﻿650 ME. H. BURY ON THE DENUDATION OP [JSoV. I9IO, 



(C) Prestwich's Hypothesis. 



Prestwich supposed that a Wealden island was formed at the 

 elose of the Cretaceous Period, over which the Chalk, though much 

 thinned by marine denudation, extended daring the Eocene Period 

 .(17, pp. 330-31) ; and consequently no Eocene beds were deposited 

 in this area, parts of which have probably remained dry land ever 

 since. In early Pliocene times a broad area separated England from 

 Belgium, and the North Downs, at least as far as Dorking, were 

 submerged (18, p. 167) ; but the extent of marine action is uncertain. 

 In the early stages of the Ked Crag a renewed upheaval united 

 the "Wealden peninsula to the Continent, and raised its central 

 mountains to 2000 or 3000 feet above the sea (18, pp. 172-73) : 

 ■even then, however, the Chalk had been so little denuded that its 

 thickness over the Crowborough district may have amounted to 

 100 feet (18, p. 169). The flanks of this mountain- chain were 

 scored in pre-Glacial times by transverse streams, more numerous 

 than at present ; but these at first had a wide lateral spread, and 

 only gradually became restricted to definite channels. The longi- 

 tudinal valleys, and with them the escarpments, were formed by 

 glacial action at a later stage. 



This last view will receive but little support at the present day, 

 nor will many be found to admit that rivers which arose at the 

 end of the Cretaceous Period could have remained without definite 

 ehaunels, as Prestwich appears to believe, until after the deposition 

 of the Southern Drift in Pliocene times. But I refrain from 

 criticizing in detail these and other points which are not really 

 essential to his theory, and pass on to more important matters. 



The absence of Wealden pebbles from the Pliocene beds is sup- 

 posed to prove (19, p. 157) that the Weald Clay and Hastings Sands 

 were not much exposed at this period, and is therefore held to 

 afford a weighty argument against extensive marine planation. 

 Topley (26, p. 293) had previously pointed out that the Wealden 

 sandstones have very little power of survival in running water, and 

 soon wear down into sand, so that Prestwich's inference from their 

 absence is hardly justified. Moreover, the Weald Clay yields an 

 inappreciable number of such pebbles, while the exposure of any 

 large surface of Hastings Beds is by no means essential to the 

 hypothesis of a marine plain. The centre of such a plain can hardly 

 be placed lower than the summit of the North Downs (882 feet O.D.), 

 and, if I am not mistaken, the addition of 100 feet of strata to 

 Crowborough Beacon (792 feet) would cover up the Hastings Beds 

 altogether. 



By a somewhat similar argument the rarity of chert in certain 

 beds of Southern Drift is taken to indicate that, when these drifts 

 were formed, the Lower Greensand was not widely uncovered 

 (18, pp. 168-70). So far as the Parnham plateau is concerned this 

 view is tenable, although there is something to be said against 

 it ; but, in applying the same reasoning to the Well Hill drift, 

 Prestwich seems to have forgotten his own account of the supposed 



