Oxfordshire’s varied underlying geology has a major influence on its correspondingly varied soils and habitats, from sites such as Dry Sandford Pit to calcareous fens in the Lye Valley. 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.
During the Jurassic Period, Oxfordshire and its surrounding counties were submerged beneath a warm, shallow sea, where corals, fish and small sea creatures thrived. Evidence of this can still be seen in many places, particularly some of the old quarries, such as Rock Edge and Magdalen quarries in Headington – both now protected Sites of Special Scientific Interest. From the beginning of the 15th century to the mid 18th century Headington quarries were the chief source of stone for the buildings in Oxford and it was even sent by river to London. Several of the colleges had their own quarries – Magdalen Pit can still be seen behind William Kimber Crescent. English Heritage and the British Geological Society produced a Building Stone Atlas of Oxfordshire in 2011.
Churned up deposits from the edge of a Jurassic coral reef form Oxfordshire’s famous ‘ragstone’. The sticky, blue Lias clays running across the north of the county near Banbury are the oldest of Oxfordshire’s rocks, and they stretch from the Dorset coastline, across the country through Oxfordshire, with the same rocks visible on the Yorkshire coast. Many of the picturesque villages found throughout Oxfordshire owe their beauty to the Jurassic Oolitic Limestones found locally in Oxfordshire. Information about minerals found in Oxfordshire can be found on Oxford University Museum of Natural History (OUMNH) web site.
Oxfordshire is particularly rich in marine fossils such as bivalves, gastropods and cephalopods, as well as the popular ammonites. There are even a few Cretaceous dinosaurs found in the county, roaming when the warm seas began to subside down the Thames Valley – the footprints of the Megalosaurus and Cetiosaurus left visible in the wet sands at Ardley Quarry near Bicester.
Some sites are recognised nationally and internationally for their scientific, educational, historic and aesthetic value and are protected from harmful development by legislation and planning policy. There are 31 geological Sites of Special Scientific Interest (SSSIs) and 45 Local Geology Sites in Oxfordshire.
Natural England is responsible for the protection and management of geological SSSIs. Local Geology Sites (formerly known as RIGS – Regionally Important Geological and Geomorphological Sites) are designated by the Oxfordshire Geology Trust, who monitor, assess and conserve them. Further information is available from the Thames Valley Environmental Records Centre (TVERC).
More information on Oxfordshire’s geology can be found on Natural England’s web-site, The Open University’s Geological Society’s Oxfordshire pages and at the Oxfordshire Geology Trust and Oxfordshire County Council Countryside pages.