OR/19/003 Results
| Kearsey, T, Lee, J R, and Gow, H. 2019. Buried valleys (onshore) - Version 1: scientific report and methodology. British Geological Survey Internal Report, OR/19/003. |
Historical dataset
The historical dataset contains interpretations of buried valleys from 96 different publications (Appendix 1) which range in age from 1926–2018. Due to their enigmatic nature and the lack of a systematic definition, buried valleys have been interpreted historically in a variety of ways. Sometimes the scientific rationale for their interpretation has been clearly outlined but this is not always the case. In some instances, different data sources interpret the same features differently. In cases of multiple interpretations no judgement has made and all interpretations are included. This enables the user to visualise the diversity of interpretations (Figure 6) and obtain a rudimentary understanding of uncertainty of their location.
The historical dataset identifies locations of buried valleys as far north as Aberdeen and as far south as Brighton, however within a specific feature interpretations often vary about the degree of interconnectivity within a feature (Figure 7). Many interpretations of buried valleys assume connectivity between individual data points enabling them to be illustrated as linear features. However, the base of many buried valleys — especially those formed or shaped subglacially, can be highly irregular reflecting a highly-dynamic relationship between flow regime and competence of the channel bed. Subsequently, interpretations of the interconnectivity of buried valleys should be considered with caution and within the context of the spatial distribution of the constraining data. The dataset can also be biased spatially by local areas of interest and boundaries between map sheets reflecting individual field geologist’s knowledge and interpretation.
Modelled thickness of buried valleys
Many of the buried valleys from the historical dataset correspond to linear features identified as areas of significant superficial thickening created from the boreholes dataset (Figure 8). In areas which include Newcastle, Northallerton, Liverpool and Manchester there is better apparent definition of the shape and depth of these features than is seen within the historical dataset. However, in upland areas, above 200 m O.D. (Ordinance Datum), there are an insufficient number of boreholes for the algorithms used to be able to identify buried valleys.
The borehole method also picks out larger non-linear areas of thickened superficial deposits which correspond to major Plio-Pleistocene basins. The most extensive of which occurs in East Anglia and corresponds to the Crag Basin. The Crag Basin was a major depositional centre between the Pliocene and early Middle Pleistocene forming a western extension of the Southern North Sea Basin (Rose et al., 2001[3]). It was drained into by several extensive river systems that drained central and eastern England with deposits of the Crag (marine) and Dunwich (fluvial) groups now considered to be ‘superficial’ deposits (McMillan et al., 2011[4] and McMillan & Merritt, 2012[5]). Several other non-linear areas of thickened superficial deposits also occur including the Cheshire Basin, Vale of York and Vale of Pickering. These correspond to areas that were either glaciated or lay adjacent to the margins of the Late Devensian glaciation and formed extensive ice dammed or proglacial lake basins (Clark et al., 2018[6]).
BGS’s digital borehole database contains many old or ambiguous boreholes in which the identification of the top of bedrock is ambiguous (Lawley and Garcia-Bajo 2009[7]). The screening process, described, removes features that were not c entred on more than 5 boreholes which contain 20 m or more of superficial deposits (Figure 9). This process was necessary to remove bull’s eyes in the dataset around isolated boreholes. Some of these deleted polygons are drumlins or areas of weathered bedrock, although, in low borehole density areas it may also remove single borehole that might indicate the presence of a buried valley. This issue is further compounded by the fact that commonly, geotechnical boreholes are terminated short of bedrock if the surface is not intersected within 20–30 m of the ground surface. So the centre of deep buried valleys may not be penetrated by boreholes. In the future as the BGS’s digital borehole database increases it may be possible to improve the resolution of this dataset and identify more buried valley features.
References
- ↑ BOOTH, K, BOOTH, S, and SLATER, C. 2012. BGS geological cross sections & Quaternary domains: user guidance notes.
- ↑ BOOTH, S, MERRITT, J, and ROSE, J. 2015. Quaternary Provinces and Domains — a quantitative and qualitative description of British landscape types. Proceedings of the Geologists’ Association, Vol. 126, 163–187.
- ↑ ROSE, J, MOORLOCK, B S P, and HAMBLIN, R J O. 2001. Pre-Anglian fluvial and coastal deposits in Eastern England: lithostratigraphy and palaeoenvironments. Quaternary International, Vol. 79, 5–22.
- ↑ MCMILLAN, A A, HAMBLIN, R J O, and MERRITT, J. 2011. A lithostratigraphical framework for onshore Quaternary and Neogene (Tertiary) superficial deposits of Great Britain and the Isle of Man. British Geological Survey, IR/06/094 (Nottingham).
- ↑ MCMILLAN, A A, and MERRITT, J W. 2012. A new Quaternary and Neogene lithostratigraphical framework for Great Britain and the Isle of Man. Proceedings of the Geologists' Association, Vol. 123, 679–691.
- ↑ CLARK, C D, ELY, J C, GREENWOOD, S L, HUGHES, A L, MEEHAN, R, BARR, I D, BATEMAN, M D, BRADWELL, T, DOOLE, J, and EVANS, D J. 2018. BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British — Irish Ice Sheet. Boreas, Vol. 47, 11–e8.
- ↑ LAWLEY, R, and GARCIA-BAJO, M. 2009. The National Superficial Deposit Thickness Model. (Version 5). British Geological Survey, Vol. (OR/09/049) 18pp.