VALLEY AND ITS KELATiON TO THE WESTLETON BEDS, ETC. 171 



Later on, as the covering of Tertiary strata and of Chalk was 

 removed, the Lower Greensand became more exposed, and its hard 

 and indestructible siliceous debris was scattered more widely along 

 the base of the old Wealden range of hills. Thus it is that on the 

 Bagshot hills of Hampshire and Berkshire we find the chert and rag- 

 stone of the Lower Greensand occurring in quantity, and in Kent 

 also scattered freely, though thinly, in places over the Chalk-plateau, 

 having lodged there after the denudation of the Tertiaries and 

 their removal around Well Hill. 



Another and somewhat later phase occurred when the drainage of 

 the Wealden Highlands became restricted to narrower bounds and 

 more defined channels. It was then that some of the main trans- 

 verse valleys received their first rudimentary channelling, as, for 

 example, in the case of the Medway, where the flint- and chert- 

 gravel above Hailing and on some hills above Stroud point to old 

 channels in the line of the present Medway valley, but at heights 

 of from 200 to 300 feet above the existing river ; or, in the case of 

 the Darent, the bank of flints and chert on the slope above Eynsford, 

 at a height of from 250 to 350 feet above that stream. Another 

 instance is that of the Wey through the gorge of the Chalk at 

 Guildford. On the east side of this passage, Lower-Greensand 

 and flint-debris cover Merrow Down at a height of 400 feet, or of 

 300 feet above the Wey. In all these cases this old gravel is 

 considerably above the Post-Glacial gravel more intimately con- 

 nected with the existing valleys, though at the same time the Challv- 

 arouiid rises to still greater heights than this Drift. 



In the foregoing instances, the valleys flanked by this old chert- 

 drift pass through the Chalk-escarpment into the W^ealden area, 

 showing that there has been continuity of action along the same 

 lines from the initial stages of the formation of these valleys to the 

 present time, and therefore that it was not, as has been suggested, 

 the present streams which have cut their way back through the 

 escarpment. 



There are other instances where the continuity has been inter- 

 rupted, and the original drainage diverted into other channels, leaving 

 the valleys, as it were, half formed, and the old channels were 

 left dry, but retaining a drift carried in from beyond their present 

 precincts. A good instance of this is to be seen in the narrow 

 valley of Smitham Bottom (fig. 8), which, commencing at Croydon, 

 runs up to the Chalk-escarpment above Merstham, where it is cut 

 off by the transverse valley draining into the Mole, and no longer 

 passes into the Wealden area. At present it is a dry vaUey, 

 excepting that during certain seasons there is an outbreak for 

 about half its length of a bourne, which continues to run for a few 

 months, but is in no way connected with the old drainage-aren. 

 Along the upper part of this valley, and extending to its end above 

 Merstham, is a thin bed of gravel composed of subangular flints, 

 and flint-pebbles, with subangular fragments of chert, raystone., and 

 ironstone derived from the Lower Greensand. It is clear, therefore, 

 that this Drift was deposited before the strata beyond the escarpment 



