page_title: Chew Valley
section_title: Geology
hierarchical_section_title: Chew Valley / History / Geology
caption_attribution_description: Map of Chew Valley. Topographical data derived from NASA SRTM, PD Roads, Rivers and location data derived from 1890 OS mapping, PD by Berne convention.
context_page_description: The Chew Valley is an area in North Somerset, England, named after the River Chew, which rises at Chewton Mendip, and joins the River Avon at Keynsham. Technically, the area of the valley is bounded by the water catchment area of the Chew and its tributaries; however, the name Chew Valley is often used less formally to cover other nearby areas, for example, Blagdon Lake and its environs, which by a stricter definition are part of the Yeo Valley. The valley is an area of rich arable and dairy farmland, interspersed with a number of villages.
  The landscape consists of the valley of the River Chew and is generally low-lying and undulating. It is bounded by higher ground ranging from Dundry Down to the north, the Lulsgate Plateau to the west, the Mendip Hills to the south and the Hinton Blewett, Temple Cloud and Marksbury plateau areas to the east. The valley's boundary generally follows the top of scarp slopes except at the southwestern and southeastern boundaries where flat upper areas of the Chew Valley grade gently into the Yeo Valley and eastern Mendip Hills respectively.
context_section_description: The western end of the area (around Nempnett Thrubwell) consists of the Harptree Beds which incorporate silicified clay, shale and Lias Limestone. Clifton Down Limestone, which includes calcite and dolomitic mudstones of the Carboniferous period, is found in the adjoining central band and dolomitic conglomerate of the Triassic period. There are two main soil types, both generally well-drained. The mudstones around the lakes give rise to fertile silty clay soils that are a dull dusky red colour because of their high iron content. The clay content means that where unimproved they easily become waterlogged when wet, and hard with cracks and fissures during dry periods. The main geological outcrops around the lake are mudstone, largely consisting of red Siltstone resulting in the underlying characteristic of the gently rolling valley landscape. Bands of Sandstone of the Triassic period contribute to the undulating character of the area. There are also more recent alluvial deposits beside the course of the River Chew. The transition between the gently sloping landscape of the Upper Chew and Yeo Valleys and the open landscape of the Mendip Hills plateau is a scarp slope of 75 to 235 metres (250–770 ft). The predominant formation is Dolomitic Conglomerate of the Triassic period. It formed as a result of desert erosion and weathering of the scarp slopes. It takes the form of rock fragments mainly derived from older Carboniferous Limestone cemented together by lime and sand which hardened to give the appearance of concrete. The northern boundary is formed by the sides of the Dundry Plateau where the most significant geological formation is the Inferior Oolite of the Jurassic period found on the higher ground around Maes Knoll. This overlays the Lower Lias Clay found on the adjoining slopes. The clays make a poor foundation and landslips are characteristic on the slopes. This area was once connected to the Cotswolds. The intervening land has subsequently been eroded leaving this outlier with the characteristics of the Cotswold Plateau. The unusual geological features have been recognised as Sites of Special Scientific Interest (SSSI) for their geological interest including Barns Batch Spinney, Hartcliff Rocks Quarry and Dundry Main Road South Quarry.
  The oldest geological formation in the valley is the Supra-Pennant Measures of the Carboniferous period. It is a significant feature towards the north-eastern part of the area and is represented by the Pensford Syncline coal basin, which formed part of the Somerset coalfield. It is a complex formation containing coal seams and is made up of clay and shales. The landscape is typically undulating and includes outcrops of sandstone. Most of the area around Stanton Drew have neutral to acid red loamy soils with slowly permeable subsoils. Soils to the eastern part of the area are slowly permeable clayey and fine silty soils. They are found on Carboniferous clay and shales typical of the Supra-Pennant Measures. They are frequently waterlogged where the topography dictates. They tend towards being acid and are brown to grey brown in colour. In the south and south east of the area there are coal measures which are sufficiently near the surface for coal mining to have taken place around Clutton and High Littleton. In the eastern area of the valley as the River Chew flows through Publow, Woollard and Compton Dando before joining the River Avon at Keynsham there are alluvial deposits of clay soils.
caption: Topographical map of the Chew Valley
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