VALLEY AND ITS RELATION TO THE WESTLETON EED3, ETC. 173 



Fig. 9. — Section in front of the Ghallc-escarpment above Merstliam. 

 w. ^• 



Sea-level. 



V. Valley of Sinitham Bottom, 

 a. Vallej Drift. 



J). Level of valley at foot of escarpment. 

 2. Lower Tertiary strata. 3. Ohalk. 



anticlinal may have formed a range from 2000 to 3000 feet high, 

 with a drainage north and south. 



As the channels of the early streams became deeper and larger, 

 and the Lower Greensand more exposed, the mass of debris carried 

 down increased, and the proportion of chert and ragstone became 

 greater. It was then that were formed the extensive plateaux of 

 gravel of the Chobham and Frimley Downs, and of the other hills we 

 have named in Berkshire, Hampshire, and Surrey. 



In the absence of organic remains of any sort in these plateau- 

 gravels, we are without a clue as to Avhether fluviatile or marine 

 action had to do with their origin. It is not improbable that they 

 are, in part, of subaerial origin ; and to compare small things with 

 great, they may have been formed in a manner analogous to the 

 fan-shaped masses of debris carried down by torrents from the 

 mountains bordering the plain of the Indus as described by 

 Mr. Drew *. This would account for the localized form of these 

 sheets of gravel, and for their absence on the intermediate Chalk- 

 hills. It is also possible that the cone may have discharged under 

 water, and so spread out to a greater extent and more uniformly, 

 and with the rough sort of bedding this gravel sometimes shows. 



In any case, whatever may have been the manner in which these 

 hill-gravels were formed, there can be no doubt of the source 

 whence the materials have been derived t- They clearly come solely 

 from the Lower Tertiary, Chalk, and Lower Greensand, lying south 

 of the area over which they are spread. Flints and flint-pebbles 

 might have been brought from otlicr directions, but the chert and 

 ragstone are characteristic of the JiOwer Greensand of Kent and 

 Surrey, and the peculiar character of some of the chert of the 



■* Quart. Journ. Gool. .Soc. vol. xxix. p. 441 (1873). 



t Prof. Kupcrt Joue.s arrived independently at the same general couclusions as 

 myself with respect to tlie age and origin of the Bagshot plateau-gravels, thoutrh 

 he ascribes more to marine action. I should hnve liked to give an abstract 

 of his papei-, but must refer the reader to the original work (Proc. Geologist's 

 Association, vol. vi. pp. 437-443, 1880). The Rev. A. Irving has also written on 

 the subject, and concluded that these beds were of estuarine origin and pre- or 

 interglacial age (Proc. Geol. Assoc, vol. viii. pp. 1(U-171, 18s3). 



