Natural England - Somerset

Somerset

Geologically, Somerset is a diverse County with the rocks and sediments spanning the last 440 million years of time. The landscape of the County is diverse, with contrasts being provided by the Mendip, Polden, Brendon and Quantock Hills and the intervening low-lying Somerset Levels and Vale of Taunton.

Doulting Railway

Railway cutting through Jurassic limestones in Somerset.

Geologically, Somerset is a diverse County with the rocks and sediments spanning the last 440 million years of time. The landscape of the County is diverse, with contrasts being provided by the Mendip, Polden, Brendon and Quantock Hills and the intervening low-lying Somerset Levels and Vale of Taunton. In the east and the west of the County, there is an intricate landscape of hills and vales that represents the southern end of the Cotswold escarpment and plateau. This landscape diversity is largely due to the variation in the underlying geology and the history of climatic environmental change that has influenced its development.

The Mendips are formed in blocks of folded Carboniferous Limestone, and represent the remnants of a much higher range of hills that existed hundreds of millions years ago. The Quantocks and the Brendon Hills at the eastern end of Exmoor are formed by thick sequences of slates and sandstones of Devonian age that were deposited by large deltas that built out into a shallow sea. All of these hill ranges are surrounded by mudstones and sandstones of Triassic age, which represent the deposits of large river systems that crossed a desert plain. In the south and east of Somerset where the marine lower Jurassic clays form undulating and rich pasture between Staple Fitzpaine and Castle Cary before the succeeding sands and limestones give rise to a more intricate landscape of scarp slopes, hills and small steep valleys that rises to the south and east. In the far south of the County, the heathy windswept plateaux and ridges of the Blackdown Hills are capped by the marine sands of the Cretaceous Upper Greensand, which in turn is overlain by the purer white limestone of the Chalk in the Chard and Crewkerne area.

Over the last two million years the area was not directly impacted by the repeated advances and retreats of the great ice sheets of the Ice Age. However, the arctic climate that prevailed during glacial periods led to the development of tundra-like conditions. During the warmer periods between glacials, sea-level rose to invade and cover the Somerset Levels on several occasions.

Silurian

The oldest rocks occurring in Somerset are of Silurian age (443-417 million years ago) and represent the most southerly known outcrop of rocks of this age in Britain. These comprise a sequence of lavas, tuffs (volcanic ash), shales and mudstones which form a narrow outcrop to the northeast of Shepton Mallet in the eastern Mendip Hills. These sediments have yielded a fairly rich fossil fauna of brachiopods and trilobites indicating that they were deposited in a shallow marine sea into which the lavas were extruded. The rocks are quarried at Moons Hill near Stoke St. Michael for aggregate.

Devonian

During the Devonian (417-354 million years ago) Britain was part of a large continental landmass that included Europe and North America, which lay approximately 10o south of the Equator. This land has, in geological studies, become known as the Old Red Sandstone Continent on account of the deposition of sediments rich in iron oxides (giving the sediments a rich red colouration). What is Somerset, was at this time located on the margin of a subsiding marine basin with the Old Red Sandstone Continent coastline stretching approximately from the Bristol Channel to the Thames estuary (or at least where they are now). Rocks from this period form the solid geology of much of Exmoor, the Quantocks and in the north of the County, the cores to the folded masses of the Mendip Hills. The rocks weather to provide soils of a warm red colour which, because of their free-draining nature, tend to support heathland and acid grassland. In the Mendips, the distinction between the rough grassland and heathland vegetation of the sandstone outcrops and the vegetation of the surrounding Carboniferous Limestone plateau is readily apparent.

The oldest part of the Devonian succession, the Lynton Slates, occur in the core of an anticline extending eastwards from Lynton on Exmoor. Successively younger Devonian rocks comprise the Hangman sandstone, Ilfracombe Slates (with thin limestones yielding corals and brachiopods), Morte Slates, Pickwell Down Sandstones, Upcott Slates, Baggy Sandstones and Pilton Shales. These rocks show features indicating that they were laid down in a shallow marine to intertidal conditions in an area where large deltaic fans of sediment were brought down by rivers from the nearby Old Red Sandstone.

Carboniferous

In Somerset, rocks of the Carboniferous Period (354-290 million years ago) are represented by the Carboniferous Limestone which forms the landform of the Mendip Hills, which rise abruptly out of the flat landscape of the Somerset Levels and Moors. In early Carboniferous times, sea spread over the whole area, progressively deepening and depositing 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 succession of the Mendips. 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. Throughout the Mendips, on slopes the Carboniferous Limestone gives rise to thin calcareous soils, some of which support limestone grassland, but deeper soils, particularly on the main plateau surface, have weathered to become acidic in their upper layer and support a rare vegetation type known as limestone heath. Weathering and erosion of the limestone over millions of years has also produced the characteristic limestone landform features of the area, such as complex underground cave and river systems, gorges, dry valleys, swallets and sink holes.

Throughout Britain, the limestones of the Carboniferous Limestone give way to marine shales, with thin limestones and sandstones before giving way to the more massive sandstones and shales of the Upper Carboniferous Millstone Grit Series. 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. Deposits from this period occur to the north of the Mendips and are only represented in Somerset by a small outcrop of the Quartzite Sandstone Formation north-west of Bridgwater.

Much of the entire Carboniferous (and Devonian) succession within the south-west of England was subjected to intense compressive forces and heat during a phase of mountain building (the Variscan Orogeny) at the end of the Carboniferous and into the early Permian.

Permian and Triassic

The last event of the Variscan Orogeny was a general uplift of the continental area from which Britain is now formed. This resulted in the formation of several mountainous areas including Dartmoor in the south, Exmoor and the Quantocks and the Mendips. Under a subtropical, arid climate, conglomerates, sandstones and breccias, which represent the remnants of the large fans of sediment coming down from the mountains, now outcrop between Exmoor and the Quantocks. In the Taunton area Permian (290-248 million years ago) red sandstones and breccias outcrop while rocks of Triassic age underlie much of Somerset and form the solid geology to the Somerset Moors and Levels.

The lower part of this succession comprises red sandstones with horizons of pebbles deposited by flash floods. In the north of the outcrop area the pebbles are mainly of Carboniferous Limestone while in the west they are Devonian. These beds grade up into the Otter Sandstones, which formed in fluvial 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. These rocks, which outcrop in a wide area around the Quantocks, along the coast to Minehead and up to the Mendips, represent desert plain and evaporated lake deposits.

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.

Jurassic

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 is in the east of the County and extends from the Shepton Mallet area southwards to Yeovil and then westwards to the Staple Fitzpaine area. The oldest Jurassic rocks, the Blue Lias, comprises clays with thin shelly limestones and outcrops in a broad arc from Burnham, via Somerton to Staple Fitzpaine, typically giving rise to flat or gently undulating land such as the levels surrounding the River Brue. The Polden Hills are capped by the Blue Lias Limestone, which in places forms a steep scarp. Where exposed or quarried the Blue Lias has yielded many marine fossils, including ammonites and marine reptiles such as ichthyosaurs.

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. South of the Mendips, the sands of the Bridport Formation, occur in the lower slope of the Jurassic escarpment, and form some isolated low hills such as Glastonbury Tor and Brent Knoll. These sands represent the deposits of a large offshore sand bank that gradually advanced during the Lower Jurassic from the north.

The Middle Jurassic, which outcrops from Wansdyke southwards to Milborne Port, typically forms a series of roughly parallel westward-facing scarps of limestone, separated by clay vales. The outcrop continues westwards from the south of Yeovil almost to Ilminster. The earliest Middle Jurassic series is the Inferior Oolite, which consists mainly of shallow marine, fossil-rich, limestones with iron-rich horizons deposited in a shallow shelf sea. The succeeding Great Oolite Group comprises the Fuller’s Earth Rock and the Forest Marble forming prominent escarpments in southeast Somerset. The clays of the Fuller’s Earth Rock were deposited in deeper water, while the succeeding limestones and clays of the Forest Marble contain fossils and structures that indicate that they were deposited in shallow, perhaps tidal waters. The Jurassic sea then deepened again and the rubbly, brown limestones of the Cornbrash were deposited. This fossil-rich limestone forms a narrow outcrop in the south-east of Somerset between Templecombe and Bruton. A deepening of the marine basin marks the beginning of the Upper Jurassic and the deposition of the overlying marine clays of the Kellaways Beds and Oxford Clay. These clays floor the lower-lying land immediately to the south-east of Wincanton and which runs up to Frome adjacent to the higher ground formed by the wooded slope of the Cretaceous Lower Greensand.

Cretaceous

The Cretaceous (142 to 65 million years ago) is represented in south Somerset by two divisions of rock, which were deposited in a shallow, sub-tropical sea which covered the entire area. The rocks from this period are now confined to the Blackdown Hills area. Here the Upper Greensand, a series of dark green to grey sands with calcareous concretions, chert and shell beds and forms the capping to the series of hills, ridges and escarpments that make up the Blackdown Hills. In the western part of this area, west of Chard, the Upper Greensand is devoid of calcareous material but the sands yield superb fossils of marine bivalves and gastropods (snails) preserved in silica. Overlying the Upper Greensand is the Chalk. At its base this comprises calcareous sands and sandy limestones rich in marine fossils such as sea-urchins, ammonites and bivalves. Over time, the Chalk sea deepened and the input of sand decreased, resulting in the deposition of the purer calcium carbonate-rich white Chalk. In Somerset, the Chalk is restricted to the Chard and Crewkerne area where it is richly fossiliferous and yields sea-urchins, brachiopods and ammonites.

Quaternary

Over the last two million years ago the climate of Britain has varied tremendously with periods of temperate climate interrupter by repeated advances and retreats of glaciers and ice sheets. Collectively these have been known as the Ice Age and the actions of the ice sheets have been instrumental in forming the landscape we see today.

However, unlike much of the rest of Britain, Somerset did not experience the full effects of the repeated advances of ice sheets, as the southward movement of the most recent ice sheets did not extend into the County. However, during these glacial periods it is likely that much of the County resembled the tundra of today’s arctic provinces.

Sea-level rises and falls associated with the repeated growth and decay of ice sheets are recorded at various locations around Somerset, particularly in the Somerset Levels and along the coast. Patches of shelly sand and clay, known as the Burtle Beds and locally as ‘batches’, that are raised above the alluvial deposits of the Somerset Levels represent the deposits of a marine incursion. Dating of the sediments indicates that they were laid down during a warmer climatic period approximately 200,000 years ago, when sea-levels were higher. The area was also invaded by the sea during the last interglacial, the Ipswichian, some 120,000 years ago. Other evidence for changes in sea-level comes from a raised-beach deposit overlying Carboniferous Limestone at Middle Hope, north of Weston-Super Mare. This has been dated to the Ipswichian Interglacial.

Deposits from the last glacial period, the Devensian occur at several places around the County and provide evidence of the harsh climatic conditions of this time. At Brean Down, south of Weston-Super-Mare, sands, silts and breccias rest on a shore platform against an ancient sea-cliff. These sediments contain the bones of reindeer, bison, arctic fox and lemming indicating the existence of a tundra-like environment in the area some 50,000 years ago. River valley deposits, exposed in a cliff section at Doniford, show features typical of deposition under arctic conditions and have also yielded the bones and tusks of mammoth, herds of which inhabited the tundra-plain during the last glacial. Remains of elephants, woolly rhinoceros, wolves and lemmings have also been found in deposits from many of the caves in the Carboniferous Limestone of the Mendip Hills.

Following the melting of the last ice sheet, the Somerset Levels developed as a large wetland area. Deposits of peat formed in these inland basins, the thickest deposits occurring in the Brue Valley, where the peat is still worked today.

Geological Highlights:

  • Lying close to the Wiltshire border in south-east Somerset is Ham Hill. The warm, buff-coloured stone that outcrops here has been quarried and worked since Roman times, although deep quarries were not commenced until the middle of the 19th Century. This relatively soft stone lends itself to being carved and it has been used extensively for building work in the local area (e.g. Sherborne Abbey and Montacute House) and further afield. The stone is restricted to a small area in the Ham Hill district and takes the form of a lenticular mass, up to 27m thick, within the Bridport Sand Formation (the Yeovil Sands), where shell debris is held together with iron-rich cement. During early Jurassic times, some 180 million years ago, when the Ham Hill Stone was deposited, southern England lay beneath a shallow sea.

  • In the late 19th numerous virtually complete ichthyosaur skeletons were quarried from the Lower Lias (Jurassic) rocks around Street in Somerset. The ichthyosaur was subsequently adopted as the official symbol of Street and it is still used on roadside signs welcoming visitors to the town.

  • One of the more important geological features in Somerset is the suite of fissure deposits within the Carboniferous Limestone of the Mendips. The fissures developed following uplift of the Carboniferous Limestone to form the Mendips during the early Permian and are filled with Triassic and Jurassic aged sediments. During the Triassic desert conditions prevailed and sediment, including the remains of creatures, from the surrounding upland areas was swept into the fissures. Of special note are fossils of the lizard Kuehneosaurus latus which is one of the earliest-known flying vertebrates.

  • Over millions of years the Carboniferous Limestone of the Mendips has been dissolved and eroded through the action of water to produce one of the most important collection of caves and cave systems in Britain. At Priddy, on the main plateau of the Mendips, the cave system contains about 16km of surveyed cave passages divided between a number of major and minor networks. The two most extensive systems are those of St Cuthbert’s Swallet and Swildon’s Hole, both of which were formed by the erosive action of water flowing beneath the water-table at considerable pressure (so-called phreatic development). Both these cave systems contain superb examples of stalagmite and stalactite development. At Wookey Hole, which is open to the public, there has been large-scale development of cave passages and many features of limestone deposition. The caves are also well known for the bones of fossil mammals, including hyanea, rhinoceros and hippopotamus, found in the cave sediments laid down over the last million years of the Ice Age.

  • The Mendip Hills were famous for their production of mineral ores, although no production remains today. The orefield is found in the Carboniferous Limestone and the Dolomitic Conglomerate. Lead (galena) and zinc (smithsonite) are the most economically important and abundant elements. The mineralisation probably took place during the late Permian and early Triassic, associated with similar mineralization in mineralisation in Devon and Cornwall. The earliest mining of the Mendips took place in at least 49AD, six years after the Romans entered Britain though there is some evidence that it may have taken place over 200 years earlier. Ingots or ‘pigs’ of lead smelted by the Romans have been found at Charterhouse, including the largest ingot, weighing in at 223lbs, of Roman lead found in the country. The industry reached its peak between 1628 and 1659 and then declined rapidly, probably due to the exhaustion of ore in the lodes near the surface. Remains of the lead industry (known as 'gruffy ground'), dating from Roman times onwards, are locally prominent and the more recent abandoned workings have developed as wetlands, ponds and grasslands of high nature-conservation value. The most extensive ruins of the smelting industry are of St Cuthberts lead works, north-west of Hunters lodge, Priddy, which closed down in 1908.

Local sites

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.