OR/16/056 Quaternary history of the study area

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Burke, H F, Gow, H V, Cripps, C, Thorpe, S, Hough, E, Hughes, L, and Horabin, C G. 2016. The 3D quaternary geology of the area around Thornton, Cheshire. British Geological Survey Internal Report, OR/16/056.

The Quaternary geology of north Cheshire is described in detail in Earp and Taylor (1986)[1] and Lee (2015)[2] and is summarised here. The study area is located on the north Cheshire coast, in the Cheshire/north Shropshire lowlands (Figure 2). This region was glaciated during the Late Devensian, approximately 20 000 years ago (Earp and Taylor, 1986[1]), with ice feeding in from the Irish Sea and the Lake District. This led to the deposition of a sequence of glacial sediments, up to approximately 60 m thick in the study area, primarily composed of till (gravelly clay) and glaciofluvial deposits (sand and gravel) with intervening laterally discontinuous glacial lake sediments (laminated clay and silt). The most widespread glacial unit is till, which was deposited by glacial ice. Till mantles the bedrock through the majority of the study area, with isolated ‘windows’ where superficial deposits are absent and the underlying Sherwood Sandstone bedrock occurs at surface.

Following deglaciation at the end of the Devensian, global sea levels rose to inundate low lying coastal areas. Sediments along the coast in the north of the study area are dominated by tidal flat deposits associated with the Mersey estuary. Alluvium and peat occur in the floodplain of the River Gowy that runs south to north through the study area to join the Mersey at Stanlow Point.

The underlying Sherwood Sandstone Group bedrock has been scoured by glacial, river and coastal erosion processes and therefore has an irregular undulating surface buried beneath Quaternary sediments. Boreholes in the area reveal thick accumulations of glacial sand and gravel that have scoured into the Sherwood Sandstone Group. These ‘buried valleys’ have no surface expression and are only revealed by borehole data. The top of the Sherwood Sandstone Group is weathered, in some cases to sand, making the distinction between weathered bedrock and glaciofluvial sediments difficult to define in some areas.

Figure 2    Location of Thornton-le-Moors, Cheshire and the topography of the surrounding area (from Lee, 2015[2]). Superficial geology (DigMapGB-50) is also shown as coloured tints: blue – glacial till, pink – glacial sand and gravel, orange – fluvial sand and gravel, yellow – river floodplain (alluvium) and coastal deposits. NEXTMap Britain elevation data from Intermap Technologies.

References

  1. 1.0 1.1 EARP, J R, and TAYLOR, B J. 1986. Geology of the country around Chester and Winsford. Memoir for 1:50 000 geological sheet 109. British Geological Survey publication.
  2. 2.0 2.1 LEE, J R. 2015. The Quaternary Geology of Thornton-le-Moors, Cheshire — a conceptual ‘systems’ approach. British Geological Survey open report no. OR/15/059.