Category:16. Bristol and Gloucester district

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This account provides a broad perspective of the geology of the Bristol and Gloucester region, covering most of Somerset, Bristol, Bath, south Gloucestershire and the Forest of Dean. Figure 1 provides a geological sketch map and key of this region showing the rock types occurring in relation to the major towns and cities. The region is one of the most geologically varied parts of the country, with almost every geological time period represented. This account outlines the geology to a depth of at least a kilometre and summarises the current and historical use of the geological resources in the area.

It is perhaps no coincidence that William Smith, the ‘Father of English Geology’ produced the World’s first geological map in this region, based on the country around Bath. The region also hosts some well known geological sites including Cheddar Gorge, the Bath Hot Springs and the Avon Gorge. Exploitation for mineral resources including lead, zinc, iron, coal and limestone (for aggregate and building stone) has taken place since Roman times. Consequently, there is a long heritage of geological research. The surface geology is reasonably well known, particularly from surface outcrops and especially in coalfield areas where records from coal mines, deep boreholes and exploration surveys provide information on the geology up to 300 m below the surface. Although geophysical seismic surveys which provide information on the rocks by sending sound waves through the ground have provided some data, the lack of deep boreholes means much of the geological structure and details of the rocks themselves below this depth remains poorly understood.




The Cotswold Hills

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 (Figure 7) and locally with the associated mudstones they reach 150m 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 (Figure 8). 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 3km around Devizes and 2.5km around Cheltenham. The detailed structure and rock types present at these depths in this area is not known.

Subcategories

This category has the following 3 subcategories, out of 3 total.
The number of included categories (C), pages (P) and files (F) is stated in brackets.