Wiltshire (including Swindon)
The geology of Wiltshire comprises a series of rocks of Jurassic and Cretaceous age that are gently tilted to the south-east, so that the oldest rocks occur in the north-west and the youngest in the south-east. There is a small outcrop of Tertiary aged rocks in the far south-east of the County.
Cretaceous chalk is one of the dominant rock types in Wiltshire, forming much of Salisbury Plain and here in West Harnham, overlooking Salisbury
The oldest rocks in the County are in the Middle Jurassic Cotswold Hills which fringe the north west of Wiltshire. Broadly, rocks get progressively younger as you travel south eastwards and reflect changing ancient environments from deep marine, to coastal and occasionally fluvial conditions. Alternating Upper Jurassic clays and limestones have produced a series of low lying vales, including the Vales of Wardour and Pewsey, and are succeeded by Lower Cretaceous sandstones, ironstones and clays. Dominating almost two thirds of Wiltshire, however, is the Upper Cretaceous Chalk which forms the vast rolling landscape of Salisbury Plain with its seemingly endless arable fields and unimproved grassland punctuated only by small hilltop woodlands. During the Tertiary and Quaternary, erosion and weathering of the chalk produced the clay-with-flints deposits that occur across parts of the plateau surface and which today often support woodland vegetation, in contrast to the thin, dry soils of the Chalk. Scattered across the Plain and the downs are Sarsen stones, the weather-worn blocks of grey sandstone derived from the former cover of Tertiary deposits.
In the far south-east of the County, around the chalk outcrop of Dean Hill, Tertiary sands and clays outcrop on the northern margin of the Hampshire Basin and the well-wooded fringes of the New Forest.
Jurassic
During the Early Jurassic Britain was largely covered by shallow shelf-seas leading to the deposition of marine sediments. However, towards the Middle Jurassic there was a significant fall in sea level, resulting in the formation of a low-lying coastal plain in what is now southern Britain. Consequently, Middle Jurassic rocks reflect a variety of depositional environments including marine, fluvial, deltaic, saltmarsh and coastal lagoonal (brackish-water and freshwater). In these environments carbonate-rich muds, limestones, silts and sands were deposited, resulting in a complex sequence of rocks covering this period of time. These Middle Jurassic rocks form the solid geology of the north-western part of Wiltshire where it forms the southern end of the Cotswolds.
The main outcrop of rocks of this age occurs from Bradford-on-Avon northwards to just west of Malmesbury and comprises the buff-yellow limestones and grey clays of the Great Oolite. The limestones were deposited in very shallow water in an environment similar to that of the modern-day Bahamas. These limestones are the source of the famous Bath Stone which was extensively quarried in the Box area.
To the east rocks reflect a gradual deepening of the Jurassic Sea: shelly Cornbrash limestone (between Malmesbury and Frome) followed by the deeper marine clays of the Kellaways Beds and Oxford Clay which underlie the vales in which Chippenham and Melksham sit. A low ridge from the north of Swindon to Devizes represents the rubbly limestones, sands and clays of the Corallian Beds. Deposited 140 million years ago the Corallian Beds once more show the return of shallow, marine sub-tropical conditions yielding fossil ammonites, bivalves, gastropods and corals.
Towards the end of the Jurassic the sea deepened depositing the Kimmeridge Clay which forms the floor of the Vales of Wardour and Pewsey. Towards the end of the Jurassic limestones and sands of the Portland and Purbeck Formations are exposed notably around Tisbury, Potterne and Swindon. These marine, estuarine and freshwater deposits mark a significant fall in sea-level towards the end of the Jurassic and yield evidence for the changing conditions by their fossils which range from marine bivalves and ammonites in the Portland Stone to freshwater snails and plants in the limestones of the Purbeck Formation. At Chilmark in the Vale of Wardour, a limestone bed (the Chilmark Oolite) in the Portland Formation has been quarried for many centuries and was used in the construction of Salisbury Cathedral in the 13th Century.
Cretaceous
Though freshwater and terrestrial conditions continued through into the early Cretaceous once more sea-level rose and there was a return to shallow marine conditions. This is marked in Wiltshire by the sands and ironstones of the Lower Greensand, which outcrop in the Vale of Wardour and in the Vale of Pewsey to the west of Devizes. Ironstone within a small outcrop of the Lower Greensand at Seend was formerly quarried and the ore smelted on the spot.
The Lower Greensand is succeeded by the Gault Clay and the Upper Greensand, their outcrop zigzagging in a narrow strip around the edge of the Chalk escarpment and flooring the Vale of Pewsey east of Devizes. Exposures of the bluish-grey Gault Clay are rare, but quarries in the Upper Greensand near Warminster show that the warm, sub-tropical sea in which it was deposited teemed with life as large numbers of fossil bivalves, sea-urchins and ammonites can be found. In the south-west of the County, the Upper Greensand outcrop gives rise to an undulating, hilly landscape. The rounded hills and relatively free-draining and slightly acidic soils provide the attractive setting for the historic parks of Stourhead and Longleat. The transition to the overlying Chalk can be observed in several old quarries located at the base of the scarp slope that dominates the skyline in much of west and north Wiltshire. This leads up onto the large chalk plateau of Salisbury Plain which dominates the centre of the County, Cranborne Chase in the south and the Marlborough Downs in the north. The very pure limestone (up to 98% calcium carbonate) of the Chalk was deposited in a warm shallow tropical sea around 70-100 million years ago. Despite its extensive outcrop, natural exposures are not common, but it can be seen in road cuttings and old quarries such as those at Harnham just to the south of Salisbury. Throughout its area of outcrop, wherever agricultural practice is suitable, the chalk supports calcareous grassland vegetation that is rich in many plant species including orchids and herbs.
Tertiary (comprising Neogene and Palaeogene)
The main outcrop of Tertiary (65-2 million years ago) rocks occurs in the south-eastern corner of the County. These outcrop in a narrow belt to the north of Dean Hill in a shallow basin that forms a small extension of the much larger Tertiary Hampshire Basin to the south. There is also a small outcrop of Tertiary sediments at Great Bedwyn and Savernake Forest south-east of Marlborough at the far western end of the London Basin. These various clays and sands were laid down in shallow marine, coastal and fluvial (river) environments and include the Upnor Formation, Reading Formation, London Clay and Bracklesham Group.
Sarsen stones, isolated remnant blocks of weathered Tertiary sandstone with a hard silica cement, can be found over the surface of the chalk. These probably represent outlying deposits of sands within the Reading Formation and indicate that these Tertiary sediments formerly extended well beyond their present more limited outcrop. Cementation of the sands probably occurred just below the ground surface under an arid or semi-arid climate, perhaps 5-10 million years ago, and the surrounding uncemented sediments have long since been washed away. In some areas the surface of the chalk is covered by clay-with-flints. This represents the insoluble residue left after chalk has been eroded and weathered away, and has probably been formed over many millions of years. The clay-with-flints often gives rise to more acidic soil conditions which contrasts with the calcareous soils derived directly from the chalk bedrock.
Quaternary
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 the climatic changes have been instrumental in forming the landscape we see today.
Ice did not reach Wiltshire during the Quaternary, although tundra-like conditions would have prevailed. Under these arctic conditions the dry valleys of the chalk probably formed. These features occur on what is a very permeable rock which does not generally support surface drainage systems. However, during the various glacial periods, deep permafrost would have made the ground impermeable and allowed gradual erosion of the frozen soil surface to occur, particularly during summer thaws. Much of the shaping of the present form of the Chalk landscape with its scarps, dip-slopes and valleys would have been produced through water erosion.
Geological Highlights:
- In the Steeple Ashton area, near Trowbridge, the Corallian outcrop has yielded the richest known assemblage of fossil Jurassic corals in Britain. Forty six species have so far been recorded, most of these being recovered from ploughed field surfaces. This very localised fauna has been famous since the earliest years of geological study and was first recorded by the English antiquary and writer John Aubrey as early as 1656. Reference to the corals was made by Aubrey in an unpublished manuscript:
At Steeple Ashton are frequently found stones resembling the picture of the unicorne's horn, but not tapering. They are about the bignesse of a cart-rope, and are of a reddish gray colour.
The manuscript was later posthumously published as the Natural History of Wiltshire in 1847. - Quarrying of the Late Jurassic limestones of the Portland and Purbeck formations in Vale of Wardour has, over the years, revealed a remarkable series of fossils. Of particular note are the vertebrate remains which include fossil fish, crocodiles, pterosaurs (flying reptiles), dinosaurs and mammals.
- The Chilmark Oolite, one of the thicker limestones within the Portland Formation, has been quarried for use as a building stone at Chilmark for many centuries. First exploited by the Romans, the 13th and 14th centuries saw the greatest use of Chilmark stone, when it is estimated that there were up to 54 quarries operating. The quarries at Chilmark provided the material for both Salisbury and Chichester Cathedrals as well as Wilton House. Stone was extracted up until the 19th century, but the building of the Kennet and Avon Canal and the Wiltshire and Berkshire canals to the north of the county gave rise to quarries that produced Bath stone better access to a cheap form of transportation.
- The limestones of the Purbeck Formation that outcrop in the Vale of Wardour provided much material in the last century for the first major study of British fossil insects, by Reverend Peter Bellinger Brodie (1815-1897), who was the curate at Wylye. The majority of the former exposures are now gone; although the Purbeck rocks can be seen along the Teffont Evias Lane Cutting. The fossils occur in the so-called “insect limestone” which was laid down in a shallow lake some 135 million years ago. Over 60 different specimens were illustrated by Brodie, many of them unique, and the diversity of the fauna is such that insects belonging to eight different Orders occur.
- At Wootton Bassett, to the south-west of Swindon is an example of an hydrogeological phenomenon which is extremely rare in Britain. This is a mud-spring, which here consists of several vents which emit liquid mud (derived from the underlying Ampthill Clay Formation) all year round and at increased rate after periods of prolonged rainfall. The vents have been shown to contain liquid mud to a depth of about 6 m. Geochemical evidence indicates that the water contained in the mud appears to originate from an aquifer in the Jurassic Coral Rag Formation, the top of which underlies the Ampthill Clay Formation at a depth of 20 m.
- Fyfield Down, between Avebury and Marlborough contains one of the highest concentrations of sarsen stones in Britain. Trains of the stones occur, lying in the position that they were transported to by processes of freezing and thawing of the ground during the Ice Age. The sarsens have been used in the construction of the megalithic monuments of Stonehenge and Avebury, but many were also broken up to provide stone for buildings and roads. The stones also represent one of the most substantial natural exposures of hard rock in lowland Britain and provide a substrate for the growth of a diverse relict lichen flora of considerable note. Some lichens are typical of highland Britain and are rare in the lowlands while others are characteristic of rocky sea coasts.
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.
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