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 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.
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.
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.
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.
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.