Bagshot Heath Country from Bog Hill
The solid strata are, however, to a considerable extent covered with a variety of geological deposits due to rain, frost, streams, and rivers. These deposits, often termed Drift, though not marked on the majority of geological maps, have a great importance for the dwellers in our county, simply because they form the actual surface and determine the character of the soil.
Clay with Flints is a formation covering a good deal of our Chalk. It is partly débris of the chalk formation and partly of clay beds which once rested on the Chalk. In places it is 20 feet thick. Some of the best timber in the county grows upon it.
Gravel covers a good deal of the surface in Berkshire. It is found both on the high ground and in the valleys. The high-level gravels are often over 10 feet thick and the valley gravels are more than 20 feet thick in several places. Windsor, Bray, Maidenhead, Cookham, Twyford, Wokingham, Reading, Theale, Pangbourne, and Newbury stand partly or wholly upon gravel.
Sarsens in Gravel, Chobham Ridges
Alluvium, the modern deposit of the rivers, covers a good deal of ground in some places, more especially in the valley of the Kennet.
Sarsens are blocks of sandstone which are found on or near the surface of the ground or in the beds of gravel. They were probably derived in part from the Reading Beds and in part from the Bagshot Beds. The illustration on page 40 shows three sarsen stones lying at the bottom of a thick bed of gravel in a gravel pit on Chobham Ridges. The locality is in Surrey, but not far from the Berkshire border, and similar examples occur in Berkshire.