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Relationship to the superficial deposits thickness model

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{{OR/19/003}}
==Background==
Buried valleys are ancient sub-aerial (river) or subglacial drainage networks that are now abandoned and have become infilled by sediment so that they have little or no surface expression in the landscape. Their concealed occurrence can have significant and often unexpected implications for groundwater (e.g. Sandersen & Jørgensen, 2003<ref name="Sandersen 2003">SANDERSEN, P B, and JØRGENSEN, F. 2003. Buried Quaternary valleys in western Denmark—occurrence and inferred implications for groundwater resources and vulnerability. ''Journal of Applied Geophysics'', Vol. 53, 229–248. </ref>; Cloutier et al., 2008<ref name="Cloutier 2008">CLOUTIER, V, LEFEBVRE, R, THERRIEN, R, and SAVARD, M M. 2008. Multivariate statistical analysis of geochemical data as indicative of the hydrogeochemical evolution of groundwater in a sedimentary rock aquifer system. ''Journal of Hydrology'', Vol. 353, 294–313. </ref>; Seifert et al. 2008<ref name="Seifert 2008">SEIFERT, D, SONNENBORG, T O, SCHARING, P, and HINSBY, K. 2008. Use of alternative conceptual models to assess the impact of a buried valley on groundwater vulnerability. ''Hydrogeology Journal'', Vol. 16, 659–674. </ref>; Oldenborger et al., 2013<ref name="Oldenborger 2013">OLDENBORGER, G A, PUGIN, A-M, and PULLANS, S E. 2013. Airborne time-domain electromagnetics, electrical resistivity and seismic reflection for regional three-dimensional mapping and characterization of the Spiritwood Valley Aquifer, Manitoba, Canada. ''Near Surface Geophysics'', Vol. 11, 63–74. </ref>), hydrocarbon (e.g. Huuse et al., 2012<ref name="Huuse 2012">HUUSE, M, LE HERON, D, DIXON, R, REDFERN, J, MOSCARIELLO, A, and CRAIG, J. 2012. Glaciogenic reservoirs and hydrocarbon systems: an introduction. Geological Society, London, Special Publications, Vol. 368, SP368. 19. </ref>) and geothermal resources (e.g. Allen & Milenic, 2003<ref name="Allen 2003">ALLEN, A, MILNEIC, D, and SIKORA, P. 2003. Shallow gravel aquifers and the urban ‘heat island’ effect: a source of low enthalpy geothermal energy. ''Geothermics'', Vol. 32, 569–578. </ref>; Allen et al., 2003<ref name="Allen 2003"></ref>). Buried valleys can also contain significant quantities of sand and gravel mineral resources which can act as traps for contaminants as well as pathways into groundwater aquifers.

The British Geological Survey (BGS) has recognised and identified buried valleys through its survey activities since the 1870’s (Mellard Reade, 1873<ref name="Mellard 1873">MELLARD READE, T. 1873. The Buried Valley of the Mersey. Proceedings of the Liverpool Geological Society, Vol. 2, 53. </ref>) although no compiled data set has ever been produced. Since the 2000’s BGS has also published a Superficial Deposits Thickness Model (SDTM) which models variation in thickness of natural superficial deposits across Great Britain (Lawley and Garcia-Bajo, 2009<ref name="Lawley 2009">LAWLEY, R, and GARCIA-BAJO, M. 2009. The National Superficial Deposit Thickness Model. (Version 5). British Geological Survey, Vol. (OR/09/049) 18pp. </ref>). However, one limitation of this methodology is that it under-represents the spatial occurrence of linear features such as buried valleys (Kearsey et al., 2018<ref name="Kearsey 2018">KEARSEY, T I, WHITBREAD, K, ARKLEY, S, MORGAN, D, BOON, D, and RAINES, M. 2018. How accurate is your model between boreholes? Using shallow geophysics to test the best method to model buried tunnel valleys in Scotland, UK. Three-Dimensional Geological Mapping — Workshop Extended Abstracts. Vancouver, Illinois State Geological Survey 39.</ref>).

This work attempts to compile what is currently known about buried valleys from historic survey activities. It also employs a semi-automated method to identify areas of significant superficial deposits thickening based on our current onshore borehole dataset. It cannot be used to say where buried valleys are not present; but it does indicate the locations where based on the recorded evidence and expert knowledge a geologist has interpreted the presence of buried valley.

==Definition of a buried valley==
The British Geological Survey has no formal definition of a buried valley. The Encyclopaedia of Geomorphology (Goudie, 2013<ref name="Goudie 2013">GOUDIE, A. 2013. Encyclopedia of geomorphology. (Routledge.) ISBN 1134482760</ref>) defines a buried valley thus: “A buried valley is the bedrock expression of a valley buried by more recent deposits.”

They usually exhibit little or no surface expression in the landscape (Figure 1) and can only be verified using boreholes (e.g. Kearsey et al., 2019<ref name="Kearsey 2019">KEARSEY, T I, LEE, J R, FINLAYSON, A, GARCIA-BAJO, M, and IRVING, A A M. 2019. Examining the geometry, age and genesis of buried Quaternary valley systems in the Midland Valley of Scotland, UK. ''Boreas'', https://doi.org/10.1111/bor.12364.</ref>) or using geophysics (e.g. Sandersen & Jørgensen, 2003<ref name="Sandersen 2003">SANDERSEN, P B, and JØRGENSEN, F. 2003. Buried Quaternary valleys in western Denmark—occurrence and inferred implications for groundwater resources and vulnerability. ''Journal of Applied Geophysics'', Vol. 53, 229–248.</ref>). They can be formed by a range of processes including;

<ol style="list-style-type:lower-roman">
<li>Sub-aerial fluvial incision (e.g. Dyer, 1975<ref name="Dyer 1975">DYER, K. 1975. The buried channels of the ‘Solent River’, southern England. Proceedings of the Geologists' Association, Vol. 86, 239–245.</ref>; Swift et al., 1980<ref name="Swift 1980">SWIFT, D J, MOIR, R, and FREELAND, G L. 1980. Quaternary rivers on the New Jersey shelf: relation of seafloor to buried valleys. Geology, Vol. 8, 276–280.</ref>; Rose, 1989<ref name="Rose 1989">ROSE, J. 1989. Stadial type sections in the British Quaternary. 45–67 in Quaternary type sections: imagination or reality? ROSE, J S C. (editor). (Rotterdam: Balkema.)</ref>; Bozzano et al., 2000<ref name="Bozzano 2000">BOZZANO, F, ANDREUCCI, A, Gaeta, M, and Salucci, R. 2000. A geological model of the buried Tiber River valley beneath the historical centre of Rome. ''Bulletin of Engineering Geology and the Environment'', Vol. 59, 1–21.</ref>; Bridgland, 2010<ref name="Bridgland 2010">BRIDGLAND, D R. 2010. The record from British Quaternary river systems within the context of global fluvial archives. Journal of Quaternary Science: Published for the Quaternary Research Association, Vol. 25, 433–446.</ref>);</li>
<li>Glacial over-deepening to form U-shaped valleys, which can then become partly-submerged and filled by younger sediment (e.g. Holtedahl, 1967<ref name="Holtedahl 1967">HOLTEDAHL, H. 1967. Notes on the formation of fjords and fjord-valleys. Geografiska Annaler: Series A, Physical Geography, Vol. 49, 188–203.</ref>; Nesje & Whillans, 1994<ref name="Nesje 1994">NESJE, A, and WHILLANS, I M. 1994. Erosion of Sognefjord, Norway. ''Geomorphology'', Vol. 9, 33–45.</ref>);</li>
<li>Incision by subglacial meltwater beneath glaciers and ice sheets (e.g. Ó Cofaigh, 1996<ref name="Ó Cofaigh 1996">COFAIGH, C Ó. 1996. Tunnel valley genesis. ''Progress in Physical Geography'', Vol. 20, 1–19.</ref>; Præg, 2003<ref name="Præg 2003">PRAEG, D. 2003. Seismic imaging of mid-Pleistocene tunnel-valleys in the North Sea Basin—high resolution from low frequencies. ''Journal of Applied Geophysics'', Vol. 53, 273–298.</ref>; Hooke & Jennings, 2006<ref name="Hooke 2006">HOOKE, R L, and JENNINGS, C E. 2006. On the formation of the tunnel valleys of the southern Laurentide ice sheet. ''Quaternary Science Reviews'', Vol. 25, 1364–1372.</ref>; Lutz et al., 2009<ref name="Lutz 2009">LUTZ, R, KALKA, S, GAEDICKE, C, REINHARDT, L, and WINSEMANN, J. 2009. Pleistocene tunnel valleys in the German North Sea: spatial distribution and morphology [Pleistozäne Rinnen in der deutschen Nordsee: Verbreitung und Morphologie]. Zeitschrift der deutschen Gesellschaft für Geowissenschaften, Vol. 160, 225–235.</ref>; Kehew et al., 2013<ref name="Kehew 2013">KEHEW, A E, POTROWSKI, J A, and JØRGENSEN, F. 2012. Tunnel valleys: Concepts and controversies—A review. ''Earth-Science Reviews'', Vol. 113, 33–58.</ref>).</li>
</ol>

They may also be polygenetic in origin, having been formed by two or more of these processes (e.g. Huisink, 2000<ref name="Huisink 2000">HUISINK, M. 2000. Changing river styles in response to Weichselian climate changes in the Vecht valley, eastern Netherlands. ''Sedimentary Geology'', Vol. 133, 115–134.</ref>; Huuse & Lykke-Andersen, 2000<ref name="Huuse 2000">HUUSE, M, and LYKKE-ANDERSEN, H. 2000. Overdeepened Quaternary valleys in the eastern Danish North Sea: morphology and origin. ''Quaternary Science Reviews'', Vol. 19, 1233–1253.</ref>; Montgomery, 2002<ref name="Montgomery 2002">MONTGOMERY, D R. 2002. Valley formation by fluvial and glacial erosion. Geology, Vol. 30, 1047–1050.</ref>).

[[Image:OR19003fig1.jpg|thumb|center|400px| '''Figure 1'''    A stylised example of a buried valley. Note how it is offset from the modern river system. The sequence of sediment infilling the buried valley is purely illustrative and in reality the fill of a buried valley can vary greatly between valleys. ]]

There is no minimum depth or width for these buried valleys as described in the literature. It is evident from reviewing the published BGS memoirs and reports that the term ‘buried valley’ is used for a large range of features which range in depth from tens to hundreds of metres. Furthermore, the depth of the features, or the evidence upon which they have been identified with, is not often recorded in the reports.

The National Superficial Deposit Thickness Model (Lawley and Garcia-Bajo, 2009<ref name="Lawley 2009">LAWLEY, R, and GARCIA-BAJO, M. 2009. The National Superficial Deposit Thickness Model. (Version 5). ''British Geological Survey'', Vol. (OR/09/049) 18pp.</ref>) is currently derived from by 293 988 boreholes (Figure 2) that prove the thickness of superficial deposits across the Great Britain. These show that the average thickness of superficial deposits is 8.35 m and that 75% of the boreholes have superficial thicknesses of less than 10.50 m (Figure 3). Buried valleys themselves are continuous linear depressions within the dataset, whereas depressions that possess no clearly-defined linearity are classified as significant superficial thickenings. The latter probably correspond to broader basinal areas within the landscape.

[[Image:OR19003fig2.jpg|thumb|center|400px| '''Figure 2'''    Distribution of boreholes that prove the thickness of superficial deposits in the Great Britain. Contains Ordnance Data © Crown Copyright and database rights 2019. Ordnance Survey Licence no. 100021290. Created using ArcGIS. Copyright © Esri. ]]

[[Image:OR19003fig3.jpg|thumb|center|500px| '''Figure 3'''    Thickness of superficial deposits in boreholes across the Great Britain. ]]

==Objectives==
The objectives of this work are twofold:

* To compile a comprehensive dataset of where BGS historic survey activities and other published work has identified buried valleys onshore in the Great Britain.
* To use our current onshore borehole dataset to create a modelled thickness of buried valleys dataset.

==Who would benefit from the dataset?==
This dataset is for users who are seeking information about the locations of buried valleys across the Great Britain. These are important features for geologists, civil engineers, hydrogeologists and environmental scientists because they can have unexpected consequences for anyone interested in the position of bedrock or the thickness of superficial deposits. Equally this dataset will be of interest to the research community of the Great Britain as buried valleys provide detailed archives of palaeoenvironmental and landscape change.

===Relationship to the superficial deposits thickness model===
The BGS creates the National Superficial Deposit Thickness Model (SDTM) as a licenced product (Lawley and Garcia-Bajo 2009<ref name="Lawley 2009"></ref>). Although the Buried Valleys (onshore) data shares some of the same input data it is not meant as a replacement but compliments the existing data.

The key differences are:

* The Buried Valleys (onshore) data is presented at a coarser scale than the SDTM Model (1:250 000 compared to 1:50 000 of the SDTM).
* The all superficial deposits between 40–161 m are grouped together in the Buried Valley dataset but are in the SDTM Model.
* The Buried Valleys (onshore) data was created through an expert driven process specifically targeted at identifying buried valleys, which has removed superficial features such as drumlins and other mounds.

Please do not use this as the Buried Valley dataset as an indicator of superficial thickness. For superficial thickness please use the National Superficial Deposit Thickness Model (SDTM).

==References==
{{reflist}}

[[Category:OR/19/003 Buried valleys (onshore) - Version 1: scientific report and methodology | 04]]

OR/19/003 Introduction - Revision history

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