Bristol and Gloucester district - The Costwold Hills

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Oolitic limestones of the Inferior Oolite Group exposed on Leckhampton Hill, near Cheltenham. P902608.
The De la Beche Unconformity. Horizontally bedded Jurassic limestones (Inferior Oolite) rest upon grey folded Carboniferous Limestone, Vallis Vale, near Frome. P902609.

This area forms the eastern part of the region and is dominated by the steep western edge of the Cotswolds, extending from Stow-on-the-Wold-in the north, to Cirencester, Tetbury and south to Bath. The area also includes the less well-defined extension to the areas around Frome and Wincanton and an isolated upland area around Crewkerne in the extreme south of the area.

Younger sedimentary bedrock

The Cotswold Hills consist largely of relatively young rocks, comprising a sequence of oolitic limestones (Inferior and Great Oolite groups) and mudstones. Oolitic limestone is a particular type of limestone formed of rounded grains of calcium carbonate precipitated in warm tropical seas.

Being strong and resistant to erosion, these limestones cap the Cotswold Hills (Plate P902608) and locally with the associated mudstones they reach 150 m thick. In the Bath area, they include the famous ‘Bath Stone’ that was extensively quarried for building stone, with shallow mines extending under many of the hills to the south and east of Bath, particularly around Combe Down and Box, and extending east to the Corsham area. These limestones peter out around Wellow and are replaced to the south mainly by mudstones. This change in rock-type is reflected in the landscape, with the high edge of the Cotswold Hills becoming less obvious to the south and merging into the lower, gentler hills around Frome. Across the east Mendips near Frome, the limestones rest directly on the underlying steeply inclined Carboniferous Limestone. South of the Mendips, a thicker sequence of finer-grained limestones and mudstones was deposited in quieter, deeper-water conditions. The limestones form an important aquifer in the Cotswold Hills, and are heavily used for public water supply. At depth beneath the limestones are the Lias deposits and then the Mercia Mudstone and Sherwood Sandstone described above. These do not occur at surface in the Cotswolds except in the deeply incised river valleys around Bath and Frome.

Older sedimentary bedrock

The Coal Measures, Carboniferous Limestone and Old Red Sandstone are present beneath the western margin of the Cotswold Hills (Plate P902609). Between Yate, Wickwar, Bath and the eastern end of the Mendip Hills, the Carboniferous Limestone is present. To the east, progressively older rocks occur at depth beneath the cover of younger sedimentary layers. The depth to the top of these older rocks occurs between 500 and 1500 m in the western Cotswolds, descending eastwards to 3 km around Devizes and 2.5 km around Cheltenham. The detailed structure and rock types present at these depths in this area is not known.