Geological history

Oxford Geology Group's guide to the geological history of Oxfordshire.

Oxfordshire in the Jurassic

Britain was very different in the Jurassic period, 208 to 146 million years ago. It was not even in the same place as today, but much further south, between 30° and 40° N. Early on, in the Lower Jurassic, it was under shallow seas, marked in the geological record by marine sediments. It was all change by the time of the Middle Jurassic, as there was a significant fall in relative sea level, resulting in the formation of a low-lying coastal plain in southern Britain. Consequently, Middle Jurassic rocks reflect a variety of environments in which sediments were deposited: shallow sea water, rivers, deltas, marshes, and coastal lagoons, brackish or freshwater. In these environments carbonate-rich muds, silts and sands were laid down, resulting in a complex sequence of rocks. It is these Lower and Middle Jurassic rocks that form the solid surface geology that underlies the gently undulating landscape of mid to north Oxfordshire today.

 

Oxfordshire rocks of the Lower/Early Jurassic.

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 limy, sandy ironstone; thanks to its relative hardness, it forms a ridge along the limit of the Lias outcrop. Clays in the lower levels of the Lias Group have been exposed along the River Evenlode near Charlbury and along the River Cherwell in the upper part of its meandering valley. A hard shelly limestone called Banbury Marble occurs in this part of the Lias.

 

Oxfordshire rocks of the Middle Jurassic.

 

 

Overlying the Lias are the various warm-yellow to buff-orange limestones, sands and clays of the Middle Jurassic Inferior Oolite and Great Oolite, both typically used for building in this part of the Cotswolds. The Inferior Oolite sands and limestones outcrop to form the Rollright Ridge; they cap Shenlow and Epwell hills, and reach as far afield as Chipping Norton and Steeple Aston. The succeeding clays and limestones of the Great Oolite Group outcrop to form the plateau between Burford and Brackley. These variable sediments were deposited in shallow, warm, clear sea or lagoon environments, and include a number of levels rich in fossils. One such formation is the Forest Marble, named after its occurrence in the Wychwood Forest district: a thin, grey, variable limestone, weathering brown and flaggy; it was probably deposited in a brackish, channelled, marshy environment. Many important fossils have emerged from the Middle Jurassic formations, including mammals, pterosaurs (flying reptiles), dinosaurs and sharks, reflecting the variable conditions under which they were deposited.

As the sea deepened, the Great Oolite was succeeded by the shelly limestones of the Cornbrash (between Fairford and Bicester) and then by the clays of the Kellaways Beds and Oxford Clay, which line the main valley floor of the Thames drainage channel. They were laid down during the Callovian period, off the London-Brabant Massif, between 165 and 160 million years ago, in a latitude equivalent to that of the modern Mediterranean Sea.  Exposures of these heavy, greenish and bluish clays are uncommon, except in places where they have been quarried for bricks.

 

Oxfordshire rocks of the Upper/Late Jurassic.

A series of Upper Jurassic rocks, known as the Corallian Group, dip to the south east; they rise above the Thames Valley in a low and irregular north-facing scarp which reaches heights of 100 -110 m above sea level.  Where capped by the Cretaceous Lower Greensand, as at Boar’s Hill, the ridge rises to 150 m. A very gentle, almost imperceptible dip slope falls away southwards to the floor of the Vale of the White Horse. The sandy limestones of the Corallian were deposited approximately 140 million years ago in shallow coastal waters, near coral reefs analogous to the modern Bahama Banks. The various strata can be seen today in the old Dry Sandford pit. Corals may sometimes be found, for example in the vicinity of Wytham Woods.

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 are rare, except in pits worked for bricks or earthenware. The old brickpits 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, in 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 were deposited in shallow seas or estuaries, and mark the top of the Jurassic succession, but they are rarely exposed.

Oxfordshire in the Cretaceous

Rocks deposited during the Cretaceous Period (142-65 Million years ago (Mya) crop out in the south-east of Oxfordshire and are marine in origin, the product of sediments laid down in sub-tropical seas that alternately shallowed and deepened over time.

Oxfordshire rocks of the Cretaceous.

The lowermost (the oldest) part of the Cretaceous rocks represented here is the Lower Greensand, which can be found in isolated outcrops along the narrow mid-vale ridge between the valley of the River Thames and the Vale of White Horse. These sediments were deposited close to land and contain a unique assemblage of fossil sponges, sea-urchins and bivalves.

 

Succeeding the Lower Greensand are the heavy, grey-blue Gault Clays which form the floor of the Vale of White Horse. The thin Upper Greensand deposits form a low feature at the foot of the prominent Chalk cuesta of Lambourn Downs and the Chilterns.

 

The very pure limestone of the chalk was deposited in an extensive warm shallow tropical sea around 70-100 Mya.

 

Oxfordshire in the Quaternary

Climate change is not something new (though the present developments may be unprecedentedly fast).  Throughout the last two million years the climate of England has oscillated between cold arctic conditions and a warmer, more temperate climate. This is the result of the waxing and waning of glaciers and ice sheets to the north. These ice sheets and the cold tundra-like conditions to the south of them has been instrumental in forming the landscape we inherit today.

Oxfordshire rocks of the Quaternary.

Much of the solid geology of Oxfordshire is covered by more recent sediments, known as drift. The area lay to the south of the icesheets in a tundra-like environment during the Devensian glaciation (110 to 12 thousand years ago [Kya].  It is thought that ice may have reached Oxfordshire during the earlier Anglian glaciation, some 400 Kya, or that 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 drift which occurs over the Jurassic bedrock between the valleys of the Windrush and Cherwell in northern Oxfordshire. This deposit contains pebbles perhaps derived from Triassic deposits found in the Birmingham area. This is still a moot point, as a rival theory suggests that this northern drift material was brought into the area by a joined Severn-Thames river system during the middle part of the Quaternary Period, before 450 Kya.

There are wide expanses of river terraces along the Thames Valley. They are made up of limestone gravels derived from the Cotswolds, and are the result of alternating periods of deposition and down cutting; the surfaces represent former floodplains that were abandoned by the river as a result of erosion down to present day level.

 

Dry valleys, developed under arctic conditions by repeated freezing and thawing of the surface soil layers, occur all along the scarp slope of the Lambourn Downs and the Chilterns. Another, ice-age deposit, the so-called clay-with-flints, occurs above the chalk, particularly in the Chilterns. This is the remains of chalk that was weathered and eroded away during the Quaternary.