Bristol and Gloucester district - 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 500 m of the Lias is preserved. These rocks generally occur within 100 m 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.

Seacliffs on the north Somerset coast near Watchet. Here the red –brown Mercia Mudstone is faulted against the paler Jurassic rocks of the Lias. P902607.

Beneath the Lias are two layers of predominantly red coloured rocks (Plate P902607) 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 450 m 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 to 1500 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 to 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.