The rocks of Gloucestershire span over 500 million years of geological time and together represent one of the most diverse areas of geology in England.
The area is dominated by three main components: the Devonian and Carboniferous rocks of the Forest of Dean, the Triassic and Early Jurassic mudstones and clays of the Severn Vale and the Jurassic limestones of the Cotswolds. There are also small outcrops of Silurian limestones at May Hill, to the north of the Forest of Dean, and near Tortworth in the south of the county.
The rocks of the Forest of Dean are situated in a large shallow basin and comprise sandstones and mudstones of the Devonian Old Red Sandstone around the edge of the Forest, overlain by Carboniferous Limestone and Upper Carboniferous Coal Measures. These Carboniferous rocks have been economically important since Roman times yielding both iron ore and coal. The rocks of the basin extend under the Severn where they outcrop in the Berkeley area. To the east of the Forest of Dean, the wide floodplain of the Severn is formed in Triassic mudstones belonging to the Mercia Mudstone Group. These are overlain by the clays and thin limestones of the Lower Jurassic Lias Group. Exposures are very limited as much of the Severn Vale is covered by deposits of sand, gravel and clay deposited during the repeated glaciations over the last 2 million years of the Ice Age or Quaternary.
East of the Severn the land rises dramatically, forming the indented escarpment of the Cotswolds. This is formed by sands, clays and limestones of Lower to Middle Jurassic age which were deposited in a warm, shallow sea. The Jurassic limestones have been used as a building stone, and provide towns such as Cheltenham and many of the Cotswold villages with their warm and mellow character. Much of the Jurassic sequence contains the fossils of the marine life of the time, including many species of brachiopod, corals, sea-urchins and marine reptiles. In the far south-east of Gloucestershire, the clays of the Upper Jurassic Oxford Clay form the valley of the Upper Thames near Lechlade.
The oldest rocks in Gloucestershire are of Cambrian age (545 - 495 million years ago) and comprise a small outcrop of shales to the north-east of Berkeley. Though poorly exposed, the shales have yielded fossil trilobites, which enables them to be dated to the Upper Cambrian.
Rocks of Silurian age (443-417 million years ago) occur in two separate areas. In the Tortworth-Charfield area, Silurian rocks are exposed as 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. There is also a very small outcrop of Silurian rocks at Tites Point on the Severn from which numerous remains of freshwater, armoured fish have been recovered.
In the far north-west of the County, on the border with Herefordshire, a thicker sequence of Silurian rocks is present, forming the May Hill Inlier (which includes May Hill and Huntley Hill). Here the three units of the classic Silurian succession can be found. The lowest unit, the Llandovery Series, comprises coarse sandstones and conglomerates and siltstones with abundant fossil brachiopods and trilobites. These sediments were originally deposited in a shallow sea, close to land with much deeper water lying to the west and north-west. The succeeding limestones and shales of the Wenlock Series contain abundant fossil brachiopods, corals and trilobites which indicate the deposition of the original sediments in a shallow shelf sea.
Finally, the Ludlow Series can be split into rocks that were deposited in the shallow shelf sea and those that were deposited in the much deeper ocean basin to the west. The May Hill area lay closer to land and shelf limestones and mudstones, rich with the fossil remains of corals, brachiopods and trilobites were deposited. The upper part of the Silurian succession contains a number of horizons yielding fossil fish.
In Gloucestershire, rocks of Devonian (417-354 million years ago) age outcrop in the Forest of Dean area and on the other side of the Severn in a narrow band running from Sharpness to near Thornbury. The Forest of Dean is an elevated mass of disturbed sandstones and limestones sandwiched between the Rivers Wye and Severn. These Devonian rocks represent continental deposits laid down on a wide coastal plain over which large rivers meandered. The lower part of this thick sequence comprises red and green shales that form the solid geology along the eastern edge of the Forest of Dean at Newnham and Blakeney, and then across the Severn to Sharpness. On the eastern side of the Forest of Dean, the succeeding grey-brown sandstones of the Brownstones form a series of prominent north-south ridges from Mitcheldean southwards past Blakeney to Lydney.
The Brownstones are overlain by the Quartz Conglomerate, which contains large amounts of pebbles made of quartz and igneous rocks. This, together with the overlying yellow-brown sandstones of the Tintern Sandstone Group outcrop on either side of the Wye Valley where it forms a more upland tract of land with poor sandy soil. These latter rocks form the Upper Old Red Sandstone succession in the area and also occur as a narrow band around the northern rim of the Forest of Dean.
Carboniferous rocks (354-290 million years old) outcrop in the Forest of Dean area and have had particular economic significance for Gloucestershire. In early Carboniferous times, sea spread over the whole area, resulting first, in the deposition of the Lower Limestone Shales, a series of mudstones with thin limestones indicating deposition relatively close to land. As the sea deepened purer carbonate-rich muds were deposited and these now form the massive pale-grey limestones and well-bedded very dark grey limestones seen in the Wye Gorge below Tintern (sometimes referred to as the Mountain Limestones). The limestones give way to the sandstones and grits of the Drybrook Sandstone, which reflects a shallowing of the Carboniferous sea and the formation of a low-lying area of submerged river deltas adjacent to a landmass.
Following a period of earth movements, uplift and erosion a low-lying shallow basin formed in which rivers discharged from higher ground to the north. Here, extensive swamp forests developed, the rotting vegetation of which accumulated as thick beds of peat which, over millions of years, were compressed to form coal. This area now forms the Upper Carboniferous Coal Measures of the Forest of Dean of Coalfield. The coalfield covers an area of approximately 90 km², is completely exposed and comprises over 11 workable seams of coal separated by beds of shale and sandstone. The lower part of the sequence is dominated by grey sandstones, with subordinate mudstones (the Trenchard and Pennant Formations). The main workable, coal seam, the Coleford High Delf Seam, occurs towards the base of the Pennant Formation. The upper part of the Coal Measures, the Supra-Pennant Formation largely comprises soft mudstones with eight workable coal seams.
During the Permian (290-248 million years ago) and Triassic (248-205 million years ago), what is now Britain formed part of a large supercontinent, known as Pangea (All Earth), located close to the equator. Dry arid, desert-like conditions were responsible for oxidation of iron compounds within the rocks, leading to the characteristic red colouration of the sandstones, breccias and mudstones that occur north of Newent and which belong to the Triassic Sherwood Sandstone Group. The overlying Mercia Mudstone Group forms the gently rolling, lower-lying countryside of north-west Gloucestershire between Huntley and Tewkesbury. The Mercia Mudstone comprises red and green mudstones, sandstones and thin horizons of gypsum and rock salt. These sediments probably represent wind-blown dust that settled in shallow salt-lakes and sun-baked mudflats on the extensive flood plain. The arid conditions under which these rocks were deposited are indicated by the occurrence of numerous layers of salt, which formed through the evaporation of mineral-rich water under the intense desert sun.
Towards the top of the Triassic 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 succession is a bone-bed, yielding fish teeth and reptile bones, which reflects the probable reworking of strandline deposits. The succeeding dark shales of the Westbury Formation indicate the presence of oxygen-deficient waters in the shallow marine basin. A return to more intertidal conditions is marked by the Cotham Beds, which at Garden Cliff, near Frampton on Severn, yields fossil crustaceans and plants. The youngest part of the Penarth Group marks a return to more marine conditions and comprises limestones with shales.
It is perhaps for the Jurassic outcrops of the Cotswolds that the geology of Gloucestershire is best known. However, the oldest Jurassic rocks belong to the Lias Group and these clays and limestones form the main valley floor to the River Severn from Blakeney, through Gloucester and up to Tewkesbury. Characteristically, the mudstones give rise to heavy clay soils prone to wetness and flooding. In places outliers of overlying harder Middle Jurassic limestone, have created isolated but prominent hill features (e.g. Oxenton, Dumbleton, and Churchdown Hills).
A steep indented escarpment marks the edge of the Cotswold Hills, running along the eastern side of the Gloucestershire. Due to the slope of the rocks, the oldest Jurassic strata outcrop in the west and get progressively younger travelling to the east. Above the Lias clays of the Severn Vale the lower part of this escarpment consists of further clays and silts and sands belonging to the Charmouth Mudstone, Dyrham Sand and Marlstone Rock (formerly the Middle and Upper Lias). These beds are generally relatively soft, protected from erosion by the overlying Middle Jurassic limestones.
The latter are divided into the Inferior and Great Oolite Groups; within each group are numerous different limestone formations, which tend to vary laterally in thickness, nature and occurrence through the Cotswolds. The Inferior Oolite Group comprises a varied sequence of mainly oolitic limestones ranging from less than 20m thick in the east and south of the County to over 100 metres in the Cheltenham-Cleeve Hill area. These limestones and lesser beds of clay, sand and marl were deposited in a shallow, sub-tropical shelf sea and yield a rich fossil fauna of brachiopods, molluscs and sea-urchins.
Much of the upland plateau area of the Cotswolds comprises the Great Oolite Group. Although the sequence varies from location to location, clays (Fuller's Earth), clay-rich limestones (Stonesfield Slate) and shelly oolitic limestones predominate. The succeeding limestones are mainly oolitic with variable amounts of shell, sand and marl. Marking the top of the Great Oolite is the Cornbrash, a persistent bed of fossiliferous rubbly limestone, which forms a narrow outcrop in the south-east of the County between Siddington and Fairford. In this area and at Lechlade, the Upper Thames Valley is floored by the grey mudstones of the overlying Upper Jurassic Oxford clay.
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 have been instrumental in forming the landscape we see today.
Much of the underlying geology of Gloucestershire is covered by more recent sediments, often collectively known as drift. The area was not affected by ice movement during the last Devensian Glacial, although tundra-like conditions would have prevailed. Patches of gravel containing pebbles derived from Triassic deposits in the Birmingham area occur high up in the Cotswolds in the Moreton area. It has been suggested that these were deposited by ice some 400,000 years ago. However, it is more likely that they represent the deposits of a joined Severn-Thames river system that had its headwaters in Wales during the middle part of the Quaternary. Diversion of the Severn by an ice sheet, during the Anglian glaciation, approximately 450,000 years ago, dismembered the Thames, which has remained within its present bounds ever since.
The Severn Vale and adjoining areas drained by the River Severn and Avon and its tributaries are characterised by widespread deposits of unconsolidated clays, silts, sand and gravel beds of both glacial and fluvial origin. Fan gravel deposits, comprising large amounts of Jurassic limestone derived from erosion along the Cotswold escarpment during the last glacial also occur. Terraces of sand and gravel at varying heights occur throughout the Severn Vale and the main river valleys. These deposits catalogue the gradual erosion of the rivers down to their present levels and the various climatic conditions under which they have flowed over the past half a million years.
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.