Natural England - Oxfordshire

Oxfordshire

At a broad scale, the geology of Oxfordshire 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.

The M40 motorway cutting through Upper Cretaceous chalk in Oxfordshire at Aston Rowant. - Peter Wakely (Natural England)

The escarpment and plateau of the Cotswold Hills dominate the north-west landscape of the County. These are formed in Jurassic shallow coastal limestones, shales and sands. It is these buff and yellow limestones that give the Cotswold buildings and landscape such a distinctive character. The plateau surface gradually shelves southwards to the valley of the upper reaches of the River Thames and Oxford, which is floored by the heavy clays of the Jurassic Oxford Clay.

Immediately to the south of Oxford and running between Faringdon and Thame, a low ridge separates the Thames valley from the Vale of White Horse to the south. This ridge is developed in the Upper Jurassic limestones and clays of the Corallian and Kimmeridge Clay and is overlain at its eastern end by the shallow marine and estuarine deposits of the Portland Group and the Purbeck Limestone. Marking the beginning of the Cretaceous Period are small outcrops of the Lower Greensand lying on top of the ridge.

The Vale of White Horse has been cut into the thick, bluish-grey Gault Clay; a marine clay of Cretaceous age and is fringed to the south by the impressive escarpment of Lambourn Downs and the southern end of the Chilterns. This line of hills is formed in the Upper Cretaceous pure marine limestone of the Chalk, which supports characteristic downland grassland and beech woodlands. Evidence for the arctic-like conditions that prevailed over the area during the ice ages of the last two million years is provided by the dry valleys, such as that at Whitehorse Hill, which occur across the outcrop of the Chalk.

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 shallow 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 Lower and Middle Jurassic rocks form the solid geology of the northern third of the County where they form the northern extension of the Cotswold Hills.

The early Jurassic marine shales, limestones and sands of the Lias Group outcrop along the indented north-west facing slope of the Cotswolds between Banbury and Burford. The most prominent part of this succession is the Marlstone Bed, which is a calcareous, sandy ironstone, and given its relative hardness forms an elevated ridge along the limit of the Lias outcrop. Clays in the lower part of the Lias Group succession have been exposed by the Evenlode near Charlbury and by the Cherwell in the upper part of its valley. A hard shelly limestone called Banbury marble occurs in this part of the Lias.

Overlying the Lias are the various buff-yellow to orange limestones, sands and clays of the Middle Jurassic Inferior Oolite and Great Oolite. The outcrop of the Inferior Oolite sands and limestones form the Rollright Ridge and caps Shenlow and Epwell hills; and also reaches down to Chipping Norton and eastward to Steeple Aston. The succeeding clays and limestones of the Great Oolite Group outcrop in a broad swathe forming a plateau surface between Burford and Brackley. These variable sediments were deposited in shallow marine to lagoon habitats and include a number of very fossiliferous horizons. One such bed of rock is the Forest Marble, a thin limestone named after its occurrence at Wychwood Forest, and which was probably laid down in a brackish, marsh environment. This has yielded important fossil mammals, pterosaurs (flying reptiles), dinosaurs and sharks, reflecting the variable conditions under which it was deposited.

Representing a progressive deepening of marine conditions the Great Oolite is succeeded by shelly limestones of the Cornbrash (between Fairford and Bicester) and the overlying clays of the Kellaways Beds and Oxford Clay which floor the main valley of the Thames in which Oxford is situated. Exposures of these heavy, greenish and bluish clays are uncommon. On the southern side of the Oxford Clay vale, the land rises to form a ridge which comprises coarse and rubbly-textured Upper Jurassic Corallian limestones and sands, overlain in places by Kimmeridge Clay. These rocks form a distinct escarpment rising from the Oxford Clay with a low and irregular north-facing scarp rising to 110m and a very gentle southern slope that gradually falls, almost imperceptibly in places, to the Vale of the White Horse to the south. The sandy limestones of the Corallian were deposited approximately 140 million years ago in shallow coastal waters close to coral reefs possibly under conditions similar to those found in the Bahama Banks today.

The overlying clays and shales of the Kimmeridge Clay indicate that there was a deepening of the sea towards the end of the Jurassic. Exposures of the Kimmeridge Clay, apart from those in pits worked for bricks or earthenware are rare. The old brickpits in the Kimmeridge Clay to the west of Hurst Hill have yielded the remains of fossil marine reptiles such as ichthyosaurs, plesiosaurs and pliosaurs. The skeleton of a terrestrial, herbivorous dinosaur known as Camptosaurus was also found in the former exposures of the Kimmeridge Clay at Chawley.

Overlying the Kimmeridge Clay, and occurring as intermittent patches along the gentle northern slope of the Vale of White Horse between Abingdon and Thame are the limestones and sands of the Portland Group and the thin limestones of the Purbeck Limestone. These shallow marine to estuarine deposits mark the top of the Jurassic succession, but are rarely exposed.

Cretaceous

Rocks of Cretaceous (142-65 million years ago) outcrop in the south-east of Oxfordshire and were deposited in a sub-tropical sea of varying depth throughout this Period. The lowermost part of the Cretaceous present comprises the Lower Greensand which occurs as isolated outcrops along the narrow ridge between Oxford and the Vale of the White Horse. These sediments were deposited close to land and contain a unique assemblage of fossil sponges, sea-urchins and bivalves. The succeeding heavy, grey-blue clays of the Gault Clay floor the Vale while the thin Upper Greensand forms a low feature at the foot of the prominent Chalk ridge of Lambourn Downs, the Ridgeway and the Chilterns. The Gault Clay has been worked for bricks at Culham. The very pure limestone of the Chalk was deposited in an extensive warm shallow tropical sea around 70-100 million years ago. The Chalk outcrop supports calcareous grassland vegetation that is rich in many plant species including orchids and rarities such as the Chiltern gentian. The woodlands of the Oxfordshire Chilterns reflect the underlying geology and support species such as beech, oak and several species of woodland orchids that are typical of chalk substrates and the more acidic overlying clay-with-flints.

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 have been instrumental in forming the landscape we see today.

Much of the underlying geology of Oxfordshire 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. There is some evidence to suggest that ice reached Oxfordshire during the Anglian glaciation some 400,000 years ago, or the County at least lay very close to the maximum advance of the ice. Part of this evidence comes from the existence of the so-called “Plateau” or Northern Drift which occurs over the Jurassic bedrock between the river valleys of the Windrush and Cherwell in northern Oxfordshire. This deposit contains pebbles that have been derived from Triassic deposits in the Birmingham area. However, it is suggested that this material was brought into the area by a joined Severn-Thames river system that had its headwaters in Wales during the middle part of the Quaternary, prior to 450,000 years ago. Diversion of the Severn by ice, during the Anglian and excavation of the Severn-Avon valley separated the Thames, which has remained within its present bounds ever since.

Within the floodplain of the Thames there are wide expanses of terraced river gravels of limestone, derived from the Cotswolds. The gravel terraces are present at various heights and these catalogue the gradual erosion of the Thames River system down to its present level and the various climatic conditions under which it has flowed over the past half a million years.

Dry valleys, developed under arctic conditions through repeated freezing and thawing of the surface soil layers occur all along the escarpment of Lambourn Downs and the Chilterns. Another, ice-age deposit, the so-called clay-with-flints occurs over the chalk, particularly in the Chilterns. This represents the remnants of part of the chalk succession that was eroded and weathered away during the Quaternary to leave behind only the clay particles and flint nodules contained within it.

Geological Highlights:

  • William Smith, who is known as the "Father of English Geology, and is credited with creating the first nationwide geologic map was born on March 23rd 1769 at Churchill in Oxfordshire, where he attended the local school. Through survey work for the canal system in Somerset and other parts of England, Smith built up an extensive knowledge of the different rock strata that he encountered. This led him in 1799 to produce a document known as the "table of the Strata near Bath" and it was for this that William Smith became known as one of the principal founders of the Science of Geology. He also produced what is considered to be the first geological map in 1801 by colouring in the geological features on a small map of England. This was the first sketch of Smith’s great geological map that came out in 1815. Recognition of Smith’s work was slow in coming, largely because of his low-born status in the genteel and upper class world of 19th century geology. His work was plagiarised, he became financially ruined and spent time in debtor's prison. It was only much later in Smith's life that he received recognition for his accomplishments.

  • The Middle Jurassic Forest Marble Beds at Kirtlington, where the rocks have been extensively quarried, is one of only five sites of this age in the world to yield fossil mammals. At least nine species of early mammals, including species related to marsupials, have been found from a clay band of probable marsh origin within the basal part of the Forest Marble Beds. The bed has also yielded the fossil remains of crocodiles, pterosaurs, dinosaurs and a diverse fish fauna including several species of small material. Forest Marble Beds at this site have also yielded a rich and diverse fauna of fish species based on teeth.

  • In and around the village of Stonesfield, part of the Middle Jurassic Great Oolite succession has been worked for the production of roofing slates, giving rise to the Stonesfield Slates. This fine-grained rock type is very localised and was probably deposited in a shallow marine environment. The mines are no longer worked, but during production they yielded abundant remains of fossil plesiosaurs, marine crocodiles, dinosaurs, pterosaurs, turtles and plants. These occur as well preserved teeth, limb bones and vertebrae. The Stonesfield Slate yielded in 1812 the first known pre-Tertiary mammals and in 1815 several bones belonging to a ‘giant lizard’. These bones were studied by the great zoologist and geologist, William Buckland's who in 1824 formally named and described them as Megalosaurus (Great Lizard) – this was the first dinosaur to be scientifically described.

  • At Ardley Quarry, dozens of footprints of dinosaurs have been exposed in part of the White Limestone (Great Oolite) flooring the floor to the quarry. The fine muddy limestone was deposited very near to the shore of a sea and probably represents the remnants of a gradually shelving mudbank. At some point approximately 165 million years ago dinosaurs walked across the mudbank while the sediment was still soft and wet. The footprints form trackways made by two different dinosaurs. The most numerous are pothole-like impressions, 60cm across, of the feet of herbivorous sauropods, possibly Cetiosaurus. The second type of trackway is made up of three-toed, bird-like footprints of a bipedal, carnivorous dinosaur. These footprints are up to 80cm long and 2 metres apart. The trackmaker was probably Megalosaurus, the only known carnivorous dinosaur of appropriate size and geological age.

  • More than 1000 remains of an early form of mammoth in association with those of straight-tusked elephant have been excavated from a gravel pit at Stanton Harcourt near Oxford. The gravels were deposited by the River Thames along a previous course slightly north of its present position some 200,000 years ago. Fossil shells, insects and vegetation from the gravels indicate that the gravels were deposited under temperate conditions during a warmer period between glacials. The presence of both elephant and mammoth has proved interesting, as until the discovery of both species together it was always assumed that mammoths were representative of cold conditions. This is clearly not the case and the deposits and their fossils at Stanton Harcourt provide an important glimpse of the habitats of these large extinct mammals.

  • The dry valley of The Manger, cut into the scarp face of the chalk escarpment where it rises to a crest at Whitehorse Hill, is a classic example of one of the most distinctive landforms of the English chalk landscape. The Manger coombe is a rounded, totally dry valley 500m long and over 50m deep with more than 10 tributary dry valleys feeding into it and thick deposits of chalk rubble and silt lying in its floor. This feature was formed in the glacial conditions of the Ice Ages when the ground was frozen. The repeated freezing and melting of the ground ice gradually eroded out the coombe forming the feature we see today. Under the present climate the valley is dry, as chalk is very porous and does not generally support surface streams.

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.