Bristol and Gloucester district - 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.

Reddish brown Triassic Mercia Mudstone exposed at Aust Cliff near Bristol. P211058.

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 (Plate P211058) 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.

Carboniferous Limestone exposed in the Avon Gorge in Bristol. P006732.

The Rock of Ages at Burrington Combe, there the Carboniferous Limestone is steeply inclined due to folding of the older sedimentary bedrock. P006946.

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 (Plate P006732) and in the many valleys, gorges and quarries in the Mendips, particularly Cheddar Gorge, Burrington Combe (Plate P006946) 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 in 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.