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.
[[Image:P902245.jpg|thumb|400px|Geological sketch map and key showing the range and distribution of different rock types in the Bristol and Gloucester region, in relation to the major towns and cities. The extent of the Bristol and Gloucester region is identified on the inset map of the United Kingdom. P902245.]]
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 P902245''' 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
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.
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.
==Severn Valley and Somerset Levels==
This area occupies much of the low-lying ground west of the Cotswolds, including the Severn Valley, the margins of the Severn Estuary, and the Somerset Levels together with the gently rolling hills and vales of south Somerset and north Dorset in the southern part of the region. The main settlements are Gloucester, Cheltenham, Stroud, Weston super Mare, Bridgwater, Glastonbury and Yeovil.
===Younger Sedimentary Bedrock===
The uppermost parts of the Younger Sedimentary Bedrock comprise grey mudstones and thin limestones overlain by sandstones. These are referred to as the Lias and were deposited in a deep sea environment around 200 million years ago. These rocks are widespread at the surface, for example in much of central Somerset, forming the Polden Hills and those around Somerton,Langport, and the Isle of Wedmore. The same layers are well exposed on the North Somerset coast between Watchet and Kilve, where the limestones contain many fossils. Around Glastonbury, these rocks were quarried for flagstones. The same layers are also at the surface along the Severn Valley, at the foot of the Cotswold Hills. Here the upper part of the Lias Group is formed of sandstones but south of the Mendips, there is a rapid change into mudstones. Across central Somerset and west into the Bristol Channel and in the Gloucester-Worcester area, over 500m of the Lias is preserved. These rocks generally occur within 100m of the surface across most of the region, but they descend to depths of 300 m in the Bridgwater area and to 200 to 400 m around Cheltenham and Gloucester.
Beneath the Lias are two layers of predominantly red coloured rocks (Figure 6) that were deposited under arid desert conditions around 255 to 210 million years ago. The upper part comprises the reddish Mercia Mudstone. Across much of Somerset it is up to 450m thick. These rocks locally include layers of gypsum, which is used as a fertiliser and the main constituent of many forms of plaster, and rock salt minerals.
However, north of Gloucester, the Mercia Mudstone reaches a thickness of 1200 to1500 m. The lower part is composed of the slightly older sandstones of the Sherwood Sandstone and these form a very important aquifer. They are only found at the surface in this area around Huntley, west of Gloucester, but they occur widely at depth beneath the overlying layers. They reach a maximum thickness of about 1500 m in the far north of this area, are absent in the Bristol-Mendip area, and reappear at depth further south around Bridgwater where they are at least 170 m thick.
===Older Sedimentary Bedrock===
The depth to the top of the older sedimentary bedrock layers in this area ranges from a few hundred metres around the margins of the Severn Estuary, to 2.6 km in the Gloucester-Cheltenham area. Farther east, the depth gradually increases to over 3 km around Devizes. To the south of the Mendips, the Carboniferous Limestone descends to about 1-1.5 km beneath the Somerset Levels, before rising back up the surface near Bridgwater on the eastern side of the Quantock Hills. However, the extent, depth and thicknesses of these older rocks including the Coal Measures beneath the Somerset Levels and the Severn Valley are poorly known.
==The Cotswold Hills==
[[Category:Regional Geology overview | 16]]
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.
[[Category:Geological_regions_of_England | ]]
===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.
[[Category:Regional Geology | 16]]
Latest revision as of 09:40, 19 April 2016
Geological sketch map and key showing the range and distribution of different rock types in the Bristol and Gloucester region, in relation to the major towns and cities. The extent of the Bristol and Gloucester region is identified on the inset map of the United Kingdom. P902245.
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 P902245 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.
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.
