Yol. 68.] GLACIAL OKTGIjST OF THE CLAT-WITH-PLINTS. 203 



at least in part, to a very ancient period.^ We seem justified, there- 

 fore, in assuming the presence of the Chiltern escarpment in 

 pre-Glacial times. 



In addition to Walter's Ash, there are sections in other places 

 showing similar structure. At Denner Hill, near JSpeeu, are pits 

 in Clay-with-Flints which do not reach the Chalk, although they 

 are situated considerably below the top of the plateau ; and one 

 at least is 30 feet deep. Here the sarsens occur in association 

 with ordinary Clay-with-Flints. 



At Lee Gate, 3 miles south-east of Wendover, a group of sarsens 

 has recently been discovered on the Clay-with-Flint flat. Some of 

 them are as much as 15 feet long, and the group is embedded in 

 Reading Clay surrounded on every side by Clay-with-Flints. One 

 of the large sarsens is broken, and the fragment, about 4 feet 

 long, lies about 4 feet from the main mass with Reading Cla}^ 

 between (Pi. XIII). It is clear that the fracture, which is along 

 a plane of weakness, dates back to the time when the group of 

 sarsens was deposited in its present place, for the whole mass (as we 

 have stated) forms one huge boulder. The only cause of fracture, 

 other than that of transport by ice, which we can think of is 

 solifluction, and there is at present no dominating high ground 

 whence the sarsens could have been moved ; nor is there any 

 trace of a Reading-Bed outlier nearer than Cowcroft, 6 miles to 

 the south-east down the dip-slope. 



Some of the sarsens, notably at Walter's Ash, showed a highly 

 glazed surface, and in one case remarkably parallel scratches were 

 seen. These features characterize a dry climate, such as is found! 

 in deserts; but they are also found under tundra-conditions, and 

 might therefore be produced in Glacial times. 



It should be noted that our objection to the 'Chalk residue*" 

 theory of the origin of the Clay-with-Flints of Buckinghamshire 

 applies to the deposit as it now exists. The flints in the detritus 

 which was swept up by the ice-sheet may verj^ well have been 

 derived from solution of Chalk ; but we consider that these flints 

 were mixed with detritus from the Eocene and moved into their 

 present position by the ice. 



[h] The Gravels associated, with the Clay-with-Flints. 



The Clay-with-Flint area is fringed towards the south-east by a 

 belt of country about 2 miles wide, parallel to the strike of the 

 Chalk, in which the high ground is covered by gravel. In addition^ 

 the gravel is frequently seen cropping out in the lateral valleys 

 along the margin of the Clay-with-Flints and evidently underlying 

 it, but as a rule there is not enough to map. There are, however, a 

 few places, as at Prestwood, where the gravel replaces the Clay- 



1 W. Whitaker, ' Geology of London ' Mem. Geol. Surv. vol. i (1889) 

 pp. 281-83. ^ ^ 



Q. J. G. S. No. 270. Q 



