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 GEOLOGY The London Clay itself is as its name implies almost wholly clay and of a very uniform character throughout, excepting near the top, which is often rather sandy. It is practically impervious to water. It extends over a broad belt of country from Windsor to Reading and from that town south to the county boundary. West of Reading it is largely covered by the next formation, the Bagshot Beds. Most of Windsor Park, Winkfield, Hawthorn Hill, Warfield, Bin- field, Hurst, Arborfield, Shinfield, Swallowfield, Mortimer, Burghfield and Beenham are on London Clay. It is but little obscured by superfi- cial deposits. Its colour is dark, usually of a bluish tint, but near the surface of the ground it is reddish or reddish-brown, the effect of the action of air and percolating water. It contains layers of septaria or cement stones, i.e. nodules of hard calcareous clay with divisions of calcite or aragonite. In the east of the county the London Clay is very thick. Several wells and borings have passed through the whole formation. At Cum- berland Lodge, Windsor Park, it was 314 feet in thickness, and at Ascot Racecourse as much as 349! feet, but it gradually thins westwards. At Wokingham it was 273 feet, Bearwood 256 feet, in the Burghfield district it is only a little over 200 feet, and at Inkpen only 52 feet in thickness. In all these cases the basement bed is included in the figures given. It forms a stiff soil. At Bracknell, Wokingham and other places the clay is worked for brick and tile making. Fossils are not common but occur in the sep- tarian nodules. The bivalves usually have the valves united and are not waterworn. Mr. Gardner considers that the climate was warmer than in the Reading Bed period. The fossils are marine, and the extent, thickness and uniform character of the greater part of the formation suggest that owing to depression the sea water had encroached much further up the estuary than in the time of the Reading Beds and even than in that of the basement bed. BAGSHOT, BRACKLESHAM AND BARTON BEDS The three formations, Bagshot, Bracklesham and Barton, may be taken together, for they are intimately connected with one another and indeed are often all included under the general name Bagshot Beds. They extend over a considerable area in Berkshire. Sunninghill, Ascot, Bagshot Heath, Easthampstead Plain, Sandhurst, Wokingham, Sulhampstead Abbots, Ufton, Padworth, Aldermaston Park, Wasing and Brimpton are on beds belonging to these formations. The surface of the ground is however to a large extent covered with gravel; indeed it is to this fact that the patches of Bagshot Beds owe their preservation. They are essentially soft formations, consisting of sands with very subordinate beds of clay, and they have consequently suffered great erosion from rain, streams, etc., so that though they i 17 3