emphasises the importance of the fault to the east of locality lb.

For the next few tens of metres westward the feldsparphyric pillow lavas are well exposed, particularly on the flat surfaces overlooking the sea. An approximate north–south strike of steeply inclined bedding can be readily established, and a continuation of the westward younging confirmed, from the shape of the pillows. These have smooth convex upper surfaces but more irregular lower surfaces which bulge and drape into underlying cavities. Note the red chert filling spaces between some of the pillows. The feldsparphyric pillow lavas are exposed on the next rocky headland (1c) but there they are cut by dykes of fine-grained basalt, best seen on the seaward end of the headland. The dykes are taken to be feeders for the next higher unit of pillow lavas because these have the same aphyric composition.

The contact between the two lava types is exposed in a fault gully about 15 m farther west (1d). A deep cleft is open to the sea but at the narrower, inland end the rock in the gully walls can he examined. '''Take great care; the sea is a long way down and the exposure is '''precarious. The eastern side of the gully is formed of feldsparphyric lava and the lowermost pillows in the western wall are also feldsparphyric. However, these are conformably overlain by aphyric pillow lavas, which then form the sequence continuing westward.

Continue west on the footpath, crossing faulted and brecciated aphyric lavas, towards the mouth of the Bennane Burn. Stratified, fine-grained elastic rocks overlying the brecciated lavas, are best examined beside a sea-water pool on the SW side of the burn ('''1e'''). Interbedded sandstone, chert and dark shale form a small cliff; the shale has yielded graptolites of middle Arenig age ('''Stone and Rushton, 1983)'''. Thus, the traverse has passed from the lower to the middle Arenig, confirming the sedimentological evidence for westward younging.

From the sea-water pool climb inland for a short distance up a cattle track into a shallow NW–SE gully with rock forming low crags along its SW side. These crags expose a familiar sequence: reddened feldsparphyric pillow lavas conformably overlie fine-grained elastic strata including a red and cream striped sandstone remarkably similar to the early Arenig example seen at Locality ('''1b'''). The feldsparphyric lavas are well exposed to the SW and provide abundant evidence of continued and consistent steep dip and SW-younging. Cheri and siliceous sandstone interbeds occur at intervals and from one of these an early Arenig graptolite fauna has been collected ('''Stone and Rushton, 1983'''). The comparison with the sequence traversed at Localities lb to Id is then further strengthened by the appearance of aphyric pillow lavas above and to the SW of the feldsparphyric pillows. The aphyric pillows are well exposed in landward-facing cliffs at the margin of a small embayment about 200 m SW from Bennane Burn ('''1f''') but from there the coastal cliffs become impassable. A major fault repeating the succession seems probable and the most likely site is the NW–SE gully followed by the cattle track at Locality le above Bennane Burn.

Return to the car park above Balcreuchan Port and proceed to Locality 2, where the southward continuation of the section can be seen.

==== 2 Bennane Lea: Balcreuchan Group (Arenig) conglomerate and chert ====
South from Balcreuchan Port the A77 has been re-routed inland for about 2 km to Bennane Lea. Some parking space is available on the seaward side of the road where it rejoins the coast and the extensive raised beach (NX 092 858). Vehicles should be left outside the cattle grid; access to the beach section is via a small sand pit. The old A77 road is now a private access route and should not be used. Further details and a full description of the sections are also provided by '''Bluck (1978)''' and in '''Bluck and Ingham (1992).'''

Exposures of Permian red sandstone beds, dipping gently south, can be examined at low tide. These lie at the edge of an extensive offshore Permian basin, the faulted eastern margin of which runs beneath the raised beach between Bennane Lea and Ballantrae. However, locally at Bennane Lea the Permian strata are unconformable on the Ballantrae Complex and the basal red sandstone contains clasts of spilitic lava ('''Stone, 1988''').

There is a marked topographical change at Bennane Lea: steep sea cliffs to the north contrast with the raised beach, backed by relic sea cliffs cut in glacial till, to the south. [[File:P219962.jpg|thumb|300px|right|Coast N. of Ballantrae, Bennane Lea. Tight, upright folds in bedded chert. The hinge of a synform is well exposed in the smaller stack on the right of the photograph. P219962]]The change takes place across a faulted junction between basaltic lava and breccia, to the north, and less-resistant ultramafic rock of the Southern Serpentinite Belt, to the south. The ultramafic rock exposed on the shore is altered and reddened, it also contains pods of gabbroic composition which are probably tectonic inclusions. The fault itself is exposed on the foreshore at Bennane Lea (subject to the vagaries of drifting sand) as a thin zone of silicification, north of which a massive tuff unit forms the first rocky outcrop. Traversing northwards a conformable contact between the tuff and underlying thinly bedded cherts can be seen. The cherts themselves contain altered radiolaria and are chaotically deformed into small-scale, disharmonic structures which seem most likely to be the result of soft sediment deformation through slumping. Slightly farther north, mass-flow conglomerates are interbedded with the cherts; the pebbles and cobbles present are mainly of spilitic lava and all can be related to lithologies exposed elsewhere in the Ballantrae Complex. Pale blocks, seemingly of limestone, at first appear out of place but contained chrome spinel grains suggest that they are likely to have originated as ultramafic rock. Alternations of bedded chert and conglomerate continue north for about 100 m, folded about several large, upright hinges which plunge steeply seawards. These are tectonic structures and can be correlated with the large anticlines and synclines clearly visible in the steep cliffs on the inland side of the old A77 road. The steep hinge plunges may bring down to the beach exposures the highest stratigraphical levels preserved, i.e. the mass flow conglomerates which are certainly not present in any of the inland outcrops.

Farther along, to the north of Bennane Cave, discontinuous layers of coarse feldspathic sandstone are interbedded with the chert. At one important exposure (NX 0909 8627) such sandstone locally forms about 30 per cent of the sequence; close by and slightly north, black siliceous mudstone, locally stained green by secondary copper minerals, is exposed between large boulders. There is a marked change of strike in this vicinity, and evidence for much minor faulting, but there is a consensus that the black mudstone stratigraphically underlies the chert and sandstone ('''Bluck and Ingham, 1992'''; '''Stone and Smellie, 1988''', table 5). Slightly farther north, and lower in the sequence, similar siliceous mudstone layers are interbedded with basalt lava and breccia. A graptolite fauna recovered from these mudstones gave a middle Arenig age ('''Stone and Rushton, 1983'''), similar to that obtained at the sea-water pool (Locality le) in the Balcreuchan Port traverse. Fracture planes within the black mudstone may be coated with green, secondary copper minerals such as malachite.

North towards Bennane Head the rock exposed at sea level for the first few hundred metres is predominantly basalt lava but the cliffs inland of the old A77 consist mainly of basalt breccia which extends seaward to form the steep cliffs of Bennane Head itself. The breccia is believed to overlie the middle Arenig sequence seen at Locality le ('''Stone and Smellie, 1988''', table 5). It can be most readily examined by continuing north for about 300 m to the southern flanks of Bennane Head (NX 091 865) where it includes beds of coarse-grained sandstone. However, the general lithology can be examined in the abundant large loose blocks which surround the black siliceous mudstone exposures. This completes the section from Balcreuchan Port to Bennane Lea, a traverse from island arc lavas into and through an oceanic island volcano-sedimentary assemblage. Return along the beach to the parking area.

==== 3 Carleton Fishery: ultramafic rock and altered dolerite ====
The next locality, Carleton Fishery, is about 5 km north along the A77. Ample parking is available in the well-signposted picnic site (123 894). Walk east for about 100 m to the old black boathouse and descend to the beach on the NE side of the rock outcrop.

The outcrop is within the Northern Serpentinite Belt and ultramafic rock, mainly dunite, is exposed between the loose boulders and shingle. Locally, layered relationships on a centimetre scale are developed between the dunite and coarser-grained, pyroxene-rich harzburgite. The main mass of the outcrop is composed of dolerite, which was probably originally intrusive into the dunite. It has suffered extensive calcium metasomatism and this has produced the fine-grained, flinty appearance of the marginal dolerite which is now composed of an assemblage of calcium-rich secondary minerals (e.g. prehnite, pectolite, hydrogrossular) known as rodingite. Large relict feldspar phenocrysts can be seen in some parts of the dolerite, and a large ultramafic enclave forms an eroded hollow on the top of the main rock mass. Another feature of interest is the network of thin (< 1.5 mm) chrysotile asbestos veins, seen in the ultramafic rock towards low water mark on the north side of the outcrop. The veins are mostly developed adjacent to the dolerite and parallel to its margins. Return to the parking area at the Carleton Fishery picnic site.

==== 4 Bonney's Dyke and Pinbain Beach: gabbro pegmatite, Balcreuchan Group melange and breccia ====
[[File:P556396.jpg|300px|thumbnail|left|Bonney's Dyke (pegmatitic gabbro sheet intruded into serpentinite) Ballantrae. Contact with serpentinite (darker rock at base). P556396.]]
About 2.5 km north from Carleton Fishery is a locality known as 'Bonney's Dyke'. Parking is available in a rough lay-by on the seaward side of the A77 (136 910).

Bonney's Dyke is a term used in the geological literature for a mass of pegmatitic gabbro within the Northern Serpentinite Belt. The name derives from Professor T G Bonney, the eminent Victorian mineralogist who recognised the igneous origins of much of the Ballantrae Complex ('''Bonney, 1878'''). When approached from the south the gabbro stands out as a paler, more resistant body within the ultramafic rock. It is exposed just above high water mark and in the intertidal zone. The outcrop is slightly arcuate owing to the cumulative effect of minor sinistral wrench faults. The pegmatitic texture is spectacular, with plagioclase and altered clinopyroxene crystals up to 3 cm across within zones of marked grain-size variation. There is no sign of chilling against either the surrounding ultramafic rock or the numerous serpentinite xenoliths. Three types of marginal contact relationship are seen:

* sharp gabbro-serpentinite contacts;

* less well-defined margins where the gabbro is in contact with coarse pyroxenite veins;

* sheared margins that are fine grained, flinty and particularly intensively Cametasomatised (rodingitised).

The south side of Bonney's Dyke shows a combination of types 1 and 2 whereas the north side is principally a sheared contact of type 3.

Pyroxenite veins occur intermittently throughout the ultramafic outcrop; they are pale green, coarse grained and up to about 50 cm across. The coarsest developments contain pyroxene crystals several centimetres across and can be seen slightly to the south of the gabbro towards low water mark.

From Bonney's Dyke walk north along the shore for about 350 m towards Pinbain Bridge. Several low intertidal outcrops expose fine-grained serpentinised harzburgite, although the movement of the beach sands may occasionally obscure them. Alternatively, drive up to a small lay-by on the seaward side of the road at Pinbain Bridge (NX 137 913). The Pinbain beach section, also described by '''Bluck (1978''') and in '''Bluck and Ingham (1992'''), exposes the contact between the Northern Serpentinite Belt and the Pinbain volcano-sedimentary sequence. A Tertiary dyke trending approximately east-west is intruded along the contact and has baked the adjacent serpentinite so that it now stands out as the more resistant lithology, a reversal of the normal situation. North of the dyke, a mélange deposit is well exposed in the intertidal zone; within its foliated muddy matrix are clasts of basalt and rarer amphibolite and schist. Prominent large, pale grey, carbonate blocks are not of organic or even sedimentary origin; residual grains of chrome spinel within the carbonate reveal that they are altered ultramafic rocks. The origin of the mélange deposit was probably by mass flow but much of the foliation through the matrix may have been imparted by subsequent tectonic shearing.

The highest point within this sequence of rocky outcrops consists of brecciated spilitic pillow lavas. Many of the pillows retain their shape despite pervasive cracks but others have completely disaggregated. There is no fine-grained matrix with this deposit and it is uncertain whether it forms a discrete unit interbedded with the mélange or whether it is part of a very large clast contained within the mélange. The latter lithology is also exposed to the north of the breccia but there the proportion of clasts is higher, and the foliation less marked, than in the exposure to the south.

Farther north a complex fault zone reintroduces tectonic slivers of serpentinite into the section. These are not exposed at beach level but form the cliff behind the isolated raised beach inland of the A77 where some dunite contains large pods of chrome spinel. The main mass of steep cliffs and rocky coastal outcrop to the north is composed of volcaniclastic sandstone, the faulted contact with the serpentinite slivers being intruded by Tertiary dykes. These are exposed at beach level immediately south of the sandstones. The latter are at the base of a thick sequence of lavas and elastic sedimentary rock which is exposed continuously for some distance north along the coast. Graptolites have been recovered at the base of the sequence from siltstones which are exposed on the inland roadside of the A77 behind the crash barriers. An early Arenig age was deduced by Rushton et al. (1986) but the specimens are generally fragmentary, very scarce, and only recovered with much patient effort.

==== 5 Kennedy's Pass: Caradocian Kilranny Conglomerate and Ardwell Formation ====
[[File:GirvanBallentraeStratigraphicalRelationships.jpg|300px|thumbnail|Schematic illustration of stratigraphical relationships above the Balantrae Complex south of Girvan. Northwards marine transgression was controlled by a sequence of faults (developed in the order 1 to 3 etc.) throwing down to the south.]]
Drive about 2.5 km north from Pinbain to Kennedy's Pass. Ample parking is available in a lay-by on the seaward side of the road (NX 149 932).

This locality allows examination of some of the late Ordovician strata which form an unconformable cover to the Ballantrae ophiolite. It is also described in an extensive field itinerary for the cover sequence given in '''Bluck and Ingham (1992)'''. The full cover sequence records north-westward marine transgression, from the late Llanvirn onwards, across the obducted ophiolite. Sedimentation was controlled by a series of faults throwing down to the south but becoming sequentially younger to the north. The overall geometry is summarised in Figure 32, based largely on the work of '''Williams (1959'''; cf. '''Ingham, 1978''' and '''Ince, 1984'''). The Barr Group rests unconformably on the ophiolitic rocks and continues up to the base of the Caradoc, whence the conformably succeeding Ardmillan Group ranges up to the high Ashgill.

At Kennedy's Pass the Kilranny Conglomerate is well exposed in the sea cliffs below the parking area. The Barr Group and the unconformable base of the sequence are here both faulted out and the Kilranny Conglomerate, low in the Ardmillan Group, is the lowest unit exposed. However, in lithology it is typical of the Barr Group conglomerates (e.g. Benan Conglomerate) which rest unconformably on the ophiolite elsewhere.

Overall the Kilranny Conglomerate is crudely stratified with both clast- and matrix-supported lithologies present. Some beds show clast imbrication and from this can be deduced a palaeocurrent flow from the north. Clasts range up to 1 m across and include red chert, basalt and gabbro (probably derived from the underlying ophiolite) and abundant felsitic and granitic rocks. Prominent amongst the latter are clasts of pink granite which have been dated by the Rb-Sr method at about 470 Ma (Longman et al., 1979). This suggests that intrusion occurred only a relatively short time before deposition (early Caradoc is approximately 455 Ma) and so rapid uplift and erosion of the source hinterland seem likely.

The conglomerate beds become younger northwards, and near the north end of the cliff exposure they are unconformably overlain by thinly bedded siltstones and greywackes of the Ardwell Formation. These are turbidites and appear to fill a channel eroded into the top of the underlying conglomerate. They are best examined about 100 m farther north where wave-polished surfaces reveal good examples of graded bedding and fine lamination. However, the most striking aspect of this section is the spectacular development of large and small chevron 'box' folds. Northwards from Kennedy's Pass the fold hinges trend approximately NE but plunge is variable; the southernmost examples plunge gently SW but, farther north, plunge passes through the horizontal and progressively steepens to about 45° NE. There is some controversy over the origin of these folds, which have been variously described as products of late Caledonian tectonism or as slump folds produced by the downslope movement of unconsolidated sediment: the former interpretation seems the most likely.

==== 6 Cow Rock, Horse Rock and Craigskelly: contrasting early Llandovery conglomerates ====
Drive north from Kennedy's Pass for about 5 km to a large car park beside the beach on the southern outskirts of Girvan (NX 182 964). There are toilet facilities here.

From the car park walk south along the beach on the seaward side of the Ainslie Manor Nursing Home (formerly the Haven Hotel). This section also forms part of an extensive itinerary in '''Bluck and Ingham (1992)'''. Outcrops between the beach and the Nursing Home consist of a coarse, mainly matrix-supported conglomerate containing abundant quartz pebbles and siltstone clasts, many quite angular. A small proportion of metamorphic lithologies is also present. The bed is known informally as the `quartz conglomerate' and is stratigraphically a part of the Scart Grit. Southwards the number and size of siltstone clasts increases and at Cow Rock, NW of the Nursing Home, the base of the conglomerate can be seen (at low tide) to channel into an underlying siltstone and fine greywacke sequence. This is the Woodland Formation, which contains a sparse shelly fauna of Llandovery age ('''Cocks and Toghill, 1973'''). The topmost few metres of the Woodland Formation, beneath the quartz conglomerate, are much disturbed by slumping.

The Woodland Formation underlies the small sandy beach extending for about 30 m SW towards the next rock outcrop, the Horse Rock. This is also formed of conglomerate but of a very different character to that previously seen. At the Horse Rock, and also on Craigskelly which can be reached at low tide, the Craigskelly Conglomerate is well exposed as a polymict and clast-supported lithology. It contains rounded pebbles of acid and basic igneous rock, some metamorphic fragments, jasper and clastic turbidite strata. The beds are quite thick, reaching about 8 m on Craigskelly, but at the Horse Rock the conglomerate is interbedded with turbidite greywacke units up to about 50 cm thick. The base of the Craigskelly Conglomerate is exposed on the SW side of the Horse Rock and may be seen at low tide subject to the vagaries of the shifting beach sand. An unconformable but sharp planar contact occurs between the conglomerate and the underlying thin greywacke and shale beds of the Shalloch Formation. The latter is of mid-Ashgill (Ordovician) age and the Craigskelly Conglomerate is taken to mark the base of the Silurian.

Palaeocurrent evidence, deduced from clast imbrication and bottom structures, indicates that the Craigskelly and quartz conglomerates were both derived from the NW. The cause of the abrupt change in character of the source terrane during the early Silurian is a matter of speculation.

Localities 1 to 6 will provide a full day's excursion and cover many points of interest within the Ballantrae Complex and its sedimentary cover. However, many of the exposures require examination at low tide and an inland site, Locality 7, is suggested as a partial alternative if the tides are unfavourable.

==== 7 Craighead Quarry: Caradocian Reef Limestone ====
Craighead Quarry exposes Caradocian reef limestone overlying lavas of the Ballantrae Complex and is a Site of Special Scientific Interest for botanical as well as geological reasons. It is described as part of an extensive excursion itinerary for the Craighead Inlier in '''Bluck and Ingham (1992)'''.

From Girvan drive north on the A77 towards Ayr and then turn right on to the B741 and continue for about 5 km. At Low Craighead Farm turn left; there is parking space for 3 or 4 cars in the entrance of the track leading to a disused quarry about 200 m beyond the farm on the left of the road (NX 235 014). Take care not to obstruct the adjacent farm track. Access to the quarry is via the track and thence by a footpath on the right which leads down on to the quarry floor. The footpath is frequently overgrown and may not be obvious.

The quarry walls expose parts of a late Ordovician limestone reef assemblage (Craighead Limestone) stratigraphically equivalent to a level within the Ardwell Formation '''(Figure 32)'''. On the NW side of the quarry a dark mass of spilitic lava can be seen overlain by a limestone breccia containing much algal debris and algal-cemented basalt clasts. The lava is thought to be an inlier of the Ballantrae ophiolite seen at Localities 1-4. The stratigraphical relationships, with Caradocian reef limestone overlying Arenig ophiolite lava, continue the trend of north-westward transgression discussed earlier. The eastern walls of the quarry reveal a variety of reef-flank limestone types containing abundant, although mainly broken, fossils; corals and crinoids are the commonest groups. There is abundant evidence for slumping. The sequence within the quarry is much disrupted by faulting, probably related in the main to Carboniferous movement on the nearby Kerse Loch Fault.
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P219962.jpg

2015-11-17T12:16:46Z

JenniferFindlay1: Coast N. of Ballantrae, Bennane Lea. Tight, upright folds in bedded chert. The hinge of a synform is well exposed in the smaller stack on the right of the photograph.

== Summary ==
Coast N. of Ballantrae, Bennane Lea. Tight, upright folds in bedded chert. The hinge of a synform is well exposed in the smaller stack on the right of the photograph.
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Heriot and Innerleithen - an excursion

2015-11-17T11:56:49Z

JenniferFindlay1:

'''By A.J. Weir. From [[Scottish Borders geology: an excursion guide]] edited by A.D. McAdam, E.N.K. Clarkson, P. Stone. Edinburgh : Scottish Academic Press (for [http://www.edinburghgeolsoc.org/ Edinburgh Geological Society]), 1992.'''

OS1:50000 Sheet 73 (Galashiels & Ettrick Forest)

BGS. 1:50000 Sheets 24 E (Biggar), 25 W (Peebles) and 32 E (Edinburgh).

=== Introduction ===
The excursion is designed to show a variety of Lower Palaeozoic rock types (greywackes, shales, cherts and tuffs), their associated sedimentary and tectonic structures, and their depositional relationships. Graptolites and trace fossils can also be collected. A small granitoid pluton is included and, in addition, the excursion affords excellent illustrations of geological control over the topography (Walton 1975). It is designed as a circular tour and can be started from Middleton as described or Innerleithen or Caddonfoot. Some localities can conveniently be combined with parts of the Tweedsmuir, Dob's Linn or Hartfell Score excursions.

From Middleton take the A7 Edinburgh-Dalkeith-Galashiels road for 2 km towards Galashiels and then turn right on to the B7007 road to Innerleithen. This road runs over flat, low-lying land, underlain by relatively soft Carboniferous sedimentary rocks of the Inverclyde and Strathclyde groups, southwards towards the Lammermuir Fault. The fault itself is not exposed but the resistant Ordovician greywackes form a prominent scarp on the south side of the fault, especially impressive from this approach.
[[File:HeriotInnerleithenExcursionMap.jpg|300px|thumbnail|Heriot and Innerleithen - excursion map.jpg]]

==== 1. Wull Muir: Viewpoint ====
The road curves sharply southwest into the line of the fault, and climbs obliquely for about 100 m to the top of the scarp, whence it again turns sharply southwards. At this point (NT 349 544) advantage should be taken of the very fine view across the southern half of the Midland Valley. Lower Carboniferous sedimentary rocks underlie the flat moorland in the foreground, but the topography becomes more varied to the north and west where the Lower Limestone Group crops out. Beyond, scattered colliery bings indicate the outcrops of the Limestone Coal Group ('Edge Coals') and the Coal Measures In the distance, beyond the Pentland Fault and the Carboniferous terrain rise the Pentland Hills. These are composed of Silurian and Old Red Sandstone rocks; the lower, rounded hills are formed of sandstone, whereas the higher steep-sided hills to the north consist of lavas. The height of the range decreases towards the north-east and the city of Edinburgh with its Castle Rock, a glacial crag-and-tail feature formed around a Carboniferous volcanic neck. The Salisbury Craig Carboniferous teschenite sill forms the prominent scarp feature to the right which, in turn, merges with the rugged mass of Arthur's Seat, another Carboniferous volcanic neck. Under favourable conditions the view extends across the Firth of Forth to Fife and westwards to the Ochil Hills, another range formed of Old Red Sandstone lavas and separated from the Pentland Hills lavas by a second major Carboniferous basin. In Fife, especially, this basin is dominated by the Midland Valley Sill of Permo-Carboniferous quartz-dolerite, which forms the distant ridge of the Lomond Hills to the north-east. Two prominent summits superimposed on this ridge are the Permo-Carboniferous vents of West and East Lomond. On an exceptionally clear day the southern margin of the Grampian Highlands, beyond the Highland Boundary Fault, may be seen.

==== 2. Broadlaw Quarry: Granodiorite ====
Leave the road and continue west along the scarp for 1 km where there is a disused quarry (NT 339 539) in the Broadlaw Granodiorite, one of a number of small, Late Caledonian granitoid intrusions ranged along the southern side of the Southern Upland Fault. The granodiorite body is not more than 1 km in length (NE-SW) and about 100 m in maximum width, elongated roughly parallel to the regional strike of the Ordovician greywacke country rocks. The contact of the granodiorite with the greywackes is nowhere exposed but an intrusive relationship was confirmed by Mould (1947)<ref>Mould, D.D.C.P. 1947. The Broadlaw 'granite'. Geol. Mag. 84, 178-80. </ref> who reported a small roadside exposure of thermally metamorphosed and hornfelsed mudstones and greywackes.
[[File:BroadlawExcursionMap.jpg|thumbnail|Broadlaw - excursion map.]]
The granodiorite exposed in the quarry is grey in colour except in the south-east corner of the quarry where it is red. Both colour variations consist essentially of quartz, plagioclase and orthoclase feldspar, and biotite; accessory minerals include zircon, pyrite and apatite. The grey granodiorite is porphyritic, with phenocrysts of plagioclase (oligoclase with labradorite cores) set in a finer-grained matrix and brown biotite mostly altered to chlorite. The slightly more acidic red granodiorite is medium-grained with plagioclase, somewhat more sodic and generally sericitized. The intrusion appears to be zoned with the granodiorite surrounded by a fine-grained quartz-diorite, sporadically exposed north-cast of the quarry. Shale and mudstone xenoliths have been converted to a hornfels consisting of quartz, andesine and biotite. Mould (1947)<ref>Mould, D.D.C.P. 1947. The Broadlaw 'granite'. Geol. Mag. 84, 178-80. </ref> also reports the occurrence of hornblende, actinolite, corundum and, rarely, sillimanite. Jointing is well developed in the granodiorite, and a number of planes of shearing and crushing can be seen. Quartz veins are fairly common, and other thin veins are filled with pyrite and more rarely with arsenopyrite.

==== 3. Broadlaw: Ordovician Chert Inlier ====
Chert and graptolitic shale crop out in an inlier originally considered to include Arenig and Caradoc horizons (Peach and Horne, 1899), in a road cutting (NT 347 533) 1 km south of the granodiorite. There is exposure on both sides of the road for about 200 m and, according to Peach and Horne (1899), black radiolarian cherts arc overlain by grey mudstones with black shale interbeds, and these in turn by greywackes with black shale partings (belonging to the early Caradoc Portpatrick Formation). The grey mudstones were thought to be equivalent to the Clenkiln Shale (Llandeilo-Caradoc) of the Moffat Shale Group, whilst one black shale band in the overlying greywacke succession yielded a slightly younger fauna including ''Diplograptus foliaceus'', ''Lasiograptus margaritatus'' and Dicellograptus more characteristic of the Hartfell Shale (Caradoc-Ashgill).

The rocks currently exposed are represented in Figure 30, although the section has deteriorated considerably through slippage and weathering. Small-scale faulting and folding can be readily seen, but Peach and Horne's (1899)<ref>Peach, B.N. and Horne, J. 1899. The Silurian rocks of Britain, 1, Scotland. Mem. Geol. Surv. U.K. </ref> interpretation of an anticlinal structure cannot be confirmed. Instead, a stack of small imbricate thrusts appears to be associated with isoclinal folds in which the uninverted limbs have been partly or wholly sheared out. The shales in particular arc affected by recumbent folds of about a metre amplitude which verge south-eastwards. This style of imbricate thrusting associated with attenuation of uninverted fold limbs is a typical deformation pattern within the, Southern Uplands (Webb 1983)<ref>Webb, B. 1983. Imbricate structure in the Ettrick area, Southern Uplands. Scott. J. Geol. 19, 387-400.</ref>. Small normal faults oblique to the imbricate thrusts are probably late tensional features.

The exposures, though restricted and discontinuous, are adequate to demonstrate that black cherts are not confined to one horizon in the core of an anticline. The best exposure of chert is at point A in Figure 30 where a stratigraphic thickness of more than 1 m is visible wherein the chert, dominantly black, occurs as bands and lenses up to 10 cm thick, separated by pale greenish-grey mudstone partings up to 4 cm thick; some at least of which may be bentonites (altered air fall tuffs). Individual chert bands may become very pale grey towards their margins. Scarce and fragmentary graptolites in the black shales include ''Climacograptus brevis'', ''Corynoides incurvus'' and an indeterminate diplograptid (identifications by I. Strachan). These indicate a high Glenkiln or low Hartfell horizon, i.e. basal Caradoc and possibly a little lower than was deduced by Peach and Horne (1899)<ref>Peach, B.N. and Horne, J. 1899. The Silurian rocks of Britain, 1, Scotland. Mem. Geol. Surv. U.K.</ref>.

Pale grey mudstones with interbedded black shale horizons underlie the cherts in inverted succession in the same exposure. The pale grey mudstones are hard and blocky, and weather to a mottled brown and yellow appearance. Fresh surfaces of the grey mudstone commonly have a brecciated aspect, with patches of contrasting shades of grey, and may be intraformational microconglomerates. Large aggregates of pyrite (up to several cm across) also occur. North of the fault (B on Figure 30) the grey mudstones are in normal contact with interbedded greywackes and shales and it is of particular significance that 25 mm bands of undoubted chert occur within the shales. It is evident from this interbedding that the cherts arc of an age with the other sedimentary rocks here, rather than being of an entirely different (Arenig) age as suggested by Peach and Horne. (1899). Some chert bands are coarser-grained and ashy (e.g. at C on Figure 30) and in this respect it is significant that ashy bands and radiolarian cherts also occur in the Glenkiln succession of the Moffat district (e.g. Peach and Horne 1899, p. 103)<ref>Peach, B.N. and Horne, J. 1899. The Silurian rocks of Britain, 1, Scotland. Mem. Geol. Surv. U.K.</ref>.
Southwards the route traverses a typical Southern Uplands landscape of well-rounded moorland with a uniformity of summit levels indicating a dissected peneplain, and deeply incised streams with flat valley floors. A small glacial meltwater overflow channel may be observed on the north side of the road near Windy Slack (NT 352 523). Further exposures of the cherts and shales may be visited opposite Garvald farmhouse (NT 354 513) some 200 m up the hillside; no fossils have as yet been reported from this locality.

==== 4. Raeshaw Wood Quarry: Llandovery Sedimentary Rocks ====
Steeply inclined and strongly jointed, pebbly greywacke and siltstone beds arc exposed in a large roadside (B 709) quarry at Raeshaw Wood (NT 357 504). The coarse-grained greywackes, which represent a conglomeratic development, the 'Raeshaw Conglomerates', within the Kilfillan Formation (Figure 30), consist of quartz grains and generally quartzo-feldspathic rock fragments together with larger clasts of lightly cleaved black shale, all contained in a fine-grained matrix. The shale clasts have yielded graptolites, including ''Climacograptus scharenbergi'', ''C. bicornis'', ''Cryptograptus tricornis'', ''Dicellograptus'' and ''Didymograptus'' ''superstes.'' (Peach and Home 1899, p. 270)<ref>Peach, B.N. and Horne, J. 1899. The Silurian rocks of Britain, 1, Scotland. Mem. Geol. Surv. U.K. </ref> denoting a Llandeilo-Caradoc age and suggesting an association with the Glenkiln Shale. Their cleaved state indicates that these clasts are not intraformational, but are earlier, and were subsequently incorporated into the conglomerate. The basiclastic matrix conforms compositionally to Kilfillan arenites. McKerrow (in Hutton & Murphy 1987)<ref>Hutton, D.H.W. and Murphy, F.C. 1987. The Silurian of the Southern Uplands and Ireland as a successor basin to the end-Ordovician closure of Iapetus. J. Geol. Soc. Lond. 144, 765-772.</ref> notes the occurrence of Glenkiln clasts in other Southern Uplands Silurian conglomerates. The clasts were derived by erosion of sea-floor mud by strong turbidity currents carrying the detritus forming the coarse greywackes. Such sequences typify inner-channel deposits on a submarine fan (Mutti and Ricci-Lucchi 1975)<ref>Mutti, E. and Ricchi-Lucchi, F. 1975. Turbidite facies and facies associations from selected associations in the Northern Appenines. IX Congr. Sedim. Nice, Field Trip A-II, 21-36.</ref>. Grading is sufficiently well-developed to allow the direction of upward stratigraphical sequence to be determined, a process of the utmost importance in lithologically uniform successions such as this, which have undergone severe folding. Grading is only one of a number of features which can be used and other sedimentary structures useful in this respect will be seen at the remaining localities.

==== 5. Hazelbank Quarry: Llandovery Sedimentary Rocks ====
The road east from Raeshaw Wood Quarry leads past Ladyside to Heriot village and the main road to Hawick and Carlisle (A7); Hazelbank Quarry (NT 425 505) is on the east side of this road 5.5 km south of Heriot. It is currently being worked in thick greywacke beds, with intervening partings of shale and siltstone, dipping ESE at 70-90°. Advance permission must be obtained from the quarry manager in order to visit the site, and hard hats are required.

The greywackes contain abundant andesitic volcanic fragments, including pyroxene and amphibole grains, and have relatively low quartz content. They are assigned to the lowest part of the Gala Group, and the quarry is the type locality for the Hazelbank Formation (Kassi 1984)<ref>Kassi, A. 1984. Lower Palaeozoic geology of the Gala area, Borders Region, Scotland. Ph.D. Thesis, St. Andrews University (unpublished).</ref>; this is partially equivalent to the Kilfillan Formation, (Gordon 1962) which corresponds to the Pyroxenous 'Group' of Walton (1955)<ref>Walton, E.K. 1955. Silurian grey wackes in Peeblesshire. Proc. R. Soc. Edinb. B 220 65, 327-57.</ref>. The greywackes are strongly jointed, and are cut by a number of small normal and reverse faults. The sequence is dominated by greywacke units up to several metres thick, many of which display amalgamation. They are coarse-grained, poorly graded, and carry a profusion of large, dark grey mudstone clasts. Some greywacke units display reversed grading towards their base and normal grading towards their top. The remaining greywacke beds (about 20% of the total sequence) are thinner and well-graded 'classic' turbidites. Taken as a whole the Hazelbank sequence represents a channelized mid-fan association (Mutti and Ricci Lucchi 1975)<ref>Mutti, E. and Ricchi-Lucchi, F. 1975. Turbidite facies and facies associations from selected associations in the Northern Appenines. IX Congr. Sedim. Nice, Field Trip A-II, 21-36.</ref>, probably less proximal than that of Raeshaw Wood Quarry. The siltstone interbeds (about 12% of the sequence) are, in effect, thin base-absent' turbidite sequences. These form 'packets' 40 cm to 2 m thick, and are interpreted as overbank deposits, laid down on and beyond levees flanking the submarine channels.

Grading in the steeply-inclined greywacke beds indicates south-eastward younging, confirmed by small-scale cross-lamination in the siltstones, which are also ripple-marked on their top surfaces. In addition, undersides (sales) of many of the greywacke beds carry more or less elongated convex impressions (sole-markings) of various kinds. Many have a lobate appearance, deepening towards rounded ends (flute casts). These may be aligned to form longitudinal ridges, or may occur as alternating rows (scaly patterns). Flute casts are formed by the moving, turbulent current scooping out small hollows in the mud of the sea bed to form moulds which arc then filled by the deposited sand. Erosion strips away the softer shale leaving the casts as projections on exposed bed bases. Other sole-markings present include straight or curved narrow ridges, often not seen to terminate. These are variously termed groove casts, drag marks or tool marks and are fillings of grooves cu t in to the sea-floor mud by objects dragged by the moving current. The unifying feature of all these sedimentary phenomena is that they only occur on the bases of the greywacke beds and so they can be used to establish way-up; at Hazelbank these sole-markings confirm the SE younging direction.

==== 6. Bower Quarry: Kilfillan Formation ====
This quarry (NT 429 501) is some 450 m south of Hazelbank Quarry, and is also in andesitic-rich greywackes of the Gala Group (Kilfillan Formation). Graptolites have been recorded; these include ''Climacograptus'' ''normalis,'' ''Diplograptus'' and other fragmentary forms, indicating a horizon within the zone of ''Parakidograptus acuminatus'', the lowest biozone of the Llandovery Series. Thus, the great (though indeterminate) thickness of greywackes and finer sediments in the present area is laterally equivalent to, at the most, a few tens of metres of black and grey shales and mudstones within the Birkhill Shale sequence at Dob's Linn and elsewhere in the Moffat area (Dob's Linn Excursion). This demonstrates the rapid facies change from continental margin clastic scdimentation to pelagic oceanic sedimentation farther out into the original depositional basin.

Sedimentary features in the greywackes of Bower Quarry indicate that the beds, which dip at 85° to the NW, are the right way up. Particularly prominent are well-developed transverse and interference-ripple patterns on the upper surfaces of many beds. The dip and younging direction are both opposite to that in Hazelbank Quarry to the north, and indicate that a synclinal axis must lie in the unexposed ground between the two quarries.

The greywackes in the two quarries show a contrast in colour, those of Hazelbank being a pale greenish-grey whereas those of Bower are distinctly red. This red colour is due to iron oxide staining, and is presumably caused by the proximity of Old Red Sandstone strata. The greywackes are otherwise compositionally and texturally identical. Larger particles include pebbles of granite, quartz-porphyry, keratophyre, spilite and andesite, together with accessory quartz and alkali feldspar clasts, shale, red mudstone and chert. These lithologies also contribute to the sand-grade assemblage but in this fraction quartz and andesitic grains are more abundant, the latter including pyroxene and amphibole crystals. Chlorite and mica-schist grains are occasionally present.

==== 7. Craigend Quarry: Llandovery Greywackes ====
Farther south-east along the A7 this quarry (NT 444 461) exposes thick and massively-bedded Gala Group greywacke sassigned by Walton (1955)<ref>Walton, E.K. 1955. Silurian grey wackes in Peeblesshire. Proc. R. Soc. Edinb. B 220 65, 327-57. </ref> to the Garnetiferous 'Group'. This youngest subdivision of the Gala Group was originally termed the Buckholm Grits by Lapworth and Wilson (1870) and the name was lately resurrected as the Buckholm Formation (Kassi 1984)<ref>Kassi, A. 1984. Lower Palaeozoic geology of the Gala area, Borders Region, Scotland. Ph.D. Thesis, St. Andrews University (unpublished).</ref> This is now regarded as an interdigitating lithofacies of the Queensberry Formation of Geikie (1871)<ref>
Geikie, A. 1871. On the order of succession among the Silurian rocks of Scotland. Trans. Edinb. Geol. Soc. 3, 74-95.</ref>. Folding and imbricate thrusting replicate the formation and give it a wide outcrop which encompasses most of the remaining Silurian localities to be visited during this excursion. The greywacke beds have a gentle dip towards the NW, shale partings are poorly developed, and internal sedimentary structures are scarce. These are again typical inter-fan channel deposits. Accessory garnets are the characteristic detrital component although they are mostly microscopic. Otherwise the greywackes are quartz-rich with minor amounts of quartzo-feldspathic and schistose rock fragments.

==== 8. Caddonfoot Quarry: Llandovery Greywackes ====
Continue south along the Galashiels road (A7) to Bowland, thence turn south-west on to the B710 and follow this road through Clovenfords. The quarry (NT 450 351) is situated on the east side of the road about 200 m north of the T-junction with the Selkirk road (A707) at Caddonfoot. This locality is again in garnetiferous Queensberry Formation greywackes of the upper Gala Group, though here they are thinly-bedded, with graded bedding and occasional sole-markings. The latter arc best developed in an especially thinly-bedded sequence in the south corner of the quarry where small-scale tool-markings, including brush and prod moulds, can be observed. The upper surfaces of many thin beds are ripple marked, and ripple-cross-lamination occurs throughout. Scour marks are absent, indicating a low-energy input and a relatively deep depositional environment. Sporadic graptolites include Monograptus turriculatus and ''M. crispus'', zonal forms for a high level within the Llandovery. They are most readily recovered from the cutting on the east side of the road, between Caddonfoot cottage and the quarry.

==== 9. Thornylee: Trace Fossils in Red Shale ====
From Caddonfoot follow the A707 and then the A72 towards Innerleithen for about 6 km and then park in a lay-by on the SW side of the road beside the River Tweed. Interbedded Gala Group greywackes and shales (Queensberry Formation; garnetiferous lithofacies) are exposed in a quarry (NT 420 363) in the steep slope across the road and north from the lay-by, and in 300 m of abandoned railway cuttings between the quarry and the road. The greywackes arc medium-grained and well-graded, and display a range of erosional sole-markings and tool-markings. These are on a small scale, indicating a moderate energy input, higher than at the previous locality but still in deep water, probably within a middle or lower fan environment. The red shale bedding surfaces arc covered by traces of the meandering feeding burrow ''Dictyodora,'' including both the larger and regularly meandering ''D. scotica'' and the smaller and more irregular ''D. tenuis''. This meandering pattern is typical of organisms scavenging a food source efficiently, keeping in contact with earlier-formed parts of the burrow and, when these arc lost, making a 180-degree turn to re-establish contact and to resume feeding. The burrows extend upwards as narrow striated walls, each possibly created by a siphon maintaining contact with the sea bed above. Other trace-fossils present include the narrow and almost straight ''Caridolites wilsoni'', which may represent a juvenile stage of ''Dictyodora.'' In general, meandering burrows arc typical of deep-water marine trace fossil associations (Benton and Trewin 1980).

==== 10. Pirn Quarry: Llandovery Turbidites with Sole-markings ====
[[File:P266075.jpg|thumb|300px|right|Pirn Quarry, Innerleithen. Big bulbous flutes. P266075]]Continue along the A72, through Walkerburn and to the outskirts of Innerleithen where a large lay-by on the north side of the road is adjacent to the overgrown quarry (NT 339 373). A succession of Gala Group greywackes belonging to the Intermediate 'Group' of Walton (1955)<ref>Walton, E.K. 1955. Silurian grey wackes in Peeblesshire. Proc. R. Soc. Edinb. B 220 65, 327-57.</ref> is exposed at this locality and has been correlated with the Fountainhall Formation of the Gala Water valley (Kassi 1984)<ref>Kassi, A. 1984. Lower Palaeozoic geology of the Gala area, Borders Region, Scotland. Ph.D. Thesis, St. Andrews University (unpublished).</ref> now, like the Garnetiferous 'Group' considered as an interdigitating siliciclastic lithofacies of the Queensberry Formation. The quartzo-feldspathic greywackes are bedded in units up to about 3 m thick. Delayed grading is typical, with isolated mudstone clasts up to 5 cm long concentrated towards the bases of several units. The succession dips steeply towards the south-cast but is overturned, thus exposing the bases of beds, and a spectacular range of sole-markings including longitudinal ridge casts, 'scaly-pattern' flute casts, drag marks, channel fills and other rarer types (Dzulynski and Walton 1965)<ref>Dzulynski, S. and Walton, E.K. Sedimentary features of flysch and greywackes. Developments in Sedimentology 7. Elservier, Amsterdam274pp.</ref>.

The main surface exposed in the south-west corner carries two intersecting sets of groove casts. Dragging relationships indicate consistent relative ages for the sets, and so denote a marked change in the current direction. [[File:P266073.jpg|thumb|300px|left|Pirn Quarry, Innerleithen. Grooves in greywacke. P266073]]The plastic mud displaced from the grooves has been rucked up into rope-like structures. Immediately below (though stratigraphically above) the grooves, one internal surface within the greywacke displays smaller grooves. These rare structures are internal grooves, the origin of which is revealed by the presence of small shale clasts which terminate a few of the structures. The bed is otherwise a pale grey, graded greywacke typical of the Gala Group at this locality. Further east another sole carries, in addition to continuous groove casts, discontinuous tool-markings including prod marks and brush marks. The outcrop of the first horizon continues into the middle of the face, and reveals numerous shale and siltstone clasts up to 20 cm across, which may be the tools responsible for inscribing many of the markings. Groove casts also occur in the north-east corner of the quarry, which is otherwise noteworthy for channel fills. These generally have a fluted structure, though one wide example lacking significant fluting is seen high up on the quarry face.

Characteristic of the greywackes here is delayed grading. A thin basal zone of coarse grains with large and dispersed shale clasts is succeeded upwards by uniformly medium-grained greywackes containing only sporadic shale clasts. This in turn is succeeded by a thin, graded top which terminates in cross-laminated and laminated siltstone and merges into the overlying pelite interval.

==== 11. Cowpeel Bridge: Llandovery Turbidites with Flute Casts ====
[[File:P266078.jpg|thumb|300px|right|Cowpeel Bridge, Peebles. Flutes in greywacke. P266078]]Continue to Innerleithen (0.5 km) on the A72; there join the Hawick road (B709) and follow it southwards for 4 km through Traquair. A cutting face on the west side of the road (NT 314 310) exposes the slightly overturned soles of several steeply-inclined, quartzo-feldspathic Gala Group greywacke beds (=Queensberry Formation: garnetiferous lithofacies.) These show a mass of well-developed 'scaly' flute casts which have been slightly exaggerated by load casting, i.e. by the heavy turbidite sand, laid down suddenly, sinking into the underlying soft mud. and so creating downward bulges. Nevertheless they unequivocally indicate current flow from the NNW (vertically upwards on the exposed bed base), and normal to the regional strike. These greywackes are deposits of high-energy turbidity currents, directly down-slope on the submarine fan.

==== 12. Grieston Quarry: Llandovery Distal Sedimentary Rocks ====
Return to Traquair and follow the old Peebles road (B7072) for 1.7 km northwestwards to Howford. Grieston quarry (NT 316 539) is reached via a rough track which leads south-west from the B7062 on the south side of Howford farm. The thinly-bedded and fine-grained greywackes arc interbedded with shales which were originally worked for roofing slates. Graptolites were first recorded by Nicol (1848<ref>Nicol, J. 1848. On the geology of the Silurian rocks in the valley of the Tweed. Quart. J. Geol. Soc. Lond. 4, 195-209.</ref>, 1850<ref>Nicol, J. 1850. Observations on the Silurian strata of thesouth east of Scotland. Quart. Geol. Soc. Lond. 6, 53-65.</ref>) and were subsequently described in modern terms by Toghill and Strachan (1970)<ref>Toghill, P. and Strachan, I. 1970. The graptolite fauna of Grieston Quarry, near Innerlei then, Peeblesshire. Palaeontology. 13, 511-21.</ref>. About 43 m of well laminated, thinly-bedded flaggy greywackes, siltstones and mudstone are exposed, and have a NW dip of 60-65° and abundant small-scale sole-markings, showing that the succession is the right-way up. Parallel, cross and convolute laminations arc frequently seen in the thinlybedded (5 cm) sandstone and siltstone beds. The fine grain size, thin beds and sedimentary structures indicate a very low energydistal environment of deposition. Graptolites occur in situ in fine-grained bluish-grey greywackes 3.7 m above the base of the succession in the east end of the quarry, and in a less prolific horizon a metre above the base. The commonest species are ''Monoclimacis griestoniensis'', ''Monograptus priodon'' and ''M. spiralis'', with rarer ''M. discus'', ''Rastrites'' geinitzianus angustidens and Pseudoplegmatograptus obesus. Micaceous greywacke slabs in the quarry talus have not been located in place, and contain a slightly different assemblage, adding common ''Monograptus drepanoformis'', ''Pristiograptus nudus'' and ''Clyptograptus?'' ''nebula'', having only rare ''M. griestoniensis'', and lacking ''M. discus''. Both assemblages indicate a horizon within the ''griestoniensis'' Zone, and represent the highest horizon yet recorded within the Gala Group; localities to the E and SE, including Caddonfoot and Thornylee, yield graptolites of the underlying ''crispus'' and ''turriculatus'' zones (Lapworth 1870<ref>Lapworth, C. 1870. On the Lower Silurian rocks of Galashiels. Geol. Mag. 7, 204-9, 279-84.</ref>; Peach and Horne 1899). The presence of relatively young strata this far north in the Southern Uplands is anomalous in terms of the regional structure and its implications have not yet been fully assessed. It is possible that the Grieston strata were originally deposited in an isolated perched basin unconformably above the developing Southern Uplands accretionary complex.

==== 13. Craigburn Quarry: Haggis Rock ====
Continue to Peebles on the B7062. If returning through Eddleston, Craigburn Quarry (NT 237 544) just before Leadburn is worth a visit to see the Haggis Rock. This quarry is described as the first locality in the next excursion (Noblehouse, Lamancha).

===References===

<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P266078.jpg

2015-11-17T11:50:08Z

JenniferFindlay1: Cowpeel Bridge, Peebles. Flutes in greywacke.

== Summary ==
Cowpeel Bridge, Peebles. Flutes in greywacke.
== Licencing ==
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File:P266073.jpg

2015-11-17T11:36:58Z

JenniferFindlay1: Pirn Quarry, Innerleithen. Grooves in greywacke.

== Summary ==
Pirn Quarry, Innerleithen. Grooves in greywacke.
== Licencing ==
__notoc__
{|class="wikitable" style="width:100% style="background:#f0f8ff; font-size:80%"
| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

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The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
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File:P266075.jpg

2015-11-17T11:32:06Z

JenniferFindlay1: Pirn Quarry, Innerleithen. Big bulbous flutes.

== Summary ==
Pirn Quarry, Innerleithen. Big bulbous flutes.
== Licencing ==
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[[Category:License tags]]

Kirkcudbright - an excursion

2015-11-17T10:32:53Z

JenniferFindlay1:

By P Stone, R F Cheeney and D E White. Excursion 5. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Kirkcudbright: a volcanic vent, Hawick Group turbidites, graptolites ==

1:50 000 Sheet 83 Newton Stewart & Kirkcudbright

BGS 1.50 000 Sheet 5W Kirkcudbright

=== Main points of interest ===
A Siluro-Devonian volcanic vent, Silurian turbidite greywackes and Wenlock graptolites, complex Caledonian folding.

=== Logistics ===
All the localities lie SW from Kirkcudbright and involve relatively easy coastal walking: about 1.5 km at Locality 1, about 3.5 km at Locality 2, and about 7 km for Localities 3 and 4 if the coastal path is walked in both directions. A shorter return route from Locality 4 requires prior permission from Ross Farm. A low tide is advantageous but much can be seen under all but the highest tidal conditions. Vehicle access is good for Locality 1, where there is ample car or coach parking. However, as access for the other localities is via minor roads and parking space is limited, the full excursion should not be attempted in any vehicle larger than a minibus. Total driving distance from and back to Kirkcudbright is about 25 km.

=== Introduction ===
Kirkcudbright is an attractive small town with a picturesque harbour situated on the estuary of the River Dee. Maclellan's Castle, a ruined tower house dating from about 1580, may be seen in the town centre. Close by the Tollbooth dates from 1627 and the old Merkat Cross still features the jougs (a form of pillory) 'for the public humiliation of offenders'. About 3 km north of the town, on the A711, the [http://www.spenergywholesale.com/userfiles/file/tongland%20tour1lo.pdf Tongland dam and hydro-electric power station] may be visited and a guided tour could form an appropriate adjunct to the geological excursion.

The Kirkcudbright area is underlain by Carghidown Formation (Hawick Group) greywackes, and siltstones. These were deposited during the Silurian period about 430 million years ago. The coastal sections to the SW of the town provide splendid outcrops, illustrating turbidite sedimentology and complex Caledonian structure, and it is these that provide the focus for the excursion. Examples of igneous intrusive rocks will also be seen and graptolites of Wenlock age may be found in the Ross Formation beds, a Hawick Group component slightly younger than the Carghidown Formation and exposed farther south. Still younger strata of the Riccarton Group crop out to the SE of Kirkcudbright and are examined in detail by Excursion 11. It would be possible to include some elements of that excursion as an extension of the itinerary described here. The complex fold structure may be further examined in the excellent coastal exposures slightly farther NW at Barlocco (NX 585 486) which are described in detail by Treagus (1992).

===Excursion===

==== 1 Shoulder O'Craig: volcanic vent ====
[[File:KirkudbrightExcursionMap.jpg|thumbnail|Locality map and outline geology for the Kirkcudbright excursion.]]
The excursion is best begun at the car park and picnic area adjacent to The Doon and Gull Craig beside Nun Mill Bay (NX 658 487). This is situated about 5 km from Kirkcudbright and is reached via the A755 and B727. About 600 m NE from the parking area an agglomerate-filled volcanic vent (Figure 23) cuts Silurian greywacke and siltstone (Carghidown Formation). The sedimentary rocks are exposed on the foreshore in Clinking Haven as steeply inclined beds striking NE and locally folded into tight, upright structures. A good array of turbidite features can be seen on the wave-smoothed surfaces and includes graded bedding and loaded bed bases. A penetrative cleavage is developed subparallel to bedding in the finer-grained lithologies but does not continue into the vent agglomerate which was therefore a post-tectonic intrusion. The agglomerate, believed to have been intruded in latest Silurian or early Devonian times, is one of a number of such vent features scattered across SW Scotland. A fresh kersantite (biotite-plagioclase lamprophyre) phase of the vent intrusion has given a K-Ar age of 410 ± 10 Ma (Rock et al., 1986a). The vent occupies the northern side of Clinking Haven forming the Shoulder O'Craig cliffs (663 491) and probably extends for a short distance inland. Lamprophyre dykes cut both the vent agglomerate and the turbidite country rock. Detailed petrographical and geochemical data for the intrusive rocks are given by Rock et al. (1986a).
[[File:KirkudbrightShoulderOCraigexcursionMap.jpg|thumbnail|Outline geology of the Shoulder O'Craig volcanic vent (Locality 1).]]

The texture of the vent agglomerate is best seen on the wave-polished surfaces on the NW side of Clinking Haven. The cliff sections provide more extensive outcrop in three dimensions and confirm that the agglomerate consists principally of variably rounded greywacke, siltstone and sporadic microdiorite or basaltic clasts set in a fine-grained matrix; the latter is largely altered to carbonate and chlorite. Clast size is very variable and ranges up to rafts of country rock a few metres in length. The preponderance of sedimentary clasts in the vent suggests that initially it emitted steam and gases for the most part and did not directly tap a source of magma. Thus the vent agglomerate should more accurately but less descriptively be termed an intrusion breccia. It is cut by a number of basalt bodies and lamprophyre dykes the larger of which are shown in Figure 23. Note the irregular and fractured biotite-olivine basalt mass which intrudes the agglomerate in the western end of the vent. It is generally clast-free and its contact with the surrounding agglomerate varies from sharp to diffuse and gradational. Oval, pillow-like textures and possible flow fractures may suggest that the intrusion was emplaced in a semi-solid state (Rock et al., 1986a). However, it does imply that the vent developed from a steam and gas escape route to a conduit for magma.

Other dykes cut the greywacke country rock and a noteworthy example occurs about 20 m beyond the NE extremity of the vent. This has been dubbed the 'Loch Ness Monster' dyke on account of its bizarre outcrop pattern. It is a kersantitic lamprophyre consisting of biotite phenocrysts set in a dark grey feldspathic matrix. The highly irregular form is thought to reflect high volatile pressure during emplacement.

==== 2 Brighouse Bay: Carghidown Formation greywackes and structure ====
[[File:KirkcudbrightBrighouseBayExcursionMap.jpg|thumbnail|Locality map and geological notes for the west side of Brighouse Bay (Locality 2).]]
From Locality 1 follow the B727 Kirkcudbright to Borgue road for about 2 km SW towards Borgue before turning left towards Brighouse Bay. About 2 km of unclassified road leads to the head of the bay where parking is available on the raised beach. The west side of the bay (Figure 24) provides extensive exposures of well-bedded greywackes of the Carghidown Formation (Hawick Group, Figure 2), but the structural complexity is considerable. The greywacke beds are repeatedly folded and sheared out, with remarkable variability in the attitude and orientation of the fold hinges. The relationships are best examined between Point of Green and Dunrod Point (NX 628 445) on the SW corner of the bay, reached by a coastal walk of about 1.5 km. This is the recommended starting point for a traverse back along the coastal section. Fold structures are abundant but one of particular interest can be seen on the west side of Point of Green (2a in Figure 24). This fold is downward-facing or inverted: the apparent antiform is in fact synclinal. The antiform plunges moderately to the NE and the curved upper surface is covered with well-developed flute casts and load structures, marking it out as an inverted bed base. A non-axial planar slaty cleavage is also clearly seen to transect the axial surface and both limbs of the fold by a few degrees clockwise. Traversing eastwards across Point of Green several other steeply plunging folds are separated by shear planes which on a large scale merge to form an anastomosing zone. At Point of Green some thick greywacke beds appear in the sequence and large flute casts are preserved on the base of the thickest (2b). [[File:P220425.jpg|thumb|300px|left|Brighouse Bay, 3 km. S. of Borgue. Looking W. Flute casts and grooves on base of greywacke beds - axial current (from north-east). Carghidown Formation, Hawick Group. P220425]]This bed can be seen to cut down through the underlying more thinly bedded greywackes, an example of channelling. Eastward towards Dunrod Point steeply plunging fold hinges are contained within the anastomosing shear system (2c). Their inter-relationship may be conveniently examined in detail at the NE corner of Dunrod Point (NX 6286 4459), a structural summary of which is shown in Figure 25. From this locality the coastal section continues NE towards the head of Brighouse Bay. Near-continuous exposure reveals much folding of the greywacke strata with variably but often steeply plunging hinges separated by sinuous shear zones. Thicker greywacke beds often show a good array of bottom structures and there is a strong, ubiquitous slaty cleavage. Note that the cleavage is axial planar to some of the folds but clockwise transecting in other examples; hinge plunge variation commonly occurs within the axial plane. These folds are believed to have formed in a transpressive stress regime when a variable component of sinistral shear was imposed on the overall NW-SE regional shortening. A detailed discussion of this phenomenon was given by Stringer and Treagus (1980). The incidence of folding decreases, and the frequency with which the section is cut by shear zones increases, towards the NE as the route leads back to the parking area at the head of the bay.
[[File:KirkcudbrightDunrodPointExcursionMap.jpg|thumbnail|Detail of structure near Dunrod Point (Locality 2).]]

==== 3 Meikle Ross: Ross Formation greywackes and siltstones ====
This small peninsula forms the southern extremity of the west side of Kirkcudbright Bay. It is reached by means of unclassified roads which link Brighouse Bay with Ross Farm (NX646 447). Manor Point forms the headland on the south side of Ross Bay (about 1.5 km SE along the footpath from Ross Farm) and from the Point about a kilometre of well-exposed coastal section extends south. It is most readily accessible if the path is followed to the southernmost point of Meikle Ross and the coastal section then traversed northwards.

The strata are well-bedded greywackes and siltstones with sporadic interbedded grey-green shale and belong to the Ross Formation (Hawick Group, Figure 2). Restricted graptolite faunas (Figure 26) of the ''M. riccartonensis'' Biozone (early Wenlock) have been found in rare hemipelagite horizons (localities 52-55 of White et al., 1992).

[[File:P259084.jpg|thumb|300px|left|Meikle Ross. Ripples - cross-bedded, Ross Formation. P259084]]Bedding is fairly regular with a general dip of about 60-70° to the SE; beds are commonly inverted and young towards the NW. Bottom structures are abundant, and ripple marks and sand volcanoes can be seen on the top surfaces of some beds. Fold hinges are far less evident than was the case at Brighouse Bay; the best examples are seen around Thunderhole Bay (Figure 22). However, the evidence for variable plunge of fold hinges is merely more subtle in this locality and can be demonstrated by a systematic examination of the lineation formed by the intersection between bedding and cleavage. The cleavage is axial-planar to the folds, as can be demonstrated along strike on the island of Little Ross (beyond the scope of this excursion) and so the lineation can be taken as parallel to local fold hinge orientation. Overall there is a smooth variation in the plunge of the lineation, from 47° to the ENE at the north end of the section to 38° to the WSW in the south passing through a maximum plunge of 64° where the lineation is parallel to the dip of the cleavage. Diachronous formation of the cleavage under a varying stress regime seems the most likely explanation for these relationships, a similar mechanism to that proposed at Brighouse Bay. Further discussion of the tectonic implications of the Meikle Ross section is given by Kemp (1986; 1987a).

==== 4 Fauldbog Bay: graptolitic hemipelagites ====
[[File:KircudbrightGraptolitesRossFormation.jpg|200|thumbnail|Examples of graptolites from the Ross Formation all x 5, except for a and b, x 10. (a, b) ''Barrandeograptus''? ''bornholmensis'' (Laursen), ''centrifugus'' Biozone. (c) ''Monoclimacis'' cf. ''vomerina vomerina'' (Nicholson), ''centrifugus'' to ''riccartonensis'' biozones. (d) ''Monoclimacis vomerina basilica'' (Lapworth), ''centrifugus'' to ''riccartonensis'' biozones. (e) ''Monoclimacis vomerina'' s.l. (Nicholson), ''centrifugus'' to ''riccartonensis'' biozones. (f, g) ''Monograptus riccartonensis'' Lapworth, ''riccartonensis'' Biozone.]]
Well-developed fold structures can be seen on the west side of Meikle Ross at Fauldbog Bay (NX 642 444), a locality which is also notable for the abundance of graptolites in the interbedded hemipelagite horizons. The west-facing coast of the bay exposes strata lithologically similar to the Ross Formation but containing some thin red mudstones characteristic of the Carghidown Formation. The boundary between the two formations is taken at a fault in the northern extremity of the bay, the strata on the east side are assigned to the Ross Formation and those on the west to the Carghidown Formation. Variable and complex folding may be seen in many parts of the shore section. Sheared zones separate the folded areas from units of more uniformly bedded strata.

To reach Fauldbog Bay continue along the coastal path around the headland and up the west coast of Meikle Ross. Good exposures of graptolitic hemipelagites of the ''Cyrtograptus centrifugus'' Biozone (basal Wenlock) can be examined in intertidal reefs at the northern end of Fauldbog Bay (NX 642 445 to NX 644 443) (localities 61-63 of White et al., 1992). Graptolites collected here include ''Barrandeograptus''? ''bornholmensis'' (Laursen), ''C''. cf. ''centrifugus'' Boucek, cf. ''C. grayi'' Lapworth, ''Monoclimacis vomerina'' basilica (Lapworth) , Mcl. ''vomerina vomerina'' (Nicholson), Mcl. ''vomerina'' c.l., ''Monograptus priodon'' (Bronn), ''M.'' aff ''priodon'', ''M. remotus'' Elles & Wood, ''Retiolites'' ''geinitzianus angustidens'' Elles & Wood and ''R. geinitzianus geinitzianus'' Barrande. Approximately 750 m to the south, (around NX 644 437) (localities 57-60 of White et al., 1992) hemipelagites in the intertidal reefs contain a restricted fauna of the ''Monograptus riccartonenis'' Biozone, mainly ''M. Riccartonensis''. A selection of graptolites recovered from these localities is shown in Figure 26. They are of Wenlock age (Figures 2 and 5).

Both sides of Fauldbog Bay expose abundant minor folding but the NW side is of particular interest. Many of the folds there are inverted (downward-facing) and enclosed within shear zones reminiscent of the Brighouse Bay section; a similar origin seems likely.

From Fauldbog Bay it is possible to cross the fields eastwards towards Ross Farm if prior permission has been obtained. Otherwise the coastal route should be retraced.
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P259084.jpg

2015-11-17T10:25:29Z

JenniferFindlay1: Meikle Ross. Ripples - cross-bedded, Ross Formation.

== Summary ==
Meikle Ross. Ripples - cross-bedded, Ross Formation.
== Licencing ==
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File:P220425.jpg

2015-11-17T10:15:10Z

JenniferFindlay1: Brighouse Bay, 3 km. S. of Borgue. Looking W. Flute casts and grooves on base of greywacke beds - axial current (from north-east). Carghidown Formation, Hawick Group.

== Summary ==
Brighouse Bay, 3 km. S. of Borgue. Looking W. Flute casts and grooves on base of greywacke beds - axial current (from north-east). Carghidown Formation, Hawick Group.
== Licencing ==
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{|class="wikitable" style="width:100% style="background:#f0f8ff; font-size:80%"
| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

====Non-commercial Use====

Use of the images downloaded from this site and reproduced digitally or otherwise may only be used for non-commercial purposes, which are:-

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Whithorn - an excursion

2015-11-10T10:19:49Z

JenniferFindlay1:

By R P Barnes. Excursion 13. From: Stone, P (editor). 1996. [[Geology in south-west Scotland: an excursion guide]]. Keyworth, Nottingham: British Geological Survey.

== Whithorn: turbidite sequences and deformation in the Hawick Group ==

OS 1:50 000 Sheet 83 Newton Stewart 6- Kirkcudbright

BGS 1:50 000 sheets 2 Whithorn, 4E Wigtown

=== Main points of interest ===
D1 deformation styles and post-D I structures in Silurian turbidites, characteristics and stratigraphical relationships of the Cairnharrow, Kirkmaiden, Carghidown and Ross formation, syntectonic dykes.

=== Logistics ===
All localities are easily reached via the A75 from Newton Stewart or Glenluce and small roads on the Whithorn peninsula; Locality 4 requires vehicular access across private land and permission should be gained. The localities are described anticlockwise around the coastline (Figure 40). Some exposure is usually available above the high water mark but most localities are best visited at medium to low tide; low water is essential at Locality 2. To visit all six localities requires a long day; if time is short, Locality 3 could be omitted. For general safety it is necessary to be fully aware of the tide movements and be wary when the rocks are wet as they are likely to be very slippery. Some localities include cliffs where special care is necessary, although no particularly difficult route for access is required. Whilst in the area visitors may wish to see something of the Whithorn archaeological excavations. The dig is investigating the 1500 year old church site founded by St Ninian and the ensuing Northumbrian monastery housing a shrine to the saint which became a centre of pilgrimage. A Viking trading centre was established on the same site and developed through a Scots take-over into a medieval town. Details of the up-to-date excavations and admission arrangements may be obtained from the Whithorn Trust, 45-47 George Street, Whithorn, DG8 8NS, Telephone 01988 500508.

=== Introduction ===
This excursion aims to demonstrate the relatively complex deformation of the Hawick Group in the coastal exposures of the Whithorn promontory. Relationships can be demonstrated between the widespread, but here relatively variable, D1 deformation, the later folding and faulting, and the intrusion of suites of syntectonic dykes.

The Hawick Group crops out over the southern part of the Whithorn promontory (Figure 40). The Kirkmaiden Formation, named after the chapel at Kirkmaiden (between Localities 1 and 2), shows the general nature of the Hawick Group. It is characterised by medium-bedded Ta(bc), Tb(c) or Tc greywacke units with thin muddy partings, and includes packets (usually less than 3 m thick) of thinly interbedded Tc greywacke and silty mudstone, and thickly bedded massive sandstone in single beds or sequences of several beds. To the north, the Cairnharrow Formation typically includes a higher proportion of more thickly bedded Tb(c) greywackes. To the south, two formations are defined on the presence of distinctive muddy interbeds in a sequence which is otherwise much like that of the Kirkmaiden Formation. The Carghidown Formation includes red mud-stone beds, thin and infrequent in the north but increasing in importance southwards, accompanied by conspicuous detrital reddened mica in some of the sandstones. Soft-sediment deformation is also extensively developed in this formation, varying fromdisrupted bedding to mélange units. On the southern headland (Burrow Head, Locality 4) the red mudstone interbeds give way, up-sequence, to grey laminated siltstone beds in the Ross Formation. The siltstones contain graptolites of early Wenlock age (White ''in ''Barnes, 1989), providing one of only two direct biostratigraphical controls over the age of the Hawick Group in this area (White et al., 1990). The other is a sparse, late Llandovery graptolite fauna from the Kirkmaiden Formation near Monreith.

Tectonostratigraphy follows the regional pattern of the Southern Uplands. The younging direction of the strata is typically northwards yet the limited biostratigraphical evidence indicates that the formations become older in this direction. This paradox can only be resolved by inferring the presence of reverse faults parallel to the ENE strike of the rocks. These subvertical structures, spaced 2-5 km apart, define a series of parallel-sided tectonostratigraphical tracts. The faults are thought to have formed as thrusts in association with D1 folding and SI cleavage formation but have since been rotated into a near-vertical attitude. The thrust front, detaching the turbidite sequence on the underlying Moffat Shale Group, effectively migrated southwards into progressively younger strata; the D1 event is thus diachronous, becoming younger southwards. This concept is discussed more extensively in the Introduction (see especially Figure 4).

D1, represented by gently plunging folds, was the only phase of deformation to have affected many of the rocks now preserved in the northern half of the Southern Uplands (Leadhills and Gala groups), later phases of deformation being apparent only rarely. To the south, in the Hawick Group, D1 was more complex and produced a wide range of fold plunge including steeply plunging sinistral folds. A second phase of deformation D2 is also relatively widespread, forming inclined, south-verging open folds and smaller recumbent folds associated with a locally well-developed subhorizontal cleavage (S2). Numerous lamprophyre and felsite dykes were emplaced into the Hawick Group at various times during the deformation history. The early, bed-parallel dykes are locally cut by the S1 cleavage, whereas other dykes have various relationships with D2 folds suggesting emplacement before or during D2 deformation. Examples of these relationships can be examined at Locality 2 of this excursion and at Locality 1 of Excursion 16.

Steeply plunging folds also occur elsewhere in the Southern Uplands, commonly adjacent to the tract-bounding faults. The Orlock Bridge Fault, for example, shows evidence of major sinistral strike-slip reactivation (Excursion 17). Many of these structures may have formed in response to a single sinistral shear event. At Locality 2 (below), folds of this nature can be shown to deform S2 and therefore to represent a third phase of deformation (D3). However, bearing in mind the diachronous nature of D1, these D3 structures may have formed at the same time as the steeply plunging D1 folds in the younger part of the Hawick Group (Figure 4).

Extensive coastal exposure through the Hawick Group on the Whithorn promontory allows study of its turbidite sedimentology and the effects of the deformation it has undergone. The tectonic structure is particularly well displayed. A study of the area by Rust (1965) was followed by British Geological Survey mapping (BGS, 1987; 1992c; Barnes et al., 1987; Barnes, 1989). The structure is dominated by D1, the effects of which are separated into two domains (Figure 40) by a D2 thrust fault (Barnes, 1989): in the northern zone folds plunge gently to moderately NE whereas in the southern zone fold plunges may be gentle, moderate or steep towards NE or SW. This excursion visits the localities where the main features of the stratigraphy and the inter-relationships between the deformation episodes can be seen.
=== Excursion ===
[[File:WhithornExcursionMap.jpg|500px|thumbnail|Locality map and outline geology for the Whithorn excursion.]]

==== 1 Black Rocks, Monreith: steeply plunging D3 folding ====
From Newton Stewart follow the A714 to Port William (just over 30 km) then turn south on the A747 coast road for 5 km, parking on the seaward side of the road (NX 357 409) just before a sharp left bend into Monreith village. From here a flight of steps leads down to the beach where Black Rocks (NX 359 408), forming a short section of exposure to the south, is accessible except at high water. Interpretation here is influenced by the style and chronology of D1 and D2 structures established at Locality 2, nearby.

Thin- to thick-bedded greywacke, often parallel-laminated even through thick beds, is interbedded with silty mudstone. These strata were formerly regarded as part of the Kirkmaiden Formation (BGS, 1987; Barnes, 1989) but are now assigned to the Cairnharrow Formation (BGS, 1992b; 1992c). They are deformed by steeply plunging folds, varying from minor folds with short limbs a few centimetres long, locally intensely developed in very thin-bedded units, to larger sinistral folds in more thickly bedded greywacke. The folds are associated with a steep, east-striking, near-axial-planar cleavage. The gently dipping S2 cleavage, which will be seen associated with recumbent D2 folds at Locality 2, is also developed in places here and the relationship between the various structures is critical to understanding the timing of the fold phases. The gently dipping S2 cleavage appears to be folded by the steeply plunging folds but, because these are at a high angle to one another, it is difficult to be certain. However, S2 is also crenulated by the steep cleavage, confirming that the steeply plunging structures are later. They can therefore be referred to the D3 phase of deformation.

Identical steeply plunging fold structures will be seen farther SE (Localities 3 to 6) associated with the late stages of Dl. Given the diachronous nature of the D1 deformation, the same phase of sinistral shear may have been responsible for the formation of steeply plunging D1 folds in the youngest strata being deformed at any one time, contemporaneous with steeply plunging refolding (D3) structures in the older, previously deformed rocks of the thrust hinterland.

==== 2 Back Bay, Monreith: Kirkmaiden Formation structure ====
[[File:WhithornBackBayExcursionMap.jpg|500px|thumbnail|Sketch section of the Back Bay cliffs (Locality 2) . Letters a to f identify points of particular interest discussed in the text. ]]
Proceed through Monreith village on the A747 and about 1 km farther on turn right for Monreith golf course. The narrow road turns sharply to the left after 500 m, at which point a bronze otter, a memorial to the author Gavin Maxwell, is situated on a cup and ring marked rock to the right of the road. Drive through the golf course to the public car park at the shore (NX 368 394) and walk along the beach towards the cliff at the east side of Back Bay. This is only accessible at mid to low tide.

The turbidites exposed in the cliff are typical of the Kirkmaiden Formation: thin-to medium-bedded greywacke with some thicker beds, and interbedded silty mud-stone. Several D1 folds are well displayed. Two sets of later folds are also present, with steeply and gently dipping axial planes and opposed vergence. Their overall conjugate geometry forms open box folds (Figure 41; cf. Barnes, 1989, frontispiece) and they are therefore jointly referred to as D2. Recumbent D2 folds of small size are well displayed near high water (A on Figure 41) where they are associated with thrust faults which climb gently up through the cliff and have a small displacement up to the south. In the cliff to the south, larger recumbent D2 folds are apparent, the northern one of which passes down into a steeply inclined D2 fold (B) which refolds an isoclinal D1 syncline. An anticline and another syncline are more obvious a few metres to the south, bedding in the synclinal hinge (C) being deformed by recumbent D2 folds and associated cleavage. To the south (D), moderately inclined bedding in the north-younging limb of the fold includes a large, steeply inclined D2 fold with subhorizontal tension gashes apparently related to the S2 cleavage. Farther towards the headland (E) a south-verging NW D1 fold pair is particularly well displayed, the shape of the fold hinges changing in response to the varying thickness of the bedding. An irregular lamprophyre dyke is also present near the headland (F).

[[File:P008561.jpg|thumb|300px|left|S. of Back Bay, Monreith. Looking NE. Offsets in lamprophyre dyke facilitated by folding. One of the several sideways steps in a 1 m. thick dyke is facilitated by an open recumbent D2 fold, suggesting emplacement synchronous with folding. P008561]]At low tide it is possible to walk around the headland into the next bay, the deep embayment in the southern part of which is called Callie's Port. Alternatively, from just above the high water mark in Back Bay, follow the narrow path which winds up on to the headland then walk around the cliff top to the SE side of Callie's Port where the beach is easily reached. Several lamprophyre dykes are present in the north of the bay, particularly in the west-facing cliff where critical relationships with fold structures indicate post-D1 but pre-D2 intrusion. One large, irregular intrusion cross-cuts an anticlinal Dl fold hinge. Several bedding-parallel intrusions occur in the adjacent strata, a 1.5 m-thick dyke being folded by a small recumbent D2 fold and cut by fractures parallel to the S2 cleavage (Barnes, 1989, plate lc). Immediately south of Callie's Port, a wide wave-cur platform presents easily accessible exposure in which both sets of D2 folds cause variations in the dip of bedding. About 100 m south of Callie's Port, a 1 m-thick lamprophyre dyke is exposed parallel to bedding but divided into a number of offset segments. At least one of the offsets is related to a recumbent D2 fold closure (Barnes, 1989, plate lb), suggesting syn-D2 emplacement.

Proceed a further 50 m south, to the end of the wave-cut platform where deep gullies have been eroded along NW-trending fractures, and look at the cliff face immediately SE to the east. Thin-bedded turbidites folded around an upright D2 fold include a bedding-parallel lamprophyre dyke, a few centimetres thick, which also passes around the fold. It is difficult in such circumstances to determine whether the dyke was intruded before or after folding. However, here the anticlinal hinge is fractured and the dyke is offset, suggesting that the dyke predates the folding. At low tide it is possible to proceed along the section to the SE where, over the next 120 m, D1 folds are deformed by recumbent D2 folds and related thrust faults.

To return to the car park, regain the cliff top and follow it almost to the golf course. After crossing a wire fence a path slants down over the steep grassy slope into the northern corner of Back Bay.

==== 3 Port Castle Bay to Carghidown: Carghidown Formation structure ====
[[File:P008562.jpg|300px|thumbnail|Shore S. of Port Castle Bay. Looking NE. Cross-cutting lamprophyre dykes. Strongly cleaved, fine-grained sandstone and silty mudstone with an approximately bed-parallel lamprophyre dyke folded by a kink band in the foreground. This dyke is displaced by a perpendicular dyke along a minor dextral fault. P008562.]]
From Monteith take the A747 south for about 9 km towards Isle of Whithorn; at a cross-roads, turn right following a sign for St Ninian's Cave and after a further 2 km stop in the car park just before the farm of Kidsdale (NX 432 367). A public footpath follows the track through the farmyard then turns right just after the house on the right, following the wooded valley for 1 km to the shore at Port Castle Bay (NX 425 358). St Ninian's Cave is along the shore to the north but the most informative geological section is to the south, towards Carghidown (435 351).

In passing, examine the small exposure of thin-bedded greywackes at the back of the beach a few metres south of the end of the path. This exposure includes gently plunging, south-verging D2 folds with a crenulation cleavage developed in the fold hinges only. Exposures towards the south of the beach show thin slickensided quartz coatings on bedding planes. These indicate subhorizontal shearing along the bedding, shown to be sinistral where the sense of movement can be demonstrated, and probably relate to deformation associated with a strike-parallel fault along the straight valley followed by the path to the beach.

The section south of the beach is similar in its sedimentology to the Kirkmaidensequence but two features distinguish it as the Carghidown Formation: 1. thin red mudstone beds occur sparsely here but increase in frequency and thickness to the south; 2. short sections in which the bedding is disrupted are commonly interspersed with coherently bedded units. Faults form most of the boundaries to the disrupted zones which are themselves veined, probably because these units were initiated as zones of soft-sediment disruption but served to localise subsequent deformation. South of the beach, the strata are cut by several lamprophyre dykes which postdate well-developed, west-plunging folds but display variable relationships with faults.

Two fault sets, NW- and NNE-trending with conjugate geometry, are present throughout the Whithorn area, the straight coast at Locality 3 being developed along a zone of the NW-trending set. These cut the bedding and most of the bed-parallel dykes with clear evidence of dextral lateral displacement, although polyphase oblique slip is shown by several generations of slickensides. Later dykes have been emplaced into these faults. One example, exposed about 50 m from the start of the main coastal section, shows evidence of intrusion during fault movement. Bedding, including a 25 cm bed-parallel dyke, is displaced by a minor fault trending 125°. A 40 cm-thick lamprophyre dyke has been emplaced into the fault and carries a fabric in its margins consistent with dextral movement on the fault. Both dykes are folded by steeply plunging, dextral kink bands. A short distance to the south, D1 folds are well displayed in a north-verging fold pair which plunges moderately SW. A thick dyke forms a cliff immediately to the south and clearly postdates the folds.

The cliff prevents further access but a walk south along the cliff top for 100 m allows the shore to be regained in the south side of a small bay. However, before descending to the shore, walk 50 m further along the cliff top to where a steeply plunging sinistral D1 fold pair is particularly well exposed. The anticlinal hinge, plunging at about 87° at the top of the cliff, curves sharply to plunge 66° west lower down. Return along the cliff top, descend to the shore and examine the steeply plunging folds on the wave-cut platform. Cleavage is best developed in the muddy beds between sandstones and is axial planar to the folds. To the south, the same cleavage is related to several minor folds, affecting thin-bedded greywacke and silty mudstone alike; this emphasises that, despite their widely variable attitude, all the folds are the products of one phase of deformation. The cliff to the south exposes several minor thrust faults which displace the D1 folds and were probably formed during the D2 event; similar relationships were seen at Locality 2.

Return to the cliff top and walk back via Port Castle Bay to Kidsdale. Alternatively, if appropriate transport arrangements can be made, the cliff top can be followed for a further 2 km SE to Locality 4, near Burrow Head. In passing, the shore can be accessed to examine the characteristic features of the Carghidown Formation, including red mudstone beds, slumped units, folds of variable attitude and cross-cutting faults and dykes.

==== 4 Burrow Head: Carghidown/Ross formation junction ====
[[File:P008407.jpg|thumb|300px|left|Burrow Head. Looking N. Red mudstones in Carghidown Formation. Unusually thick red mudstone beds in steeply-dipping, locally disrupted greywackes of the Carghidown Formation, Hawick Group. P008407]]From Kidsdale, return to the A747 and proceed 4 km SE to Isle of Whithorn. Turn right shortly after entering the village, following the sign for Burrow Head caravan park. Stop at the reception office to gain permission for access, then drive to the south part of the site and park near the cliff top (NX 451 342). Most of the exposure here is above high water mark, although the critical section across the junction between the two formations is difficult to reach when the tide is high; access to the foreshore is otherwise not difficult. Burrow Head (NX 453 341) is important because it is the only place in Wigtownshire where the junction between the fossiliferous Ross Formation and the unfossiliferous Carghidown Formation can be examined. The same junction in Kirkcudbrightshire (Excursion 5) occurs in a more complete sequence where the Ross Formation is transitional between the Hawick and Riccarton groups (Kemp and White, 1985). The Ross Formation was formerly regarded as part of the Riccarton Group (Clarkson et al., 1975) because it includes the characteristic finely laminated dark grey siltstone. However, it is more similar in its overall sedimentology to the Hawick Group and is now regarded as the highest formation thereof (White et al., 1992). Structurally it also has more in common with the Hawick Group, being strongly deformed and, at Burrow Head, forming part of the zone characterised by widely variable D1 fold plunge as seen at Locality 3. At Burrow Head the distinctive siltstone occurs sparingly, in beds from 1 cm to 1 m thick, interbedded with turbidites which are in other respects similar to those of the Kirkmaiden Formation. The disrupted bedding and red mudstone beds characteristic of the Carghidown Formation are confined to stratigraphical levels below the laminated silt-stone bed which marks the base of the Ross Formation. However, the sporadic occurrence of reddened mica in sandstone above the lowest siltstone indicates that the boundary is to some extent gradational. The junction is situated in a broad zone of south younging strata, implying that the Carghidown Formation is older than the Ross Formation. Unfortunately, the section at Burrow Head is cut by many faults, mainly of the conjugate, dextral and sinistral sets trending NW and NNE but also by some strike-parallel faults. These may be associated with minor folds and serve to make the continuity of the succession rather uncertain.

From where the vehicles are parked, walk round the coast to a stream at the east margin of the caravan park. The stream is incised into the raised beach and enters the sea along a small north-trending sinistral fault. Descend to the shore west of the stream close to two lamprophyre dykes, one north-trending and the other parallel to bedding. Bedding dips steeply SW and can be shown from sedimentary structures to be the right way up. The southward younging nature of the succession is consistent with a minor, north-verging fold pair plunging 10° SW, immediately west of the stream. Note that the plunge changes to about 30° NE across the fault.

Two red mudstone beds, about 20 cm thick, occur on the foreshore either side of the stream; the intervening greywacke beds contain reddened mica which is conspicuous (particularly with the aid of a hand-lens) on freshly broken surfaces. These features indicate that the strata are part of the Carghidown Formation, which makes up the coastal section for 4 km to the NW (see Locality 3).Return to the cliff top and walk on to the small headland about 50 m to the SE. Here the turbidite sequence is essentially similar to that seen previously, but instead of red mudstone beds it includes grey laminated siltstone, indicating that these strata are part of the Ross Formation. The first siltstone, about 30 cm thick, crosses obliquely through the centre of the headland; two more beds, each about 45 cm thick, occur close together about 20 m to the SE. Locally, the intervening sandstones contain red mica. The southward younging of the sequence is again shown by grading in the sandstone beds, congruent with a north-verging D1 fold pair which plunges 32° NE on the east side of the headland.

The transition between the two formations is exposed in the west side of the headland. However, the transition zone is confused by a fold pair with a hinge separation of about 25 m and some faulting. The occurrence of red mica in strata which are clearly part of the Ross Formation suggests that the disruption is minor.

The shore can be accessed from the cliff top at various places to west and east of this locality, allowing a wider examination of the Carghidown and Ross formations. Grey silt-stone can be found in various places and may yield graptolites if carefully split along the lamination. Graptolites of the early Wenlock ''Cyrtograptus centrifigus ''and ''Monogmptus riccartonensis ''biozones have been collected (Barnes, 1989; White et al., 1992). Apart from the frequent disruption of bedding in the Carghidown Formation, the structure of the two formations is essentially the same, with beds dipping steeply south and frequent folds varying from moderate SW- to moderate NE-plunging. Steeply plunging sinistral D1 fold pairs are also common, related to the S1 cleavage which transects the gentle to moderately plunging folds but is axial planar to the steeply plunging folds.

==== 5 Isle of Whithorn: Carghidown Formation structure ====
From the Burrow Head caravan park return to Isle of Whithorn; turn right into Main

Street and drive around the harbour, parking on the wharf or in the small car park at the south end of the harbour (NX 478 362).

[[File:P001124.jpg|thumb|300px|right|Isle of Whithorn viewed looking south-east across Isle of Whithorn Bay towards the South Pier of the harbour. Geologically it is part of the extensive outcrop of Lower Palaeozoic strata which forms the Southern Uplands of Scotland. Greywackes are the dominant rock type in the area though a laminated siltstone of the Riccarton Group is exposed at Isle of Whithorn in an intensely deformed zone which also contains red mudstones. P001124]]The Isle of Whithorn (really a peninsula) and the area to the north are largely formed of Carghidown Formation turbidites like those at Localities 3 and 4, with common red mudstone beds. The south end of the Isle includes a tectonically complex sliver of Ross Formation. Several folds and strike-parallel faults cause changes in the younging direction of the strata. Lamprophyre dykes up to 3 m thick are commonly bed-parallel or fault-parallel, but include a few of ESE trend which are not related to any obvious structural weakness.

Well-bedded turbidites, dipping steeply SE but overturned to young NW, are exposed in the shore below the car park. About 70 m to the south these pass into a 20 m wide zone disrupted by slumping. Massive sandstone blocks and disorientated bedded slabs lie in a mudstone matrix with patches of red mudstone. Units of very thin-bedded greywacke and silty mudstone contain small slump folds with widely variable axial orientations. The mélange is cut by lamprophyre dykes which lie parallel to bedding in the adjacent strata, indicating that this attitude is tectonically controlled, not simply constrained by bedding as the easiest path for intrusion. The mélange passes southwards back into steeply SE-dipping turbidites, initially southward younging indicating the presence of a fold or fault either within or at the southern edge of the melange. After 50 m there is a return to north-younging across a bedding-parallel fault.

Hereabouts a narrow concrete path leads NE across the foreshore; follow it to where it crosses the high water mark. Immediately south of the path, a small anticlinal fold pair in very thin-bedded greywackes shows extreme variation in the plunge of the prominent anticlinal hinge, from 81° NE to 45° SW over a distance of 1-2 m along the axial plane. Cleavage, approximately axial planar, is continuous throughout the fold.

A short distance to the south a narrow bay called Chapel Port West has been deeply eroded along a strike-parallel fault. Cross to the opposite side of the Isle where a comparable bay (Chapel Port East) south of the ruins of St Ninian's Church marks the trace of the same fault. South of the fault the turbidite dominated sequence contains thick red mudstone beds. Irregular ESE-trending mica-lamprophyre dykes cut obliquely across bedding and show sinistral displacements ranging from a few centimetres to 3 m on faults lying nearly parallel to bedding. A red mudstone 6 m thick contains several such faults; a lamprophyre dyke is repeatedly offset sinistrally by 30 to 50 cm. Horizontal slickensides can be seen on some of the fault surfaces and are also commonly seen in thin veins along bedding, indicating that this late (post-lamprophyre) sinistral shear was mainly taken up by movement on discrete faults and by slip along bedding.

About 50 m SE of the bay on the east side of the Isle, a major anticlinal fold, plunging about 30°NE, is well exposed across the foreshore. The same fold is also exposed on the west coast, although here it is associated with minor folds and plunges more gently. Follow the western shore south; after about 40 m, a very thick sandstone bed with large flute casts on its base is well exposed above high water in an open, east-trending fold pair. A further 40 m south, several strike-parallel faults are apparent in an intensely deformed zone, 50 m wide, marked by discontinuous and contorted sandstone beds and blocks, with some gently plunging coherent folds, in a sheared matrix. Discontinuous beds of laminated, grey siltstone occur in the deformed zone and yield a sparse lower Wenlock graptolite fauna. These features are characteristic of the Ross Formation as seen at Burrow Head, although here at Isle of Whithorn the precise stratigraphical relationships are unclear.

Return to the car park by walking north around the shore or by following one of several paths across the Isle.

==== 6 Cairnhead Mote: polyphase D1 folding ====
From Isle of Whithorn, take the B7063 north towards Wigtown. After 1.5 km, shortly before the road reaches the coast, the farm of Cairnhead is visible set back east of the road. Park at the road and, with permission from the farm, follow the track

through the farmyard for about 700 m until a small east-facing bay is reached. Cairnhead Mote (NX 486 383), and the exposures of interest, are situated on a small headland (Cairn Head) not far to the south.

Bedding within the Carghidown Formation is here disrupted in a manner consistent with soft sediment deformation. However, two folds with axes plunging steeply west are clearly seen crossing the foreshore ENE of the mote; they form a sinistral fold pair and have minor folds locally in the short limb. Close inspection of bedding will reveal earlier tight folds which, despite poor preservation, can be seen to be refolded. A tectonic fabric is apparent in the early fold hinges, suggesting that they are not simply slump folds. This fabric is not discernable elsewhere, possibly due to the intense cleavage associated with the steeply plunging folds, or perhaps because it formed only in association with the early fold hinges. This evidence for two phases of folding was first described by Rust (1965). Elsewhere, throughout much of the Hawick Group outcrop, 'early' folds are characterised by widely variable hinge plunge and a mildly transecting cleavage; some isolated, steeply plunging sinistral folds appear to be related to the same cleavage. It is only at Cairn Head, where the two fold styles are superimposed, that one can be seen to postdate the other, but even here it seems likely that both are part of the D1 event. The variable relationships are probably due to a change in the stress regime during deformation, from one dominantly orthogonal to one involving a major component of sinistral shear.

Return directly to the road along the track, or follow the coast north around Cairn Head and then round the south side of Portyerrock Bay, meeting the road just north of Cairnhead Farm. If the latter route is taken the foreshore exposures of the Carghidown Formation can be examined. Changes in the direction of younging of the near-vertical bedding, shown by sole markings and grading in the sandstone beds, indicate the presence of major folds, although many of the fold hinges have been replaced by bed-parallel faults. Many minor fold pairs are present, the hinge plunges varying from moderate east to moderate west. This variation is sometimes seen in the same fold, producing markedly arcuate hinges. Several lamprophyre dykes are also present, usually bed-parallel and less than 2 m thick but including an 8 m-thick dyke through Cairn Head.
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P001124.jpg

2015-11-10T10:16:05Z

JenniferFindlay1: Isle of Whithorn viewed looking south-east across Isle of Whithorn Bay towards the South Pier of the harbour. Geologically it is part of the extensive outcrop of Lower Palaeozoic strata which forms the Southern Uplands of Scotland. Greywackes are the d...

== Summary ==
Isle of Whithorn viewed looking south-east across Isle of Whithorn Bay towards the South Pier of the harbour. Geologically it is part of the extensive outcrop of Lower Palaeozoic strata which forms the Southern Uplands of Scotland. Greywackes are the dominant rock type in the area though a laminated siltstone of the Riccarton Group is exposed at Isle of Whithorn in an intensely deformed zone which also contains red mudstones.
== Licencing ==
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====Copyright====

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Use of the images downloaded from this site is at the users own risk. NERC gives no warranty as to the quality of the images or the medium on which they are provided or their suitability for any use.

====Ordnance Survey topography====
Maps and diagrams in Earthwise use topography based on Ordnance Survey mapping. The National Grid and other Ordnance Survey data ©Crown Copyright and database rights 2015. Ordnance Survey Licence No. 100021290 EUL.
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File:P008407.jpg

2015-11-10T10:10:37Z

JenniferFindlay1: Burrow Head. Looking N. Red mudstones in Carghidown Formation. Unusually thick red mudstone beds in steeply-dipping, locally disrupted greywackes of the Carghidown Formation, Hawick Group.

== Summary ==
Burrow Head. Looking N. Red mudstones in Carghidown Formation. Unusually thick red mudstone beds in steeply-dipping, locally disrupted greywackes of the Carghidown Formation, Hawick Group.
== Licencing ==
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{|class="wikitable" style="width:100% style="background:#f0f8ff; font-size:80%"
| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

====Non-commercial Use====

Use of the images downloaded from this site and reproduced digitally or otherwise may only be used for non-commercial purposes, which are:-

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[[Category:License tags]]

Whiteadder Water - an excursion

2015-10-27T16:17:32Z

JenniferFindlay1:

'''By A.D. McAdam. From Scottish Borders geology: an excursion guide edited by A.D. McAdam, E.N.K. Clarkson, P. Stone. Edinburgh : Scottish Academic Press (for [http://www.edinburghgeolsoc.org/ Edinburgh Geological Society]), 1992.'''

O.S. 1:50000 Sheet 74 Kelso

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002365 BGS 1:50 000 Geological Survey of Scotland map. 34 Eyemouth Drift]

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002366 BGS 1:50 000 Geological Survey of Scotland map. 34 Eyemouth Bedrock]

=== Introduction ===
The Cementstone Group, the lowest series in the Carboniferous of Berwickshire, is exposed in many of the sections cut in the valley sides of the Whiteadder Water. It is difficult to relate these sections one to another, because of the lack of distinctive horizons, the absence of progressive upward lithological change and the variable structure (Greig 1988, pp 45-49)<ref>Greig, D.C. 1988. Geology of the Eyemouth district. Memoir of the British Geological Survey, Sheet 34 (Scotland), 78pp.</ref>
[[File:WhiteadderWaterExcursionMap.jpg|600px|thumbnail|center|Whiteadder Water - excursion map.]]

==== 1. East Blanerne: Cementstone Facies ====
[[File:P218956.jpg|thumb|300px|right|Chirnside, East Blanerne. Looking SE. Mudstones and cementstones of Lower Carboniferous dipping to south at 12 degrees. South-east view. P218956]]One of several good cliff sections cutting cementstone facies in the Whiteadder Water, lies 2 km WNW of Chirnside by the B6355 road where there is limited roadside parking (NT 845 570). Access can be gained from the west end of the cliff and the section examined by scrambling along the base. The cliff is some 5 to 10 m high and is surmounted by a few metres of red sand and gravel deposits resting on boulder clay. The strata dip generally to the south-east exposing some 30 m of section. The beds are dominantly grey in colour with some green and reddish beds. The strata are mainly composed of grey mudstones and silty mudstones, generally poorly-bedded and blocky, with beds of siltstone and sandstone, and ribs and bands of cementstone. Faulting affects the south-east end of the section. The bivalves ''Modiolus latus''? And ''Curvirimula'', ''Spiororbis'', ostracods and fish fragments have been reported.

==== 2. Willie's Hole: Cementstone Facies, Plants ====
Return to Chirnside and continue south through Allanton on the B6437 road. Take the first left past Broomdykes, left again at the next T-junction and straight on at the next junction. A good track goes down to the riverside where there is parking by the footbridge and ford. Walk west along the track across the alluvial plain for 500 m to a cutting beside the low riverside cliff, waterfall and pool known as Willie's Hole (NT 878 547). A 1.5 m pale brown fine-grained sandstone, dipping at 12 degrees to the south, forms a low waterfall across the river. Above the sandstone are exposed 5 m of grey mudstones and silty mudstones with pale brown-grey nodular cementstone ribs less than 10 cm in thickness. Six metres of similar beds occur below the sandstone but with thicker cementstone bands up to 30 cm, one making a low weir across the river. Well-preserved plants are found at this locality, including ''Alcicornopteris'' and Archaeocalamites. Also recorded are the bivalves ''Lithophaga'' and ''Modiolus'', together with `''Estheria''', ostracodes, an early scorpion ''Eoscorpius''?, rare fish fragments and eurypterid fragments. The shrimp ''Tealliocaris'' occurs in a silty bed towards the top of the section. Cater et al (1989)<ref>Cater, J.M.L., Briggs, D.E.G. and Clarkson, E.N.K. 1989. Shrimp-bearing sedimentary successions in the Lower Carboniferous (Dinantian) Cementstone and Oil Shale Groups of northern Britain. Trans. R. Soc. Edinb: Earth Sci. 80, 5-15.</ref>interpret the environmental setting of this shrimp-bed as a shallow 2-metre stagnant pond on the marine influenced coastal flood plain and the overlying sediments as deposited on dessicating mud-flats and in shallow ponds into which minor mouth-bars prograded. Going back along the riverside track for 300m gives a fine view of Steeple Haugh, on the north bank, a 13m section inaccessible because of a deep pool. It has beds lower than or equivalent to those at Willies Hole, but with well-developed channel sandstones.

==== 3. Hutton Castle: Cementstone Facies, Gypsum ====
[[File:P906517.jpg|thumb|300px|right|Cementstone Group. Hutton Castle Barns BH [NT85SE/1], 248 yards south 16 degrees east of Hutton Castle Barns and 20 yards north of bank of Cabby Burn; depth 70 - 75 fm. P906517]]Farther downstream on the south bank of the river north-east of Hutton Castle Barns, there is an excellent 30-m cliff section (NT 893 547). It can be approached from the tracks running north towards Edington Mill, from which a distant view of the cliff in red facies can be seen downstream. From the bend in the track half-way down the steep hill, the section can be reached by scambling down the slope and working back upstream. A pale brown-grey cross-bedded channel sandstone with water seeping from its diachronous base forms the topmost 4 m or so of this cliff. Below the sandstone the cliff shows typical cementstone facies of grey mudstones and silty mudstones with numerous cementstone ribs, a few prominent ones up to 30 cm thick but generally less than 10 cm thick. Bands of bedded sandstone, generally less than a metre thick, are mainly purple or dark brown in colour. Fossiliferous horizons occur at several levels and have yielded gastropod fragments, ''Modiolus'', ‘''Estheria’'' and ostracods. The beds dip at 18 degrees to SSW. An unusual feature of this section is the presence of white or pink gypsum, as nodules, as satin spar veins along the bedding and as large vugs.

==== 4. Paties Cove: Reddened Cementstone Facies ====
The next southward loop of the Whiteadder Water forms an even more spectacular cliff, the east end of which is named Patie's Cove (NT 902 544). It can be reached by going north from Hutton to Hutton Castle Mill, from which are good views of reddened cementstone facies cliffs across the valley, and then upstream along the alluvium. Much of the main cliff is inaccessible, but the cementstone facies is evident and there is a prominent channel sandstone some 3.5 m thick in the middle of the section. The beds show various shades of red, purple, brown and green. Most striking is the change in colour of the beds from the general greyish hues seen in the previous locality to the overall reddish tints at Patie's Cove. Gypsum is notably absent. Such major changes in so short a distance illustrates the difficulty of building up a stratigraphy in these beds. ''Spirorbis'' and ostracods have been found in the lower beds. The beds dip at 15 degrees to SW.

==== 5. Foulden: Site of Special Scientific Interest ====
The nature of the Cementstone Group strata was fully investigated by a multi-disciplinary study on a locality excavated near Foulden Newton Farm (NT 922 553). In this area, sediments of the Cementstone Group, mainly siltstones mudstones and cementstones, were deposited on an extensive flood-plain, crossed by small meandering rivers and passing southwards into coastal mudflats. Semi-permanent lakes surrounded by lycopod swamps lay scattered on the flood-plain and in one of these the Foulden Fish Bed, the principal subject of the investigation, was deposited. During its existence of perhaps a few thousand years, the lake was never more than a few metres deep and, though initially floored by a shell-bed, the bottom became too soft to support benthos. Through time the salinity of the water fluctuated, the lake became shallower and it finally silted up altogether. At its maximum extent the lake supported a rich fauna including the 2 m long fish ''Megalichthys''. The relative proportions of the fauna changed with the salinity and there were two mass-mortality events. Results were published in a special part of the Transactions of the Royal Society of Edinburgh (1985)<ref>Wood, S.P. and Rolfe, W.D.I. 1985. Introduction to the palaeontology of the Dinantian of Foulden, Berwickshire, Scotland. Trans. R. Soc. Edinb: Earth Sci. 76, 1-6. (This issue, part 1, pp 1-100, contains 12 papers devoted to the faunas, environment and ecology of Foulden).</ref> on the general palaeontology (Wood and Rolfe), the sedimentology (Anderton), the plants (Scott and Meyer-Berthaud), the miospores (Clayton), certain arthropods (Waterston, Briggs and Clarkson, Almond), the ostracods (Pollard), various fish (Forey and Young, Gardiner, Andrews) and the palaeoecology (Clarkson). Although protected the site can be visited and some of the 30 m section in grey cementstone facies can still be examined.

===References===

<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P906517.jpg

2015-10-27T16:15:19Z

JenniferFindlay1: Cementstone Group. Hutton Castle Barns BH [NT85SE/1], 248 yards south 16 degrees east of Hutton Castle Barns and 20 yards north of bank of Cabby Burn; depth 70 - 75 fm. P906517

== Summary ==
Cementstone Group. Hutton Castle Barns BH [NT85SE/1], 248 yards south 16 degrees east of Hutton Castle Barns and 20 yards north of bank of Cabby Burn; depth 70 - 75 fm. P906517
== Licencing ==
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File:P218956.jpg

2015-10-27T16:04:47Z

JenniferFindlay1: Chirnside, East Blanerne. Looking SE. Mudstones and cementstones of Lower Carboniferous dipping to south at 12 degrees. South-east view. P218956

== Summary ==
Chirnside, East Blanerne. Looking SE. Mudstones and cementstones of Lower Carboniferous dipping to south at 12 degrees. South-east view. P218956
== Licencing ==
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[[Category:License tags]]

Tweedsmuir - an excursion

2015-10-27T16:00:21Z

JenniferFindlay1:

'''By J. D. Floyd and P. Stone. From Scottish Borders geology: an excursion guide edited by A.D. McAdam, E.N.K. Clarkson, P. Stone. Edinburgh : Scottish Academic Press (for [http://www.edinburghgeolsoc.org/ Edinburgh Geological Society]), 1992.'''

O.S. 1:50000 Sheets 72 Upper Clyde Valley, 73 Galashiels and Ettrick Forest and 79 Hawick and Eskdale

B.G.S. 1:50000 Sheets 16W Moffat, 16E Ettrick and 24W Biggar

Route: Figure 35

=== Introduction ===
This excursion traverses across strike through the Northern date Ordovician) and Central (early Silurian) belts of the Southern Uplands (Figure 1). A variety of turbidite lithological assemblages and structural styles can be observed as the route is followed through some of the finest scenery In the south of Scotland. Several of the minor roads used are narrow, serpentine and unsuitable for vehicles larger than private cars or perhaps a minibus.

==== 1.Coulter Quarry: Marchburn Formation Greywacke, Haggis Rock ====
The excursion commences at Coulter on the A702 between Biggar and Abington. At the sharp bend of the main road over the Coulter Water bridge (NT 024 340) take the minor road to the south-east marked "No through road". This leads past Coulter Primary School and straight on for 1 km; Coulter Quarry is on the left (NT 028 334) with parking available either opposite the quarry or a little further on, across the bridge, on the left. The main quarry section exposes well-bedded greywackes and siltstones of the Marchburn Formation (Llandeilo-Caradoc) which strike NE-SW with a steep, normal dip to the NW. The greywackes show all of the characteristics of deposition from turbidity flows with well-developed graded bedding and a variety of erosional bottom structure casts on bed bases. These indicate a variable current regime with flow from either the NE or the NW. In the central part of the quarry section are exposed the most massive greywacke beds, ranging up to several metres in thickness and commonly showing bulbous load casts and associated flame structures at their bases. These beds may also have a very coarse, granular basal greywacke unit colloquially known as "Haggis Rock". This distinctive lithology contains a wide range of clast types including jasper, felsite granite, gabbro and dolerite, which impart a speckled, multicoloured appearance to the rock reminiscent of the traditional Scottish dish from which it takes its name. Toward the northern and southern margins of the quarry the proportion of thin-bedded greywacke and siltstone in the sequence increases but, curiously, the best example of "Haggis Rock" occurs at the base of the thin-bedded unit on the north side of the quarry. Slightly farther north, in the roadside outcrops just beyond the quarry, the thin-bedded greywackes are folded into a large, upright anticline-syncline pair. This fold pair is south verging, typically so for the northern part of the Southern Uplands, with hinges (slightly faulted in the anticline) plunging gently north-east. An associated cleavage is only very weakly developed as a rather irregular spaced fracture in the finer-grained lithologies. Elsewhere in the quarry small fold pairs, only 2-3 m in wave length and amplitude, are incongruous with the larger, upright fold pair and may represent a different deformational event. The quarry section can be followed intermittently southwards for about 250 m towards Coulter Craigs. Thin to medium-bedded greywackes dominate at first but near the southern limit of exposure the greywacke horizons appear to be interbedded with red siliceous mudstone an, chert. Such assemblages are generally regarded as amongst the earliest sedimentary deposits in the Southern Upland succession and here arc probably of Llanvirn or Llandeilo age. The overlying greywackes in the quarry section are generally assigned to the Llandeilo to Caradoc ''N. gracilis'' graptolite zone.

==== 2. Stobo Quarry: Lowther Shales ====
South-east from Coulter Quarry, across the strike trend of the Southern Uplands, the Stobo and Wrae areas lie within the outcrop of a succession of particularly well-laminated siltstones and mudstones known as the Lowther Shales. These form part of the Shinnel Formation (Caradoc) and have been worked in numerous quarries between Stobo and Moniaive (50 km along strike to the south-west) as an inferior roofing slate. From Coulter Quarry return to the A702 and thence to Biggar; at the eastern end of the town turn right on to the minor road signposted for Broughton. In this village turn right on to the A701 (towards Moffat) but after only a short distance turn left on to a minor road following signs for "Peebles via Dreva." From the road there is a good view north-eastward towards the Stobo quarry complex which is reached by a left turn on to a rough track at Altarstone (NT 156 359). The various quarry sections at Stobo jointly expose a continuous section of at least 150 m, across a greywacke, siltstone and shale sequence. The darker variety of the shales, which have a pervasive bedding-parallel fissility, are known as the Stobo slates and have yielded Caradocian graptolite fauna of D. clingani zone age (Eckford and Ritchie 1931; Leggett 1980). Bedding strikes about 060 degrees and is vertical to steeply inclined with a consistent sense of younging towards the NW. In the lowest (farthest south-east) part of the Stobo section thin, fine-grained greywacke beds occur sporadically with the generally pale grey shale: Darker shale horizons are associated with thin nodular chert bands containing radiolarian remains. Upwards through the succession there is a tendency for the greywacke beds to become thicker and more numerous. However, the fine grained shales remain dominant across the quarry until, if its north face, the proportion of greywacke increases abruptly with only thin mudstone interbeds. The greywackes are thickly-bedded and are usually fine to medium-grained. They are often massive, possibly due to amalgamation. Bottom structures are very common on the bases of the greywacke beds with load-casts being the most abundant. Where a current direction can be deduced from flute-casts it is invariably indicative of flow approximately along strike from the north-east. Some of the thicker greywacke beds contain large numbers of shale rip-up clasts.

==== 3. Wrae Hill: Caradoc Limestone Breccia and Lava ====
A different aspect of the Lowther Shales can be seen in the various disused quarries at Wrae, about 6 km along strike to the south-west from Stobo (Figure 36). These quarries are reached by rejoining the A701 at Broughton and proceeding south-west towards Moffat for about 3 km. Roadside parking space at the foot of Wrae Hill, beneath the quarries, is very limited (NT 119 326). On the eastern slope of Wrae Hill the Wrae Limestone is interbedded with the Lowther Shales and was formerly worked in a number of small quarries. These exposures are of historical significance for geology since it was here, in 1792, that Sir James Hall discovered fossil shells in the limestone. This led his friend James Hutton to conclude, in Theory of the Earth (Hutton 1795), that the "schistus" of the Southern Uplands were once water-lain sediments which had been elevated from the bottom of the sea. The country-rock in the Wrae area is a succession of particularly veil-cleaved laminated siltstones and mudstones, the Lowther Shales. These have been worked as an inferior slate is in the North Quarry at Wrae which has a large scree of broken shaly debris creeping downslope below the workings. The regional strike of bedding is 070 degrees and dip is about 75 towards the NW. Sedimentary structures indicate that the beds are right-way-up. The Upper Quarry, 200 m farther south, probably worked the limestone which consisted entirely of resedimented blocks, some many metres in size, set in a pebbly mudstone matrix. Although now mostly quarried-out, a few scattered blocks of the limestone can still be found lying around on the floor of the quarry, some with recognisable shelly fragments on weathered surfaces. The limestone is very hard and recrystallised so that it is almost impossible to extract fossils without destroying them. The fauna of brachiopods, trilobites and gastropods is of shallow water (reefal) aspect and has been transported into deeper water by some sort of mass-flow mechanism. Its age was originally described as Upper Caradoc by Peach & Horne (1899), but Bergstrom (1971) has assigned it to the Llandeilo based on conodonts recovered from the limestone. The gap in exposure represented by the worked-out limestone is about 8 m wide. Traversing southwards and down the succession, 17 m of pebbly mudstone with gritty patches and pebbles of mudstone, limestone and quartz is seen. This is underlain in turn by 5 m of greenish-grey calcareous tuff containing pebbles of fine-grained porphyritic lava. The tuff rests on a wedge of pale grey fine-grained non-porphyritic alkaline lava (quartz-keratophyre) which forms a prominent 5 m thick feature just above the fence. From its apparently conformable and intimate contact with the underlying Lowther Shales, this unit has been interpreted by Eckford & Ritchie (1931) and Leggett (1980) as a lava flow rather than a slab-like olistolith. The whole succession of lava, tuff, pebbly mudstone and limestone breccia is about 35 m thick and is underlain and succeeded by Lowther Shales of uniform appearance. It was considered by Leggett (1980) to be a submarine slide deposit derived from a volcanic island fringed with a limestone reef. In the Lower Quarry, 150 m downslope from the Upper Quarry, a mudflake breccia horizon is interbedded with greywackes. Greywackes are more abundant than siltstones in this quarry and the stone was probably used for building dry-stone dykes. This quarry is now in the middle of a thick forest and access is difficult.

==== 4. Tweedsmuir: Greywackes "Pyroxenous Group" ====
Continue towards Moffat on the A701 for a further 10 km as far as the village of Tweedsmuir. Turn south across the River Tweed on the unclassifed road signposted to St Mary's Loch. NOTE: This road is not suitable for caravans or vehicles larger than a minibus. Park near the crossroads a few metres south of the bridge. In the gorge of the River Tweed both upstream and downstream from the bridge, thick-bedded pebbly greywackes of the Llandovery "Pyroxenous group" are well displayed with a strike trend of 035 degrees and dipping 65 degrees towards the NW. Close inspection of bedding planes a few metres upstream from the bridge will reveal, from the evidence of such characteristic greywacke features as graded-bedding and flame-structures, that the beds are in fact overturned and young towards the southeast. These beds form part of a zone, about 2 km wide across strike, along the northern edge of the Central Belt in the Tweedsmuir area, where the dominant direction of younging is towards the south-east in contradiction of the regional pattern. At this locality and others in the Tweed and its tributaries a short distance along strike to the south-west, Peach and Horne (1899) record scarce graptolites indicative of the zone of P. acuminatus, the basal zone of the Llandovery. The graptolites are not easily found and occur in thin bands within a grey and dark grey shale and greywacke succession. The paucity of graptolites, together with the grey shale and greywacke lithology, contrasts sharply with the beds of similar age at Dob's Linn, only some 14 km across strike to the south-east, where graptolites are abundant in a black shale lithology (see Dob's Linn Excursion).

==== 5. Cramalt Road Section, Megget: Greywacke, Gala Group ====
Continue eastwards on the unclassified road from Tweedsmuir towards St Mary's Loch. This road traverses the valley of the Talla Water and along the side of the Talla Reservoir, built in 1895-1905 as the first major water supply scheme for the City of Edinburgh. The road at the head of the valley is particularly steep, winding and narrow where it climbs over the watershed to the valley of the Megget Water. The view from the top of the hill, from the bridge over Talla Linn northwest along the glacially gouged Talla valley, must be one of the most magnificent in the south of Scotland and is worth a brief stop to appreciate. Glacial moraine deposits are liberally scattered on the eastern side of the watershed. Proceed eastwards along the road for a further 7 km to a lay-by near Cramalt (NT 200 229) beside the new Megget Reservoir (filled in 1983). This carefully landscaped gravity dam forms the latest and largest addition to Edinburgh's water resources. View points with explanatory display boards are situated overlooking the dam and it is possible to walk out along the top of the latter the better to appreciate its construction. A new road has replaced the old route now flooded by the reservoir, and this affords excellent exposures in the greywackes and siltstones of the Queensberry Formation (Gala Group) along the north side of the reservoir. On the north side of the road a few hundred metres to the west of the Cramalt Burn a roadside crag shows a fine array of bottom structures, mainly linear grooves, on the bases of the greywacke beds. However, a more complete section may be inspected further east, beyond the Cramalt Burn and close to the large lay-by (NT 200 229) on the reservoir side of the road. At this locality (Figure 37), thick-bedded pebbly greywackes strike 040 degrees and dip 80 degrees to the NW. These massive greywackes occupy a large channel cut into the underlying dark-grey laminated siltstones, and contain numerous irregular ripped-up fragments of dark mudstone and siltstone. Large flute-casts visible at the eastern end of the section demonstrate that the beds young towards the north-west. Examination of the beds near the west end of the section reveals an excellent example of channelling by a greywacke unit. The channel infill cuts down into the underlying laminated siltstones and thin greywackes for over 3 m in a relatively short distance, with flutes on the base of this and several other adjacent beds showing that currents were generally flowing from south-east to northwest. Very coarse-grained breccias of sedimentary clasts fill likely de-watering pipes which cut through the massive channel-fill greywackes. The laminated siltstone at the western end of the section is locally deformed by tight, small-scale anticline-syncline pairs plunging about 20° to the NE. The greywackes and laminated siltstones at Cramalt are the equivalent of the greywackes which succeed the Moffat Shales at Hartfell (Hartfell Score excursion) and are thus probably of M. gregarius zone (Llandovery) age or younger. Trace fossils such as Megagrapton and Planolites are visible on the bases of some of the bedding surfaces at the western end of the section (Benton 1982).

==== 6. Cappercleugh Quarry: Gala Group ====
Follow the unclassified road eastward from the Megget Dam

towards St Mary's Loch. About 0.5 km east of Henderland Farm a small quarry is located on the north side of the road, adjacent to a large unpaved lay-by and partially obscured by trees (NT 235 232). This small quarry affords an interesting example of the use of sedimentary structures in deducing the presence of a fold. At first sight the greywackes exposed seem to have a fairly consistent NE-SW strike, the beds being either vertical in attitude or dipping steeply to the NW. However, on closer inspection, the bases of beds on the eastern side of the quarry are marked by load-accentuated grooves which show that the sequence youngs to the SE. In contrast, on the western side of the quarry the bases of the beds are marked by very small bottom-structures which, together with welldeveloped grading within the beds, show that there the sequence youngs to the NW. An anticlinal structure is required within the quarry to explain these relationships but, where exposure is more or less complete along the back of the quarry the greywackes consistently young to the NW. One possible solution is shown in Figure 38 where a low-angle fault or thrust separates the upper and lower quarry sections. The younging evidence in this area is ambiguous. Is there additional structural complication? What do you think?

FIG 38. Structural problem, Cappercleuch Quarry.

From St Mary's Loch the excursion may be continued in one of three ways. If the A708 is followed south-westward towards Moffat a visit can be made to the classic section through the Moffat Shale Group described in the Dob's Linn Excursion. Alternatively the A708 may be followed north eastwards to the Gordon Arms where a left turn towards Innerleithen will link with the Heriot and Peebles Excursion. The third possibility involves a right turn at the Cordon Arms on to the B709 towards the Tushielaw Inn.

==== 7. Tushielaw: Hawick Group ====
[[File:P220053.jpg|thumb|300px|right|Hawick road section, 2 km. SE of Tushielaw, Ettrick. Looking E. Folded greywackes. Southern half of Hawick road section showing folds 3 (anticline), 4 (syncline) and 5 (imbricated) in greywacke sandstones of the Gala Group. P220053]]From Cappercleugh Quarry continue along the unclassified road to St Mary's Loch and thence by the A708 (via the Gordon Arms) and B709 to the Tushielaw Inn. About 2 km farther south-east, on the B711 towards Hawick, a fine section in tightly folded Hawick Group (upper Llandovery) greywackes and siltstones is exposed on the north side of the road (318 164). This section (Figure 39), described in detail by Webb (1983), consists of a series of anticlines and synclines plunging at 40° towards the NE. A well-developed axial-planar slaty cleavage can be seen in places. Although the fold hinges are not always obvious, due to the fusion or amalgamation of beds in the cores of the folds, plentiful sole structures on beds within the limbs of the folds allow the structure to be easily demonstrated. Directional evidence from flutes indicates a predominance of currents flowing from east to west with a few flowing from south to north.

Continuing south-east on the B711 will join the A7, Hawick. Alternatively retrace the route to the Gordon Arms crossroads whence the A708 leads to Moffat (turn left), Selkirk (turn right) whilst the B709 (straight on) is the beginning of a scenic route to Edinburgh via Innerleithen an Peebles.

===References===

<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P220053.jpg

2015-10-27T15:55:36Z

JenniferFindlay1: Hawick road section, 2 km. SE of Tushielaw, Ettrick. Looking E. Folded greywackes. Southern half of Hawick road section showing folds 3 (anticline), 4 (syncline) and 5 (imbricated) in greywacke sandstones of the Gala Group. P220053

== Summary ==
Hawick road section, 2 km. SE of Tushielaw, Ettrick. Looking E. Folded greywackes. Southern half of Hawick road section showing folds 3 (anticline), 4 (syncline) and 5 (imbricated) in greywacke sandstones of the Gala Group. P220053
== Licencing ==
__notoc__
{|class="wikitable" style="width:100% style="background:#f0f8ff; font-size:80%"
| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

====Non-commercial Use====

Use of the images downloaded from this site and reproduced digitally or otherwise may only be used for non-commercial purposes, which are:-

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When using the images please credit 'British Geological Survey' and include the catalogue reference ('P Number') of the item to allow others to access the original image or document.
Non-commercial users of the images from this site are restricted to downloading no more than 30 images, without seeking further permission from [mailto:enquiries@bgs.ac.uk enquiries@bgs.ac.uk]

====Commercial Use====

For commercial use of these images for which higher resolution images are available, individual permissions and/or licences arrangements should be agreed by contacting [mailto:enquiries@bgs.ac.uk enquiries@bgs.ac.uk]

Commercial use will include publications in books (including educational books), newspapers, journals, magazines, CDs and DVDs, etc, where a cover charge is applied; broadcasts on TV, film and theatre; and display in trade fairs, galleries, etc. If you are in doubt as to whether your intended use is commercial, please contact [mailto:enquiries@bgs.ac.uk enquiries@bgs.ac.uk]

====Warranty====
Use of the images downloaded from this site is at the users own risk. NERC gives no warranty as to the quality of the images or the medium on which they are provided or their suitability for any use.

====Ordnance Survey topography====
Maps and diagrams in Earthwise use topography based on Ordnance Survey mapping. The National Grid and other Ordnance Survey data ©Crown Copyright and database rights 2015. Ordnance Survey Licence No. 100021290 EUL.
|}
[[Category:License tags]]

St. Abb's Head - an excursion

2015-10-27T15:22:52Z

JenniferFindlay1:

'''By D.C.Greig. From Scottish Borders geology: an excursion guide edited by A.D. McAdam, E.N.K. Clarkson, P. Stone. Edinburgh : Scottish Academic Press (for [http://www.edinburghgeolsoc.org/ Edinburgh Geological Society]), 1972.'''

O.S. 1:50 000 Sheet 67 Duns and Dunbar

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002365 BGS 1:50 000 Geological Survey of Scotland map. 34 Eyemouth Drift]

[http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1002366 BGS 1:50 000 Geological Survey of Scotland map. 34 Eyemouth Bedrock]

==== Introduction ====
[[File:P000813.jpg|thumb|300px|right|Oblique aerial view of St. Abb's from the east. Berwickshire. The rocks in the foreground are intrusive porphyrites and agglomerates. The rocks on the shore are intrusive porphyrites and unbedded agglomerates, probably within a volcanic vent, of Lower Old Red Sandstone age. Beyond the trees in middle distance the high ground is formed of Silurian greywackes and siltstones. The vent, while not clearly defined, is recognized as such by the chaotic distribution of igneous and pyroclastic rocks on the well-exposed shore and cliffs, and by the extensive occurrence of very coarse agglomerates unlike any of the bedded sequence of St. Abb's Head. P000813]]he principal object of this excursion is to study the volcanic rocks of Lower Old Red Sandstone (Lower Devonian) age which form the peninsula of St Abb's Head, on the Berwickshire coast north of St Abb’s and Coldinghame. The excursion could occupy a full day, but as it does not cover a very extensive area it can conveniently be cut short as required. The complete circuit covers a distance of about 8 km, of which about 4 km is over hilly open pasture, 1 km over rough rocky seashore, and nearly 3 km on a surfaced hill-road. The steep grassy slopes above the precipitous cliffs of St Abb's Head demand sturdy boots. The excursion is affected by the state of the tide at only one or two localities, possibly requiring adjustments to the route

St Abbs lies 7 km south-east of Fast Castle and is reached by the B6438 road from Coldingham. The headland, just north of the village, is a National Nature Reserve, managed jointly by the National Trust for Scotland and the Scottish Wildlife Trust [http://www.nts.org.uk/Property/St-Abbs-Head-National-Nature-Reserve/Property-description NTS-SWT]. Car and coach parking are located beside the information centre and coffee shop at Northfield Farm (NT 913 674), limited car parking is also possible near the Lighthouse (NT 913 692). The excursion is described as starting and finishing at the information centre, where an illustrated guide to the Reserve and other literature are available.

The lavas of St Abb's Head are generally andesitic, but the constituent minerals are so altered that classification is often in doubt. In thin section the feldspars appear, when fresh, to be mainly oligoclase or andesine, and the ferromagnesians, usually altered to iron-ore, may show the crystal form of hornblende or augite. More basic rock, with labradorite and olivine, is quite common. Micas, particularly biotite, are often present and in somerocks are abundant. The rocks are generally microporphyritic, and under the microscope many show well developed flow-structure. The upper and lower parts of the flows are often highly amygdaloidal, the vesicles being filled with silica and/or calcite. In general type the lavas are similar to the mica-andesites of the Cheviot Hills (Geikie 1864<ref>Geikie, A. 1864. The Geology of Eastern Berwickshire. Mem. Geol. Surv. Scot.</ref>, 1897<ref>Geikie, J.1897. Geology and Petrology of St Abb's Head. Proc. R. Soc. Edinb.14, 177-193.</ref>, Greig 1988<ref>Greig, D.C. 1988. Geology of the Eyemouth district. Mem. Brit. Geol. Surv. Hutton, J. 1795. Theory of the Earth. William Creech, Edinburgh.</ref>).

[[File:StAbbsHeadExcursionMap.jpg|thumb|600px|right|St Abb's Head]]

==== 1. Northfield: Viewpoint ====
Follow the signposted footpath from the car park, which leads to the cliff-top north-west of Northfield House. From this point the precipice of White Heugh immediately catches the eye. A small outcrop of Lower Devonian conglomerate forms the rounded grassy summit of Bell Hill and the red pock-marked cliff to the left of White Heugh. Silurian greywackes and shales form a narrow outcrop on the lower part of this cliff and on the foreshore to north-east.

==== 2. Craig Robin: Silurian and Devonian Sediments on Shore ====
The foreshore can be reached from the path beyond locality 1. The large stack known as Craig Robin, is made up of closely veined and broken Silurian siltstone, locally weathered to a characteristic yellow colour. It lies within a NE-SW zone of similar breccia, 70 m wide, which marks the boundary between the extensive vent-breccia of St Abbs and the Silurian greywackes below White Heugh. The latter are exposed on cliff and shore between 50 and 100 m north-west of Craig Robin, where they are seen to be sheared and squeezed, but well bedded with a dip of 60° to NW. Beyond them, close to a prominent gully marking the St Abb's Head Fault, and above them on the cliff, lies an outcrop of breccia or conglomerate considered, from its content of greywacke fragments, to be of Lower Devonian age. The boundary between greywacke and conglomerate was thought by James Geikie (1887) to be a north-easterly fault. Beyond the gully, lava which forms White Heugh is seen to be underlain by Lower Devonian sediments ranging from fine-grained shaly sandstone to coarse greywacke-breccia. The base of the lava is irregular but follows approximately the bedding of the sediments, which dip to NNW at 50°. These last exposures can be examined with ease only near low tide.

==== 3. White Heugh: St Abb's Head Fault ====
Climb back to the cliff-top to south-west and follow the path northwards to the landward end of White Heugh. From the col below the path a short descent to north-east affords a view of the detail of the St Abb's Head Fault. The steep grassy slope follows the dip in sandstones with shaly partings and mudstone pellets, part of the Lower Devonian sequence seen at the base of White Heugh and forming Bell Hill to the west. The cliff-exposure to the left is divided by near-vertical planes into four parts. On the left is a triangular wedge of irregularly jointed biotite-lamprophyre, an extension of a dyke exposed in a disused quarry 80 m to west, beyond the boundary fence of the Reserve<ref>The dyke-rock, which cuts Lower Devonian conglomerate, has been radiometrically dated at 400 Ma, close to the Siluro-Devonian boundary (Rock and Rundle, 1986) </ref> This is followed to north-east by a zone 4 m wide of steeply dipping red shaly sandstones, which are taken to be part of the Lower Devonian basal sediments. Small bodies of breccia and intrusive rock occur at the margins of this zone. The third division is a vertical band of breccia 2.5 m wide, difficult to examine but probably largely of broken lava, the essential fault-zone between the Lower Devonian and Silurian sediments to south-west and the lavas to north-east. The lavas make up the fourth division of the exposure.

==== 4. Hardencarrs Heugh and Burnmouth Harbour: Lavas and Agglomerates ====
The northward continuation of the cliff path crosses a large body of fine-grained lava, which makes up most of Hardencarrs Heugh. It is well exposed on the northern (inland) face of White Heugh, where steep platy jointing is conspicuously developed. Between Hardencarrs Heugh and Burnmouth Harbour are many small exposures of red lava full of tiny amygdales, locally clearly forming a homogeneous agglomerate, but the distinction between that and unbroken lava is difficult to determine and may not be very significant. Some of the clean wave-washed rock at Burnmouth Harbour is very obviously agglomeratic.

==== 5. Horsecastle Bay: Agglomerate, Brecciated Lavas, Lamprophyre Dyke ====
[[File:P005893.jpg|thumb|300px|right|St. Abb's Head, Horsecastle Bay. View N. Grits and lava. Bedded tuffaceous grits overlie massive lava in the foreground, and are overlain on the headland by a later flow with a clearly irregular base. Such interrelations of bedded grits are not uncommon in the predominantly lava sequence of St. Abb's Head. Lower Old Red Sandstone. [http://194.66.252.158/asset-bank/action/viewAsset?id=5142 P005893]]]
From Burnmouth Harbour follow H.W.M. for 200 m to a little embayment on the north side of the headland of Horse Castle, where the agglomeratic country rock is cut by 3-m dyke of highly micaceous lamprophyre. It follows a curved north-easterly course, is cut by faults, and has two thin offshoots on its south-eastern side. It could be the same intrusion as that noted earlier at White Heugh and on Bell Hill, and is the only minor intrusion so far recognised in the lava sequence. Most of the rock between Burnmouth Harbour and Horsecastle Bay is roughly bedded brecciated lava. Thin layers of unbroken lava are present, being clearly distinguishable on the stack of Horsecastle Rocks, and one at least 4.5m thick caps the cliffs at Horse Castle, near the lamprophyre dyke. Typical broken lava is well exposed on the south- eastern side of Horsecastle Bay, where many large tabulate or rounded fragments of lava lie parallel to the major bedding fractures. The rock is thought to result from the spasmodic solidification of lava during the continuing movement of the flow as a whole.

==== 6. Horsecastle Bay: Tuffaceous Grits and Lavas ====
The beach of Horsecastle Bay is traversed by a fault trending north-east, which separates the brecciated lavas already seen from a thick series of well differentiated lavas and tuffaceous grits which occupies all the groundnorth-westwards to St Abb's Head and Pettico Wick. The exposures on the north-western side of the bay are particularly instructive and readily accessible. Tuffaceous grits form well defined planar beds from a few centimetres up to 2 m thick, and consist largely of small fragments of lava, like the rocks above and below, in a matrix in places highly siliceous but locally like a sericitic mudstone. No 'foreign' fragments have been recognised in any of the many developments of these grits. They are thought to be in the main the water-laid product of erosion of the lavas, but to some extent they appear to have a more directly pyroclastic origin, manifested by the development of a welded texture among the fragments. The major group of grits here is 27 m thick. At its base there is a gradual downward transition to unbroken lava through a small thickness of decreasingly fragmented rock. On the southern face of East Hurker, on the north side of the bay, the reddened top of the grits is seen to have been irregularly gouged by the next lava-flow. The 'col' which connects East Hurker to the main cliffs marks the line of a N-S fault, which can be clearly seen to displace the grits and lavas along the western side of Horsecastle Bay. A westward downthrow of 6 m may be estimated.

==== 7. Kirk Hill: Lavas with Agates ====
The prominent mass of Kirk Hill, rich in archaeological association, appears to be formed of one lava-flow, that which overlies the thick grits of Horsecastle Bay. Some 60 m of rock is preserved but the top of the flow may have been eroded away. Among the crags above the footpath, on the south-western face of the hill, the lava is locally rich in ovoid amygdales up to at least 50 mm long, among which moderately good specimens of agate may be seen.

==== 8. Cauldron Cove: Faulted Grits and Lavas ====
[[File:P005892.jpg|thumb|300px|left|St. Abb's Head, Cauldron Cove. View SW. Grits and lava. Bedded tuffaceous grit (c. 27 m. thick) dips to left and overlies massive lava (lower left). The lava appears to be thrust over bedded grits in right foreground. Lower Old Red Sandstone. P005892]]Follow the base of Kirk Hill northwards and north-eastwards to regain the sea-cliffs at Cauldron Cove. On the south-east side of the cove 4.5 m of bedded tuffaceous grits form a crag at the top of a very steep grassy slope, close below the lava of Kirk Hill. Below these grits, beyond about 4.5 m of concealed strata, a further 18 m of thinly bedded grits are exposed, more massive towards their base, resting on massive lava. Keep well to the right in descending the steep grass here, to avoid the dangerous slopes overhanging the vertical cliffs which bound the inlet. The beds dip ESE at between 28 and 40°. This 27-m group of tuffaceous grits occupies the marked depression west of Kirk Hill, and can be traced to old workings at the south-eastern end of Mire Loch. Its postulated recurrence at Horsecastle Bay, despite the dip of the strata, is attributed to the effect of faults like that seen at the bay, which repeatedly step the beds upwards as they are followed down-dip. One or two of these faults may be recognised on the sea-cliffs of Kirk Hill. Cauldron Cove marks the line of a group of north-east faults, one of which, reversed and hading to south-east, coincides with a cave at the head of the inlet. The base of the 27- m grits is exposed on the south-east side of the fault; to north-west only higher beds of the grits are seen.

==== 9. Lighthouse Cliffs: Lava-Tuff Topography ====
From Cauldron Cove climb the steep slope to west, essentially a dip-slope following the top of the lava beneath the 27-m grits, then continue by the cliff-tops to the Lighthouse. Bearing in mind the general south-eastward dip of the lavas it is possible along the way to observe on the cliffs below a succession of thick flows and interbedded grits, the latter much thinner than the lavas and more readily eroded. The landward topography can be interpreted in terms of this succession, but there is much fault- complication. From the cliff-top just west of the entrance to the Lighthouse several steep faults are seen to cut the cliffs and stacks to the north. An extensive even slope seen below, falling eastwards to sea-level, marks the top of a lava-flow, on which there are some small outlying patches of the overlying tuffaceous grits. By a short detour to the west this slope can be followed to the inlet immediately north-west of the Lighthouse, where, at the base of the west-facing cliff, the grit is exposed to a thickness of 0.5 m, with lava above and below. From the cliff-top the upper lava is seen to form the apparently vertical basal part of the Lighthouse headland, and to be capped by a red band which is in places overhung by the succeeding lava. This red band reaches the cliff-top at a depression just west of (outside) the Lighthouse fence, where it is overlain by 0.5 m of boulder clay. If the depression is followed southwards a small working in tuffaceous grit will be readily located, just above the north-west corner of a walled garden. The flaggy rock, exposed to about 1m and dipping at 34° to SE is similar to the grits seen earlier and evidently forms the red band below the Lighthouse. The succession of lava-flows and grits seen on the cliffs provides a key to the topography of the area. In general terms the central parts of the lava-flows form lines of upstanding crags, whereas the amygdaloidal tops and bottoms and the inter-flow grits form intervening elongate depressions. The resulting characteristic trap-feature topography is however often obscured by the rounded weathering of the lavas, by the incidence of faults, or by the limited extent of rock units.

==== 10. Nunnery Point: Viewpoint, Sea-birds ====
Follow the road westward from the Lighthouse gate for 150 m then continue north-westward by the cliff-tops. Near the headland to north-west are the rectangular foundations of a building, formerly described as a ‘chapel’, on the site of St Ebba's convent, but now thought to be mediaeval. Traces of buildings on the summit of Kirk Hill are more likely to indicate the site of the 7th-century convent. From the headland the view to the north extends in favourable weather to the conical plug of North Berwick Law, the Lomond Hills of Fife, the Bass Rock, Largo Law in Fife, and the flat profile of the Isle of May, all intrusive igneous bodies of Carboniferous age. The cliffs below, and towards the Lighthouse, largely composed of Lower Devonian lavas, are cut by several steep open fractures, some of which define the faces of the offshore stacks. In early summer the visitor cannot fail to be impressed by the vast numbers of sea-birds nesting on the cliffs and wheeling over the sea. The most abundant species are guillemots and razorbills, kittiwakes, herring-gulls, and fulmars. Shags are common on the lower rocks, and a few puffins may be seen.

==== 11. Lighthouse Road: Amygdaloidal Lava and Intrusive Tuffs. ====
From the Lighthouse gate the road runs westwards for 150 m, then drops southward through a little valley which is thought to follow the outcrop of the scoriaceous top of a lava-flow and the softer tuffaceous sediment above it. Small exposures of the sediment are sometimes visible close to the road, and at the sharp right-hand bend above Mire Loch the underlying lava is exposed. Here it contains many vesicles, generally filled with greenish silica and some calcite, and often drawn out in various directions by the movement of the lava. This rock must be close to the top of the flow. Rock lower in the flow is widely exposed beside the road as it descends north-westward to Pettico Wick. It is mainly of purplish grey colour, is generally amygdaloidal in varying degree, and shows a distinct dip of between 30 and 50° to ESE. About half-way down the long straight the lava is cut by several brick-red ramifying dyke-like bodies, which under the microscope are seen to be composed of irregular fragments of highly amygdaloidal lava set in a finely crystalline siliceous matrix. They are thought to represent intrusions into the cooling rock of igneous fluids laden with rock fragments. Similar bodies are to be seen at several other localities on the peninsula.

==== 12. Pettico Wick: Silurian Greywackes and Shales ====
[[File:P000817.jpg|thumb|300px|right|Pettico Wick, St. Abb's. Looking westwards to Silurian cliffs. Berwickshire. The rocks are greywackes and siltstones of the Llandovery series folded by repeated broad flexures with an east-north-eastward trend. The complex syncline of Broadhaven Bay is well displayed on the left, the broad north-western limb continuing to the right, as the photograph suggests, across the two minor headlands. The foreground rocks are Lower Old Red Sandstone lavas of St. Abb's Head, separated from the greywackes by a north-west to south-east fault in the bay below. P000817]]The little natural harbour of Pettico Wick straddles the St Abb's Head Fault. Lower Devonian lavas of the St Abb's Head outcrop form the steep north-eastern cliffs, whereas the rocks on the south-western side of the bay are clearly well-bedded sediments, greywackes and shaly siltstones of Silurian (Llandoverian) age, as seen in the narrow outcrop at locality 2. The exact position of the fault is obscured by superficial deposits, but its general line, along Mire Loch to south-east, is very clear.

At Pettico Wick the Silurian rocks are disposed in a complex multiple syncline with a horizontal north-easterly axis, best studied below half-tide. This syncline is typical of the many major folds of the Silurian outcrop, which extends on the coast effectively to Siccar Point and over a wide area inland to south-west. Graptolites have been collected from finer-grained bands on the descent to the beach, and just round the corner on the left near HWM. Sedimentation structures can be seen in places on the upper and lower surfaces of the greywackes, current-bedding is locally developed, and axial-plane cleavage is seen in some of the finer-grained rocks.

It is about 2 km from Pettico Wick along the road from the Lighthouse, to the car park at which the itinerary began. The walk affords fine views to east of the terrain traversed on the excursion. The rocks seen at a few places along the way are all within the greywacke outcrop. In general form and in local detail the topography displays the erosive effects of ice moving south-eastwards, most conspicuously in the over-deepening of the Mire Loch.

==== 13 St Abbs Harbour: Quartz-Dolerite Dyke ====
An instructive exposure of a quartz-dolerite dyke of Late Carboniferous-?Permian age occurs immediately south of the southern wall of St Abbs Harbour, 800 m from the NNR car park. For most of its length the Harbour wall stands on this dyke, which is about 14 m wide. The well-exposed southern margin of the dyke is fine-grained and shows banding, irregularity of detail, and in corporation of xenoliths up to 30 cm across.

===References===

<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P005892.jpg

2015-10-27T15:06:45Z

JenniferFindlay1: St. Abb's Head, Cauldron Cove. View SW. Grits and lava. Bedded tuffaceous grit (c. 27 m. thick) dips to left and overlies massive lava (lower left). The lava appears to be thrust over bedded grits in right foreground. Lower Old Red Sandstone. P005892

== Summary ==
St. Abb's Head, Cauldron Cove. View SW. Grits and lava. Bedded tuffaceous grit (c. 27 m. thick) dips to left and overlies massive lava (lower left). The lava appears to be thrust over bedded grits in right foreground. Lower Old Red Sandstone. P005892
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File:P000813.jpg

2015-10-27T14:52:54Z

JenniferFindlay1: Oblique aerial view of St. Abb's from the east. Berwickshire. The rocks in the foreground are intrusive porphyrites and agglomerates. The rocks on the shore are intrusive porphyrites and unbedded agglomerates, probably within a volcanic vent, of Lower...

== Summary ==
Oblique aerial view of St. Abb's from the east. Berwickshire. The rocks in the foreground are intrusive porphyrites and agglomerates. The rocks on the shore are intrusive porphyrites and unbedded agglomerates, probably within a volcanic vent, of Lower Old Red Sandstone age. Beyond the trees in middle distance the high ground is formed of Silurian greywackes and siltstones. The vent, while not clearly defined, is recognized as such by the chaotic distribution of igneous and pyroclastic rocks on the well-exposed shore and cliffs, and by the extensive occurrence of very coarse agglomerates unlike any of the bedded sequence of St. Abb's Head. P000813
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| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

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Southerness To Borron Point - an excursion

2015-10-27T14:42:16Z

JenniferFindlay1:

By A A McMillan. Excursion 10. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

== Southerness To Borron Point: Lower Carboniferous of the Solway Basin ==

OS 1:50 000 Sheet 84 Dumfries, Castle Douglas & surrounding area

BGS 1.50 000 sheets 5E Dalbeattie, 6 (Annan)

=== Main points of interest ===
[[File:P001159.jpg|400px|thumbnail|Taken from about 200 m. south-south-west of the Triangulation Point west of Southerness, Kirkudbrightshire, looking north. Rocks of the Carboniferous Southerness Limestone Formation. Thin-bedded marine limestone and mudstone dipping gently eastwards on eastern side of anticlinal dome. These rocks of the Southerness Limestone Formation are part of the Kirkbean Outlier. Criffel, part of a large granodiorite pluton forms the background. An outlier is a group of rocks surrounded by outcrops of rocks of older age.]]
Lower Carboniferous stratigraphy and structure; limestones, sandstones and siltstones. algal beds, shallow marine fossils and trace fossils.

=== Logistics ===
This coastal itinerary begins at Southerness, reached via a minor road from the A710 between New Abbey and Dalbeattie. Parking for large coaches is available only at Southerness, from which Localities 1-4 (Figure 35) can be visited. At low tide, it is possible to walk 2.5 km north-eastwards from Southerness across the sandy Gillfoot Bay to Powillimount to reach Localities 5-10, (Figure 36). Alternatively, cars and minibuses only can be parked at the shore beyond Powillimount Farm for Localities 5-10. For the best appreciation of the sections, the shore should be walked at low tide. Total return walking distance (excluding the optional crossing of Gillfoot Bay) is about 1 km at Southerness and 5 km from Powillimount to Hogus Point. As with all coastal sections, the rocks may be wet and slippery.

=== Introduction ===
Lower Carboniferous rocks of Dinantian age are well exposed along the Southerness shore and between Powillimount and Borron Point. Unlike the strata which lie close to the North Solway Fault (as at Rockcliffe, Excursion 3) the sections at Southerness are representative of a wide range of depositional environments from fluviatile to open shallow marine conditions.

The rocks contain a rich macrofossil assemblage which enables them to be correlated with the equivalent Lower to Upper Border Group strata of the Langholm district, ranging in age from Courceyan to Asbian (Table 1). They have been described in detail by Craig (1956) and Deegan (1970).

The oldest strata, the Kirkbean Cement-stone Formation, are found in scattered inland exposures on the south flanks of Criffell, notably in Kirkbean Glen (NX 975 591), where they rest partly on a thin development of basaltic lavas (Birrenswark Volcanic Formation) and partly on Upper Old Red Sandstone strata; this sequence rests with marked angular unconformity on Silurian turbidites. Access to the Kirkbean section ''is ''unfortunately restricted and this excursion concentrates on the younger rocks exposed at the coast.

===Excursion===
[[File:SouthernessExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Southerness shore section (Localities 1-4).]]

==== 1 Southerness shore: Southerness Limestone Formation ====
Cars and coaches may be parked at Southerness village (NX 977 543). From there proceed on to the foreshore by the old lighthouse and walk west for 0.4 km to the faulted eastern boundary of the Southerness Limestone Formation (Locality 1 a, Figure 35). The type section (Craig 1956) of the Southerness Limestone Formation occupies a 0.3 km stretch of coast from here to a gently NNE-plunging anticline (Locality lb). Localities 2 and 3 are specific points of interest within the type section which exposes some 135 m of fossiliferous, thinly bedded calcareous mudstones, silt-stones and limestones. At least four prominent thick beds of sandstone are present. Easterly dips vary from 5 to 45°.

Although a number of east—west faults displace the strata, a reasonably complete section across the east limb of the anticline can be measured (Deegan, 1970). In the upper part of the formation Deegan identified three sedimentary cycles ranging in thickness from 8 to 15 Prominent algal horizons are developed at similar positions in two of the three cycles. An idealised cycle may be summarised as follows:

* Flaggy sandstone with plant remains
* Interbedded limestone and micaceous mudstone
* Interbedded limestone and calcareous mudstone
* Nodular algal band
* Thin muddy limestone with calcareous mudstone
* Fine-grained rippled sandstone
* Flaggy micaceous sandstone with ripple marks and plant remains

The cyclicity reflects varying depositional environments and may be attributed to gradual subsidence combined with variations in terrigenous sediment input possibly related to periodic dip-slip movement on basinal bounding faults. Each cycle commences with sandstones, probably deposited in a littoral environment, which are succeeded by calcareous beds formed under shallow subtidal conditions. Sandy limestones containing ooliths indicate that the sediments were affected by wave action. However, the algal beds show little sign of reworking and probably represent slightly deeper water sedimentation below the wave base. A traverse of the shore section enables the cyclicity identified by Deegan to be followed. Specific highlights include the Syringothyris Limestone (Craig, 1956) and algal beds.

==== 2 Syringothyris Limestone ====
A good exposure of the ''Syringothyris ''Limestone, estimated by Craig (1956) to be 16.7 m thick, is seen some 600 m west of the lighthouse (Locality 2, Figure 35). The limestone comprises several beds of argillaceous limestone and calcareous mudstone, and contains a varied marine fauna of brachiopods including ''Syringothyris cuspidata, ''bivalves, polyzoa and crinoid ossicles. Faunal similarities with the Harden Beds of Langholm were noted by Lumsden et al. (1967) and indicate that the Southerness Limestone Formation probably spans the boundary between the Lower and Middle Border Groups of Langholm (Table 1).

==== 3 Algal stromatolite beds ====
Above the Syringothyris Limestone, about 550 m west of the lighthouse, two distinctive algal stromatolite bands, 1.2 m and I m thick, are present within the sequence. Craig (1956) and Failicher (1977) referred the stromatolites to the genus ''Somphospongia. ''They resemble the dome type described by Leeder (1975) from the Lower Border Group of the Northumberland Basin. Individual domes are up to 30 cm in diameter with a relief of 10 to 15 cm and are set in a calcareous mudstone matrix. They are composed of alternating micritic and detrital laminae. Calcareous algal filaments commonly wrap fragments of shell including gastropods and ostracods. The irregular, nodular xterior of the algal growths indicates limited reworking. Basing his observations on studies of modern stromatolites, Leeder (1975) inferred that domed types formed in a low intertidal to shallow sub: tidal depositional environment.

==== 4 Southerness Lighthouse: Gillfoot Sandstone Formation ====
Walk back towards Southerness, across the fault (Locality la) which downthrows east and brings the Gillfoot Sandstone Formation in against the Southerness Limestone Formation.

Between 120 and 150 m of strata assigned to the Gillfoot Sandstone Formation are exposed on the shore between here and a position south of Powillimount Farm (NX 9880 5620). The formation conformably overlies the Southerness Limestone Formation, and Craig (1956) placed the top of the formation at the base of a breccia forming the base of the succeeding Powillimount Sandstone Formation (Locality 5).

The Gillfoot Sandstone Formation comprises white and purplish, flaggy, quartzose sandstones; conglomerates with intraformational fragments; red flaggy siltstones; and mudstones. A few red to grey, thin-bedded, sandy limestones with scattered detrital fossil remains are also present. Conglomerates, which form about 20 per cent of the succession, have a calcareous matrix and contain intraformational fragments in addition to pebbles of vein quartz, greywacke and microdiorite derived from the Southern Uplands hinterland to the north. Some more feldspathic layers are dominated by microdiorite and felsic rock debris derived from Lower Palaeozoic minor intrusions.

The formation ''is ''sparsely fossiliferous. A derived fauna, collected by Craig from thesandstone on which the lighthouse stands, suggests that the strata are of Arundian age and equivalent to part of the Middle Border Group of Langholm (Lumsden et al., 1967).

The lithologies indicate a more marginal depositional setting than that of the underlying formation. The conglomerates may have been transported from the hinterland and deposited by periodic sheetfloods flowing over low-lying supratidal areas. Textures in some of the sandstones indicate wave action and a littoral environment. Shallow subtidal environments may be indicated by the presence of thin fossiliferous, sandy limestones.

==== 5 Powillimount Shore: Powillimount Sandstone Formation ====
[[File:PowillimountExcursionMap.jpg|300px|thumbnail|Locality map and outline geology for the shore section between Powillimount and Hogus Point (Localities5-10).]]
Access to the Powillimount shore is via the road which links Powillimount Farm with the Kirkbean to Southerness road. A small car park at the shore beyond the farm (Figure 36) is suitable for cars and minibuses only. Immediately below the car park some 160 m of strata exposed on the shore between Powillimount Bay (NX 9880 5610) and Thirlstane (NX 9925 5690) are assigned to the Powillimount Sandstone Formation. The top 25 m are distinguished as the Thirlstane Sandstone Member, a prominent ridge of thick-bedded sandstone with spectacular penecontemporaneous deformation structures (Craig, 1956; Deegan, 1970; Ord et al., 1988).

The base of the Powillimount Sandstone Formation is drawn a short way SW of the car park, at the base of a grey calcareous breccia, above the highest bed of purple mudstone in the Gillfoot Sandstone Formation. The strata form part of the SE limb of a major NE-trending anticline and on the coast are further folded about a tight, gently plunging syncline—anticline pair, the axial planes of which trend NNE. Faunal assemblages are similar to those in the Middle Border Group of Langholm. Aspects of the fauna immediately below the Thirlstane Sandstone resemble those of the overlying Arbigland Limestone Formation.

[[File:P001161.jpg|300px|thumbnail|left|Taken from about 100 m. north of the Thirl Stane natural arch, Powillimount, looking north-north-east. Thirlstane Sandstone Member of the Powillimount Sandstone Formation, Carboniferous. Allokinetic liquefaction structures consisting of domes and volcanoes, bedding deformation increasing towards the north end of the outcrop (beyond the geologist). Strata of the Arbigland Limestone Formation occupy the background and are faulted against the Thirlstane Sandstone. Liquefaction is the transformation of loose sediment into a fluid mass preliminary to movement e.g. in a turbidity current or subaqueous slumping or sliding. P001161]]Lithologies include calcareous and quartzose sandstone, sandy limestone with beds of dark grey fissile mudstone, and calcareous mudstone. Locally, thin coals and associated seatearths are present. Sandstone beds are laterally extensive, ranging in thickness from 0.3 to 3 m. They are well sorted and commonly exhibit ripple cross-lamination. Many contain abundant carbonaceous plant remains and are extensively burrowed, particularly by ''Chondrites. ''Limestones range from arenaceous to argillaceous and contain detrital fossil remains, ooliths and rolled algal nodules. One distinctive oncolite bed, 0.3 m thick, contains rounded algal-coated lithic and fossil fragments. The individual oncolites are generally spherical and up to 1 cm across. Oncolites are produced by the accretion of sediment on to mobile grains through the action of algae, and their presence indicates constant agitation of the sea floor by wave action.

The characteristic lithologies, especially rolled algal nodules and detrital fragments, point to a shallow-marine environment exposed to gentle wave action. Deegan (1970) proposed that the sediments were deposited in a tidal lagoon protected from the effects of severe storms by some form of offshore sand barrier. The presence of thin coals and seatearths indicates periodic shallowing of lagoonal waters and the development of vegetated low-lying supratidal flats.

==== 6 Thirlstane natural arch: Thirlstane Sandstone Member ====
Walk about 50 m NE from the car park to a prominent sandstone ridge (NX 991 565) which can be followed to beyond the Thirlstane natural arch. The ridge is formed by the 25 m-thick Thirlstane Sandstone Member. At the base of the Thirlstane Sandstone the contact with the underlying strata is irregular, and intraformational fragments and large plant remains are present in the lowest beds. The fine natural arch at Thirlstane and nearby exposures to seaward display various sedimentary structures in a pinkish grey, medium-grained, well-sorted, quartzose sandstone. The rocks are characterised by large-scale trough cross-bedding and a spectacular development of liquefaction structures which increase in frequency and magnitude from south to north along the outcrop. It is worthwhile examining the extraordinary degree to which the original bedding has been disrupted. In a detailed study of these structures Ord et al. (1988) recognised different types of structure including oversteepened and recumbently folded cross-stratification, domes, sand volcanoes and zones of anastomosing, vertical cracks. They attributed the magnitude and frequency of the liquefaction structures to causes such as local seismicity, and deduced the presence of a syndepositionally active fault lying north of the present outcrop.

Deegan (1970) proposed that the Thirlstane Sandstone formed as an offshore sand barrier which initially enabled the inshore lagoonal environment of the Powillimount Sandstone Formation to develop. The evidence is equivocal, however and, as Ord et al. suggest, an alluvial origin is also a possibility.

==== 7 Thirlstane to Arbigland Garden: Arbigland Limestone Formation ====
All the strata exposed between Thirlstane (Locality 7) and Hogus Point (NX 997 589) (Locality 10) are assigned to the Arbigland Limestone Formation, estimated to be some 300 m thick (Craig, 1956). The conformable junction between the Thirlstane Sandstone Member and the Arbigland Limestone Formation (noted by Smith, 1910) is often obscured by shifting sands; the most obvious contact is a normal fault, (Locality 7) 200 m NE of the Thirlstane natural arch (NX 993 569). The fault plane appears to have a reverse throw and may have been rotated through vertical. Between this fault and an ESE-trending hinge fault opposite Arbigland Garden, the strata strike parallel to the coast. North of the second fault a narrow zone of disrupted bedding is succeeded by a series of beds striking ESE. Perched prominently on these is a huge erratic block of Criffell granodiorite known locally as the 'Devil Stone'. Tradition has it that the Devil bit off this chunk of Criffell and spat it out on the shore. Conventional glacial theory would suggest that the block was eroded and transported to its present position by a glacier during the last ice age.

The lithologies of the lower part of the Arbigland Limestone Formation resemble those of the Powillimount Sandstone Formation. Key features to look out for include thick-bedded, bioturbated, calcareous sandstones with coalified plant casts, thin sandy limestones locally with ooliths and algal debris, dark grey carbonaceous mudstones, and thin coal partings.

==== 8 Arbigland Bay to Borron Point: Arbigland Limestone Formation and pericline ====
Walk from the Devil Stone across the sandy Arbigland Bay. Diversions from the geology here include the fine gardens of Arbigland House, built by William Craik in 1755, and Paul Jones Cottage, where John Paul Jones (1747-92), founder of the American Navy, was brought up. The gardens may be visited (vehicular access from the Kirkbean to Southerness road) on certain weekdays.

North of the bay a small, shallow, periclinal basin structure is present (Locality 8). North of here the strike of the strata swings from east–west through ENE to NE, to become subparallel with the line of the coast around Borron Point (NX 995 580). The strata are cut by numerous normal faults of ESE and SE orientation.

==== 9 Arbigland Bay to Borron Point: faunal assemblages and sedimentary features of the Arbigland Limestone Formation ====
A traverse of the shore between Arbigland Bay and Borron Point is worthwhile for anyone with an interest in the sedimentology and faunal characteristics of shallow marine shelf sediments. [[File:P511132.jpg|300px|thumbnail|left|Lithostrotion vorticale: a coral from Arbigland Bay. P511132]]The strata here were termed by Deegan (1970) the Middle Arbigland Beds. They are richly fossiliferous and contain a fine compound rugose coral fauna including spectacular massive hemispherical colonies of ''Lithostrotion clavaticum, ''first recorded by Smith (1910). Some are in life position with individual corallites visible on the upper surface. Overturned colonies show only the outer walls of the corallites radiating from the central columella. Please do '''not hammer them out.'''

Deegan (1970) described the strata below McCulloch's Castle (NX 996 577) (Locality 9) as mainly thin argillaceous and sandy limestones interbedded with calcareous mudstones and several prominent thick beds of massive bioturbated sandstone. Many beds of sandstone and sandy limestone have been extensively and repeatedly reworked by sediment feeders, and burrow forms such as ''Chondrites, Diplocrtzterion ''and ''Rhizocorallium ''are commonly seen. The limestones and mudstones have an abundant and diverse fauna including corals, brachiopods, bivalves, gastropods, crinoids, bryozoa and orthocones. Faunal equivalence with the Upper Border Group of Langholm (Lumsden et al., 1967), of Asbian age, is considered most likely. In the Langholm district, the Glencartholm Volcanic Beds form the base of the Upper Border Group and these rocks have been equated with the Clattering Band of Bewcastle (Day 1970) which George et al. (1976) place at the base of the Asbian. It follows that if correlation of the Middle Arbigland Beds is extended to the Clattering Band then the lower part of the Arbigland Limestone Formation (Locality 7) is probably Holkerian (as noted by George et al.).

At Borron Point stratigraphically higher rocks are downthrown by two small faults. Here the strata are steeply inclined and locally overturned. The sequence is characterised by thick-bedded bioturbated sandstone, and ripple cross-laminated fine-grained sandstone interbedded with calcareous mudstone and a few argillaceous limestones. Shallow sandstone-filled scours and washouts are common.

==== 10 Hogus Point: northernmost exposures of the Arbigland Limestone Formation ====
[[File:P220465.jpg|300px|thumbnail|right|Taken from 300 m. S. of Hogus Point, Carsethorne. Looking NNE, across the estuary of the River Nith. Arbigland Limestone Formation. Plunging anticlinal-synclinal pair in thin-bedded limestones and sandstones. P220465]]Those who wish to visit the northernmost exposures of the formation at Hogus Point (NX 997 588) should continue along the shore north from Borron Point. Alternatively the excursion can be completed at Borron Point and the route retraced to Powillimount Farm.

The strata at Hogus Point are reasonably well exposed. They are disposed about a tight, gently NE-plunging anticline—syncline fold pair. The rocks comprise thick units of thin-bedded silty, calcareous mud-stone, well-bedded sandstone and thin-bedded sandy limestone. Although Craig (1956) considered the strata at Hogus Point to be the highest in the sequence, a fauna collected more recently indicates a position no higher than the beds lying south of Borron Point. The indications are that the rocks lie on the upthrow side of a SE-orientated normal fault, the trace of which may lie north of Tallowquhairn Farm (NX 993 584).
Lithological characteristics of the sequence between Thirlstane and Arbigland Garden together with the sparse, locally detrital, fauna are consistent with a restricted lagoonal environment in which there was limited reworking of sediment. Overall, however, the Arbigland Limestone Formation was probably deposited within the intertidal to subtidal zone. An open, shallow-marine environment is indicated by the abundant fauna in the Middle Arbigland Beds (Deegan, 1970).
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

File:P220465.jpg

2015-10-27T14:19:03Z

JenniferFindlay1: Taken from 300 m. S. of Hogus Point, Carsethorne. Looking NNE, across the estuary of the River Nith. Arbigland Limestone Formation. Plunging anticlinal-synclinal pair in thin-bedded limestones and sandstones. P220465

== Summary ==
Taken from 300 m. S. of Hogus Point, Carsethorne. Looking NNE, across the estuary of the River Nith. Arbigland Limestone Formation. Plunging anticlinal-synclinal pair in thin-bedded limestones and sandstones. P220465
== Licencing ==
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====Ordnance Survey topography====
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[[Category:License tags]]

File:P511132.jpg

2015-10-27T14:13:55Z

JenniferFindlay1: Lithostrotion vorticale: a coral from Arbigland Bay. P511132

== Summary ==
Lithostrotion vorticale: a coral from Arbigland Bay. P511132
== Licencing ==
__notoc__
{|class="wikitable" style="width:100% style="background:#f0f8ff; font-size:80%"
| Download of 1000 x 1000 pixel images is free for all non-commercial use - all we ask in return is for you to acknowledge BGS when using our images. Click our Terms and Conditions link below for information on acknowledgement text, and to find out about using our images commercially.

====Copyright====

The images featured on this site unless otherwise indicated are copyright material of the Natural Environment Research Council (NERC), of which the British Geological Survey is a component body. The British Geological Survey encourages the use of its material in promoting geological and environmental sciences.
The images may be reproduced free of charge for any non-commercial use in any format or medium provided they are reproduced accurately and not used in a misleading or derogatory context.
Where any images on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
The permission to reproduce NERC protected material does not extend to any images on this site which are identified as being the copyright of a third party. Authorisation to reproduce such material must be obtained from the copyright holders concerned.

====Non-commercial Use====

Use of the images downloaded from this site and reproduced digitally or otherwise may only be used for non-commercial purposes, which are:-

* Private study or research for a non-commercial purpose
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When using the images please credit 'British Geological Survey' and include the catalogue reference ('P Number') of the item to allow others to access the original image or document.
Non-commercial users of the images from this site are restricted to downloading no more than 30 images, without seeking further permission from [mailto:enquiries@bgs.ac.uk enquiries@bgs.ac.uk]

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====Ordnance Survey topography====
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[[Category:License tags]]

Southerness To Borron Point - an excursion

2015-10-20T11:18:38Z

JenniferFindlay1:

By A A McMillan. Excursion 10. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

== Southerness To Borron Point: Lower Carboniferous of the Solway Basin ==

OS 1:50 000 Sheet 84 Dumfries, Castle Douglas & surrounding area

BGS 1.50 000 sheets 5E Dalbeattie, 6 (Annan)

=== Main points of interest ===
[[File:P001159.jpg|400px|thumbnail|Taken from about 200 m. south-south-west of the Triangulation Point west of Southerness, Kirkudbrightshire, looking north. Rocks of the Carboniferous Southerness Limestone Formation. Thin-bedded marine limestone and mudstone dipping gently eastwards on eastern side of anticlinal dome. These rocks of the Southerness Limestone Formation are part of the Kirkbean Outlier. Criffel, part of a large granodiorite pluton forms the background. An outlier is a group of rocks surrounded by outcrops of rocks of older age.]]
Lower Carboniferous stratigraphy and structure; limestones, sandstones and siltstones. algal beds, shallow marine fossils and trace fossils.

=== Logistics ===
This coastal itinerary begins at Southerness, reached via a minor road from the A710 between New Abbey and Dalbeattie. Parking for large coaches is available only at Southerness, from which Localities 1-4 (Figure 35) can be visited. At low tide, it is possible to walk 2.5 km north-eastwards from Southerness across the sandy Gillfoot Bay to Powillimount to reach Localities 5-10, (Figure 36). Alternatively, cars and minibuses only can be parked at the shore beyond Powillimount Farm for Localities 5-10. For the best appreciation of the sections, the shore should be walked at low tide. Total return walking distance (excluding the optional crossing of Gillfoot Bay) is about 1 km at Southerness and 5 km from Powillimount to Hogus Point. As with all coastal sections, the rocks may be wet and slippery.

=== Introduction ===
Lower Carboniferous rocks of Dinantian age are well exposed along the Southerness shore and between Powillimount and Borron Point. Unlike the strata which lie close to the North Solway Fault (as at Rockcliffe, Excursion 3) the sections at Southerness are representative of a wide range of depositional environments from fluviatile to open shallow marine conditions.

The rocks contain a rich macrofossil assemblage which enables them to be correlated with the equivalent Lower to Upper Border Group strata of the Langholm district, ranging in age from Courceyan to Asbian (Table 1). They have been described in detail by Craig (1956) and Deegan (1970).

The oldest strata, the Kirkbean Cement-stone Formation, are found in scattered inland exposures on the south flanks of Criffell, notably in Kirkbean Glen (NX 975 591), where they rest partly on a thin development of basaltic lavas (Birrenswark Volcanic Formation) and partly on Upper Old Red Sandstone strata; this sequence rests with marked angular unconformity on Silurian turbidites. Access to the Kirkbean section ''is ''unfortunately restricted and this excursion concentrates on the younger rocks exposed at the coast.

===Excursion===
[[File:SouthernessExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Southerness shore section (Localities 1-4).]]

==== 1 Southerness shore: Southerness Limestone Formation ====
Cars and coaches may be parked at Southerness village (NX 977 543). From there proceed on to the foreshore by the old lighthouse and walk west for 0.4 km to the faulted eastern boundary of the Southerness Limestone Formation (Locality 1 a, Figure 35). The type section (Craig 1956) of the Southerness Limestone Formation occupies a 0.3 km stretch of coast from here to a gently NNE-plunging anticline (Locality lb). Localities 2 and 3 are specific points of interest within the type section which exposes some 135 m of fossiliferous, thinly bedded calcareous mudstones, silt-stones and limestones. At least four prominent thick beds of sandstone are present. Easterly dips vary from 5 to 45°.

Although a number of east—west faults displace the strata, a reasonably complete section across the east limb of the anticline can be measured (Deegan, 1970). In the upper part of the formation Deegan identified three sedimentary cycles ranging in thickness from 8 to 15 Prominent algal horizons are developed at similar positions in two of the three cycles. An idealised cycle may be summarised as follows:

* Flaggy sandstone with plant remains
* Interbedded limestone and micaceous mudstone
* Interbedded limestone and calcareous mudstone
* Nodular algal band
* Thin muddy limestone with calcareous mudstone
* Fine-grained rippled sandstone
* Flaggy micaceous sandstone with ripple marks and plant remains

The cyclicity reflects varying depositional environments and may be attributed to gradual subsidence combined with variations in terrigenous sediment input possibly related to periodic dip-slip movement on basinal bounding faults. Each cycle commences with sandstones, probably deposited in a littoral environment, which are succeeded by calcareous beds formed under shallow subtidal conditions. Sandy limestones containing ooliths indicate that the sediments were affected by wave action. However, the algal beds show little sign of reworking and probably represent slightly deeper water sedimentation below the wave base. A traverse of the shore section enables the cyclicity identified by Deegan to be followed. Specific highlights include the Syringothyris Limestone (Craig, 1956) and algal beds.

==== 2 Syringothyris Limestone ====
A good exposure of the ''Syringothyris ''Limestone, estimated by Craig (1956) to be 16.7 m thick, is seen some 600 m west of the lighthouse (Locality 2, Figure 35). The limestone comprises several beds of argillaceous limestone and calcareous mudstone, and contains a varied marine fauna of brachiopods including ''Syringothyris cuspidata, ''bivalves, polyzoa and crinoid ossicles. Faunal similarities with the Harden Beds of Langholm were noted by Lumsden et al. (1967) and indicate that the Southerness Limestone Formation probably spans the boundary between the Lower and Middle Border Groups of Langholm (Table 1).

==== 3 Algal stromatolite beds ====
Above the Syringothyris Limestone, about 550 m west of the lighthouse, two distinctive algal stromatolite bands, 1.2 m and I m thick, are present within the sequence. Craig (1956) and Failicher (1977) referred the stromatolites to the genus ''Somphospongia. ''They resemble the dome type described by Leeder (1975) from the Lower Border Group of the Northumberland Basin. Individual domes are up to 30 cm in diameter with a relief of 10 to 15 cm and are set in a calcareous mudstone matrix. They are composed of alternating micritic and detrital laminae. Calcareous algal filaments commonly wrap fragments of shell including gastropods and ostracods. The irregular, nodular xterior of the algal growths indicates limited reworking. Basing his observations on studies of modern stromatolites, Leeder (1975) inferred that domed types formed in a low intertidal to shallow sub: tidal depositional environment.

==== 4 Southerness Lighthouse: Gillfoot Sandstone Formation ====
Walk back towards Southerness, across the fault (Locality la) which downthrows east and brings the Gillfoot Sandstone Formation in against the Southerness Limestone Formation.

Between 120 and 150 m of strata assigned to the Gillfoot Sandstone Formation are exposed on the shore between here and a position south of Powillimount Farm (NX 9880 5620). The formation conformably overlies the Southerness Limestone Formation, and Craig (1956) placed the top of the formation at the base of a breccia forming the base of the succeeding Powillimount Sandstone Formation (Locality 5).

The Gillfoot Sandstone Formation comprises white and purplish, flaggy, quartzose sandstones; conglomerates with intraformational fragments; red flaggy siltstones; and mudstones. A few red to grey, thin-bedded, sandy limestones with scattered detrital fossil remains are also present. Conglomerates, which form about 20 per cent of the succession, have a calcareous matrix and contain intraformational fragments in addition to pebbles of vein quartz, greywacke and microdiorite derived from the Southern Uplands hinterland to the north. Some more feldspathic layers are dominated by microdiorite and felsic rock debris derived from Lower Palaeozoic minor intrusions.

The formation ''is ''sparsely fossiliferous. A derived fauna, collected by Craig from thesandstone on which the lighthouse stands, suggests that the strata are of Arundian age and equivalent to part of the Middle Border Group of Langholm (Lumsden et al., 1967).

The lithologies indicate a more marginal depositional setting than that of the underlying formation. The conglomerates may have been transported from the hinterland and deposited by periodic sheetfloods flowing over low-lying supratidal areas. Textures in some of the sandstones indicate wave action and a littoral environment. Shallow subtidal environments may be indicated by the presence of thin fossiliferous, sandy limestones.

==== 5 Powillimount Shore: Powillimount Sandstone Formation ====
[[File:PowillimountExcursionMap.jpg|300px|thumbnail|Locality map and outline geology for the shore section between Powillimount and Hogus Point (Localities5-10).]]
Access to the Powillimount shore is via the road which links Powillimount Farm with the Kirkbean to Southerness road. A small car park at the shore beyond the farm (Figure 36) is suitable for cars and minibuses only. Immediately below the car park some 160 m of strata exposed on the shore between Powillimount Bay (NX 9880 5610) and Thirlstane (NX 9925 5690) are assigned to the Powillimount Sandstone Formation. The top 25 m are distinguished as the Thirlstane Sandstone Member, a prominent ridge of thick-bedded sandstone with spectacular penecontemporaneous deformation structures (Craig, 1956; Deegan, 1970; Ord et al., 1988).

The base of the Powillimount Sandstone Formation is drawn a short way SW of the car park, at the base of a grey calcareous breccia, above the highest bed of purple mudstone in the Gillfoot Sandstone Formation. The strata form part of the SE limb of a major NE-trending anticline and on the coast are further folded about a tight, gently plunging syncline—anticline pair, the axial planes of which trend NNE. Faunal assemblages are similar to those in the Middle Border Group of Langholm. Aspects of the fauna immediately below the Thirlstane Sandstone resemble those of the overlying Arbigland Limestone Formation.

[[File:P001161.jpg|300px|thumbnail|left|Taken from about 100 m. north of the Thirl Stane natural arch, Powillimount, looking north-north-east. Thirlstane Sandstone Member of the Powillimount Sandstone Formation, Carboniferous. Allokinetic liquefaction structures consisting of domes and volcanoes, bedding deformation increasing towards the north end of the outcrop (beyond the geologist). Strata of the Arbigland Limestone Formation occupy the background and are faulted against the Thirlstane Sandstone. Liquefaction is the transformation of loose sediment into a fluid mass preliminary to movement e.g. in a turbidity current or subaqueous slumping or sliding. P001161]]Lithologies include calcareous and quartzose sandstone, sandy limestone with beds of dark grey fissile mudstone, and calcareous mudstone. Locally, thin coals and associated seatearths are present. Sandstone beds are laterally extensive, ranging in thickness from 0.3 to 3 m. They are well sorted and commonly exhibit ripple cross-lamination. Many contain abundant carbonaceous plant remains and are extensively burrowed, particularly by ''Chondrites. ''Limestones range from arenaceous to argillaceous and contain detrital fossil remains, ooliths and rolled algal nodules. One distinctive oncolite bed, 0.3 m thick, contains rounded algal-coated lithic and fossil fragments. The individual oncolites are generally spherical and up to 1 cm across. Oncolites are produced by the accretion of sediment on to mobile grains through the action of algae, and their presence indicates constant agitation of the sea floor by wave action.

The characteristic lithologies, especially rolled algal nodules and detrital fragments, point to a shallow-marine environment exposed to gentle wave action. Deegan (1970) proposed that the sediments were deposited in a tidal lagoon protected from the effects of severe storms by some form of offshore sand barrier. The presence of thin coals and seatearths indicates periodic shallowing of lagoonal waters and the development of vegetated low-lying supratidal flats.

==== 6 Thirlstane natural arch: Thirlstane Sandstone Member ====
Walk about 50 m NE from the car park to a prominent sandstone ridge (NX 991 565) which can be followed to beyond the Thirlstane natural arch. The ridge is formed by the 25 m-thick Thirlstane Sandstone Member. At the base of the Thirlstane Sandstone the contact with the underlying strata is irregular, and intraformational fragments and large plant remains are present in the lowest beds. The fine natural arch at Thirlstane and nearby exposures to seaward display various sedimentary structures in a pinkish grey, medium-grained, well-sorted, quartzose sandstone. The rocks are characterised by large-scale trough cross-bedding and a spectacular development of liquefaction structures which increase in frequency and magnitude from south to north along the outcrop. It is worthwhile examining the extraordinary degree to which the original bedding has been disrupted. In a detailed study of these structures Ord et al. (1988) recognised different types of structure including oversteepened and recumbently folded cross-stratification, domes, sand volcanoes and zones of anastomosing, vertical cracks. They attributed the magnitude and frequency of the liquefaction structures to causes such as local seismicity, and deduced the presence of a syndepositionally active fault lying north of the present outcrop.

Deegan (1970) proposed that the Thirlstane Sandstone formed as an offshore sand barrier which initially enabled the inshore lagoonal environment of the Powillimount Sandstone Formation to develop. The evidence is equivocal, however and, as Ord et al. suggest, an alluvial origin is also a possibility.

==== 7 Thirlstane to Arbigland Garden: Arbigland Limestone Formation ====
All the strata exposed between Thirlstane (Locality 7) and Hogus Point (NX 997 589) (Locality 10) are assigned to the Arbigland Limestone Formation, estimated to be some 300 m thick (Craig, 1956). The conformable junction between the Thirlstane Sandstone Member and the Arbigland Limestone Formation (noted by Smith, 1910) is often obscured by shifting sands; the most obvious contact is a normal fault, (Locality 7) 200 m NE of the Thirlstane natural arch (NX 993 569). The fault plane appears to have a reverse throw and may have been rotated through vertical. Between this fault and an ESE-trending hinge fault opposite Arbigland Garden, the strata strike parallel to the coast. North of the second fault a narrow zone of disrupted bedding is succeeded by a series of beds striking ESE. Perched prominently on these is a huge erratic block of Criffell granodiorite known locally as the 'Devil Stone'. Tradition has it that the Devil bit off this chunk of Criffell and spat it out on the shore. Conventional glacial theory would suggest that the block was eroded and transported to its present position by a glacier during the last ice age.

The lithologies of the lower part of the Arbigland Limestone Formation resemble those of the Powillimount Sandstone Formation. Key features to look out for include thick-bedded, bioturbated, calcareous sandstones with coalified plant casts, thin sandy limestones locally with ooliths and algal debris, dark grey carbonaceous mudstones, and thin coal partings.

==== 8 Arbigland Bay to Borron Point: Arbigland Limestone Formation and pericline ====
Walk from the Devil Stone across the sandy Arbigland Bay. Diversions from the geology here include the fine gardens of Arbigland House, built by William Craik in 1755, and Paul Jones Cottage, where John Paul Jones (1747-92), founder of the American Navy, was brought up. The gardens may be visited (vehicular access from the Kirkbean to Southerness road) on certain weekdays.

North of the bay a small, shallow, periclinal basin structure is present (Locality 8). North of here the strike of the strata swings from east–west through ENE to NE, to become subparallel with the line of the coast around Borron Point (NX 995 580). The strata are cut by numerous normal faults of ESE and SE orientation.

==== 9 Arbigland Bay to Borron Point: faunal assemblages and sedimentary features of the Arbigland Limestone Formation ====
A traverse of the shore between Arbigland Bay and Borron Point is worthwhile for anyone with an interest in the sedimentology and faunal characteristics of shallow marine shelf sediments. The strata here were termed by Deegan (1970) the Middle Arbigland Beds. They are richly fossiliferous and contain a fine compound rugose coral fauna including spectacular massive hemispherical colonies of ''Lithostrotion clavaticum, ''first recorded by Smith (1910). Some are in life position with individual corallites visible on the upper surface. Overturned colonies show only the outer walls of the corallites radiating from the central columella. Please do '''not hammer them out.'''

Deegan (1970) described the strata below McCulloch's Castle (NX 996 577) (Locality 9) as mainly thin argillaceous and sandy limestones interbedded with calcareous mudstones and several prominent thick beds of massive bioturbated sandstone. Many beds of sandstone and sandy limestone have been extensively and repeatedly reworked by sediment feeders, and burrow forms such as ''Chondrites, Diplocrtzterion ''and ''Rhizocorallium ''are commonly seen. The limestones and mudstones have an abundant and diverse fauna including corals, brachiopods, bivalves, gastropods, crinoids, bryozoa and orthocones. Faunal equivalence with the Upper Border Group of Langholm (Lumsden et al., 1967), of Asbian age, is considered most likely. In the Langholm district, the Glencartholm Volcanic Beds form the base of the Upper Border Group and these rocks have been equated with the Clattering Band of Bewcastle (Day 1970) which George et al. (1976) place at the base of the Asbian. It follows that if correlation of the Middle Arbigland Beds is extended to the Clattering Band then the lower part of the Arbigland Limestone Formation (Locality 7) is probably Holkerian (as noted by George et al.).

At Borron Point stratigraphically higher rocks are downthrown by two small faults. Here the strata are steeply inclined and locally overturned. The sequence is characterised by thick-bedded bioturbated sandstone, and ripple cross-laminated fine-grained sandstone interbedded with calcareous mudstone and a few argillaceous limestones. Shallow sandstone-filled scours and washouts are common.

==== 10 Hogus Point: northernmost exposures of the Arbigland Limestone Formation ====
Those who wish to visit the northernmost exposures of the formation at Hogus Point (NX 997 588) should continue along the shore north from Borron Point. Alternatively the excursion can be completed at Borron Point and the route retraced to Powillimount Farm.

The strata at Hogus Point are reasonably well exposed. They are disposed about a tight, gently NE-plunging anticline—syncline fold pair. The rocks comprise thick units of thin-bedded silty, calcareous mud-stone, well-bedded sandstone and thin-bedded sandy limestone. Although Craig (1956) considered the strata at Hogus Point to be the highest in the sequence, a fauna collected more recently indicates a position no higher than the beds lying south of Borron Point. The indications are that the rocks lie on the upthrow side of a SE-orientated normal fault, the trace of which may lie north of Tallowquhairn Farm (NX 993 584).
Lithological characteristics of the sequence between Thirlstane and Arbigland Garden together with the sparse, locally detrital, fauna are consistent with a restricted lagoonal environment in which there was limited reworking of sediment. Overall, however, the Arbigland Limestone Formation was probably deposited within the intertidal to subtidal zone. An open, shallow-marine environment is indicated by the abundant fauna in the Middle Arbigland Beds (Deegan, 1970).
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

Southerness To Borron Point - an excursion

2015-10-20T11:17:52Z

JenniferFindlay1:

By A A McMillan. Excursion 10. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

== Southerness To Borron Point: Lower Carboniferous of the Solway Basin ==

OS 1:50 000 Sheet 84 Dumfries, Castle Douglas & surrounding area

BGS 1.50 000 sheets 5E Dalbeattie, 6 (Annan)

=== Main points of interest ===
[[File:P001159.jpg|400px|thumbnail|Taken from about 200 m. south-south-west of the Triangulation Point west of Southerness, Kirkudbrightshire, looking north. Rocks of the Carboniferous Southerness Limestone Formation. Thin-bedded marine limestone and mudstone dipping gently eastwards on eastern side of anticlinal dome. These rocks of the Southerness Limestone Formation are part of the Kirkbean Outlier. Criffel, part of a large granodiorite pluton forms the background. An outlier is a group of rocks surrounded by outcrops of rocks of older age.]]
Lower Carboniferous stratigraphy and structure; limestones, sandstones and siltstones. algal beds, shallow marine fossils and trace fossils.

=== Logistics ===
This coastal itinerary begins at Southerness, reached via a minor road from the A710 between New Abbey and Dalbeattie. Parking for large coaches is available only at Southerness, from which Localities 1-4 (Figure 35) can be visited. At low tide, it is possible to walk 2.5 km north-eastwards from Southerness across the sandy Gillfoot Bay to Powillimount to reach Localities 5-10, (Figure 36). Alternatively, cars and minibuses only can be parked at the shore beyond Powillimount Farm for Localities 5-10. For the best appreciation of the sections, the shore should be walked at low tide. Total return walking distance (excluding the optional crossing of Gillfoot Bay) is about 1 km at Southerness and 5 km from Powillimount to Hogus Point. As with all coastal sections, the rocks may be wet and slippery.

=== Introduction ===
Lower Carboniferous rocks of Dinantian age are well exposed along the Southerness shore and between Powillimount and Borron Point. Unlike the strata which lie close to the North Solway Fault (as at Rockcliffe, Excursion 3) the sections at Southerness are representative of a wide range of depositional environments from fluviatile to open shallow marine conditions.

The rocks contain a rich macrofossil assemblage which enables them to be correlated with the equivalent Lower to Upper Border Group strata of the Langholm district, ranging in age from Courceyan to Asbian (Table 1). They have been described in detail by Craig (1956) and Deegan (1970).

The oldest strata, the Kirkbean Cement-stone Formation, are found in scattered inland exposures on the south flanks of Criffell, notably in Kirkbean Glen (NX 975 591), where they rest partly on a thin development of basaltic lavas (Birrenswark Volcanic Formation) and partly on Upper Old Red Sandstone strata; this sequence rests with marked angular unconformity on Silurian turbidites. Access to the Kirkbean section ''is ''unfortunately restricted and this excursion concentrates on the younger rocks exposed at the coast.

===Excursion===
[[File:SouthernessExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Southerness shore section (Localities 1-4).]]

==== 1 Southerness shore: Southerness Limestone Formation ====
Cars and coaches may be parked at Southerness village (NX 977 543). From there proceed on to the foreshore by the old lighthouse and walk west for 0.4 km to the faulted eastern boundary of the Southerness Limestone Formation (Locality 1 a, Figure 35). The type section (Craig 1956) of the Southerness Limestone Formation occupies a 0.3 km stretch of coast from here to a gently NNE-plunging anticline (Locality lb). Localities 2 and 3 are specific points of interest within the type section which exposes some 135 m of fossiliferous, thinly bedded calcareous mudstones, silt-stones and limestones. At least four prominent thick beds of sandstone are present. Easterly dips vary from 5 to 45°.

Although a number of east—west faults displace the strata, a reasonably complete section across the east limb of the anticline can be measured (Deegan, 1970). In the upper part of the formation Deegan identified three sedimentary cycles ranging in thickness from 8 to 15 Prominent algal horizons are developed at similar positions in two of the three cycles. An idealised cycle may be summarised as follows:

* Flaggy sandstone with plant remains
* Interbedded limestone and micaceous mudstone
* Interbedded limestone and calcareous mudstone
* Nodular algal band
* Thin muddy limestone with calcareous mudstone
* Fine-grained rippled sandstone
* Flaggy micaceous sandstone with ripple marks and plant remains

The cyclicity reflects varying depositional environments and may be attributed to gradual subsidence combined with variations in terrigenous sediment input possibly related to periodic dip-slip movement on basinal bounding faults. Each cycle commences with sandstones, probably deposited in a littoral environment, which are succeeded by calcareous beds formed under shallow subtidal conditions. Sandy limestones containing ooliths indicate that the sediments were affected by wave action. However, the algal beds show little sign of reworking and probably represent slightly deeper water sedimentation below the wave base. A traverse of the shore section enables the cyclicity identified by Deegan to be followed. Specific highlights include the Syringothyris Limestone (Craig, 1956) and algal beds.

==== 2 Syringothyris Limestone ====
A good exposure of the ''Syringothyris ''Limestone, estimated by Craig (1956) to be 16.7 m thick, is seen some 600 m west of the lighthouse (Locality 2, Figure 35). The limestone comprises several beds of argillaceous limestone and calcareous mudstone, and contains a varied marine fauna of brachiopods including ''Syringothyris cuspidata, ''bivalves, polyzoa and crinoid ossicles. Faunal similarities with the Harden Beds of Langholm were noted by Lumsden et al. (1967) and indicate that the Southerness Limestone Formation probably spans the boundary between the Lower and Middle Border Groups of Langholm (Table 1).

==== 3 Algal stromatolite beds ====
Above the Syringothyris Limestone, about 550 m west of the lighthouse, two distinctive algal stromatolite bands, 1.2 m and I m thick, are present within the sequence. Craig (1956) and Failicher (1977) referred the stromatolites to the genus ''Somphospongia. ''They resemble the dome type described by Leeder (1975) from the Lower Border Group of the Northumberland Basin. Individual domes are up to 30 cm in diameter with a relief of 10 to 15 cm and are set in a calcareous mudstone matrix. They are composed of alternating micritic and detrital laminae. Calcareous algal filaments commonly wrap fragments of shell including gastropods and ostracods. The irregular, nodular xterior of the algal growths indicates limited reworking. Basing his observations on studies of modern stromatolites, Leeder (1975) inferred that domed types formed in a low intertidal to shallow sub: tidal depositional environment.

==== 4 Southerness Lighthouse: Gillfoot Sandstone Formation ====
Walk back towards Southerness, across the fault (Locality la) which downthrows east and brings the Gillfoot Sandstone Formation in against the Southerness Limestone Formation.

Between 120 and 150 m of strata assigned to the Gillfoot Sandstone Formation are exposed on the shore between here and a position south of Powillimount Farm (NX 9880 5620). The formation conformably overlies the Southerness Limestone Formation, and Craig (1956) placed the top of the formation at the base of a breccia forming the base of the succeeding Powillimount Sandstone Formation (Locality 5).

The Gillfoot Sandstone Formation comprises white and purplish, flaggy, quartzose sandstones; conglomerates with intraformational fragments; red flaggy siltstones; and mudstones. A few red to grey, thin-bedded, sandy limestones with scattered detrital fossil remains are also present. Conglomerates, which form about 20 per cent of the succession, have a calcareous matrix and contain intraformational fragments in addition to pebbles of vein quartz, greywacke and microdiorite derived from the Southern Uplands hinterland to the north. Some more feldspathic layers are dominated by microdiorite and felsic rock debris derived from Lower Palaeozoic minor intrusions.

The formation ''is ''sparsely fossiliferous. A derived fauna, collected by Craig from thesandstone on which the lighthouse stands, suggests that the strata are of Arundian age and equivalent to part of the Middle Border Group of Langholm (Lumsden et al., 1967).

The lithologies indicate a more marginal depositional setting than that of the underlying formation. The conglomerates may have been transported from the hinterland and deposited by periodic sheetfloods flowing over low-lying supratidal areas. Textures in some of the sandstones indicate wave action and a littoral environment. Shallow subtidal environments may be indicated by the presence of thin fossiliferous, sandy limestones.

==== 5 Powillimount Shore: Powillimount Sandstone Formation ====
[[File:PowillimountExcursionMap.jpg|300px|thumbnail|Locality map and outline geology for the shore section between Powillimount and Hogus Point (Localities5-10).]]
Access to the Powillimount shore is via the road which links Powillimount Farm with the Kirkbean to Southerness road. A small car park at the shore beyond the farm (Figure 36) is suitable for cars and minibuses only. Immediately below the car park some 160 m of strata exposed on the shore between Powillimount Bay (NX 9880 5610) and Thirlstane (NX 9925 5690) are assigned to the Powillimount Sandstone Formation. The top 25 m are distinguished as the Thirlstane Sandstone Member, a prominent ridge of thick-bedded sandstone with spectacular penecontemporaneous deformation structures (Craig, 1956; Deegan, 1970; Ord et al., 1988).

The base of the Powillimount Sandstone Formation is drawn a short way SW of the car park, at the base of a grey calcareous breccia, above the highest bed of purple mudstone in the Gillfoot Sandstone Formation. The strata form part of the SE limb of a major NE-trending anticline and on the coast are further folded about a tight, gently plunging syncline—anticline pair, the axial planes of which trend NNE. Faunal assemblages are similar to those in the Middle Border Group of Langholm. Aspects of the fauna immediately below the Thirlstane Sandstone resemble those of the overlying Arbigland Limestone Formation.

[[File:P001161.jpg|300px|thumbnail|left|Taken from about 100 m. north of the Thirl Stane natural arch, Powillimount, looking north-north-east. Thirlstane Sandstone Member of the Powillimount Sandstone Formation, Carboniferous. Allokinetic liquefaction structures consisting of domes and volcanoes, bedding deformation increasing towards the north end of the outcrop (beyond the geologist). Strata of the Arbigland Limestone Formation occupy the background and are faulted against the Thirlstane Sandstone. Liquefaction is the transformation of loose sediment into a fluid mass preliminary to movement e.g. in a turbidity current or subaqueous slumping or sliding. P001161]]Lithologies include calcareous and quartzose sandstone, sandy limestone with beds of dark grey fissile mudstone, and calcareous mudstone. Locally, thin coals and associated seatearths are present. Sandstone beds are laterally extensive, ranging in thickness from 0.3 to 3 m. They are well sorted and commonly exhibit ripple cross-lamination. Many contain abundant carbonaceous plant remains and are extensively burrowed, particularly by ''Chondrites. ''Limestones range from arenaceous to argillaceous and contain detrital fossil remains, ooliths and rolled algal nodules. One distinctive oncolite bed, 0.3 m thick, contains rounded algal-coated lithic and fossil fragments. The individual oncolites are generally spherical and up to 1 cm across. Oncolites are produced by the accretion of sediment on to mobile grains through the action of algae, and their presence indicates constant agitation of the sea floor by wave action.

The characteristic lithologies, especially rolled algal nodules and detrital fragments, point to a shallow-marine environment exposed to gentle wave action. Deegan (1970) proposed that the sediments were deposited in a tidal lagoon protected from the effects of severe storms by some form of offshore sand barrier. The presence of thin coals and seatearths indicates periodic shallowing of lagoonal waters and the development of vegetated low-lying supratidal flats.

==== 6 Thirlstane natural arch: Thirlstane Sandstone Member ====
Walk about 50 m NE from the car park to a prominent sandstone ridge (NX 991 565) which can be followed to beyond the Thirlstane natural arch. The ridge is formed by the 25 m-thick Thirlstane Sandstone Member. At the base of the Thirlstane Sandstone the contact with the underlying strata is irregular, and intraformational fragments and large plant remains are present in the lowest beds. The fine natural arch at Thirlstane and nearby exposures to seaward display various sedimentary structures in a pinkish grey, medium-grained, well-sorted, quartzose sandstone. The rocks are characterised by large-scale trough cross-bedding and a spectacular development of liquefaction structures which increase in frequency and magnitude from south to north along the outcrop. It is worthwhile examining the extraordinary degree to which the original bedding has been disrupted. In a detailed study of these structures Ord et al. (1988) recognised different types of structure including oversteepened and recumbently folded cross-stratification, domes, sand volcanoes and zones of anastomosing, vertical cracks. They attributed the magnitude and frequency of the liquefaction structures to causes such as local seismicity, and deduced the presence of a syndepositionally active fault lying north of the present outcrop.

Deegan (1970) proposed that the Thirlstane Sandstone formed as an offshore sand barrier which initially enabled the inshore lagoonal environment of the Powillimount Sandstone Formation to develop. The evidence is equivocal, however and, as Ord et al. suggest, an alluvial origin is also a possibility.

==== 7 Thirlstane to Arbigland Garden: Arbigland Limestone Formation ====
All the strata exposed between Thirlstane (Locality 7) and Hogus Point (NX 997 589) (Locality 10) are assigned to the Arbigland Limestone Formation, estimated to be some 300 m thick (Craig, 1956). The conformable junction between the Thirlstane Sandstone Member and the Arbigland Limestone Formation (noted by Smith, 1910) is often obscured by shifting sands; the most obvious contact is a normal fault, (Locality 7) 200 m NE of the Thirlstane natural arch (NX 993 569). The fault plane appears to have a reverse throw and may have been rotated through vertical. Between this fault and an ESE-trending hinge fault opposite Arbigland Garden, the strata strike parallel to the coast. North of the second fault a narrow zone of disrupted bedding is succeeded by a series of beds striking ESE. Perched prominently on these is a huge erratic block of Criffell granodiorite known locally as the 'Devil Stone'. Tradition has it that the Devil bit off this chunk of Criffell and spat it out on the shore. Conventional glacial theory would suggest that the block was eroded and transported to its present position by a glacier during the last ice age.

The lithologies of the lower part of the Arbigland Limestone Formation resemble those of the Powillimount Sandstone Formation. Key features to look out for include thick-bedded, bioturbated, calcareous sandstones with coalified plant casts, thin sandy limestones locally with ooliths and algal debris, dark grey carbonaceous mudstones, and thin coal partings.

==== 8 Arbigland Bay to Borron Point: Arbigland Limestone Formation and pericline ====
Walk from the Devil Stone across the sandy Arbigland Bay. Diversions from the geology here include the fine gardens of Arbigland House, built by William Craik in 1755, and Paul Jones Cottage, where John Paul Jones (1747-92), founder of the American Navy, was brought up. The gardens may be visited (vehicular access from the Kirkbean to Southerness road) on certain weekdays.

North of the bay a small, shallow, periclinal basin structure is present (Locality 8). North of here the strike of the strata swings from east–west through ENE to NE, to become subparallel with the line of the coast around Borron Point (NX 995 580). The strata are cut by numerous normal faults of ESE and SE orientation.

==== 9 Arbigland Bay to Borron Point: faunal assemblages and sedimentary features of the Arbigland Limestone Formation ====
A traverse of the shore between Arbigland Bay and Borron Point is worthwhile for anyone with an interest in the sedimentology and faunal characteristics of shallow marine shelf sediments. The strata here were termed by Deegan (1970) the Middle Arbigland Beds. They are richly fossiliferous and contain a fine compound rugose coral fauna including spectacular massive hemispherical colonies of ''Lithostrotion clavaticum, ''first recorded by Smith (1910). Some are in life position with individual corallites visible on the upper surface. Overturned colonies show only the outer walls of the corallites radiating from the central columella. Please do '''not hammer them out.'''

Deegan (1970) described the strata below McCulloch's Castle (NX 996 577) (Locality 9) as mainly thin argillaceous and sandy limestones interbedded with calcareous mudstones and several prominent thick beds of massive bioturbated sandstone. Many beds of sandstone and sandy limestone have been extensively and repeatedly reworked by sediment feeders, and burrow forms such as ''Chondrites, Diplocrtzterion ''and ''Rhizocorallium ''are commonly seen. The limestones and mudstones have an abundant and diverse fauna including corals, brachiopods, bivalves, gastropods, crinoids, bryozoa and orthocones. Faunal equivalence with the Upper Border Group of Langholm (Lumsden et al., 1967), of Asbian age, is considered most likely. In the Langholm district, the Glencartholm Volcanic Beds form the base of the Upper Border Group and these rocks have been equated with the Clattering Band of Bewcastle (Day 1970) which George et al. (1976) place at the base of the Asbian. It follows that if correlation of the Middle Arbigland Beds is extended to the Clattering Band then the lower part of the Arbigland Limestone Formation (Locality 7) is probably Holkerian (as noted by George et al.).

At Borron Point stratigraphically higher rocks are downthrown by two small faults. Here the strata are steeply inclined and locally overturned. The sequence is characterised by thick-bedded bioturbated sandstone, and ripple cross-laminated fine-grained sandstone interbedded with calcareous mudstone and a few argillaceous limestones. Shallow sandstone-filled scours and washouts are common.

==== 10 Hogus Point: northernmost exposures of the Arbigland Limestone Formation ====
Those who wish to visit the northernmost exposures of the formation at Hogus Point (NX 997 588) should continue along the shore north from Borron Point. Alternatively the excursion can be completed at Borron Point and the route retraced to Powillimount Farm.

The strata at Hogus Point are reasonably well exposed. They are disposed about a tight, gently NE-plunging anticline—syncline fold pair. The rocks comprise thick units of thin-bedded silty, calcareous mud-stone, well-bedded sandstone and thin-bedded sandy limestone. Although Craig (1956) considered the strata at Hogus Point to be the highest in the sequence, a fauna collected more recently indicates a position no higher than the beds lying south of Borron Point. The indications are that the rocks lie on the upthrow side of a SE-orientated normal fault, the trace of which may lie north of Tallowquhairn Farm (NX 993 584).
Lithological characteristics of the sequence between Thirlstane and Arbigland Garden together with the sparse, locally detrital, fauna are consistent with a restricted lagoonal environment in which there was limited reworking of sediment. Overall, however, the Arbigland Limestone Formation was probably deposited within the intertidal to subtidal zone. An open, shallow-marine environment is indicated by the abundant fauna in the Middle Arbigland Beds (Deegan, 1970).
<References/>
{{EGwalks}}
[[Category:6. The South of Scotland]]

Siccar Point - an excursion

2015-10-20T09:35:08Z

JenniferFindlay1:

[[File:Lothian Geology cover.jpg|thumb|100px|right|[http://www.edinburghgeolsoc.org/p_sales.html#lothiangeology Buy the book]]]

=== Siccar Point: Hutton's classic uncorformity ===

'''By G.Y. Craig. From: Lothian geology and excursion guide. Edited by A.D. McAdam and E.N.K. Clarkson. 1996'''

O.S. 1 :50000 Sheet 67 Duns and Dunbar

B.G.S. 1:50000 Sheet 34 Eyemouth

Historically the Siccar Point unconformity is world-famous because its discoverer, James Hutton, was the first geologist to grasp the true significance of such a structure. Although this was not the first unconformity that Hutton had observed-the others were in Arran in 1786 and Jedburgh in 1787-it is certainly the most spectacular. His view of the rocks of the Earth as being the products of an essentially cyclical, oft-repeated process was triumphantly demonstrated at Siccar Point in 1788.

[[File:SiccarPointExcursionMap.jpg|600px|thumbnail|center|Siccar Point - excursion map]]

[[File:P000823.jpg|300px|thumbnail|left|Siccar Point, eastward view. Berwickshire. Unconformity between Upper Old Red Sandstone and the underlying Silurian (Llandovery) age rocks. Dark red sandstones and breccias of the Upper Old Red Sandstone age dip at low angles to the north and rest unconformably on an uneven surface of vertical greywackes and siltstones of Silurian (Llandovery) age. The red sandstones form the shore in the middle distance; the cliffs beyond, between Fast Castle and St. Abb's Head, are formed of steeply-dipping folded greywackes. P000823]]Siccar Point (NT 813 710) lies on the coast 4 km east of Cockburnspath. Turn east off the A1, a little over 2 km south of Cockburnspath on to the A1107. This road crosses the post-glacial gorge of the Pease Burn almost at once and the quarry road to Siccar Point turns off 450 m on the left after the narrow bridge over the gorge. Keep to the right fork of the quarry road, cross the grid bridge and continue along an extremely fine glacial drainage channel into Old Cambus Quarry. Continue through the north-east gate in the quarry and strike obliquely left up the hillside towards the far corner of the field, 60 m below which lies Siccar Point and Hutton\’s unconformity. From the cliffs a fine panorama can be seen to the north-west of the Upper Old Red Sandstone grading up into the grey sandstones of the Lower Carboniferous (Cove Excursion). The lighthouse in the middle distance at Barns Ness lies on the Lower Limestone Group (Catcraig Excursion) and in the far distance the Bass Rock juts out from the sea with North Berwick Law lying inland slightly to the west. Both are plugs of phonolitic trachyte (North Berwick Excursion).

Siccar Point speaks eloquently for itself and needs little description. It is spectacular at any stage of the tide. An inclined uneven basement of vertical greywackes and shales of Llandovery. Silurian, age youngs to the WNW and is covered unconformably by gently dipping dull-red breccia and sandstone of Upper Devonian or Lower Dinantian age. The breccia is composed of greywacke fragments. The breccia and sandstones were formed under flood conditions. The strong imbricate structure of the clasts in the breccia shows that the direction of derivation of the material was from the NNE and not from the cliffs above.

Both Hutton and Playfair deserve to be quoted. Hutton described it (1795, I, 458) as follows:

:: "Having taken boat at Dunglass burn, we set out to explore the coast; and, we observed the horizontal sandstone turn up near the Pease burn, rising towards the schistus. We found the junction of that schistus with the red sandstone and marly strata on the shore and sea bank, at St. Helens, corresponding in general with what we had observed in the burns to the westward. But, at Siccar Point, we found a beautiful picture of this junction washed bare by the sea. The sandstone strata are partly washed away, and partly remaining upon the ends of the vertical schistus; and. in many places, points of the schistus strata are seen standing up through among the sandstone, the greatest part of which is worn away. Behind this again we have a natural section of those sandstone strata, containing fragments of the schist us.

:: After this nothing appears but the schistus rocks, until sandstone and marl again are found at Redheugh above the vertical strata. From that bay to Fast Castle we had nothing to observe but the schistus, which is continued without interruption to St. Abb\’s Head. Beyond this, indeed, there appears to be something above the schistus; and great blocks of a red whinstone or basaltes come down from the height and lie upon the shore; but we could not perceive distinctly how the upper mass is connected with the vertical schistus which is continued below.
[[File:P000818.jpg|300px|thumbnail|left|Siccar Point viewed from the south-west. Berwickshire. Basal breccia or conglomerate of Upper Old Red Sandstone dipping at a low angle to left (north), resting on vertical shaly siltstone and thin greywacke of Silurian (Llandovery) age. The breccia consists of angular fragments of greywacke in a red, sandy matrix. Siccar Point has outstanding historical associations, stemming from a visit paid by James Hutton in 1788, accompanied by Sir James Hall and Professor John Playfair. Unconformity between Upper Old Red Sandstone and the underlying Silurian (Llandovery) age rocks. Hutton's classic world-famous unconformity. He was the first geologist to grasp the true significance of such a structure. P000818]]
:: Our attention was now directed to what we could observe with respect to the schisti, of which we had most beautiful views and most perfect sections. Here are two objects to be held in view. in making those observations; the original formation or stratification of the schisti, and the posterior operations by which the present state of things has been procured. We had remarkable examples for the illustration of both those subjects.

:: With regard to the first, we have every where among the rocks many surfaces of the erected strata laid bare, in being separated. Here we found the most distinct marks of strata of sand modified by moving water. It is no other than that which we every day observe upon the sands of our own shore, when the sea has ebbed and left them in a waved figure, which cannot be mistaken. Such figures as these are extremely common in our sandstone strata; but this is an object which I never had distinctly observed in the alpine schisti; although, considering that the original of those schisti was strata of sand, and formed in water, there was no reason to doubt of such a thing being found. But here the examples are so many and so distinct, that it could not fail to give us great satisfaction.

:: We were no less gratified in our view with respect to the other object, the mineral operations by which soft strata, regularly formed in horizontal planes at the bottom of the sea had been hardened and displaced. Fig. 4 represents one of those examples; it was drawn by Sir James Hall from a perfect section in the perpendicular cliff at Lumesden burn. Here is not only a fine example of the bendings of the strata, but also of a horizontal shift or hitch of those erected strata."

Hutton's clinical description is in marked contrast to that of the eloquent prose of Playfair (1805, 71-72).

:: "The ridge of the Lammermuir Hills in the south of Scotland, consists of primary micaceous schistus, and extends from St Abb's-head westward, till it joins the metalliferous mountains above the source of the Clyde. The sea-coast affords a transverse section of this alpine tract at its eastern extremity, and exhibits the change from the primary to the secondary strata, both on the south and on the north. Dr Hutton wished particularly to examine the latter of these, and on this occasion Sir James Hall and I had the pleasure to accompany him. We sailed in a boat from Dunglass, on a day when the fineness of the weather permitted us to keep close to the foot of the rocks which line the shore in that quarter, directing our course southwards, in search of the termination of the secondary strata. We made a high rocky point or headland, the Siccar, near which, from our observations on the shore, we knew that the object we were in search of was likely to be discovered. On landing at this point, we found that we actually trode on the primeval rock, which forms alternately the base and the summit of the present land. It is here a micaceous schistus, in beds nearly vertical, highly indurated, and stretching from south-east to north-west. The surface of this rock runs with a moderate ascent from the level of low-water, at which we landed, nearly to that of high-water, where the schist us has a thin covering of red horizontal sandstone laid over it; and this sandstone, at the distance of a few yards farther back, rises into a very high perpendicular cliff. Here, therefore, the immediate contact of the two rocks is not only visible, but is curiously dissected and laid open by the action of waves. The rugged tops of the schistus are seen penetrating into the horizontal beds of sandstone, and the lowest of these last form a breccia containing fragments of schistus, some round and others angular, united by an arenaceous cement.

:: Dr Hutton was highly pleased with appearances that set in so clear a light the different formations of the parts which compose the exterior crust of the earth, and where all the circumstances were combined that could render the observation satisfactory and precise. On us who saw these phenomena for the first time, the impression made will not easily be forgotten. The palpable evidence presented to us, of one of the most extraordinary and important facts in the natural history of the earth, gave a reality and substance to those theoretical speculations, Which, however probable, had never till now been directly authenticated by the testimony of the senses. We often said to ourselves, What clearer evidence could we have had of the different formation of these rocks, and of the long interval which separated their formation, had we actually seen them emerging from the bosom the deep? We felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited in the shape of sand or mud, from the waters of a superincumbent ocean. An epocha still more remote presented itself, when even the most ancient of these rocks instead of standing upright in vertical beds. lay in horizontal planes at the bottom of the sea, and was not yet disturbed by that immeasurable force which has burst asunder the solid pavement of the globe. Revolutions still more remote appeared in the distance of this extraordinary perspective. The mind seemed to grow giddy by looking so far into the abyss of time; and while we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much farther reason may sometimes go than imagination can venture to follow. As for the rest, we were truly fortunate in the course we had pursued in this excursion; a great number of other curious and important facts presented themselves, and we returned, having collected, in one day, more ample materials for future speculation, than have sometimes resulted from years of diligent and laborious research."

Other localities might seem to be an anticlimax after Siccar Point, but three in the general area are worth mentioning. The first is an anticline of Silurian greywackes and shales exposed in the old quarry (NT 801 653) at Grantshouse, some 5 km south of the junction of the A1107 with the A1. The core of the fold is cut by a minor reversed fault trending parallel to the fold axis and downthrowing to the south-east. Cleavage is developed in the shales and slickensiding seen in bedding planes. Graded bedding, flute marks and groove moulds are among the more common sedimentary structures.

Hutton owned two farms, not one as is commonly recorded, and lived in one of them, Slighhouses (NT 823 593) between 1754 and 1767. This farm can be reached by turning off the A 1 some 6 km south-east of Grantshouse on to the B6437 then west on the B6438 for 3 km (NT 822 603) to turn south on an unclassified road. The second is an upland farm, Nether Monynut (NT 728 645), and lies some 4 km north-west of Abbey St Bathans. On a fine day this part of the Border country can be delightful, but if it is wet ... !

{{EGwalks}}

[[category:6. The South of Scotland]]

File:P000823.jpg

2015-10-20T09:15:07Z

JenniferFindlay1: Siccar Point, eastward view. Berwickshire. Unconformity between Upper Old Red Sandstone and the underlying Silurian (Llandovery) age rocks. Dark red sandstones and breccias of the Upper Old Red Sandstone age dip at low angles to the north and rest unco...

== Summary ==
Siccar Point, eastward view. Berwickshire. Unconformity between Upper Old Red Sandstone and the underlying Silurian (Llandovery) age rocks. Dark red sandstones and breccias of the Upper Old Red Sandstone age dip at low angles to the north and rest unconformably on an uneven surface of vertical greywackes and siltstones of Silurian (Llandovery) age. The red sandstones form the shore in the middle distance; the cliffs beyond, between Fast Castle and St. Abb's Head, are formed of steeply-dipping folded greywackes.
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Rhins of Galloway - an excursion

2015-10-13T09:52:41Z

JenniferFindlay1:

By By J A McCurry and P Stone. Excursion 15. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

== Rhins of Galloway:a coastal traverse across the Northern and Central belts of the Southern Uplands ==

OS 1:50 000 sheets 76 Girvan and 82 Stranraer; Glen Luce surrounding area

BGS 1:50 000 Sheet 1 and 3 with parts of 7 and 4W The Rhins of Galloway

=== Main points of interest ===
[[File:RhinsOfGallowayExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Rhins of Galloway.]]
Ordovician and Silurian structure; turbidite sedimentology (conglomerate, greywacke and shale) and stratigraphy; Caledonian folding and thrusting.

=== Logistics ===
The itinerary suggested for this relatively remote area can be used as the basis for a 2- or 3-day excursion centred on Portpatrick. Some of the localities in Excursions 17 and 18 can also be visited from here. All the localities are readily accessible to small vehicles, but anything larger than a minibus will not be able to negotiate the narrow roads. All the localities are coastal, so more rock will be seen at low tide. Localities 2, 9 and 10 are the most tide dependent and should be visited out of sequence if necessary. Most of the localities require scrambling over steep rock outcrops which may be wet and slippery; the longest walk required is about 4 km.

=== Introduction ===
[[File:RhinsOfGallowayLowerPalaeozoicStratigraphy.jpg|400px|thumbnail|Summary of Lower Palaeozoic stratigraphy on the Rhins of Galloway.]]
The coast sections of the 45 km-long Rhins of Galloway peninsula provide an unsurpassed traverse through the Lower Palaeozoic outcrop of south-west Scotland. The sections chosen (Figure 45) build up a complete cross-strike traverse through the imbricate thrust sheet of late Ordovician and early Silurian turbidite strata. Biostratigraphical ages quoted are based largely on recent determinations by A W A Rushton and S P Tunnicliff summarised in Stone (1995). The summary of tectonostratigraphy given in Figure 46 encapsulates the stratigraphical paradox of the Southern Uplands: within each fault-bounded tract the exposed strata become younger towards the NW, though the tracts themselves become sequentially younger towards the SE. The strike faults separating the tracts are therefore thrust faults which originally propagated mostly towards the SE, carrying older beds over younger. Each thrust slice steepened upwards before its sole thrust stuck and was replaced at a lower level by a new thrust. In some interpretations one of the faults is given special importance as a possible terrane boundary along which a large displacement has occurred. This structure, the Orlock Bridge Fault, is examined in Excursion 17. In the Rhins of Galloway traverse described here Localities 1-5 are within the Ordovician Northern Belt of the Southern Uplands and Localities 6-10 are in the Silurian Central Belt to the south.

The introductory section of this guide which deals with the Lower Palaeozoic regional geology is particularly relevant to this excursion. Also of great value is the BGS 1:50 000 Rhins of Galloway map, but note the alternative interpretation at Locality 9. Much of the evidence to be seen is pertinent to the debate over the origin of the Southern Uplands: forearc accretionary prism or backarc to foreland basin thrust belt. The regional palaeocurrent pattern (at least in the Ordovician) and the interdigitating siliceous and volcaniclastic turbidites at locality 4 provide evidence of a volcanic source to the south of a backarc Southern Uplands (Stone et al., 1987); the opposing younging and vergence of D1 thrusting and folding on either side of the Port Logan Fault (Localities ''7, ''8 and 9) provide evidence of obduction accretion comparable with that in the Washington—Oregon forearc (McCurry and Anderson, 1989). Descriptions for Localities 1, 2 and 5 have been prepared by P Stone, for Localities 3, 7, 9 and 10 by J McCurry, and for Localities 6 and 8 jointly.

===Excursion ===
[[File:RhinsOfGallowayFinnartsBayExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Finnarts Bay area (Locality l).]]

==== 1 Finnarts Bay: Kirkcolm Formation ====
Finnarts Bay is on the east side of Loch Ryan. If approaching from the east on theA75 turn north on the A751 and follow the A77 through Cairnryan. About 4 km north of Cairnryan a large, disused quarry is seen on the right; shortly after take the left turn signposted for the Fish Farm and at the bottom of the hill turn abruptly left to park on the extensive raised beach (NX 051 725). If approaching from the north, follow the A77 down the valley of Glen App and, when the road reaches the coast and swings south, turn right towards the Fish Farm. The itinerary (Figure 47) has two sections, a coastal traverse and a nearby road section along the A77. For the latter part there is parking for one or two cars at the entrance to the rough track leading into the old quarry (NX 053 720) or in the lay-by on the opposite (seaward) side of the road. If a larger number of vehicles is involved it is safer to leave them parked on the raised beach and walk back ro the quarry entrance and road section.

''Coastal traverse ''At the south-west end of the raised beach an extensive rocky area is exposed below the high tide mark. This outcrop consists of very thinly bedded, fine-grained greywacke and siltstone. At low tide and subject to the movement of beach shingle, a low northward extension of the main outcrop shows an intrusive porphyritic dyke (a in Figure 47). The dyke is up to 2 m across and contains abundant feldspar phenocrysts set in a fine-grained microdioritic groundmass. It has sharp margins against the host siltstone and contains no trace of tectonic foliation. It was clearly intruded after deformation. The surrounding sedimentary strata, best seen on the larger exposures to the south of the dyke (b in Figure 47), form a thinly bedded interval within the Kirkcolm Formation and contain a sparse ''gracilis'' Biozone graptolite fauna (Figure 46). Seemingly chaotic deformation has affected the siltstone and greywacke, with folding on a variety of scales and styles. Brittle dislocation of the thin beds in both an extensional and a compressional sense is also apparent. The folding may well be poly-genetic, with tectonic deformation superimposed on syndepositional slump-related folding. Within the chaotically folded zones diagenetic carbonate concretions are fairly common, forming disc-shaped bodies up to I m across. These weather brown and may project from the eroded surface of the host strata. Significantly, when such concretions are surrounded by chaotically deformed layers they may preserve internal undisturbed lamination. The concretions are therefore thought to have formed before deformation occurred which suggests that much folding was post-diagenetic.

About 20 m south into the next small bay another felsic dyke is intruded into the laminated siltstone (c). However, in this case the dyke is pervasively foliated with a fabric parallel to, and apparently continuous with, that seen in the surrounding sedimentary strata. If possible, dig out the sand to expose the contact of dyke and siltstone; locally it is quite irregular but, whereas the dyke cuts across the overall bedding trend at an angle of about 10°, the cleavage fabric is continuous across the contact. Clearly the dyke was intruded prior to the imposition of the cleavage. The contrast with the dyke examined earlier, which is of very similar composition, suggests that dyke intrusion spanned the cleavage-forming deformation.

Slightly farther south within the same bay thick greywacke beds up to 1.5 m across appear in the sequence. These are medium-grained quartzo-feldspathic representatives of the Kirkcolm Formation. Another thick greywacke bed forms part of the cliff at the south end of the bay, exposed beneath the World War II gun emplacement (Loch Ryan was a convoy assembly point and flying boat base, hence the fortifications). Bedding dip in the cliff section (d) is about 50° SE but sedimentary structures such as ripple crests and shale flames show that the beds are inverted. A well-developed slaty cleavage dips more steeply than the bedding and therefore cannot be a simple axial-planar cleavage. Various explanations are possible. Either the bedding and cleavage have been jointly rotated or the cleavage has been imposed on bedding already tilted out of the horizontal; both situations require at least two deformation episodes. Alternatively a nonaxial-planar cleavage, developed coevally with the folding, could locally give this relationship. Examples of the latter phenomenon are widespread in the southern part of the Rhins of Galloway.

Seaward from the inverted greywacke beds the chaotically deformed siltstone lithology reappears. It is intruded by a porphyritic dyke (feldspar phenocrysts in a microdiorite groundmass) up to 2 m across which runs oblique to bedding and has irregular margins. Examination of the dyke margins shows that the cleavage in the silt-stone continues for up to a centimetre into the chilled margins. This dyke was clearly intruded before the end of cleavage-forming deformation.

Continue south along the beach and scramble up the low cliff on to the slightly higher rock platform; this is easiest at the inland end of the cliff: A narrow track leads south but after about 20 m leave it and scramble across the rock outcrops to seaward. The thinly bedded siltstones are here deformed in a slightly more orderly fashion and, if the beds are traced out, a sequence of steeply plunging ‘S’ folds can be established (e). A spaced cleavage can be seen subparallel to the axial plane of the folds. Note the similarity between the attitude of this cleavage and that in the inverted greywacke beds seen previously. The associated steeply plunging 'S' folds may be the result of the late sinistral shear (D3) imposed on this part of the Southern Uplands.

Regain the track and continue south for a short distance and descend into the next small inlet. The most prominent feature here (f) is a 2 m-thick felsic dyke containing abundant feldspar phenocrysts. At first sight the dyke appears compound, with a discrete central zone, but examination of thin sections has proved it to be a homogeneous porphyritic microdiorite. There is no cleavage in the dyke and so, like the first example seen, it is probably post-tectonic.

[[File:P008425.jpg|300px|thumbnail|left|Finnarts Bay, Loch Ryan. Flute casts on the base of a greywacke bed. A linear bottom structure produced by current flow, in this case from top right to bottom left. When bedding is restored from subvertical to horizontal the original current flowed from the south. P008425]]The less agile should retrace their route to the parking area, but the more sure-footed can scramble over the next two rocky spurs; the distance is only about 50 m but is quite arduous at anything but very low tide. The reward is a magnificent array of flute casts on the base of a greywacke bed, slightly overturned to dip steeply SE (g). The rock face must therefore be viewed looking north. The linear nested flutes indicate a current flow from top right to bottom left. If you imagine the bedding plane restored to the horizontal, an original eroding current flowing from the SE is suggested.

Return to the raised beach at Finnarts Bay. If several vehicles are being used the party should walk back to the A77 and '''proceed with care '''(the road carries '''much heavy traffic '''to and from the Irish ferry terminals) to the entrance track for the large disused quarry (NX 053 720). One or two vehicles can be parked in the rough track entrance or on the hard shoulder area on the opposite (seaward) side of the road. The quarry itself is structurally complex and has loose, dangerous faces. It should not be entered. Fortunately the more accessible cliffs forming the roadside section running south from the quarry (Figure 47) are both secure and instructive.

''Roadside traverse ''This section is only slightly lower in the sequence than the flute casts viewed earlier at sea level. The entrance to the quarry coincides with an overturned Fl synclinal hinge zone with short limb partly eliminated by faulting (h). Closest to the quarry entrance the greywacke beds are the right way up and dip moderately south whereas, to the south in the roadside section, beds dip steeply south but are slightly overturned and young north. The direction of younging can be readily established from the array of sedimentary structures. The greywacke beds range from about 10 cm to over 1 m in thickness. Many beds are clearly graded and contain weak cross-lamination in places. Bed bases commonly carry small flute casts and more bulbous load casts.

The roadside section continues south becoming progressively lower and more obscured by bushes. Initially the attitude of the greywacke beds remains uniform with steep dip to SE and northwards younging. However, about 100 m SE from the quarry the greywackes are folded about an Fl anticline—syncline pair with both hinge zones broken and faulted (i). The anticline lies to the north of the syncline in a structural pattern characteristic of much folding throughout the Southern Uplands (Types 1 and 2, Figure 3). No cleavage is developed in association with these folds and cleavage is in fact absent from the whole roadside section.

The greywackes exposed on the shore and roadside are all in the Kirkcolm Formation (Figure 46). They are quartzo-feldspathic greywackes with a few accessory grains of spilite, schist, garnet and zircon; the quartz content averages about 45 per cent. Full compositional details are provided by Kelling (1962), Floyd and Trench (1989), Evans et al. (1991) and Stone (1995).

==== 2 Lady Bay: Glen App Fault and Corsewall Formation ====
Leave Finnarts Bay and drive south on the A77 to Stranraer; pass through the town following signs for Leswalt and Kirkcolm and leave on the A718 heading NW. After passing the Stranraer golf course turn right at the Craigencross roundabout, still following the A718 for Kirkcolm. About 3.5 km beyond the roundabout it is worth stopping briefly beside the sea at St Mary's Croft (NX 034 659). Here, on the foreshore, are exposed the Permian breccias which fill the 1500 m-deep Stranraer basin (Stone, 1988 and references therein). The east side of the basin is formed by a major fault which defines the east coast of Loch Ryan. This contrasts with the west side of the basin, where the Permian (and some Westphalian) strata lie unconformably above the Lower Palaeozoic greywackes; the structure is a classic half-graben (Figure 45). The breccias consist of greywacke pebbles up to about 6 cm across, contained in a matrix of coarse red sand. Thin interbeds of red sandstone and siltstone are also present. The most extensive outcrop is near the low water mark.

Continue north on the A718 through Kirkcolm. About 2 km beyond the village, fork right following signs for Corsewall Point and about 1 km beyond the fork turn right following signs for Lady Bay. The road is metalled as far as Low Portencalzie Farm but thereafter deteriorates into a rough track. Nevertheless continue on down towards the sea where there is a paved parking area with adjacent picnic tables.

It is essential to visit this locality around low tide. The Glen App Fault zone is exposed, subject to the vagaries of shifting sand and shingle, at the north end of the bay (027 718) below high water mark. All the rocks in the vicinity are pervasively reddened, a reminder of the nearby Permian sequence of red sandstone and breccia. A complex array of shear zones and quartz veins cuts through the reddened greywackes defining a fault zone about 5 m across and trending approximately NE towards Finnarts Bay and Glen App on the far side of Loch Ryan. The character and geometry of the structures within the fault zone suggest that ductile deformation occurred with a sinistral sense of shear, followed by small-scale brittle effects with a dextral shear sense.

Thickly bedded greywackes abut the fault zone on its north side and it is well worth scrambling over the cliffs a little way northwards to examine their sedimentary features. A well-trodden track provides a route. The greywackes are all part of the Corsewall Formation and are compositionally immature, with abundant clasts of igneous lithologies including spilite, gabbro and serpentinite probably derived from an ophiolitic source. The quartz content is only about 14 per cent on average. Compositional details are given by Kelling (1962), Evans et al. (1991) and Stone (1995). The lithic composition gives the greywackes a dark appearance which contrasts with the pale grey Kirkcolm Formation greywackes seen at Locality 1.

Immediately north of Lady Bay the Corsewall Formation greywackes are thickly bedded (tip to 1.5 m) graded turbidites. Beds are steeply inclined and the grading, together with the weak cross-lamination in the top part of some beds and the bottom structures on their bases, establishes a consistent direction of younging towards the north. Slightly farther north (up sequence) the thick greywacke beds are separated by thinly bedded intervals of greywacke and siltstone showing classical turbidite structures such as grading, cross-lamination and rippled bed tops. Some of the thick, coarser interbeds have pockets of pebbly greywacke along their bases.

==== 3 Corsewall Point: Corsewall Conglomerate ====
[[File:RhinsOfGallowayCorsewallPointExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Corsewall Poinr area (Locality 3).]]
Return to the parking area in Lady Bay and retrace the route to the KirkcolmCorsewall Point road. Turn right and continue towards the lighthouse. Just over 1 km NW from the Lady Bay road junction be sure to take the right fork; thereafter the route is fairly obvious, though occasionally gated. At Corsewall Point park on the right side of the road by the wildlife information board (NW 982 727). From there walk NW on to the fault-bounded promontory (3a on Figure 48). On this and the two promontories to the east (3b and 3c) the sedimentary features of the conglomeratic member of the Corsewall Formation (late Llandeilo—early Caradoc) can be examined. This site has been mapped and logged in detail by Holroyd (1978) who interprets its rocks as an inner fan channel sequence deposited at the base of a deep-sea slope. The steeply SE-dipping beds are slightly overturned and young north. The sequence consists of extrabasinal conglomerates interbedded with coarse, massive sandstone units. Individual units are up to 4 m thick. Tertiary igneous activity is evidenced by thin cross-cutting dolerite dykes and, offshore to the north, by the microgranitic plug of Ailsa Craig.
[[File:P008478.jpg|left|thumbnail|Corsewall Point, Well Isle. Corsewall Conglomerate. A very coarse, boulder conglomerate of Ordovician age. P008478]]

The conglomerates are variably clast and matrix supported and consist of well-rounded pebbles and boulders up to 1.5 m in diameter set in a sandy matrix (3a''' '''and 3b). Granites and acid volcanic clasts predominate, but spilites, gabbros, greywackes and cherts are also found. Although mostly disorganised, the conglomerates display increased organisation eastwards across the three promontories. Organisation is shown by the alignment of the long axes of clasts parallel to bedding and, in places, by a crude lamination in the matrix. Both normal and reverse grading are present, but rare. Bedding is lenticular and channelised, in places clearly eroding the underlying unit (3c). The massive sandstone units are coarse grained and frequently contain outsize extrabasinal clasts. Rare sole markings indicate palaeoflow from the NW. Further east the boulder conglomerates are less common so that massive and graded sandstone units predominate. At Ochley Point (NW 986 728), about 300 m east of the parking area, fine-grained sandstone, laminated silt-stone and mudstone form interbedded units up to 1.5 m thick in an overall fining-and thinning-up sequence (3d). This facies assemblage represents an interchannel environment on a submarine fan.

Work by Elders (1987) on the provenance of the granite clasts has highlighted the tectonic importance of the site. He identified a suite of five granitic clast types within the conglomerate. The most distinctive of these is a weakly foliated, muscovite-bearing biotite granite dated at 1265 Ma. Two of the other granites yielded ages of 600 Ma and 475 Ma. Combining these dates with petrographic and geochemical evidence, Elders identified north-west Newfoundland as the only area with a plutonic and tectonic history to match that of the clasts. The Corsewall conglomerate was obviously deposited close to source and a sinistral strike-slip movement of 1500 km during closure of the Iapetus Ocean would therefore be needed to account for its present position. This controversial conclusion has not been universally accepted; Kelley and Bluck (1989) refute it based on radiometric work of their own from the Southern Uplands (see discussion, Elders, 1990); whilst Owen and Clarkson (1992) argue that faunal evidence supports at most a few hundred kilometres of strike-slip movement along the Southern Upland Fault (but see also McKerrow and Elders, 1989). In view of its importance this locality has been designated a Site of Special Scientific Interest (McCurry, 1994).

==== 4 Killantringan: Portpatrick Formation ====
[[File:RhinsOfGallowayKillantringanExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Killantringan area (Locality 4).]]
From Corsewall Point return via minor roads to the A718 north of Kirkcolm. Drive south through the village to the Craigencross roundabout and proceed straight on along the Glenstockadale road; at the end turn left on to the A764 towards Portpatrick. After about 3 km turn right on to a single-track road signposted for Killantringan Lighthouse. This road forms part of the Southern Upland Way footpath and is marked accordingly. When the road reaches the coast cars should be parked on the right-hand side overlooking Killantringan Bay (NW 982 567). At low tide a broad sweep of sandy beach allows easy access to the cliffs; when the tide is high the sea reaches the foot of the cliffs and isolates a number of small coves. These may then be accessed via the cliff top path.

[[File:P008516.jpg|300px|thumbnail|left|Killantringan Bay. Synclinal fold hinge in greywackes of the Portpatrick Formation, Ordovician. P008516]]Walk north on to the sea cliffs, either at beach level or by way of the cliff path, where thinly bedded greywackes and silt-stones of the Portpatrick Formation are extensively exposed (4a in Figure 49). Sporadic thicker greywacke beds are also present and isoclinal fold structures can be picked out in places by careful examination. However, the probability of folding is most readily deduced from the sedimentary younging indicators, mostly grading, which show local reversals. Despite these, the dominant younging direction is to the north. Structural complexity is illustrated at one place (4b) where a synclinal hinge is exposed in a section which the sedimentary younging indicators show is broadly anticlinal. A strong slaty cleavage dips moderately to the SE, compatible with the bedding attitude in an axial-planar relationship. A fragmentary graptolite fauna recovered from this vicinity indicates a ''linearis ''Biozone age (Figure 46).

Return past the parking area and move on to the rocky outcrops around high water mark to the west. A different facies of the Portpatrick Formation is exposed with thicker greywacke beds ranging up to 1 m. Cleavage is still strong but is generally confined to the finer-grained, upper part of the greywacke beds and is markedly curved, refracting through the bed as the grain size varies. Some beds preserve bottom structures on their bases (the south side of the bed so view looking north) including some large flute casts indicating current flow from the SW (4c). The greywackes are dark and immature, and on fresh surfaces it may be possible to discern with a hand lens the abundant detrital mafic minerals, dominantly pyroxene and amphibole. This composition, rich in andesitic debris, is characteristic of the Portpatrick Formation.

Leave the south end of Killantringan Bay and walk south and inland to join the Southern Upland Way. The route skirts the inlet of Portamaggie where the wreck of the Craigantlet may still be visible; the ship ran aground in February, 1982. Continue for about 200 m south from Portamaggie and then drop down on the right-hand sidetowards the coastal rocks and cliff line. Thickly bedded Portpatrick Formation greywackes are well exposed; bedding is uniformly upright and youngs north. Many of the greywacke beds are coarse and gravelly at the base and show marked truncation of the cross-laminated tops of underlying beds. Bed bases also preserve abundant bottom structures, including flute casts which uniformly indicate current flow from the SW (4d). A few tens of metres farther south the cliff section recommences, coincident with a decrease in average bed thickness. Most of the greywackes have the characteristic dark colour of the Portpatrick Formation but thin interbeds (up to 25 cm) of pale grey quartz arenite can be seen (4e). These are correlated with the Glenwhargen Formation which develops as a thick sequence of quartzose greywackes farther east (see Excursion 14). The interbeds illustrate the interfingering of two different turbidite fan systems derived from very different source areas. Portpatrick Formation greywackes contain on average about 15 per cent quartz grains whereas the Glenwhargen Formation arenite contains about 65 per cent quartz. Full compositional details are given by Stone (1995) and a sedimentological analysis is included in Kelling et al. (1987). Return to the parking area via the Southern Upland Way.

==== 5 Portpatrick: Portpatrick Formation, folding and fault zone ====
Drive back along the lighthouse road to the A764 and turn right for Portpatrick. After about 4km turn right again into the village. Part of the described route requires a lowish tide and it may be necessary to arrange the sequence of localities to accommodate this. If time allows, a walk south along the cliff path to Dunskey Castle (NX 004 534) is a worthwhile diversion from the[[File:P001122.jpg|300px|thumbnail|right|Morroch Bay, Wigtownshire viewed from the north-west. The bay and the smaller Port of Spittal Bay beyond are eroded along the outcrop of shale sequences which alternate with Portpatrick Formation (Ordovician) greywackes. The rocks are all steeply inclined ranging from vertical in the foreground to overturned towards Port of Spittal Bay. The bay has a low raised beach and old cliff line. P001122]]geology. The path is reached via steep steps just beyond the SE margin of the main car park on the south side of the harbour. The castle is about 500 in from the top of the steps. It occupies an impressive position on the cliff edge and is built mainly of local greywacke, with dressed corner stones and lintels of Permian red sandstone most probably brought in from Dumfries. It was built around 1510 by Adair of Kilhilt on the site of an earlier stronghold but was in a ruinous condition by 1684 and has remained so.

Folded Portpatrick Formation greywackes are well exposed in and around Portpatrick Harbour and exposure is more or less complete southwards to Morroch Bay. At Morroch Bay the basal beds of the Portpatrick Formation conformably overlie the Moffat Shale Group and are interbedded with shales containing ''clingani ''Biozone graprolites (Figure 46). Full details are given in Excursion 18.

[[File:P008429.jpg|300px|thumbnail|left|Portpatrick Quarry. Monocline in greywackes. The attitude of the Portpatrick Formation greywackes changes from sub-horizontal at the top of the cliff to vertical at the base. P008429]]A spectacular exposure of the greywackes is provided by an old quarry, 250 m SE from the southern harbour car park. The quarry is paved and landscaped. The main face, viewed looking east, exposes a magnificent F1 monoclinal fold. Remember, as you view the fold, that north is to your left. On the left the beds are vertical at ground level but higher in the cliff face assume a more gentle northward dip. The axial plane of the monocline dips moderately south (towards your right) and so the upper, gently dipping limb of the fold descends to ground level in that direction. A fine selection of turbidite features includes gravelly bed bases, shale rip-up clasts and cross-laminated bed tops. These make it easy to confirm the younging directions. In the north of the quarry the bedding at ground level is vertical and youngs north; in the south of the quarry it is the right way up and dips gently northwards.

Continue NW across the intertidal rocks towards Portpatrick Harbour. Bedding dip is variable and by applying younging criteria it can be seen that the beds are right way up and are folded about several open F1 synclines and anticlines. The hinges are mostly replaced by faults or shear zones but one good example of an open syncline is preserved slightly farther north in the back wall of the outer harbour and, at lowish tide, is accessible from the beach. The hinge plunges about 15° NE. A strong slaty cleavage is developed throughout this section, striking NE, and is either vertical or dips steeply SE.

Ascend the steps at the back of the harbour near the synclinal hinge. Turn left at the top and walk to the NW corner of the inner harbour (NW 997 542). Thence continue west past the paddling pool and the Southern Upland Way start/finish indicator. A rocky gully continues west and the remains of a red sandstone archway at its seaward end marks the outfall of an old (perhaps Victorian) sewer. As you approach the archway '''take care on the slippery rocks '''and note the abundant quartz veins. The modern sewer follows the same line as its ruined predecessor and this outcrop is occasionally polluted. The gully utilised by the sewer pipes follows a major fault zone, the internal structure of which is exposed at low tide beneath the red sandstone archway. A black shaly siltstone is pervasively sheared and cut by several generations of quartz veins which are themselves sheared and folded. At least some of the deformation appears to have been ductile and the fold style suggests an overall sense of sinistral shear. An important feature of this fault is its separation of two contrasting structural domains. To the south lies the open folding just traversed; to the north the bedding is upright and youngs almost consistently northwards into the Killantringan section (Locality 4). This uniform section extends for almost 3 km, interrupted only sporadically by tight Fl fold pairs.

From Portpatrick Harbour return to the car park, and take the A77 Stranraer road to Ardwell Bay. Time may be available ''en route ''for diversions to Morroch Bay or Cairngarroch Bay. The former exposes a conformable contact between the Portpatrick Formation and the Moffat Shale Group (Excursion 18, Locality 3). The second locality allows examination of the Orlock Bridge Fault zone (Excursion 17, Locality 1).

==== 6 Ardwell Bay: Gala Group, contrasting fold styles ====
From Portpatrick travel via the A77 and B7042 to join the A716 at Sandhead. Just south of Sandhead turn right at the signpost for the early Christian site at Kirkmadrine and continue on to the Clachanmore crossroads (NX 084 467). (If Morroch Bay or Cairngarroch Bay have been visited continue south along minor roads for 10 km or 5 km respectively to Clachanmore crossroads). At the crossroads continue SW for 2 km along the road and then the rough track (which may be gated) to Ardwell Bay. Parking is available in a paved area with a picnic site (NX 071 449). This journey crosses the major sinistral Orlock Bridge Fault, which separates Ordovician greywackes to the north from Silurian greywackes to the south. Ardwell Bay is on the south (Silurian) side of the fault (Figure 45) and exposes quartz-rich greywackes, the Stinking Bight beds (Gala Group 5) of ''gregarius ''Biozone age (Figure 46).

At the north end of the sandy bay (NX 071 453), where a fence meets the coast, a series of eight upright F I folds are exposed over a 25 m section. [[File:P008482.jpg|300px|thumbnail|left|Ardwell Bay, N. side. Anticline in greywackes of the Garheugh Formation, Silurian. Fold hinge plunges gently south-west. P008482]]These tight to open folds are developed in interbedded mudstones and sandstones with beds of less than 50 cm thickness. The folds display characteristic F1 geometry with curvilinear hinges plunging gently to moderately SW and axial surfaces inclined steeply SE. A vertical fault in one synclinal hinge has a minor downthrow to NW. Cleavage fans are centred on the axial surface of the folds and show strong refraction between sandstones and mud-stones in all the hinges. This indicates that the cleavage is contemporaneous with folding, not superimposed later. In the second anticlinal hinge from the south a `finite neutral point' is particularly well formed in the mudstones. This represents a point of zero stress during fold deformation of the adjacent sandstones, now marked by the bifurcation of the cleavage in the mudstone. Clockwise rotation of the S1 cleavage by 10-20° out of the axial surface in plan view is particularly well displayed. Bedding/ cleavage intersection lineations are less steep than fold plunge (or have a reversed plunge) on SE-younging limbs and plunge more steeply than the hinge on NW-younging limbs. This has produced downward facing bedding/cleavage relationships on some NW-younging limbs. Clockwise transecting S1 cleavage is common throughout the Silurian rocks of the Rhins and is believed to result from sinistral transpression acting during D1 deformation.

Return south along the beach past the parking area and continue to the south side of Ardwell Bay. The strata here are overturned so that the gently dipping bedding planes are the inverted bases of greywacke turbidite beds. These carry an impressive array of bottom structures and it is worth spending some time examining them under low tide conditions. Thereafter continue south and take the footpath up the slope past the ruined fortification of Doon Castle on Ardwell Point (labelled ''Broch ''on some OS maps). [[File:P008490.jpg|300px|thumbnail|right|S. side of The Hoolies, Ardwell Bay. Steeply-plunging fold in thin-bedded greywackes of the Garheugh Formation, Silurian. P008490]]This path leads round the top of the cliff skirting the Hooies inlet (NX 069 446). The agile can scramble down to beach level where dark graptolitic shales with pale bentonite layers are apparently interbedded among the Gala Group greywackes. Graptolites may be collected from several of the intertidal outcrops and prove the ''gregarius ''Biozone (Gala 5; Figure 46). If the graptolite localities are visited it is possible, with care, to climb out of the bay on the south side. Otherwise follow the cliff path around the back of the bay and then descend to the promontory on its south side. The bedding on the south side of the Hooies is steeply inclined and youngs to the south but on the headland it is folded about vertical hinges in a sinistral sense. The slaty cleavage developed subparallel to the bedding is also folded about the same hinges whilst maintaining its angular relationship to the bed-cling planes. Two generations of deformation are thus evident here: the main cleavage forming event was probably related to the fold and thrust episode (D1) with subsequent sinistral shear (D3) responsible for the steeply plunging hinges. If the steeply plunging hinge zone is followed seaward as far as possible the final exposure shows bedding, cleavage and fold hinge all cut across by a thin (and definitely post-tectonic) lamprophyre dyke. This instructive outcrop therefore also provides evidence for the local relationship of deformation and intrusion.

From the Hooies return via the cliff path to Ardwell Bay.

==== 7. Drumbreddan Bay: Gala and Moffat Shale groups ====
[[File:RhinsOfGallowayDrumbreddanBayExcursionMap.jpg|400px|thumbnail|Locality map and outline geo logy for the Drumbreddan Bay area (Locality 7) .]]
Drive back to Clachanmore crossroads, turn right and proceed SE for 1.5 km to a prominent left-hand bend. Turn right at the bend and continue first south and then SW for 2 km to Drumbreddan Farm where permission to park and to visit Drumbreddan Bay should be obtained. Walk west through the farm for 150 m to the track which leads past the cattle sheds (083 439). Follow this track SW for 600 m to the coast.

This locality ''is ''part of the Drumbreddan Bay Imbricate Zone and provides remarkable structural and sedimentological exposure across three imbricate thrust slices of Moffat Shale, each overlain by Gala Group greywackes of the Grennan Point Formation (Gala 6, Figure 46). The Moffat Shales acted as a decollement during thrusting and so form the lowest beds exposed. A rich graptolite fauna may be recovered from the shales (Excursion 18, Locality 4). Bedding youngs north but is overturned with a steep dip SE. Like most Gala Group lithologies the Grennan Point Formation has a siliceous petrography.

The southernmost imbricate thrust slice is exposed in a 100 m-wide promontory between two bays at the end of the track (7a in Figure 50). As the Moffat Shales exposed at the SE edge of the promontory are only visible at low spring ride, proceed NW across the bay to examine the more accessible exposure within the Grennan Bay Site of Special Scientific Interest (Treagus, 1992). Along the NW edge of the bay (NX 077 437) (7b) Birkhill Shales (Moffat Shale Group) of the gregarius and convolutus biozones young into the overlying Grennan Point Formation to the NW. The black fissile Birkhill Shales contain numerous pale bentonite layers (originally volcanic ash), one of which has been dextrally imbricated in response to a minor post-D2 steeply plunging dextral fold. The progradational sedimentary sequence from black shales into coarse well-graded turbidite beds with load, scour and tool (sole) markings, takes place over 8 m. Transitional parallel-laminated, shales and silt-stones with rare cross-lamination yield graptolites of convolutus Biozone age. Flute casts indicate flow derivation from the NE, a trend supported by the general NE—SW alignment of groove casts.

The nature of turbidite sedimentation can be examined in more detail at the edge of Grennan Point (NX 076 437) (7c). The coarse- to fine-grained turbidites are well graded and in places display complete Bouma Ta, sequences. Convolute and cross-lamination are present in Tc divisions, and load structures are common on bedding soles. Amalgamation of beds is observed in places. Thickening-upward cycles range from 5 m to 20 m, and 80 m above the base of the formation a 20 m-thick sequence of shales and thin base-absent (Tcde, Tce) turbidites is present. The facies associations at Grennan Point indicate progradation from a basin plain and outer fan into a mid-fan lobe environment. The latter predominates throughout the succession apart from a temporary regression to outer fan deposits marked by the shaly section 80 m above the base.

At the north end of Grennan Point (NX 075 438) an inclined Fl synclinal hinge with a steeply inclined axial surface (7d) is deformed by the fault forming the base of the next imbricate thrust slice (7e). Beyond this fault a 100 m-thick outcrop of Moffat Shale is again overlain by the Grennan Point Formation. The structures associated with major D1 thrusting can be seen in detail. The fault at the base of the slice appears to dip steeply SE and contains brecciated greywacke lenses within a sheared black shale (7e). The S1 cleavage is in places disturbed, so is earlier than at least some of the movement. No conclusive movement indicators are apparent, although the synclinal hinge SE of the fault, and subvertical slickensides within the fault zone, suggest a south-easterly downthrow compatible with the regional stratigraphy. The fault is itself displaced by a series of small post-D2 sinistral wrenches trending NNE.

Within the Moffat Shales beyond the fault a series of neutrally verging F1 folds with wavelengths less than I m plunge gently NE (70. A few steeply plunging folds are also present. At this locality S1 cleavage is axial planar to the folds (cf. Locality 6 Ardwell Bay). About 20 m NW of the fault an antiformal hinge within the Moffat Shales has had its SE limb removed by a subvertical fault (thrust?) developed preferentially along a 12 cm bentonite horizon(7g). The incompetent nature of bentonite relative to black shale is demonstrated by extreme thickening of the bentonites in the fold hinge. In the most north-westerly 25 m of this Moffat Shale outcrop a 12 m-thick structural inlier of black Lower Hartfell Shales and pale grey Barren Mudstones of the Upper Hartfell Shales (both of Ordovician age) are exposed NW of a steep SE-dipping fault (7h). Graptolites collected from the Lower Hartfell Shales indicate a ''wilsoni'' or ''clingani'' Biozone age. These beds young north into the Silurian Birkhill Shales thus exposing the Ordovician–Silurian boundary. Graptolites ranging from the persculptus to the ''atavus'' Biozone have been identified in the Birkhill Shales here. A synform in the Birkhill Shales south of the fault is compatible with the regional stratigraphy in suggesting a downthrow to the SE. However, local biostratigraphical evidence indicates some, possibly late, movement with downthrow to the NW (cf. Excursion 18, Locality 4).

The boundary between the Moffat Shale sequence and overlying Grennan Point Formation to the NW is a poorly exposed fault (7i). The fault has a steep south-easterly dip and separates black Moffat Shales from grey siltstones. Brecciated lensoid greywackes set in a sheared silty matrix are present for 8 m north of the boundary, beyond which are seen NW-younging brecciated greywackes of the Grennan Point Formation.

==== 8 Port Logan: Gala Group ====
[[File:P008467.jpg|350px|thumbnail|right|Port Logan, sea cliffs on S. side of bay. Tertiary dolerite dyke. Cuts orthogonally across thin greywacke beds of the Lower Silurian Port Logan Formation. Greywackes are vertical, strike 060 degrees south-east. Dyke trends north-west to south-east. P008467]]From Drumbreddan Farm drive along minor roads to join the A716 at Ardwell. Follow this road south for 2.5 km and then take a right turn for Port Logan. This road leads past the renowned Logan Botanical Gardens, which are well worth a visit. At Port Logan drive along the seafront and park by the pier at the SW corner of the bay (NX 095 404). Beneath the bay, but not exposed, is the Port Logan Fault, a major DI tectonic boundary separating north-younging, SE-verging folds and thrusts to the north, from south-younging, NW-verging fold and thrust structures to the south.

The outcrop extending SW from the car park is formed by well-bedded greywackes of the Port Logan Formation (Gala 8, Figures 45 and 46). These rocks retain a diagenetic mineralogy (Merriman et al., 1991), making the grade of metamorphism here one of the lowest recorded in the Rhins of Galloway area (see Introductory chapter for further discussion). Turbidite features are well developed, both in terms of Bouma divisions and of bottom and top bed-surface structures. A short walk SW along the coast into the next small bay will in addition traverse the following features:

# A cross-cutting, Tertiary dolerite dyke 2 m thick forms a vertical wall trending NW within a small inlet. Note the unusual honeycomb weathering of the dyke surface.
# A zone of open folding with one well-preserved open synclinal hinge and evidence of bedding imbrication in the limbs.
# Sequences of dark, interlaminated siltstone and shale up to 5 m thick interbedded with the greywacke.
# A spectacular array of ripple marks on large, steeply dipping slabs at the south side of the small bay (NX 091 401).

Retrace the coastal route to Port Logan.

==== 9 Clanyard Bay: Gala and Moffat Shale groups ====
[[File:RhinsOfGallowayClanyardBayExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the Clanyard Bay area (Locality 9).]]
To reach the next locality, Clanyard Bay, continue SE along the B7065 for 3.5 km to an offset crossroads. Turn right and drive for 800 m to just beyond the point where the road bends sharply south. Branching off to the right is the track to Low Clanyard Farm (NX 107 376). Park carefully at the road adjacent to the track, or drive to the farm where permission should be obtained to visit Clanyard Bay. This locality provides evidence for the reversed sense of D1 thrusting and allows a comparison with the Drumbreddan Bay Imbricate Zone (Locality 7).

The Moffat Shale outcrops at Clanyard Bay are locally rich in graptolites and the locality is also described in Excursion 18, Locality 5.

One important difference in interpretation is adopted here to that shown on the BGS 1:50 000 Rhins of Galloway geological map and described by Stone (1995). The Cairnharrow Formation shown therein as a part of the Hawick Group is considered by McCurry (1989) to be the most southerly Gala Group unit (Gala 9, Figure 46). McCurry places the northern boundary of this unit to the north of Clanyard Bay and thus overlaps with a part of the Gala 8 unit shown on the BGS map. The critical area is examined in this excursion at locality 9 and visitors may judge for themselves.

From Low Clanyard Farm follow the track first NW and then west for 800 m past the derelict mill to Clanyard Bay. Walk to the north end of the bay (NX 101 381) where there is an exposure of Moffat Shale 60 m thick (Figure 51). This forms part of the 1.5 km-wide Clanyard Bay Imbricate Zone within the south-younging sequences south of Port Logan. The Moffat Shale exposure is divided in two by an east-west-trending felsitic dyke 6 m thick. The northern margin of the dyke is formed by a steep SE-dipping fault. South of the dyke the Moffat Shales are folded by a large synformal F1 hinge plunging gently west (9a in Figure 51). This hinge folds a faulted contact between intensely sheared Birkhill Shale in the core of the fold and Barren Mudstone (Upper Hartfell Shale) away from the core. The fault is interpreted as an early thrust formed within the Moffat Shale decollement prior to folding. The northern limb of the synform is displaced by a late-D1 fault dipping steeply NW. At the core of the synform the Birkhill Shales range from ''atavus ''to a possible ''gregarius ''Biozone age. The surrounding Barren Mudstones are variably red, grey, blue or brown in colour and contain numerous irregularly shaped siderite nodules of diagenetic origin. The red mudstones have been reduced to produce a buff or green reaction rim around each nodule. Both F1 and post-F1 (probable F2) isoclinal folds are present in the Barren Mudstones.

North of the dyke Birkhill Shales are exposed for 30 m and have an anomalous WNW bedding strike. Graptolites indicate an age range spanning the ''atavus ''to the ''sedgwickii ''biozones in an overall northyounging sequence. At the northern boundary of the Birkhill Shales two prominent felsitic dykes, each 1 m thick, are spaced about 2 m apart. The shales are intensely sheared and contain brecciated greywacke lenses. This sheared zone continues for about 7 m north of the dykes to a distinct gouge plane with an associated S1 fabric intruded by a thin (10 cm) felsitic dyke (9b). The attitudes of the fabric and breccia zones marginal to the fault indicate a downthrow to the NW. An adjacent large synclinal F1 hinge plunging gently NE has had its SE limb sheared and removed by the fault consistent with NW downthrow. There is therefore evidence for two phases of thrusting associated with the Moffat Shale decollement: the first was probably pre-El folding and the second was syn- or post-Fl folding. [[File:P521155.jpg|300px|thumbnail|right|A fossil specimen of Monograptus limatulus. A fossil graptolite. (Graptolithina.) Clanyard Bay, south side, Wigtownshire, Scotland. This specimen of Monograptus limatulus is from the convolutus Biozone of the Silurian and was found on the south side of Clanyard Bay. British Geological Survey Biostratigraphy Collection number GSE 14773. Monograptus is characterized by a rhabdosome with a single stipe which could be straight or curved. It is uniserial (stipes consisting of a single series of thecae) and scandent (stipes grow upwards from the sicula with the thecae growing outwards). Monograptus is entirely a Silurian form. Graptolites were most common where major upwelling currents brought nutrients from deep water into the shallows, for instance at the edge of the continental shelf. Figd.Geology of the Rhins of Galloway sheet 1 and 3, fig 8 h. P521155]]The NW downthrow is the opposite to that suggested earlier within the Drumbreddan Bay Imbricate Zone (Locality 7) and is atypical of the Southern Uplands generally. It is consistent though with the SE-younging and NW-vergence of DI structures south of the Port Logan Bay Fault (McCurry and Anderson, 1989).

From the north end of Clanyard Bay (100 382) continue WNW across the southyounging turbidites of the Clanyard Bay Formation (Gala 9, Figs. 45, 46). Graptolites collected from rare black shale interbeds indicate a ''turriculatus ''or ''crispus ''Biozone age. After 30 m an upright Fl anticlinal hinge with a moderate westerly plunge is reached (9c). Some of the irregular joints developed in the hinge contain rare radiating sheaves of haematite. About 5 m north of the anticlinal hinge the beds are once again intensely brecciated in association with a number of moderate to steeply plunging post-Fl folds of wavelength 1-2 m.

Continue WNW for 70 m across a fence and over a prominent grassy ridge to a small bay (9d). On an island in the bay a superbly exposed NW-verging chevron fold pair is developed in overturned south-younging strata. The regular geometry of the fold pair represents in microcosm the structure demonstrated throughout the south-younging sequences south of the Port Logan Fault. The extreme angularity of the hinges and the straightness of the limbs characterise major fold structures, which dramatically increase in wavelength to over 1.5 km away from the boundaries of individual thrust sheets. The overturned bedding on the long limbs of folds is inclined steeply NW whereas the right-way-up flat limbs are inclined gently NW. These flat limbs are often intensely folded.

In the cliff at the north end of the bay another NW-verging Fl fold pair affecting thinly bedded turbidites can be examined in detail. A thrust is developed in the synclinal hinge and progressively excises the short limb westwards. The same style of chevron folding continues north for 400 m to the sole thrust of the sequence just north of Dunbuck (NX 094 382). Within this zone the south-younging strata are intensely brecciated and disrupted by numerous faults. There are few precise movement indicators on the faults, but fold geometry, younging directions and stratigraphy all suggest downthrow to NW. Note once again the major regional difference in structure between these sections and the north-younging sequences seen further north.

Return to the south end of Clanyard Bay, to about 150 m beyond the mouth of the burn at Clanyard Mill. Here the Moffat Shales are divided in two by an ENE fault which forms a small gully through the exposure (9e). North of the fault gully a complex sequence of Upper Hartfell Barren Mudstones and Birkhill Shales (visible only at low tide) appears to young north despite disruption by a series of strike-parallel faults. To the south of the fault gully Birkhill Shales are isoclinally folded with hinges plunging gently to moderately NE or SW. Some folds display clockwise cleavage transection of up to 8°. Graptolites indicate ''typhus ''and ''convolutus ''Biozone ages (Excursion 18, Locality 5). At the south margin of the Moffat Shale outcrop a prominent east—west felsite dyke dips steeply north. Beyond the dyke intensely brecciated south-younging greywackes are present.

==== 10 West Tarbet: Hawick Group ====
[[File:RhinsOfGallowayWestTarbetExcursionMap.jpg|400px|thumbnail|Locality map and outline geology for the West Tarbet area (Locality 10).]]
From Low Clanyard continue south and then east on the minor road to join the B7041 at Kirkmaiden. Drive south on the B7041 for 3 km and take the left-hand fork for the Mull of Galloway. On reaching the narrow isthmus at West Tarbet (Figure 52) turn right and park at the side of the road next to the track (142 309). This locality must be visited within three hours either side of low tide.

The platform between the bays at East and West Tarbet '''(10a '''in Figure 52) is covered by a layer of rounded pebbles and may be a late-glacial kame terrace. A small area of raised beach is visible in the bay below. The isthmus has formed by erosion along the major ENE trending Tarbet Fault. From the platform walk down to the northern foreshore of the bay at West Tarbet (NX 140 309). The position of the fault is indicated by a felsitic mass intruded along it and exposed in the centre of the bay '''(1013). '''The pale green Hawick Group lirhologies exposed both north and south of the fault are turbidites, but much finer grained than those of the Gala Group seen at the previous three localities, and with a calcareous matrix. They are part of the Carghidown Formation and north of the fault form the Leucarron Member of McCurry (1989). This is a sequence of fine-grained -Tace, Tade, Tee, and Tde turbidites with thick mudstone interbeds intercalated with packets of coarser channelised Tade, Tace turbidites.

For 120 m along the northern shore of the bay the strata are contorted into a series of unusual tight-to-open, post F1 folds '''(10c). '''With increased distance from the Tarbet Fault the folds develop a strong sinistral vergence and plunge moderately to steeply SE. They approach a parallel style, with little or no hinge thickening, and have a chevron geometry. Towards the fault they become increasingly brecciated, develop strong coaxial refolding and plunge moderately NE. Although they fold the SI cleavage, only a few hinges have developed a weak coaxial crenulation cleavage. [[File:P008512.jpg|350px|thumbnail|left|West Tarbet, Mull of Galloway. Steeply-plunging minor folds in greywackes of the Hawick Group, Silurian. P008512]]Examples of this planar crenulation can be seen in folds within the cliffs 30 m east of the prominent shallow cave (10d); since the cleavage is axial planar to the folds its orientation varies with the hinge orientation. Commonly both hinge and cleavage are coaxially refolded by later uncleaved folds of the same deformation, showing that this weak cleavage is an early-formed feature. These distinctive folds are probably linked with isomorphic folds seen elsewhere in the southern Rhins which are spatially associated with major late D I thrusts. The folds at West Tarbet are restricted to a 120 rn-wide zone between the Tarbet Fault and a major north—south-trending sinistral wrench fault. The latter structure has been eroded to form the prominent Mull Glen (NX 139 310) within which the folds are spectacularly exposed '''(10e).'''

Proceed over the promontory on the west side of the glen, crossing overturned, south-younging beds on the steep limb of a major NW-verging F1 fold pair. On the far side of the promontory, a bedding sole is covered in large well-formed flute casts (10f).''' '''Like most sole current structures in the Leucarron Member these indicate palaeoflow from the SE. This contrasts with the NW- and NE-derived currents that typify the Hawick Group elsewhere in the Southern Uplands. Ripples on a bedding surface on the opposite side of a small bay (10g) indicate palaeoflow at a high angle to that of the flute casts. Thisstrong divergence in flow direction between sole markings and ripples is not uncommon.

From West Tarbet visitors may wish to continue to the Mull of Galloway lighthouse. This windswept spot is the most southerly point in Scotland and affords splendid views across to Ireland, the Isle of Man and English Lake District. There are numerous exposures of thinly bedded Carghidown Formation greywackes but the sea cliffs around the headland are high and extremely precipitous and should not be approached too closely.
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[[Category:6. The South of Scotland]]