This is a geologically complex area spanning over 440 million years of geological time. The oldest rocks are found in the north of South Gloucestershire in the Tortworth area where Silurian mudstones, limestones and lavas form an isolated outcrop.
Grey, bedded Lower Carboniferous Limestone forms the Mendip Hills, which dominate the countryside in the southern part of the area as well as the spectacular Avon Gorge which cuts right through the Carboniferous Limestone outcrop that underlies north-west Bristol. The Bristol-Somerset Coalfield to the east of Bristol is dominated by the Upper Carboniferous Coal Measures which give rise to an area of undulating landscape where evidence of the former coal-mining industry has now all but disappeared.
Away from the ridges and hills of the Carboniferous Limestone outcrop, much of the area is gently undulating and is underlain by the red Triassic mudstones and Lower Jurassic clays and limestones. The eastern side of the area, the escarpment of the southern end of the Cotswolds rises up from the surrounding undulating plateau of pastures and arable land. Formed by Middle Jurassic clays and limestones, the characteristic buff-yellow limestones form the main steep edge to the escarpment and also cap Dundry Hill to the south of Bristol.
The influence of the dramatic climatic changes that have occurred during the Ice Age of the last two million years are not particularly evident in the area. However, deposits of till and gravels in the Clevedon area indicate that ice from the Bristol Channel moved over the land, probably about 500,000 years ago whilst clays and sands at Kenn and in the Gordano Valley show that the sea invaded these low-lying areas at various times over the past 250,000 years.
The oldest rocks of the area are of Silurian age (443-417 million years ago) and occur in the Tortworth-Charfield area where they form part of the Tortworth Inlier, a series of much older rocks that, through folding and faulting, have been brought to the surface within much younger rocks. Here, although exposures are very poor, the Silurian rocks comprise shales, sandstones and limestones with a couple of beds of lava. These rocks contain brachiopods and trilobites which indicate that they were deposited in a shallow shelf sea.
Rocks of the Carboniferous Period (354-290 million years ago) are represented by the Carboniferous Limestone and the succeeding Coal Measures. The Carboniferous Limestone outcrops at a number of locations, notably the western extension of the Mendips at Weston-Super-Mare, to the north-east of Congresbury, the ridge running between Clevedon and Bristol City centre and to the north in a narrow right-angle running from Thornbury to Cromhall and south to Chipping Sodbury. In the Lower Carboniferous, sea spread over the whole area, which, as it deepened, lead to the wide-spread deposition of carbonate-rich muds which now form the massive pale-grey limestones and well-bedded very dark grey limestones of the bulk of the Carboniferous Limestone. The limestones are very fossiliferous and yield evidence in the form of fossil crinoids (sea-lilies), corals and brachiopods of the abundant marine life of the time. In the eastern part of the Bristol Coalfield and at Brandon Hill, Bristol the upper part of the Carboniferous Limestone succession is replaced by the sandstones and grits of the Upper Drybrook Sandstone.
The remainder of the Carboniferous succession is exposed in the folded and faulted Bristol-Somerset Coalfield. The Carboniferous Limestone is overlain by a thick sequence of shales and sandstones with coal seams which are attributed the Upper Carboniferous Coal Measures. This change reflects a shallowing of the Upper Carboniferous sea and the formation of a low-lying area of submerged river deltas adjacent to a landmass on which lush tropical vegetation grew. Outcrops of the Coal Measures are present in four distinct basins at Radstock, Pensford, Parkfield-Coalpit and Nailsea. At these locations, the succession broadly comprises a lower series of shallow marine shales with clays and coals. These are then succeeded by a thick succession of sandstones, the Pennant Sandstone, which represents a massive delta that developed over the area, probably formed by the erosion of land to the north during a period of uplift. Thin coal seams present in the sandstone indicate that swamp conditions over the delta were occasionally established. The upper part of the Coal Measures overlying the Pennant Sandstone is characterised by thick red clays with thin coals which suggests the onset of more arid conditions.
Much of the entire Carboniferous (and Devonian) succession within south-west of England was subjected to tremendous compressive forces and heat during a phase of mountain building (the Variscan Orogeny) at the end of the Carboniferous and into the early Permian. It was this period of crustal movement that led to the folding of rocks to form the Mendips and the complicated geological structure of the Bristol-Somerset Coalfield.
The last event of the Variscan Orogeny was a general uplift of the continental area from which Britain is now formed and the subsequent erosion of large amounts of land during the arid, desert climate of the Permian Period (290-248 million years ago) and the succeeding Triassic Period (248-205 million years ago). During this time the Mendips and the Bristol would have formed upland, mountainous areas from which large amounts of sediment were eroded and transported by seasonal rivers to the desert plains surrounding them. The arid conditions were responsible for oxidation of iron compounds within the rocks, leading to the characteristic red colouration and the descriptive name for the sequence as the New Red Sandstone.
Triassic rocks occur widely throughout the Bristol-South Gloucestershire area forming much of the solid geology. The lower part of the sequence comprises red sandstones with horizons of pebbles deposited by flash floods that crossed the desert plain. These beds pass up into the Otter Sandstones, which formed in river channels and as desert dunes. The final part of the Triassic sequence is formed by the Mercia Mudstone Group, which comprise red and green mudstones, sandstones and thin horizons of gypsum and rock salt. Where the Mercia Mudstones adjoin the Mendips, which would have risen high above the desert plain, they pass laterally into large fans of eroded sediment accumulated along the flanks of the Mendips.
At the top of the Triassic succession is the Penarth Group which comprises a sequence of shales and limestones that record the inundation of the Triassic desert plain by a shallow sea. At the base of the Penarth Group is a bone-bed, yielding fish teeth and reptile bones, which reflects the probable reworking of strandline deposits. This is present at the well-known section at Aust Cliff by the first Severn Bridge, where material can be located following cliff falls. Marine and intertidal conditions persisted to the end of the Triassic with the deposition of the Westbury Formation and Cotham Marble respectively, and an eventual return to open marine conditions depositing the limestones and shales of the White Lias at end of the Triassic.
Marine conditions continued through into the Jurassic (205-142 million years ago), which is marked by a gradual deepening of the sea which covered large expanses of Britain. The main outcrop of the Jurassic extends along the entire eastern side of the area, although the Lower Lias, the oldest part of the succession occurs as isolated outcrops overlying the Triassic throughout the North Somerset and Bristol area. The Lower Lias comprises clays with coarse shelly limestones, the succession indicating that the shelf sea deepened over time. Around Radstock, shallow water conditions were maintained for a longer period of time and here shelly and iron-rich limestones were deposited.
The overlying Middle Lias consists of two distinct rock units, the lower comprising the silts and sands of the Dyrham Formation and the upper, the thinner, shelly iron-stained Marlstone Rock. Fossils of marine creatures are common and include ammonites, belemnites and bivalves. These rocks form the lower part of the Cotswold escarpment along its length and also outcrop around Dundry Hill, south of Bristol. The Upper Lias is represented by thin clays at the base followed by the sands of the Bridport Formation (the Cotswold Sands).
The Middle Jurassic, comprising the Inferior Oolite and the Great Oolite forms the main part of the Cotswold escarpment and the western section of the Cotswold plateau and caps Dundry Hill. The Inferior Oolite consists mainly of limestones with iron-rich horizons deposited in a shallow shelf sea. The limestones are richly fossiliferous and yield many species of fossil brachiopod, sea-urchins and corals. Towards the end of the deposition of the Inferior Oolite succession the whole of the Cotswolds area was folded, uplifted and eroded. The upper beds of the Inferior Oolite were then deposited over this eroded surface following a return to shallow marine conditions. The succeeding Great Oolite Group comprises a series of rocks that vary laterally across the south of England. In the North Somerset and South Gloucestershire area the Great Oolite comprises the clays of the Fuller’s Earth, deposited in relatively deeper water. These are overlain by the thick oolitic limestones of the Bath Oolite and the Coombe Down Oolite which were deposited in very shallow water in an environment similar to that of the modern-day Bahamas. Both these limestones are the source of the famous Bath Stone which was formerly extensively quarried in the Combe Down area.
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets and associated climatic changes have been instrumental in forming the landscape we see today.
There is some evidence to indicate that ice moved into the area during the Ice Age. Till, or boulder clay, and gravels derived from a melting ice sheet are present in the Kenn lowlands, south of Clevedon and along the Carboniferous Limestone outcrop on the northern side of the Gordano Valley. These deposits were laid down by ice that moved into the area from the Severn Estuary, probably over 500,000 years ago. Interglacial deposits indicating marine incursion of the low-lying areas at Kenn and Weston-in-Gordano during a period of higher sea-level are also preserved in these areas. These contain the fossils of bivalves and shells which suggest that they were deposited under a continental climate approximately 250,000 years ago. These sites also provide evidence of the sea-level rise that occurred following the melting of the last great ice sheet some 13,000 years ago.
Along the valleys of the Severn and the Bristol Avon, terraces composed of alluvium and gravels are present at various heights. These catalogue the gradual erosion of the rivers down to their present levels and the various climatic conditions under which they have developed over the past half a million years. These terraces are present at various heights. At Ham Green to the south-east of Bristol, a clear terrace surface at the mouth of the Avon Gorge is present at a height of 30m. The terrace is underlain by gravels containing large amounts of material of Jurassic age and flints derived from the Chalk. It is suggested that the terrace represents a period when the Avon flowed at this altitude, possibly over 500,000 years ago.
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.