The Upper Greensand runs across the county as a narrow and irregular band about 90 feet thick, and consists of green sands and grey marl, with beds of stone in places. It is of marine origin, and provides a supply of excellent water, and consequently many villages stand upon or close to it. Ashbury, Childrey, Wantage, Hendred, and Harwell are examples.
The Chalk. This is far the most important geological formation in Berkshire, for it occupies a large portion of the surface of the county, and in the eastern part, when not at the surface, it is to be found underground. It is a light-coloured limestone, usually soft and earthy, but in parts very hard. Its full thickness is over 700 feet, and being a porous rock, the rain which falls on its great surface sinks in and furnishes a water-supply over its whole area whether the chalk be at the surface or underground. It was deposited in a sea which not only covered our district but spread over much of Europe. There was, however, probably land to the west which included Cornwall, parts of Wales, and of Ireland. The upper part of the Berkshire Chalk contains many layers and nodules of flint.
There is a long break in our geological record after the newest beds of the Chalk found in Berkshire had been deposited, for both the top of the Chalk and the bottom of the next series are wanting here, and in order to fill the interval we have to study rocks in other parts of England, in Belgium, and in Denmark. During this great interval in time the chalk sea retired, and much of Britain became land.
The Reading Beds repose upon a water- and weather-worn surface of chalk. They consist of clays and sands, and were deposited in the bed of a great river. Their thickness is from 70 to 90 feet, and good water may be obtained from the sands. In the lower part we find a bed of oysters, and rather higher up there is in some places a bed of leaves, known as the “Reading Leaf-Bed,” a specimen of which is shown below. It will be noticed that the leaves are crowded together, and were no doubt buried in the mud of the river.
Specimen from the Reading Leaf-Bed
The Basement Bed of the London Clay comes next in order and the fossils are marine, showing that the sea was again spreading over our area. It is from 6 to 16 feet in thickness, and consists of loam and clay with green sand and pebbles. A set of shells from this bed is arranged in the Reading Museum.
The London Clay is a marine formation of very uniform character, a stiff clay, blue underground, but becoming brown near the surface, owing to the action of surface water. It contains layers of cement-stones. The thickness in the east of the county is nearly 350 feet, but the formation thins to the west, and is under 50 feet thick at Inkpen. Fossils are not uncommon, and there is a fair collection of Berkshire London Clay fossils in the Wellington College Museum. It is not a water-bearing formation. Most of Windsor Park is on London Clay, and a number of places the names of which end with “field” are upon this formation, such as Arborfield, Binfield, Burghfield, Shinfield, Swallowfield, Warfield, and Winkfield.
The Bagshot Beds, named after Bagshot Heath, consist of sand with a few beds of clay. The maximum thickness is nearly 350 feet. They are probably mainly of marine origin, but formed near the estuary of a large river. Fossils are rare in this formation in Berkshire, but a few specimens will be found in the Museums at Reading and at Wellington College. The Bagshot Beds are a water-bearing formation, but the water is not always of a satisfactory character. The scenery of the sandy Bagshot country is well shown by the view opposite.
Some indefinite time after the deposition of the Bagshot Beds considerable earth movements took place in the south of England, and Berkshire became, and has since remained, dry land. The Bagshot Beds are consequently the last marine formation in our district, and we thus complete our account of the solid geology of the county.