THE WEALD 595 



the past history of this area. It is that the chalk never covered 

 the Weald, but that the sandstones, sands, clays, etc., of the 

 latter once formed an island or bank, laved by the sea in the 

 depths of which the chalk was being deposited ; so that when 

 movements of the earth's crust gradually forced up the wealden 

 strata to form the present anticlinal, the chalk fringing it 

 was uplifted at the same time to form its skirting. The pro- 

 cess is indicated by the slight dip north and south of the two 

 lines of Downs : in the valley of the Ouse the steady rise from 

 the sea at Newhaven to the escarpment of the chalk is well 

 marked ; so is also the gradual drop from the North Downs 

 to the Thames Valley. 



Everything points to the beds subjacent to the chalk having 

 been pushed up by the anticlinal thrust well above the general 

 level of the chalk strata (see dip of greensand in Plate I) ; 

 and it can be shown that this region, after its emergence, has 

 in the main been subjected only to agencies of the simple 

 character that prevail to-day. 



There is no need to call in the forces of the sea nor of the 

 glacial epoch ; we have only to consider how it happens that 

 the chalk, which at one time only represented the lower slopes 

 of a mountain region, has now come to form an escarpment, 

 from which the panorama of the Weald is viewed, as a wide 

 basin, below it. 1 



There is an analogous case of an anticlinal area in North 

 America, where a portion of the Appalachian range presents 

 the same kind of ' transformation ' scene. High mountains 

 thrust up by a similar wrinkling of the crust, and then worn 

 away faster than the adjacent slopes, which formed their 

 setting, have changed the mountains into a valley, while the 

 more durable flanks have become the highlands. 2 



This ' differential erosion ' is exactly what has happened 

 in the case of the Weald. 



1 Col. Greenwood, in his book Rain and Rivers, 1857, strongly insists on the 

 fact that the Weald is really a kill, not a valley. It is only the great plain of the 

 Weald Clay surrounding the central part that can in any sense be described as a 

 valley. 



' The process in this instance is thus described in Chamberlin and Salisbury's 

 Geology : " In the structural adjustment which goes with the erosion of folds, it 

 often happens that the valleys come to be located on the anticlines, after they have 

 been worn down, while the outcrops of the hard layers on the flanks of the 

 anticlines become the mountains." 



