Category:16. Bristol and Gloucester district: Difference between revisions

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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.
[[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.


==Geologically recent surface deposits==
The Bristol and Gloucester region lay just outside the extent of the ice-sheets that have dominated the evolution of the British landscape over the past 2-3 million years; so there are no extensive glacial sediments preserved within this region despite it experiencing prolonged periods of extreme cold. The dominant geological process during this time has been one of uplift of the land; this has led to the erosion of the landscape, with the main rivers cutting deep valleys and depositing sand and gravel deposits along their valley floors. Since the end of the last glacial event in Britain around 11,000 years ago, extensive deposits of clay and peat up to 10m thick have built up on the Somerset Levels and along the Severn Estuary.


==Bedrock geology==
Beneath the recent surface deposits, or with just a cover of soil where such deposits are absent, are older rocks which geologists broadly split into two distinct types:
* The sedimentary bedrock geology is composed of quite hard rocks which were deposited a few hundred to tens of millions of years ago as layers of sediments in shallow seas, deserts and vastriver systems in times when Britain lay closer to the Equator and the climate and landscape were very different from those of today.
* The basement geology, which underlies the bedrock, is over 415 million years old and mainly comprises harder, denser rocks which have been strongly compacted and folded. They include both rocks originally deposited as sediments and others that are products of volcanic activity.


In the course of the past 415 million years there have been periods when the area of the UK formed a landmass and was being eroded, and other periods when it was sinking and new layers of sediment were being deposited. The history of erosion and deposition has not been the same in all parts of the UK. In the Bristol and Gloucester region the older sedimentary bedrock was deposited from 415 to 305 million years ago as a series of sandstones, limestones and coal-bearing mudstones. These rocks give the region its most distinctive geological features including its famous gorges, caves and river valleys, as well as most of its natural resources including coal, hard rock for construction, lead, zinc and iron ore. These older rocks were uplifted, folded and faulted to form a complex series of up-folds (termed anticlines) and down-folds (synclines) during a major mountain building event around 280 million years ago (see Figure 2). Although only accounting for around a third of the surface outcrop in this region, these older rocks dominate the subsurface geology at depths below about 500 m.


Subsequently, a younger sequence of sedimentary rocks, including limestones, sandstones and mudstones, was laid down to form a variable, but relatively thin cover of gently tilted layers referred to in this account as the younger sedimentary bedrock. This younger sedimentary bedrock was deposited from 250 to 145 million years ago and includes a sequence of reddish sandstones (Sherwood Sandstone) and mudstones (Mercia Mudstone), and the honey-coloured Jurassic limestones and mudstones of the Cotswolds. In the time between the end of deposition of the older sedimentary bedrock and the start of the deposition of the younger sedimentary bedrock, uplift and erosion took place. As a result, when deposition restarted the younger sediments were laid down on a variety of different older sediments. This situation where younger rocks rest directly on rocks of different older ages because of uplift and erosion is referred to by geologists as an unconformity.


Small areas of the even older basement rocks more than 415 million years old are also encountered at the surface most notably in the area around Tortworth between Bristol and Gloucester. These comprise tilted, strongly folded and faulted sedimentary rocks with layers of volcanic lavas.


==Individual areas==
To describe the geology in more detail the region is divided into three areas: a triangular area of mainly uplands between the Mendip Hills in the south, extending northwards to the Forest of Dean and the Wye Valley; the low-lying ground of the Severn Valley and Somerset Levels; and the eastern part of the region dominated by the Cotswold Hills.


==Mendip Hills to the Forest of Dean==
This area extends from the Mendip Hills in the central part of the region north to Thornbury, extending east to Chipping Sodbury, Radstock and Frome. It includes the uplands of the Mendips, Broadfield Down, the ridges between Clevedon, Bristol, Almondsbury and Thornbury, together with the coalfields of east Bristol and north Somerset, and across to the Forest of Dean and the Wye Valley. This area is typified by characteristic upland ridges and hills formed of older sedimentary bedrock, surrounded by low-lying areas underlain by softer younger sedimentary bedrock.


===Younger Sedimentary Bedrock===
In this area, the younger sedimentary rocks were deposited over a rugged landscape of older rocks, which were gradually buried as sedimentation continued. Initially, the younger rocks were deposited in the lower lying areas, lapping around the margins of the older rugged uplands but as these upland areas were progressively buried, the younger rocks were deposited over a wider region. Consequently, the thickness
and type of rocks laid down is highly variable, depending on the depth of the basin and the source of the sediment at the time.


Patchy remnants of the uppermost youngest layers include limestones and mudstones, known as the Lias, which occur at the surface around Radstock, Whitchurch, Saltford and Filton.
[[Category:Regional Geology overview | 16]]
 
[[Category:Geological_regions_of_England | ]]
The lower parts of this sequence are dominated by reddish mudstones, (Mercia Mudstone), deposited in very dry desert-like conditions 240 to 200 million years ago. These mudstones include some thin sandstones up to 7 m thick, but in Bristol the mudstones give way to a sandstone up to 50 m thick. These layers are banked up against older uplands, such as the Mendips and Broadfield Down, and fossil scree deposits accumulated at these margins composed of blocks of limestone from the upland areas; these have been used as local building stone. The mudstones reach a thickness of about 150 m around Dundry and Chew Stoke, thinning to less than 100 m around Southmead and Filton. These layers are well exposed below the old Severn Bridge at Aust Cliff (Figure 3) and at Radcliffe near Bristol. Sandstone layers form minor aquifers meaning they can be used to supply some drinking water, but in general the mudstones do not allow water to flow through them. The fossil scree deposits contain cave systems such as that at Wookey Hole.
 
===Older Sedimentary Bedrock===
The youngest and uppermost layers of the Older Sedimentary Bedrock are the Coal Measures that occupy the areas of down-folds (synclines). These rocks formed in relatively shallow seas, with vast quantities of sand and mud gradually building up to form large river deltas. When the tops of these deltas were exposed, massive swampy forests grew up and the vegetation from these forests was later buried and compressed to produce layers of coal. The coals formed a valuable resource and have been mined particularly in areas where these rocks occur at or near the surface. Several coalfields are present in the region.
 
The Bristol Coalfield underlies much of eastern Bristol north of the A4, extending east to Coalpit Heath and north to Cromhall. Further south, the North Somerset Coalfield is centred on Radstock, but extends south to the Mendip Hills and north to Pensford, although partially concealed by younger rocks. In the north, the Forest of Dean Coalfield occupies much of the area between Berry Hill and Bream, Cinderford and Drybrook, with coal-bearing rocks extending to depths of about 200 to 300 m below the surface. Other smaller coalfields occur around Nailsea, and beneath Avonmouth. Coal is still mined in the Forest of Dean on a small scale, but the Bristol and Somerset coalfields are no longer productive. However, across the region there still remains a legacy of abandoned coal mines, which are up to 300 m deep. The Coal Measures are locally preserved south of the Mendips, but in the absence of deep boreholes, it is unclear whether they are present beneath the younger rocks across much of central Somerset.
 
The most distinctive rocks in this area are the underlying thick grey Carboniferous Limestone, formed from compacted and cemented calcium carbonate shells and grains laid down in a tropical sea about 360 to 330 million years ago. They are found at the surface in the Mendip Hills, on Broadfield Down and forming the prominent ridges that run between Clevedon, Portishead, Bristol and Thornbury. The limestones also occur around Chepstow, form cliffs in the Wye Valley; and encircle the Forest of Dean Coalfield. Being hard massive rocks, they are well exposed in the Avon Gorge (Figure 4) and in the many valleys, gorges and quarries in the Mendips, particularly Cheddar Gorge, Burrington Combe (Figure 5) and Ebbor Gorge. In total, over a kilometre thickness of this limestone was deposited in the south of the area, thinning to a few hundred metres in the northern part of the Forest of Dean. These limestones also extend beneath the higher coal-bearing overlying layers at depths of 1-3 km in the down-folds of the North Somerset Coalfield, and at 200 to 400 m depth beneath the Forest of Dean Coalfield.
 
Being soluble, the limestones have been dissolved by rainwater percolating through them to form many caves, dry valleys and sinking streams. Landscape where drainage is underground through caves in this way are called karst landscapes. Such features are especially well developed in the Mendip Hills, around Chepstow and in the Forest of Dean. The limestones form an important aquifer locally, both for water flowing through caves and fissures, which are used for public supply (for example at Cheddar), but also for much deeper groundwater flow that feeds the famous thermal springs at Bath and Hotwells.
 
The Carboniferous Limestone is extensively quarried for roadstone aggregate in the eastern Mendips and parts of the Forest of Dean, north of Bristol and around Wick. The limestones locally contain mineral veins and are fractured. Lead and zinc deposits dominate in the central Mendips, with numerous shallow mines dating back to Roman times, whilst in the western Mendips, iron and zinc deposits are common. Iron ore has been extensively dug in the Forest of Dean.
 
The oldest sedimentary bedrock layers are reddish brown sandstones and mudstones deposited within rivers in an arid desert environment about 400 to 360 million years ago and called the Old Red Sandstone. These layers are well exposed west of the River Severn, but also come to the surface in the centre of the four up-folds (anticlines) that make up the Mendip Hills, as well as in similar structures near Westbury-on-Trym and Clevedon and along the ridge between Clevedon and Portishead where the layers are exposed on the coast. These rocks generally do not allow water to pass through them and so the drainage is at surface although small supplies of groundwater are abstracted in some areas. These rocks are up to 2400 m thick west of the River Severn, and about 800 m are exposed in the Bristol area, although the lower layers are not seen, so that the total thickness here is likely to be even greater. In the Mendips, up to 500 m of these sandstones are exposed in the core of the up-folds where they form the highest ground. Because these rocks are strongly folded, the depth to which they extend is very variable and is generally poorly understood.
 
All the Older Sedimentary Bedrock layers have been bent into a series of east-west oriented folds cut by numerous geological faults across the southern half of the area. The up-folds (anticlines) form prominent ridges and hills, of which the Mendips are the largest. Farther north, across the Severn valley and into the Forest of Dean, the folding and faulting becomes less severe, although still significant. Consequently the geological structure at depth is extremely complicated and difficult to predict where there are no deep boreholes.
 
===Basement rocks===
In the core of some of the up-folds and in very small areas north of the Bristol Coalfield even older rocks are locally found at the surface for example around Tortworth. These sediments were deposited between 540 and 415 million years ago and are mostly grey mudstone, with layers of sandstone and rare limestone together with lavas and tuffs which were formed from volcanic eruptions. These volcanic layers are now extracted for roadstone near Frome. Similar mudstones also occur in the northwest of the district in the Usk Valley. These basement rocks are harder and more intensely tilted, folded and faulted than the overlying layers of sedimentary bedrock. They are thought to underlie the whole region, but often at the considerable depth of several kilometres.
 
==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==
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.
 
 
[[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.

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