﻿Yol. 66.~] THE WESTERN END OE THE WEALD. 679 



traced by the chert that they have left on the Downs. In this way 

 it attained a volume which enabled it to keep pace, by cutting down 

 the Chalk, with the excavation of the Weald Clay by the Medway. 

 Next to the Darent the stream which survived longest was what I 

 may call the Merstham River — a branch of the Wan die ; assisted 

 perhaps by a fault, as well as by a change of strike, it succeeded in 

 excavating in the Lower Greensand a valley 4 miles long, and 

 some of the chert which it carried down is perhaps still to be found 

 in the gravels of Smitham Bottom. Dr. Hinde (6, p. 224) denies this, 

 and regards the chert in question as having been washed down from 

 the Clay-with-Flints and other drifts on the Chalk slopes bounding 

 the valley ; but, however this may be, the presence of chert in 

 these drifts has still to be accounted for, and accords best with a 

 belief in early consequent streams from the Wealden area : nor can 

 I see any reason to doubt that the Wandle did in fact drain this 

 portion of the Wealden area until a fairly recent period. 



About the other consequent streams which the Mole succeeded 

 in capturing there is not much to be said. Between Merstham 

 and Dorking the summit of the Downs presents extensive surfaces 

 of a plain gently sloping to the north, some of which correspond 

 to Prestwich's ' Chalk Plateau,' but in other places the Chalk is 

 covered by more recent deposits which seem to have been planed off 

 by some denuding agency. The -chert found on Headley Heath and 

 Walton Heath may probably be attributed to early consequent 

 rivers, one of which may be indicated by the obsequent valley of 

 Pebble Coombe ; but anj T such streams in this region must, owing 

 to the obliquity of the Mole, have been either cut off altogether 

 from the Wealden area, or reduced to infinitesimal dimensions, at 

 a very early stage. It is important to notice that Betchworth 

 Clump lies only a little to the south-west of Walton Heath, and 

 that the present drainage of the latter is towards the east ; it is, 

 therefore, nearly impossible to ascribe the drift of this heath to an 

 oscillation of the Mole. 



It remains for me to try and discover some reason why the Mole, 

 though possessing an undoubted advantage over the Wandle, only 

 cut it off finally from the W T ealden area by a flank movement in 

 the Weald Clay instead of by direct attack in the Lower Greensand. 

 If we could suppose that the latter had always had as narrow an 

 outcrop as at present, no explanation would be needed ; but this 

 cannot be admitted. On the subaerial hypothesis the Greensand 

 was first exposed near the central watershed, where the strata 

 are but slightly inclined, and where therefore its outcrop must 

 have been broad ; and then the escarpment retreated slowly 

 for a distance of 12 or 13 miles before arriving at its present 

 position. Surely, during all the length of time that this implies, 

 there must have been abundant opportunity for the growth of sub- 

 sequent streams ? The comparative thinness of the strata at this 

 point, and local differences in composition, would certainly have 

 their effect, but I very much doubt whether they afford a sufficient 



