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. 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.