GEOLOGY Near Faringdon and Cumnor the Kimeridge Clay has suffered denuda- tion before the overlying deposit, the Lower Greensand, was laid over it, and consequently its thickness has been much reduced. It was estimated at from 70 to 80 feet near Cumnor by Prestwich. PORTLAND BEDS Next in succession above the Kimeridge Clay we come to the Port- land Beds, a formation which once extended over a large tract in north Berkshire and the adjoining counties. It has however suffered greatly from denudation, and only fragments remain here and there to show its former extent. In Berkshire only one very small patch occurs at the surface. It caps the rising ground south of Shrivenham, and the village of Bourton stands on it. The formation is calcareous the upper part consists of soft, thin bedded, chalky oolite and hard, bluish limestone with pebbles of quartz and lydite. The lower part is sandy, and the thickness of the whole is perhaps 20 feet. Though this patch is very small there can be no doubt as to the age of the rock, for the characteristic Portland fossils Ammonites giganteus and Cardium dissimile have been found here. They are marine shells, and the formation appears to have been a series of sands and calcareous mud deposited on the bottom of a shallow sea. The gradual depression which went on during the periods of the Oxford Clay, Corallian and Kimeridge Clay had come to an end, and a period of elevation was begin- ning. The result of this was that the shore was closing in and Berkshire and the greater part of England were gradually becoming land, part of what has been termed the Purbeck continent. In lakes, lagoons and rivers of this continent the Purbecks, the closing formation of the Oolites, and the Wealden, the beginning of the Cretaceous system, were deposited. It is possible that patches of Portland or even of Purbeck strata may lie buried under the newer formations in Berkshire, but there is at present no satisfactory evidence of this. The rocks of Oolitic age above described dip away to the east, but it is not improbable that they lie in the form of a basin or synclinal and that they soon curve up again, in which case the various formations, Kimeridge Clay, Corallian and Oxford Clay, would be successively cut out or end off against the overlying rocks. The evidence of the Richmond boring which has been already mentioned favours this view, for there the above mentioned formations were all absent and the section passed from Cretaceous into Bathonian rocks which are older than the Oxford Clay. LOWER GREENSAND We cannot tell to what extent freshwater deposits such as are found in other parts of England may have been laid down in this county 7
