MediaWiki - User contributions [en]

History of geological research, Orkney and Shetland

2020-08-17T17:42:28Z

Dr. J.M.Ridgway:

{{OSRG}}
__FORCETOC__

== Shetland ==
=== Early accounts ===
The earliest references to the rocks of Shetland are records of the occurrence of useful minerals, ores and semi-precious stones. Thus George Low, who visited the islands in 1774, recorded the presence of talc and ‘ironstone’ (?serpentine) in Unst, fuller’s earth and copper ore in Fetlar and bog iron ore on Vaila. A record of the presence of iron ore in Dunrossness and of copper ore being mined in southern Shetland first appeared in the Statistical Account of Scotland (Sinclair 1793<ref name="Sinclair">SINCLAIR, J. 1791–99. The Statistical Account of Scotland. 21 vols. Edinburgh. </ref>, vol. V, p.186, and Vol. VII, p.393). The first attempt at a systematic account of Shetland geology is that of Jameson (1798<ref name="Jameson 1798">JAMESON, R. 1798. An Outline of the Mineralogy of the Shetland Islands, and of the Island of Arran. Edinburgh. </ref>, 1800<ref name="Jameson 1800">JAMESON, R. 1800. Mineralogy of the Scottish Isles. 2 vols. Edinburgh (Shetland, vol. 2, 185–224). </ref>), who commented on the dependence of the topography on the strike of the rocks and recognised that the Walls Sandstone is related to the sandstones and conglomerates seen along the coast of south-east Mainland. Jameson described many of the metamorphic minerals of Shetland and all the metallic ores, except chromite, which are known today. Further accounts of Shetland mineralogy and geology were produced by Traill (1806)<ref name="Traill">TRAILL, T S. 1806. Observations, chiefly mineralogical, On the Shetland Islands, made in the course of a tour through these islands in 1803. A Journal of Natural Philosophy, Chemistry and the Arts. (Editor Wm. Nicholson). 15, 353–67. </ref> and Fleming (1811<ref name="Fleming">FLEMING, J. 1811. Mineralogical Account of Papa Stour, one of the Zetland Islands. Mem. Wernerian Nat. Hist. Soc., 1, 162–75.</ref> and in Shirreff 1817<ref name="Shirreff">SHIRREFF, J. 1817. General View of the Agriculture of the Shetland Islands. Edinburgh. </ref>). These also deal with the working of copper and iron ores in south-east Mainland and the occurrence of copper in Fair Isle.

The most important of the early works on the geology of Shetland are those of Hibbert (1819–22)<ref name="Hibbert 1819">HIBBERT, S. 1819–20. Sketch of the Distribution of Rocks in Shetland. Edinb. Phil. Jnl, 1, 296-314; 2, 67–79, 224–42. </ref>, which include the first geological maps of the islands. Hibbert dealt with the distribution and structural relationship of the Shetland rocks, and also noted the presence of brucite and chromite. He drew attention to the great induration of the Walls Sandstone, which he called ‘primitive quartz rock’. Ami Boué’s (1820)<ref name="Ami">BOUÉ, AMI. 1820. Essai géologique sur L’Écosse. Paris. </ref> account of Shetland geology incorporated the observations of the earlier workers, and attempted the first correlation of the metamorphic rocks of Shetland with those of Scotland, equating the gneisses of Mainland, Yell and Unst with the Lewisian of Sutherland and the Hebrides, and the chlorite-schists and clay slates of Shetland with the Dalradian. He recognised the similarity between the sandstones and conglomerates of Shetland and the sedimentary rocks of the Moray Firth-Caithness-Orkney terrain and pointed out that the rocks of Esha Ness and Papa Stour resemble the products of extinct volcanoes. Other early works dealing with Shetland are those of MacCulloch (1821<ref name="Macculloch 1821">MacCULLOCH, J. 1821. The Elements of Practical Geology, London. 644–55. </ref>, 1831<ref name="Macculloch 1831">MacCULLOCH, J. 1831. A System of Geology. 2 vols. London, 1, 33, 70, 158, 208-10, 283; 2, 151, 158, 162, 169, l77, 193-7. </ref>) and Nicol (1844)<ref name="Nicol">NICOL, J. 1844. Guide to the Geology of Scotland. Edinburgh. (The Zetland or Shetland Islands, pp. 243–246). </ref>.

=== Old Red Sandstone ===
Fossil plants of Old Red Sandstone age were first discovered at Lerwick and Bressay (Tufnell 1853<ref name="Tufnell">TUFNELL, H. 1853. Notice of the discovery of fossil plants in the Shetland Islands. Q. Jnl geol. Soc. Lond., 9, 49. </ref>; Hooker 1853<ref name="Hooker">HOOKER, J D. 1853. Note on the Fossil Plants from the Shetlands. Q. Jnl geol. Soc. Lond., 9, 49–50. </ref>; Murchison 1853<ref name="Murchison 1853">MURCHISON, R I. 1853. Note of the Age and Relative Position of the Sandstone containing Fossil Plants at Lerwick in the Shetland Isles. Q. Jnl geol. Soc. Lond., 9, 50–1. </ref>, 1859<ref name="Murchison 1859">MURCHISON, R. I. 1859. On the Succession of the Older Rocks in the Northernmost Counties of Scotland: with some Observations on the Orkney and Shetland Islands. Q. Jnl geol. Soc. Lond., 15, 353–418. </ref>) and these were thought to belong to the Upper Old Red Sandstone. Later Peach and Home (1879a<ref name="Peach 1879a">PEACH, B N, and HORNE, J. 1879a. The Old Red Sandstone of Shetland. Proc. R. Phys. Soc. Edinb., 5, 80–7. </ref>, in Tudor 1883<ref name="Tudor">TUDOR, J. 1883. The Orkneys and Shetland; Their Past and Present State. London. </ref>; Geikie 1879<ref name="Geikie 1879">GEIKIE, A. 1879. On the Old Red Sandstone of Western Europe. Trans. R. Soc. Edinb., 28, 345–452. </ref>) found plants of Psilophyton type in the Walls Sandstone and thus proved that this formation is of Old Red Sandstone age. They also established a stratigraphic sequence in the Old Red Sandstone of east Shetland and described the Old Red Sandstone volcanic rocks of west Shetland (Peach and Home 1879a<ref name="Peach 1879a"></ref>, 1884<ref name="Peach 1884">PEACH, B N, and HORNE, J. 1884. The Old Red Volcanic Rocks of Shetland. Trans. R. Soc. Edinb., 32, 359–88. </ref>). Fossil fish from south-east Mainland were first recorded by Flett (1908)<ref name="Flett 1908">FLETT, J S. 1908. On the age of the Old Red Sandstone of Shetland. Trans. R. Soc. Edinb., 46, 313–9. </ref> who suggested that the beds of this area may range in age from the Middle to the Upper Old Red Sandstone. Finlay (1926b<ref name="Finlay 1926b">FINLAY, T M. 1926b. The Old Red Sandstone of Shetland. Part I: South-eastern Area. Trans. R. Soc. Edinb., 54, 553–72. </ref>, 1930<ref name="Finlay 1930">FINLAY, T. M. 1930. The Old Red Sandstone of Shetland. Part II. North-western Area. Trans. R. Soc. Edinb., 56, 671–94. </ref>) produced comprehensive accounts of the Old Red Sandstone sedimentary, volcanic and plutonic rocks of Shetland and recorded the discovery of the important fish bed at Exnaboe in south-east Mainland. Much additional information was obtained during the mapping of the islands by the Geological Survey which took place between 1929 and 1934, the most important, perhaps, being the discovery by Knox of the Melby Fish ‘Bed’. The fauna of this ‘bed’ [The Melby Fish Bed recorded by Knox is now known to consist of two beds, separated by about 110 m of sediment (see [[Orkney and Shetland, summary of geology#Old Red Sandstone|Melby Fish Beds]])] was described by Watson (1934)<ref name="Watson">WATSON, D M S. 1934. Report on Fossil Fish from Sandness, Shetland. Mem. geol. Surv. Summ. Prog. for 1933, Part 1, 74–6. </ref> and shown to be similar to that of the Sandwick Fish Bed of Orkney. More recently Westoll (1937<ref name="Westoll 1937">WESTOLL, T. S. 1937. The Old Red Sandstone Fishes of the North of Scotland, particularly of Orkney and Shetland. Proc. Geol. Ass., 48, 13–45. </ref>, 1951<ref name="Westoll 1951">WESTOLL, T S. 1951. The Vertebrate-bearing Strata of Scotland. Rep. XVIII Int. Geol. Congr., Part 11, Great Britain, 1948, 5–21. </ref>) has produced a zonal correlation of the fish-bearing east Shetland sequence with the Devonian sequences in the Baltic and East Greenland regions. The account of the Old Red Sandstone in this handbook is largely based on the recent work of Mykura (l972b<ref name="Mykura 1972b">MYKURA, W. 1972b. The Old Red Sandstone sediments of Fair Isle, Shetland Islands. Bull. geol. Surv. Gt Br., No. 41, 1–31. </ref>, Mykura and Phemister 1976<ref name"Mykura 1976">MYKURA, W, and PHEMISTER, J. 1976. The geology of western Shetland. Mem. geol. Surv. Gt Br. </ref>).

=== Post-orogenic intrusions and mineralisation ===
Investigations into the plutonic and hypabyssal rocks of West Shetland by Peach and Home (1884)<ref name="Peach 1884">PEACH, B. N. and HORNE, J. 1884. The Old Red Volcanic Rocks of Shetland. Trans. R. Soc. Edinb., 32, 359–88. </ref> and Finlay (1930)<ref name"Finlay 1930">FINLAY, T M. 1930. The Old Red Sandstone of Shetland. Part II. North-western Area. Trans. R. Soc. Edinb., 56, 671–94. </ref> were greatly amplified by the Geological Survey mapping. Detailed petrographic studies of the riebeckite-felsite dykes of Northmaven were made by Phillips (1926)<ref name="Phillips 1926">PHILLIPS, F C. 1926. Note on a Riebeckite-Bearing Rock from the Shetlands. Geol. Mag., 63, 72–7. </ref> and Phemister (1950)<ref name="Phemister 1950">PHEMISTER, J, SABINE, P A. and HARVEY, C O. 1950. The riebeckite-bearing dikes of Shetland. Mineralog. Mag., 29, 359–73. </ref>. Mykura and Young (1969)<ref name="Mykura 1969">MYKURA, W. and YOUNG, B. R. 1969. Sodic scapolite (dipyre) in the Shetland Islands. Rep. Inst. geol. Sci., No. 69/4. </ref> and Mykura (l972c)<ref name="Mykura 1972c">MYKURA, W. 1972c. Igneous intrusions and mineralization in Fair Isle, Shetland Islands. Bull. geol. Surv. Gt Br., No. 41, 33–53. </ref> described the scapolite associated with shear zones and basic intrusions in the Sandsting Complex and in Fair Isle. Accounts of the plutonic complexes of Sandsting and Northmaven were produced by Mykura and Phemister (1976)<ref name="Mykura 1976">MYKURA, W, and PHEMISTER, J. 1976. The geology of western Shetland. Mem. geol. Surv. Gt Br. </ref> and by Pringle (1970)<ref name="Pringle">PRINGLE, I R. 1970. The structural geology of the North Roe area of Shetland. Geol. Jnl, 7, 147–70. </ref>.

=== Metamorphic rocks ===
Prior to 1929 the main work on the metamorphic and synorogenic igneous rocks of Shetland was carried out by Heddle (1878<ref name="Heddle 1878">HEDDLE, M F. 1878. The County Geognosy and Mineralogy of Scotland, Orkney and Shetland. Truro. </ref>, 1901<ref name"Heddle 1901">HEDDLE, M F. 1901. The Mineralogy of Scotland. 2 vols. Edinburgh. </ref>) whose primary task was the description of the minerals and mineral localities throughout the islands. Heddle recorded the presence of many rare minerals, some minerals new to science, and also the first known British occurrence of relatively common minerals such as chloritoid. He also produced detailed accounts of the geology of certain parts of Shetland and discussed the age and possible correlation of the Shetland metamorphic rocks. He suggested that the latter form a group found nowhere else in Scotland, being apparently older and more highly metamorphosed than the conglomerates, schists and gneissose rocks which overlie the Lewisian (i.e. presumably the Moinian) but not so ancient as the Lewisian gneiss.

More recent work on the metamorphic rocks of Shetland commenced with the petrographic studies of serpentines, metagabbros, epidotic granites and andalusite-schists by Phillips (1927-28), and was continued with the mapping of the entire outcrop by the Geological Survey (Summ. Prog. 1930 to 1935<ref name="Summ 1930">SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1930 (for 1929). Mem. geol. Surv. Gt Br., 81–6. </ref>)<ref name="Summ 1931">SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1931 (for 1930). Mem. geol. Surv. Gt Br., 65–74. </ref> <ref> name="Summ 1932">SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1932 (for 1931). Mem. geol. Surv. Gt Br., 61–3, 79, 157–66. </ref><ref name="Summ 1933">SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1933 (for 1932). Mem. geol. Surv. Gt Br., 77–80, 95–7. </ref><ref name="Summ 1934">SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1934 (for 1933). Mem. geol. Surv. Gt Br., 70–6, 91–2. </ref><ref name="Summ 1935">SUMMARIES OF PROGRESS OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN. 1935 (for 1934). Mem. geol. Surv. Gt Br., 67–9, 82–3. </ref>. As a result of this survey Read (1933<ref name="Read 1933">READ, H H. 1933. On quartz-kyanite rocks in Unst, Shetland Islands, and their bearing on metamorphic differentiation. Mineralog. Mag., 23, 317-28. </ref>, 1934a<ref name="Read 1934a">READ, H H. 1934a. On the segregation of quartz-chlorite-pyrite masses in Shetland igneous rocks during dislocation-metamorphism, with a note on the occurrence of boudinage-structure. Proc. Lpool. geol. Soc., 16, 128–38. </ref>, b<ref name="Read 1934b">READ, H H. 1934b. On zoned associations of antigorite, talc, actinolite, chlorite, and biotite in Unst, Shetland Islands. Mineralog. Mag., 23, 519–40. </ref> and c<ref name="Read 1934c">READ, H H. 1934c. The Metamorphic Geology of Unst in the Shetland Islands. Q. Jnl geol. Soc. Lond., 90, 637–88. </ref>, 1937<ref name="Read 1937">READ, H H. 1937. Metamorphic Correlation in the Polymetamorphic Rocks of the Valla Field Block, Unst, Shetland Islands. Trans. R. Soc. Edinb., 59, 195–221. </ref>) published a series of classic papers dealing with the geology and polymetamorphism of Unst, the phases of retrograde metamorphism in the Valla Field Block of north-west Unst, and the development of zoned bodies from ultrabasic intrusions. Further work on the ultrabasic rocks of Unst was carried out by Amin (1952<ref name="Amin 1952">AMIN, M S. 1952. Metamorphic Differentiation of Talc-Magnesite-Chlorite Rocks in Shetland. Geol. Mag., 89, 97–105. </ref>, 1954<ref name="Amin 1954">AMIN, M S. 1954. Notes on the Ultrabasic Body of Unst, Shetland Islands. Geol. Mag., 91, 399–406. </ref>), on augen gneisses by Fernando (1941)<ref name="Fernando">FERNANDO, L J D. 1941. Petrology of Certain Felspathized Rocks from Herma Ness, Unst, Shetland Islands. Proc. Geol. Ass., 52, 110–30. </ref> and on chloritoid—schists by Snelling (1957<ref name="Snelling 1957">SNELLING, N J. 1957. A contribution to the mineralogy of chloritoid. Mineralog. Mag., 31, 469–75. </ref>, 1958<ref name="Snelling 1958">SNELLING, N J. 1958. Further Data on the Petrology of the Saxavord Schists of Unst, Shetland Isles. Geol. Mag., 95, 50–6. </ref>). Our present knowledge of the stratigraphy, structure and metamorphic history of eastern Shetland is, however, based largely on the work of Flinn, who, in a series of important papers, has described and interpreted the structural and metamorphic history of Delting (1954) and south Shetland (1967a), the nappe structure of Unst and Fetlar (1958, 1959) and the tectonic evolution of the Funzie Conglomerate of Fetlar (1956) and the Muness Phyllite of Unst (1952). In conjunction with Miller (Miller and Flinn 1966<ref name="Miller 1966">MILLER, J A, and FLINN, D. 1966. A Survey of the Age Relations of Shetland Rocks. Geol. Jnl, 5, 95–116. </ref>) he has produced the first integrated account of the geology of Shetland, in which the major tectonic, metamorphic and igneous events are placed in a time perspective and in which the main groups of metamorphic rocks are tentatively correlated with their possible eiuivalents on the Scottish mainland and in Scandinavia. May (1970)<ref name="May 1970">MAY, F. 1970. Movement, Metamorphism and Migmatization in the Scalloway Region of Shetland. Bull. geol. Surv. Gt Br., No. 31, 205-26. </ref> has established a sequence of metamorphic, migmatitic and structural events in the rocks of the Scalloway region, and, with Flinn (Flinn and others 1972<ref name="Flinn 1972">FLINN, D., MAY, F., ROBERTS, J. L. and TREAGUS, J. E. 1972. A revision of the stratigraphic succession of the East Mainland of Shetland. Scott. Jnl Geol., 8, 335–43. </ref>), he has re-interpreted the stratigraphy of the East Mainland Succession. The metamorphic rocks west of the Walls Boundary fault have been described by Pringle (1970)<ref name="Pringle"></ref>, Phemister (1976)<ref name="Phemister 1976">PHEMISTER, J. 1976. The Lunnister Metamorphic Rocks, Northmaven, Shetland. Bull. geol. Surv. Gt Br., in press. </ref> and Mykura (in Mykura and Phemister 1976<ref name="Mykura 1976"></ref>).

=== Pleistocene and Recent ===
The first integrated account of the glaciation of Shetland is that of Peach and Home (1879b)<ref name="Peach 1879a"></ref>, who suggested that the islands were first overridden by Scandinavian ice from the north-east, but that they later nourished their own ice cap. The former presence of Scandinavian ice was confirmed by the presence of an erratic of tonsbergite in south-east Mainland, which was recorded by Finlay (l926a)<ref name="Finlay 1926a">FINLAY, T M. 1926a. A Töngsbergite Boulder from the Boulder-clay of Shetland. Trans. Edinb. geol. Soc., 12, 180.</ref>. Robertson (1935)<ref name="Robertson 1935">ROBERTSON, T. 1935. The Glaciation of Aithsting, South Nesting, Whalsay and the Out Skerries. Geological Survey Records, (unpublished). </ref> and Flinn (1964<ref name="Flinn 1964">FLINN, D. 1964. Coastal and Submarine Features Around the Shetland Islands. Proc. geol. Ass., 75, 321–39. </ref>, 1967b<ref name="Flinn 1967b">FLINN, D. 1967b. Ice front in the North Sea. Nature, Lond., 215, 1151–4. </ref>) have, however, suggested that during the last glacial maximum Scandinavian ice crossed only the extreme south and north of the Shetland Islands and that over the rest of the area a locally-nourished ice cap prevailed. The changes in sea level around Shetland since the last glaciation have been studied by Flinn (1964)<ref name="Flinn 1964">FLINN, D. 1964. Coastal and Submarine Features Around the Shetland Islands. Proc. geol. Ass., 75, 321–39. </ref> and Hoppe (1965)<ref name="Hoppe 1965">HOPPE, G. 1965. Submarine peat in the Shetland Islands. Geogr. Annlr., 47A, 195–203. </ref>, and a ﬂoral sequence in the Shetland peats was established by Lewis (1907<ref name="Lewis 1907">LEWIS, F J. 1907. The Plant Remains in the Scottish Peat Mosses. III. The Scottish Highlands and The Shetland Islands. Trans. R. Soc. Edinb., 46, 33–70. </ref>, 1911<ref name="Lewis 1911">LEWIS, F J. 1911. The Plant Remains in the Scottish Peat Mosses. IV. The Scottish Highlands and Shetland, with an Appendix on Icelandic Peat Deposits. Trans. R. Soc. Edinb., 47, 793–833. </ref>). Chapelhow (1965)<ref name="Chapelhowe">CHAPELHOW, R. 1965. On Glaciation in North Roe, Shetland. Geogrl. Jnl, 131, 60–70. </ref> recorded the presence of an interglacial peat deposit in north-west Mainland and Mykura has since found a similar bed in west Mainland. These peats have given <sup>14</sup>C dates of 35 000 to 40 000 BP and Birks and Ransom (1969)<ref name="Birks">BIRKS, H J B, and RANSOM, MAREE, E. 1969. An interglacial peat at Fugla Ness, Shetland. New Phytol., 68, 777–96. </ref> have suggested, on pollen analytical evidence, that they may be of Hoxnian age.

=== Economic geology ===
Records of early workings for copper and iron at and near Sand Lodge and at Garths Ness (south Mainland) appear in many accounts, the most important of which are by Traill (1806)<ref name="Traill">TRAILL, T S. 1806. Observations, chiefly mineralogical, On the Shetland Islands, made in the course of a tour through these islands in 1803. A Journal of Natural Philosophy, Chemistry and the Arts. (Editor Wm. Nicholson). 15, 353–67. </ref>, Fleming (in Shirreff 1817<ref name="Shirreff"></ref>), Heddle (1880)<ref name="Heddle 1880">HEDDLE, M F. 1880. The Geognosy and Mineralogy of Scotland. Mainland [Shetland], Foula, Fair Isle. Mineralog. Mag., 3, 18–56. </ref> and Dron (1908)<ref name="Dron">DRON, R. W. 1908. Iron and copper mining in Shetland. Trans. geol. Soc. Glasg., 13, 165–9. </ref>. The available data were summarised by Flett (in Macgregor and others 1920<ref name="Macgregor 1920">MacGREGOR, M, LEE, G W, WILSON, G V, with contributions by T ROBERTSON and J S FLETT. 1920. The iron ores of Scotland. Mem. geol. Surv. Gt Br. Min. Resources, 11, 217–20. </ref> and in Wilson 1921<ref name="Wilson 1921">WILSON, G V. 1921. The lead, zinc, copper and nickel ores of Scotland. Mem. geol. Surv. Gt Br. Min. Resources. Gt Br., 17. </ref>). A description of the Sand Lodge workings in 1929 was given by O’Dell (1939)<ref name="Odell">O’DELL, A C. 1939. The historical geography of the Shetland Islands. Lerwick. </ref>. The magnetite ore at Clothister Hill near Sullom Voe was discovered by D. Haldane and the exploratory work connected with the deposit has been described by Groves (1952)<ref name="Groves">GROVES, A W. 1952. Wartime Investigations into the Haematite and Manganese Ore Resources of Great Britain and Northern Ireland. Monograph 20–703, Ministry of Supply Permanent Records of Research and Development. </ref>. The history of the chromite workings in Unst has been documented by Sandison (1948)<ref name="Sandison">SANDISON, C G D. 1948. Historical account of the mineral workings in Unst, Shetland. Scottish Council (Development and Industry), Paper No. 49(1) (unpublished). </ref>. There is also an account of the more recent explorations for this mineral by Rivington (1953)<ref name="Rivington">RIVINGTON, J B. 1953. Recent Chromite Exploration in Shetland. Min. Mag. Lond., 89, 329–37.</ref>. Summaries of the workings in and distribution of non-metallic minerals, such as serpentine, talc and magnesite, have appeared in the economic memoirs of the Geological Survey (see Strahan and others 1916<ref name="Strahan 1916">STRAHAN, A., FLETT, J S, and DINHAM, C G. 1916. Potash-felspar—phosphate of lime—alum shales—plumbago or graphite—molybdenite—chromite—talc and steatite (soapstone, soap-rock and potstone)—diatomite. Mem. Geol. Surv. Gt Br. Min. Resources, 5. </ref>; Macgregor and others 1940<ref name="Macgregor 1940">MacGREGOR, M, and others. 1940. Synopsis of Mineral Resources of Scotland. Mem. geol. Surv. Gt Brit. Min. Resources, 33. </ref>; Wilson and Phemister 1946<ref name="Wilson 1946">WILSON, G V, and PHEMISTER, J. 1946. Talc, other magnesium minerals and chromite associated with British serpentines. Geol. Surv. Gt Br. Wartime Pamphlet, No. 9. </ref>), and in reports of the Mineral Resources Panel of the Scottish Council (1954a<ref name="Scottish 1854a">SCOTTISH COUNCIL (DEVELOPMENT AND INDUSTRY). 1954a. Report of the Mineral Resources Panel on Serpentine and Olivine-rock in Scotland. </ref>, b<ref name="Scottish 1954b">SCOTTISH COUNCIL (DEVELOPMENT AND INDUSTRY). 1954b. Report of the Mineral Resources Panel on Talc in Scotland. </ref>). A detailed assessment of the talc-magnesite deposits near Cunningsburgh was carried out by Bain and others (1971)<ref name="Bain">BAIN, J A, BRIGGS, D A, and MAY, F. 1971. Geology and mineralogical appraisal of an extensive talc-magnesite deposit in the Shetlands. Trans. Instn. Min. Metall., 80, B, 77–84. </ref>, and the distribution, composition and petrography of the worked and workable limestones of Shetland have been described in Geological Survey memoirs (Robertson and others 1949<ref name="Robertson 1949">ROBERTSON, T, SIMPSON, J B, and ANDERSON, J G C. 1949. The Limestones of Scotland. Mem. Geol. Surv. Gt Br. Min. Resources, 35, 171–5. </ref>; Muir and others 1956<ref name="Muir 1956">MUIR, A, HARDIE, H G M., MITCHELL, R L, and PHEMISTER, J. 1956. The Limestones of Scotland: Chemical Analyses and Petrography. Mem. geol. Surv. Gt Br. Min. Resources, 37. </ref>).

=== Geophysics and off-shore geology ===
The Institute’s regional geophysical survey of the Shetland Islands was carried out by McQuillin and Brooks (1967)<ref name="McQuillin 1967">McQUILLIN, R, and BROOKS, M. 1967. Geophysical surveys in the Shetland Islands. Geophys. Pap. No. 2, Inst. geol. Sci., 1–22.</ref> and Sheet 16 of the IGS 1:250 000 Aeromagnetic Map of Great Britain and Northern Ireland, which includes the sea area around Orkney and Shetland, was published in 1968. Flinn (l969a)<ref name="Flinn 1969a">FLINN, D. 1969a. A geological Interpretation of the Aeromagnetic Maps of the Continental Shelf around Orkney and Shetland. Geol. Jnl, 6, 279–92.</ref> has interpreted the data from this and has produced a map showing the possible geology and structure of the areas north of Scotland. Exploratory marine geophysical surveys by ‘Watts and Bott (Bott and Watts 1970<ref name="Bott" >BOTT, M H P, and WATTS, A. B. 1970. Deep Sedimentary Basins proved in the Shetland-Hebridean Continental Shelf and Margin. Nature, Lond., 225, 265–8.</ref>; Watts 1971<ref name="Watts">WATTS, A B. 1971. Geophysical investigations on the continental shelf and slope north of Scotland. Scott. Jnl Geol. , 7, 189–218.</ref>) have further added to our knowledge of the geology of the Shetland seas, and bathymetric investigations by Flinn (1964<ref name="Flinn 1964">FLINN, D. 1964. Coastal and Submarine Features Around the Shetland Islands. Proc. geol. Ass., 75, 321–39.</ref>, 1969b<ref name="Flinn 1969b">FLINN, D. 1969b. On the development of coastal proﬁles in the north of Scotland, Orkney and Shetland. Scott. Jnl Geol., 5, 393–9.</ref>) have contributed to the understanding of possible pre- and post-glacial movements of the land relative to sea level.

== Orkney ==
The earliest geological records of Orkney deal with the presence of lead ore (Speed 1666<ref name="Speed">SPEED, J. 1666. England, Wales, Scotland and Ireland described.</ref>) and minerals and rocks such as granite, limestone, ﬂagstone and millstone (Sinclair 1795<ref name="Sinclair 1795">SINCLAIR, J. 1791-99. The Statistical Account of Scotland. 21 vols. Edinburgh.</ref>) which were being worked on these islands. Jameson (1800)<ref name="Jameson 1800">JAMESON, R. 1800. Mineralogy of the Scottish Isles. 2 vols. Edinburgh (Shetland, vol. 2, 185–224).</ref> noted the presence of granitic rocks, overlain by breccia and ﬂagstones near Stromness, and recorded basalt ‘veins’ (i.e. dykes) at Yesnaby and basalt under sandstone at Shapinsay. He also described the conglomerate (‘sandstone breccia’) at Hegglie Ber in Sanday. A record of early collections of fossil fish appears in the Proceedings of the Orkney Natural History Society (1905), which tells of about 100 specimens exhibited in 1837 in the Stromness Museum.

A large collection of fossil fish by Professor Traill was identified by Agassiz (1833–43)<ref name="Agassiz 1833">AGASSIZ, L. 1833–43. Récherches sur les Poissons Fossiles, Neuchatel.</ref> who established the correlation of the Orkney ﬂagstones with those of Caithness. Localities from which fish were collected are mentioned by Clouston in the ‘New Statistical Account of Scotland’ (vol. 15, 1845). Widespread interest in the Orkney fish was awakened by Hugh Miller (1849<ref name="Millar 1849">MILLER, H. 1849. Footprints of the Creator. Edinburgh.</ref>, 1858<ref name="Miller 1858">MILLER, H. 1858. The cruise of the Betsey. Edinburgh.</ref>) who recorded and described several species of fish found by himself and a number of local collectors, and was able to identify forms which also occurred in the ﬂagstones of Caithness. Murchison (1859) noted that the Old Red Sandstone of Orkney could be divided into a lower ﬂagstone series and an upper sandstone series, and Geikie (1879)<ref name="Geikie 1879">GEIKIE, A. 1879. On the Old Red Sandstone of Western Europe. Trans. R. Soc. Edinb., 28, 345–452.</ref> recognised the unconformity at the base of the Hoy volcanic rocks, and ascribed the Hoy sandstones to the Upper Old Red Sandstone. He also regarded the sandstones of South Ronaldsay as the lateral equivalents of the similar rocks near John o’Groats and concluded that the Old Red Sandstone rocks of Orkney, Caithness, Ross and Cromarty were laid down in a great intermontane basin which he termed ‘Lake Orcadie’.

The geology of the northern islands of Orkney was first described by Heddle (1880)<ref name="Heddle">HEDDLE, M F. 1880. The Geognosy and Mineralogy of Scotland. Mainland [Shetland], Foula, Fair Isle. Mineralog. Mag., 3, 18–56.</ref> who showed that, as on Mainland and South Ronaldsay, the ﬂagstones are conformably overlain by a series of sandstones which are locally conglomeratic. He also noted that the strata forming these islands were folded into a well-marked syncline. Peach and Home (1880<ref name="Peach 1880">PEACH, B N, and HORNE, J. 1880. The Glaciation of the Orkney Islands. Q. Jnl geol. Soc. Lond., 36, 648–63.</ref>, in Tudor 1883<ref name="Tudor">TUDOR, J. 1883. The Orkneys and Shetland; Their Past and Present State. London.</ref>) produced the first comprehensive accounts of the geology of Orkney and confirmed most of the conclusions arrived at by Geikie and Heddle. They also described the glaciation of the islands (1880). Our knowledge of the fossil fish of Orkney is largely based on the monograph by Lankester and Traquair (1868–1914)<ref name="Lankester">LANKESTER, E R, and TRAQUAIR, R H. 1868–1914. The ﬁshes of the Old Red Sandstone of Britain. Mon. Pal. Soc.</ref>, and on a series of papers by Traquair, which have also formed the basis of correlation of the Sandwick Fish Bed with the Achanarras Limestone of Caithness and the fish beds of Cromarty. Flett (1897)<ref name="Flett 1897">FLETT, J S. 1897. On the discovery in Orkney of the John o’Groats Horizon of the Old Red Sandstone. Proc. R. phys. Soc. Edinb., 13, 255-7.</ref> produced the first detailed stratigraphy of Orkney, in which he recognised the Rousay Beds as a separate formation. He recognised the presence of the John 0’ Groats fish fauna in the Eday Beds and also described in detail the lamprophyric minor intrusions of Orkney (Flett 1900)<ref name="Flett 1900">FLETT, J S. 1900. The Trap Dykes of the Orkneys. Trans R. Soc. Edinb., 39, 865–905.</ref>. Work on the Stromness basement rocks and the geology of Stronsay was carried out by Steavenson (1928a<ref name="Steavenson 1928a">STEAVENSON, A. G. 1928a. Some Geological Notes on Three Districts in Northern Scotland. Trans. geol. Soc. Glasg., 18, 193–233.</ref> and b<ref name="Steavenson 1928b">STEAVENSON, A G. 1928b. The Geology of Stronsay Parish, Orkney. Proc. Orkney Nat. Hist. Soc., 1–19.</ref>) and the peat deposits of Mainland and Westray were examined by Erdtman (1924–29)<ref name="Erdtman 1924">ERDTMAN, G. 1924. Studies in the Micropalaeontology of the post-glacial Deposits in Northern Scotland and the Scotch Isles. Jnl Linn. Soc., Botany, 46, 449–504.</ref>.

The Geological Survey mapped the islands between 1927 and 1929 and a descriptive memoir was published in 1935. The officers engaged in this survey were Wilson, Edwards, Jones, Knox and Stephens. The fish remains collected during the survey were determined by Watson and the plants by Lang. Much of the present account is based on this memoir. Additional work on the fish remains has been carried out by Miles and Westoll (1963)<ref name="Miles">MILES, R S, and WESTOLL, T S. 1963. Two New Genera of Coccosteid Arthrodira from the Middle Old Red Sandstone of Scotland and their stratigraphical Distribution. Trans. R. Soc. Edinb., 66, 179–210.</ref> and the latter has also correlated the Orkney sequence with the marine Devonian succession (Westoll 1951)<ref name="Westoll 1951">WESTOLL, T S. 1951. The Vertebrate-bearing Strata of Scotland. Rep. XVIII Int. Geol. Congr., Part 11, Great Britain, 1948, 5–21.</ref>. More recently Kellock (1969)<ref name="Kellock 1969">KELLOCK, E. 1969. Alkaline basic igneous rocks in the Orkneys. Scott. Jnl Geol., 5, 140–53.</ref> has re-examined the alkaline igneous rocks associated with the Eday Beds. Fannin (1969<ref name="Fannin 1969">FANNIN, N G T. 1969. Stromatolites from the Middle Old Red Sandstone of western Orkney. Geol. Mag., 106, 77–88.</ref>, 1970<ref name="Fanin 1970">FANNIN, N G T. 1970. The sedimentary environment of the Old Red Sandstone of western Orkney. Ph.D. thesis, University of Reading (unpublished).</ref>) has carried out a detailed sedimentological study in the Stromness Flags, and Dr. J M Ridgway has similarly investigated the Eday Beds. Ridgway, J.M. The Sedimentology and Palaeogeography of the Eday Group, Middle Old Red Sandstone, Orkney. Ph.D thesis, University of London (unpublished). /ref) The [GS regional geophysical survey of Orkney was carried out by McQuillin (1968)<ref name="McQullian 1968">McQUILLIN, R. 1968. Geophysical surveys in the Orkney Islands. Geophys. Pap. No. 4, Inst. geol. Sci., 1–18.</ref>.

== Bibliography ==
[[Bibliography, Orkney and Shetland | Full bibliography list]]

[[Category:Orkney and Shetland]]

Middle Old Red Sandstone of Orkney, Eday Beds

2020-08-17T17:31:38Z

Dr. J.M.Ridgway: All attributions to "Miss" J M Ridgway should be changed to "Dr." J M Ridgway, in line with all other references and attributions. Same goes for ref to my Ph.D work in Brit regional Geology:oOkney and Shetland mem oir

{{OSRG}}
__FORCETOC__
== Introduction ==
[[File:P916222.jpg|thumbnail|Middle Old Red Sandstone Fish. P916222.]]
[[File:P915567.jpg|thumbnail|Geological sketch-map of Orkney. P915567.]]
[[File:P915584.jpg|thumbnail|Geological map of Eday and parts of adjoining islands, showing structure of the Eday Syncline (b) Section A—B across the Eday Syncline. P915584.]]
[[File:P915585.jpg|thumbnail|Hypothetical section showing the possible lithological variations within the Eday Beds from north to south. P915585.]]
[[File:P915586.jpg|thumbnail|Measured section showing cyclic sequence in the Eday Flags, Rushacloust, west coast of Eday. P915586.]]
The Eday Beds consist of massive yellow or red sandstones which contain two major units in which sandstones alternate with either flags or marls. The subdivisions of the group are as follows (Maximum thickness in Eday, as measured by Dr. J. M. Ridgway, are as follows: L.E.S. 220 m, E.F. 33 m, M.E.S. 395 m, E.M. 103 m, U.E.S. 315 m. )

: [[Middle Old Red Sandstone of Orkney, Eday Beds#Upper Eday Sandstone|Upper Eday Sandstone]]
: [[Middle Old Red Sandstone of Orkney, Eday Beds#Eday Marls|Eday Marls]]
: [[Middle Old Red Sandstone of Orkney, Eday Beds#Middle Eday Sandstone|Middle Eday Sandstone]]
: [[Middle Old Red Sandstone of Orkney, Eday Beds#Eday Flags|Eday Flags]] (with [[Middle Old Red Sandstone of Orkney, Eday Beds#Eday Volcanic Rocks|Eday Volcanic Rocks]])
: [[Middle Old Red Sandstone of Orkney, Eday Beds#Lower Eday Sandstone|Lower Eday Sandstone]]
: [[Middle Old Red Sandstone of Orkney, Eday Beds#Passage Beds|Passage Beds]]

The total thickness of the Eday Beds generally exceeds 1000 m. The lower half of the group may be equivalent to the John 0’ Groats Sandstone of Caithness and the entire group has been included in the upper half of the Givetian stage of the topmost Middle Devonian.

Fish remains are rare in the sandstones, but the Eday Flags have yielded a highly characteristic fish fauna which includes ''Microbrachius dicki'' ([[Media:P916222.jpg|P916222]], 2) ''Pentlandia macroptera'' ([[Media:P916222.jpg|P916222]], 1), ''Tristichopterus alatus'' (P916222, 2) and ''Watsonosteus fletti''. This fauna has now been recorded in a number of localities in Orkney. It closely corresponds to that obtained from the fish beds at John O’ Groats. It also has some species in common with the Old Red Sandstone of south-east Shetland.

The Eday Beds form the greater part of the island of Eday, where they crop out in the relatively unfaulted core of the Eday Syncline ([[Media:P915584.jpg|P915584]]). They also give rise to considerable, though somewhat faulted, outcrops on Sanday, Stronsay, Shapinsay, East Mainland, Burray and South Ronaldsay. The outcrops extend over a distance of 58 km from north to south, and this makes it possible to assess the lithological variations within the group in this direction.

=== Passage Beds ===
The thickness of the passage beds underlying the Lower Eday Sandstone varies greatly. In Eday the transition from Rousay to Lower Eday lithology is fairly rapid and obvious but in the southern isles and on Mainland there is an apparent northward and westward thinning out of the Passage Beds from South Ronaldsay (possible maximum 260 m) and Deerness towards the western part of East Mainland, the south coast of West Mainland and north Hoy.

In the northern isles the Passage Beds consist of thin, in places reddish, sandstones interbedded with beds of marl and with calcareous flags. The latter have, in several islands, yielded ''Dipterus valenciennesi''. On the east coast of Deerness (East Mainland) the beds consist of thin units (up to 2 m thick) of thinly bedded purplish- or yellowish-weathering siltstone and sandstone alternating with thicker beds and lenses of massive sandstone which become more closely spaced upwards. In South Ronaldsay the Passage Beds form a thinly bedded sequence of yellow and reddish marls, flags and sandstones and on the shores of Long Hope in south-east Hoy they are made up of thin beds of hard fine-grained sandstone alternating with layers of green ‘marly’ flags with thin sandstone ribs.

The variation in the thickness and lithology of the Passage Beds suggests that they pass laterally by intercalation into either the Rousay flagstone or Eday sandstone facies. Such an interdigitation suggests an environment of deposition along the margin of a lake flat which was crossed by or impinged upon by the channels, flood plains and deltas of rivers. At first these distributaries were confined to small belts within the area, but later they advanced and spread out laterally to cover the entire district.

=== Lower Eday Sandstone ===
Over most of its southern outcrop the Lower Eday Sandstone consists of a bright yellow medium- to fine-grained cross-bedded sandstone, with relatively few small scattered pebbles, and with individual sets that rarely exceed 60 cm in thickness. Complete, unfaulted, sections of the sandstone are seen in Eday, where it ranges in thickness from 100 to 220 m and consists of two distinct portions. The lower of these, which forms from half to three-quarters of the sequence, is a medium- to coarse-grained predominantly reddish purple trough-cross-bedded sandstone. It contains scattered pebbles and lenses of conglomerate which commonly form lag deposits in the bottoms of the troughs. The pebbles are up to 7 cm in diameter, usually subrounded, and consist of pink granite and pegmatite, granitic gneiss, fine-grained quartzite, beige and red chert, clear and milky vein quartz, and lesser amounts of sandstone and hypabyssal igneous rock fragments. The upper part is a yellow, medium-grained sandstone which is generally devoid of pebbles and is characterised by large-scale planar and trough-cross-bedding. In the west of Eday this sandstone is resistant to weathering and, as it splits into large rectangular blocks, it has been quarried for freestone at Fersness Bay in Eday and on Fara. Palaeocurrent data indicate that in Eday the Lower Eday Sandstone was laid down by swift currents which entered the area from the south and south-west. On Sanday the lower facies consists of pebbly coarse- to medium-grained, yellow, orange and green sandstones. The pebbles have a maximum diameter of 13 cm and are either isolated or form lags in the basal layers of cross-bedded units. The upper facies consists of medium-grained white and pale green, often mottled, sandstones with some small isolated pebbles.

As the sandstone is traced south-eastward through Stronsay into Shapinsay it becomes progressively less pebbly and predominantly yellow in colour. On Mainland good sections are seen on the south and south-east coasts of Deerness and there are extensive exposures along the north shore of Scapa Flow ([[Media:P915567.jpg|P915567]]). In Deerness the sandstone is 200 to 215 m thick, and consists of massive, predominantly yellow, medium-grained sandstone with generally small- to medium-scale cross-bedding and only rare scattered pebbles. A feature of the sandstone exposed in Deerness and many other outcrops is the high proportion of sets with some form of convolute or slumped bedding. The thickness of sandstone exposed along the north shore of Scapa Flow, on the downthrow side of the North Scapa Fault, is estimated at 550 m, which is much greater than is usual in this group. It is possible that the sandstone here includes not only the Lower Eday Sandstone but also higher members of the Eday succession. This would imply that the horizon of the Eday Flags is entirely represented by sandstone. Alternatively, the increased thickness of sediment could be due to vertical movements along the fault taking place contemporaneously with the deposition of the Lower Eday Sandstone. Tectonic activity of this kind would account for the abundance of disturbed bedding structures in the sandstone in this part of Orkney.

In South Ronaldsay the Lower Eday Sandstone consists of soft, medium- to fine-grained yellow cross-bedded sandstone with some bands of grey or yellow marly shale and siltstone and very rare pebbly lenses. The bands of fine calcareous sediment are most abundant in the southern outcrops, and on the east coast. Between Halcro Head (ND 476 856) and Wind Wick (ND 457 870), fish-bearing rhythmic units similar to those in the Eday Flags are common in the upper part of the sandstone, suggesting that this part of the sequence is perhaps better placed in the lower part of the Eday Flags. Similar rhythmic units with calcareous flags containing fish remains (usually ''Dipterus valenciennesi'', and ''Pentlandia'' sp.) have been recorded within the lower strata of the Lower Eday Sandstone of Flotta and these may therefore be considered as Passage Beds. In east Hoy the group consists of soft, yellow and reddish cross-bedded sandstone, and in north-west Hoy, where there are small outcrops of the sandstone at Sea Geo (ND 261 027) on the north coast and beneath the Upper Old Red Sandstone tuffs along the west coast between Rack Wick (ND 197 990) and Rora Head (ND 182 992), it is characteristically soft, sulphur-yellow, fine- to medium-grained, with thin cross-bedded sets, thin bands of marly flags and rare pebbly lenses. The Lower Eday Sandstone status of the Rack Wick outcrop is, however, open to question.

=== Eday Flags ===
The Eday Flags are a group of mixed sediments of very variable thickness, which appear to pass laterally by interdigitation into the Middle and possibly also the Lower Eday Sandstone ([[Media:P915585.jpg|P915585]]). They consist of rhythmic units which range in thickness from two metres to several tens of metres. Most of these units have the following two phases:

: 1. A fining-upward phase of buff, yellow or, more rarely, red sandstone and sandy siltstone. This phase ranges in thickness from less than 1 m to over 25 m.

: 2. A coarsening-upward phase of grey, black, and locally purple ‘flagstone’ which generally has a finely laminated fish-bearing ‘quiescent water’ facies at the base succeeded by most of the other units and lithological features found in the lacustrine parts of the cycles in the Stromness and Rousay Flags.

A measured section of three Eday Flagstone cycles in Eday is shown in [[Media:P915586.jpg|P915586]]. The cycles of this formation differ in several ways from the rhythmic units of the Stromness and Rousay Flags. The sandstone phases are in many instances much thicker and the exposed sections of many cycles have one or more thick, often red, channel-fill sandstones. These sandstones in places contain scattered pebbles and they are generally coarser-grained than the sandstones in the Stromness and Rousay Flags. Within the dark calcareous finely laminated fish-bearing siltstone-shale sequences near the base of the flagstone phases, one commonly finds bands of non-laminated pale grey silty mudstones which are 10 to 30 cm thick. These non-laminated beds may be the distal deposits of intermittent turbidity currents. Sun cracks and syneresis cracks are common in most siltstones and fine sandstones, but stromatolites have not been recorded. The thin sandstone ribs within the flagstone phase very commonly have load-cast bases, many are slumped, and some exhibit graded bedding.

The Eday Flags are thickest and best developed in and near Deerness (south-east Mainland), where they are up to 150 m thick and have a volcanic horizon near their base. Excellent exposures are seen on the west shore of Newark Bay (HY 567 036). In South Ronaldsay the thickness of the Eday Flags is also about 150 m, but the group contains a progressively higher proportion of sandstone as it is traced southward and westward from the north shore. In this island the flagstone phases of the cycles become more red, green and marly towards the south. Traced northwards from Deerness the formation diminishes rapidly in thickness to 100 m in Shapinsay, less than 50 m in Stronsay and south-west Eday, until in the Calf of Eday and Sanday the Lower and Middle Eday Sandstones are separated by only 10 m or so of flaggy sediments. This apparent thinning appears to be a facies change resulting from the progressive northward thickening and coarsening of the sandstone phases and the corresponding thinning out of the flagstone phases in the higher cycles of the group. This process of interdigitation can be most readily demonstrated on Eday. Thus at Rushacloust on the south-west shore of the island, the formation consists of a lower, 18 m-thick, section which consists of three cycles with relatively thin sandstones and well-developed grey fish-bearing flags, and an upper section, about 25 m thick, composed largely of red, cross-bedded pebbly sandstone but containing three thin beds of siltstone and flagstone. The number of cycles with fish beds and thin sandstones decreases northwards to two at Fersness Bay in the centre of the island and to one on the shores of Calf Sound and on Sanday. The thin beds of flagstone in the higher cycles die out completely northwards, but on the shore of Fersness Bay an isolated fish bed crops out some 110 m above the highest remaining cycle with a flagstone phase.

=== Eday volcanic rocks ===
A number of isolated outcrops of fine- to medium-grained basic igneous rocks associated with thin beds of tuff have been recorded in the basal beds of the Eday Flags at Deerness, Shapinsay, and Copinsay (HY 595 020). The igneous rocks were interpreted as lava flows by Wilson and others (1935)<ref name="Wilson 1935">WILSON, G. V., EDWARDS, W., KNOX, J., JONES, R. C. B. and STEPHENS, J. V. 1935. The Geology of the Orkneys. Mem. geol. Surv. Gt Br.</ref>, but Kellock (1969)<ref name="Kellock">KELLOCK, E. 1969. Alkaline basic igneous rocks in the Orkneys. Scott. Jnl Geol., 5, 140-53.</ref> has upheld Flett’s original concept (Flett 1898b)<ref name="Flett 1898b">FLETT, J. S. 1898b. The Old Red Sandstone of the Orkneys. Trans. R. Soc. Edinb., 39, 383-424.</ref> that the rocks exposed at Muckle Castle (Deerness) and the Black Holm of Copinsay are intrusions. The former is probably a volcanic plug and the latter a sill. The basalts of Haco’s Ness (Shapinsay), Point of Ayre (Deerness), and the inland exposures in Deerness are lava flows with scoriaceous tops, pipe-amygdales and sandfilled fissures. At least two flows are present and though one is very thin the other is at least 7 m thick. The intrusive rocks and the lavas are alkaline olivine-dolerites and basalts which contain interstitial analcime, natrolite and alkaline feldspar. They are petrographically similar to the Carboniferous teschenites and basanites of the Midland Valley of Scotland. The field and petrographic evidence, however, leaves no doubt that the lavas and intrusions are coeval and that both are of Middle Old Red Sandstone age.

=== Middle Eday Sandstone ===
The Middle Eday Sandstone ranges in thickness from possibly 400 m in Eday and Sanday to only about 90 min East Mainland and South Ronaldsay. This southward decrease in thickness appears to be due to the lateral passage of the sandstone facies into both Eday Flag and Eday Marl type lithology ([[Media:P915585.jpg|P915585]]). Within the beds mapped as Middle Eday Sandstone there is a marked reduction in overall grain-size and an increase in the proportion of marly sediment from both the north and the south towards Deerness. In Eday and south-west Sanday the sandstone consists essentially of reddish purple, principally trough-cross-bedded, medium- to coarse-grained gritty sandstone with scattered pebbles and conglomerate lenses. Conglomerates are particularly well developed at Hegglie Ber on the south-west coast of Sanday. The pebbles consist predominantly of coarse granite, quartzite and Vein quartz but throughout the entire thickness of the sandstone and in the overlying groups there are also an appreciable number of rounded clasts of porphyritic and spherulitic rhyolite and scoriaceous basic lava. A few silty and marly beds are present in the lower part of the sequence.

In Stronsay the formation is composed of red and yellow sandstones, with only a few scattered pebbles and with marly beds, which are particularly prevalent towards the base. In Deerness the only undoubted outcrop of the Middle Eday Sandstone is that exposed at Newark Bay (HY 569 040) where the strata immediately above the Eday Flags consist of a series of fining-upward cycles, each made up of a sandstone unit 50 cm to 2 m thick overlain by deep purple poorly laminated sandy siltstone and siltstone with thin ribs of commonly convoluted sandstone. Apart from their duller purple colour these beds are similar to the typical sediments of the Eday Marls. The sequence exposed at Newark Bay appears to pass upwards into more massive sandstones. West and south of Deerness, at Scapa Bay and Burray, the higher sediments of the formation are exposed, these being red, white, pink and yellow sandstones with thin marly bands. Still further south, in South Ronaldsay, the sediments ascribed to this formation consist largely of red and yellow cross-bedded sandstones with only rare pebbles and with thin bands of grey, red and yellow ‘marl’.

=== Eday Marls ===
This formation consists principally of thick beds of bright red and pale green calcareous sandy siltstone and pale green siltstone, alternating with thinner beds of hard, cross-bedded red and yellow sandstone. The sandstone bands and lenses become thicker and more closely spaced in the upper part of the sequence. The thickness of the group is over 200 m in South Ronaldsay, at least 150 min East Mainland and then decreases northwards to about 100 m in Eday and even less in Sanday. Again, much of this thinning may be due to the interfingering of the marl facies with the adjoining sandstone facies ([[Media:P915585.jpg|P915585]]).

The diagnostic feature of the formation is the bright red colour of the fine-grained sediments. The rhythmic units with their high proportion of marly siltstones are fining-upward cycles of the type generally thought to be formed by meandering streams with shifting channels and extensive overbank deposits. In Eday the channel sandstones in the lower part of the sequence are less than 2 m thick and are separated from each other by up to 15 m of red sandy micaceous siltstone which is poorly sorted and has some ripple marks, sun cracks and traces of bioturbation. There are also bands and zones with calcareous concretions. As the sequence is followed upward the ratio of sandstone to marl increases and there is a gradual passage into the overlying Upper Eday Sandstone. The Eday Marls also crop out in East Mainland where they form the Head of Holland Peninsula [490 120] just east of Kirkwall, as well as in Burray, Hunda [432 969] and the north-west of South Ronaldsay. In these areas the beds form cyclic units which are similar to those in the lower part of the Eday Marls sequence of Eday.

=== Upper Eday Sandstone ===
The Upper Eday Sandstone crops out in two areas: a northern area which comprises the outcrops in Eday and south-west Sanday, and a southern area around the south shore of Scapa Flow, which includes the outcrops on Burray, north-westem South Ronaldsay and Flotta. In the southern area the group consists of beds of soft, cross-bedded, red, pink and yellow sandstone alternating with bands of red and green ‘marl’. The latter are most common and thickest at the base of the formation. In Eday, where the group reaches a thickness of more than 300 m, it falls into two lithological divisions. The lower division consists of beds of red and yellow sandstones with scattered pebbles, which pass upwards into more massive trough-cross-bedded pebbly sandstones alternating with beds of marl. The pebbles and sedimentary structures in the latter are closely comparable to those found in the Middle Eday Sandstones. The upper division shows a reversion to finer-grained sediments and consists of bright red and purple sandstones with thin bands of suncracked sandy marl.

=== Conditions of deposition ===
No modern account of the lithology and palaeogeography of the Eday Beds has as yet been published, and the following suggestions as to the environment of deposition and direction of palaeocurrents are based on observations made by the author and Dr N. G. T. Fannin during a brief spell of field work in 1972.

The sandstones and red ‘marls’ of the Eday Beds were deposited in the channels, alluvial fans and flood plains of fairly large rivers which entered the area from the south-west and either filled up the Orcadian lake or at least encroached upon its margin. The presence within the Eday Flags of grey thinly bedded lacustrine flagstones, which alternate with channel sandstones and lake delta deposits, indicates that there was a time when the waters of a lake again covered the greater part of the Orkney area. This lake was fed by swifter and more active rivers than those which entered the lake at the time the Stromness and Rousay flags were laid down. They regularly built out deltas into the lake and these pushed its margin north-eastwards beyond the limits of the present area. The lithology of the higher beds of the Eday group indicates that a fluvial regime was soon re-established, first in the north, but then over the whole area. The Eday Sandstones were probably laid down by swift, braided and straight rivers which formed alluvial fans, but the Eday Marls were formed in the channels and alluvial plains of slower meandering streams. The appearance of swift-flowing rivers is probably the result of differential vertical movements along faults which were responsible for the tectonic uplift of the source area or areas.

This article is based to a large extent on the work of Dr N. G. T. Fannin (Fannin 1970)<ref name="Fannin 1970">FANNIN, N. G. T. 1970. The sedimentary environment of the Old Red Sandstone of western Orkney. Ph.D. thesis, University of Reading (unpublished).</ref> and the section on the Eday Beds incorporates data supplied by Dr. J. M. Ridgway ref name="Ridgway 1974" , The Sedimentology and Palaeogeography of the Eday Group, Middle Old Red Sandstone, Orkney. Ph.D. thesis, University of London (unpublished)./ref. In consequence the classiﬁcation used in this account differs in some respects from that published by Wilson and others (1935)<ref name="Wilson 1935"></ref>. Information provided by Mr U. McL. Michie of the Institute’s Geochemical Division has been used throughout article.

== Bibliography ==
[[Bibliography, Orkney and Shetland | Full bibliography list]]
[[Category:Orkney and Shetland]]