Speaking generally, we pass from older to newer geological formations as we go from the north-west towards the south-east, and we find that the Oxford Clay is the oldest formation which comes to the surface in Berkshire.
The Oxford Clay forms a strip of low land along the banks of the Isis from the Cole to the Cherwell near Oxford. It was originally mud deposited in a sea which extended over a great part of England. It is dark coloured, often shaley, with a little clayey limestone. A large oyster is one of its common fossils. Its thickness is about 450 feet, and it is not a water-bearing formation. The Oxford Clay dips underground to the east and is covered by newer rocks, the first of which is the Corallian.
Corallian Rock, Shellingford
The Corallian forms a very well-marked band running across the county from the Cole to the Thames. Wytham Hill is formed of it, and Shrivenham, Coleshill, Faringdon, Buckland, Fyfield, Appleton, and Cumnor are situated upon it. It is essentially a calcareous formation with some hard limestone beds, and has a thickness of from 50 to 80 feet. It was formed in the sea; probably a shallow sea with shoals, sand, and coral banks. Fossil corals are abundant, and many specimens of Ammonites and other marine shells are to be found. There are some good examples of these from Marcham in the Reading Museum. Supplies of good water may often be obtained from this formation. The Corallian beds are quarried for building stone and road material in many places.
The Kimmeridge Clay, which comes above the Corallian, is, like the Oxford Clay, a bed of hardened marine mud. It has now become a shaley clay, and is about 140 feet thick. It forms a narrow east and west band across the county. Much of the Vale of White Horse is on this clay, and the town of Abingdon stands upon it. It is not a water-bearing formation.
The Portland Beds. A small patch of this formation is found resting upon the Kimmeridge Clay in Berkshire. It caps the rising ground south of Shrivenham, and the village of Bourton stands upon it. Its thickness is about 20 feet.
After the deposition of the Portland rocks, which are of marine origin, there is reason to believe that our district became land and a part of a continent, but no relics of this period remain here. They were all swept away when the land sank again and the Cretaceous sea flowed over Berkshire.
The Lower Greensand—our next deposit—was formed after a long interval, and, owing to earth movements which had taken place during that interval, it rests upon the older rocks in an irregular manner. It is a marine formation, and only occurs in patches, the largest of which extends from Uffington to near Faringdon. Its greatest thickness is about 60 feet, and it consists of sand with some ironstone and chert, pebble beds, and a calcareous sponge gravel. The sponge gravel, so-called from the number of fossil sponges it contains, is dug for garden paths and walks, and is exported to long distances. The fossil sponges in the gravel are abundant and beautifully preserved, and they seem to have lived on the spot. The ironstone was at one time worked near Faringdon. At New Lodge, in the parish of Winkfield, the Lower Greensand was reached in a boring at a depth of 1234 feet. A good supply of water was obtained, but it contains a large quantity of common salt.
The Gault, the next formation, consists of grey clay in the lower part and of a silty marl in the upper part, with a total thickness of some 220 feet. It crosses the county as a band, from one to three miles in width, from Ashbury to the Thames between Abingdon and Wallingford. It is a marine formation, and does not give a water-supply.