Category:16. Bristol and Gloucester district

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.

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.