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 



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