B. H. Chandler — Dry Chalk Valley Features. 539 



out' (or become extinct) at a very few hundred yards south of the 

 present Chalk escarpment, and this short length is not compatible with 

 the excavation of a valley 100 feet deep, such as the present wind gap. 



A feature that appears to have been overlooked, by the various 

 geologists who have written about this subject, is that the beheading of 

 ' dry ' Chalk dip valleys, by the formation of strike Gault valleys behind 

 them, would cause a lowering of the saturation level of the Chalk; 

 and as the Gault would probably be rapidly excavated, the lowering 

 of the saturation line of the Chalk would be proportionate^ rapid.' 



Now the early dip stream initiating the ' dry ' valley system would 

 doubtless flow at about the level and the gradient of the line of 

 saturation in the Chalk at that time, so that when the valley was 

 beheaded, and the saturation level lowered (owing to the Gault 

 Stream), the springs in the 'dry' valley would be thrown out at 

 successive! J- lower levels, and by the upper parts of the valley being 

 dry earlier than the lower, and the well-known action of springs in 

 cutting backward, thus forming a steep bank behind, it is here 

 proposed that the present progressive increased steepness of the valley 

 floor upward may be explained. 



The characteristic deposits of gravel, consisting of angular and little- 

 worn flints, few Tertiary pebbles, and occasional pieces of Lower 

 Greensand chert and ironstone, in a clayey matrix, found on the floors 

 of these valleys, seem to show that very little of it was formed by 

 ordinary river action, but that the deposit was due to the dissolution 

 of the chalk from around the flints by a stream of water without 

 much transporting power, such as might take place by a flow parallel 

 to and below the valley floor," and each such flow by lowering the 

 chalk and leaving the unworn flints would deepen the deposit of 

 ' dry valley gravel ' found in these situations ; added to which the 

 occasional existence of a ' bourne ' would account for the presence of 

 bedding rarely shown by this gravel. 



Anything more unlike ordinary river gravel than this ' dry' valley 

 gravel it is difiicult to imagine, and as bournes do occasionally break 

 out it seems reasonable to suppose that when the bourne is not visible 

 at the surface it is flowing some distance underneath, dissolving the 

 chalk and leaving the insoluble constituents. The Lower Greensand 

 and other water-worn material are the relics of the early stream (or 

 they may be from the immediate ' plateau '), and the clayey part of 

 the drift is the insoluble residue of the chalk and the rainwash from 

 the slopes on either side. 



Features due to the early river action (as distinguished from the 

 later or bourne features) may be found in the general configuration 

 of the valley, and more especially in the upper (and older) parts of 

 the sides where steep banks occur alternately on either hand, due to 

 the river cutting into its banks on the outer part of its curves. 



1 The saturation line consisting of two curves meeting at some distance in from 

 the escarpment on the dip side, a gentle slope down the dip, and a steeper one falling 

 towards the escarpment. It is the lowering of the point of intersection of these two 

 curves, and the movement of this point in the direction of the dip, that is here spoken 

 of as the lowering of the saturation level in the Chalk. 



2 F/rfe Prestwich, Water-hearing Strata of London, 1851, p. 132. 



