﻿652 MR. H. BURT ON THE DENUDATION OF [Nov. I9IO, 



we take (a) the dotted line, showing the probable southward con- 

 tinuation of the Chalk Plateau, and (b) a line drawn through the 

 junction of the Chalk and Upper Greensand, we shall find that they 

 cut one another at no great distance from the present escarpment. 

 The exact point of section will depend on the curve which we give 

 to the second line ; but it would be unreasonable, I think, to place 

 it more than 5 miles south of the escarpment in fig. 1, while in 

 fig. 2 this distance might even be reduced to 1| miles. There is 

 strong reason, therefore, to believe that, when the plateau was 

 formed, the southern limit of the Chalk occupied a position which 

 agrees closely with Uamsay's diagrams. 



What, then, is the nature of this plateau? I dismiss at once the 

 idea, vaguely suggested by Prestwich, that it was the work of pre- 

 Glacial rivers, because it does not seem possible that these rivers 

 could effect such widespread and uniform planation. Prestwich's 

 sections appear at first sight to show a great difference of angle 

 between the plateau and the pre-Eocene Chalk surface ; but the 

 presence elsewhere of thin Eocene outliers upon the former indicates 

 that this difference is to some extent due to a flexure which has 

 affected both formations, especially along a line somewhat to the 

 north of the plateau. Yet this plateau cannot be regarded as simply 

 the old pre-Eocene surface laid bare by removal of the Eocene 

 strata : first, because this would mean that the Lower Greensand 

 was already exposed within a few miles of the present escarpment, 

 and the Eocene deposits ought to contain chert pebbles ; secondly, 

 because the Tertiary outliers in the neighbourhood of Caterham and 

 Reigate are themselves bevelled off so as actually to form a part of 

 the plain. The simplest explanation, therefore, of the Chalk Plateau 

 seems to be to regard it as the product of the Diestian sea, while 

 admitting that in some cases it differs but slightly from the old 

 pre-Eocene plain, and that in others it has been modified by early 

 consequent rivers as well as (in all probability) by solution of the 

 underlying Chalk. 



(D) Warpings of the Plain. 



Although the formation of a marine plain can hardly, from the 

 evidence so far adduced, be regarded as definitely established, yet 

 it will be convenient at this stage to assume its existence, and, 

 deferring further proofs to the third section of this paper, to examine, 

 so far as the limited data will allow, the warpings to which that 

 plain was subjected during upheaval. In the eastern half of the 

 North Downs (Botley Hill to Folkestone) the dotted line joining the 

 summits (summit-line) in fig. 2 (p. 642) will probably be accepted, by 

 all those who admit planation at all, as approximately representing 

 the plain itself. It shows a remarkably uniform fall towards the 

 east, and is reached by at least one hill in each interspace between 

 the rivers. To the west of Botley Hill there is a similar fall towards 

 the west as far as Earnham, and the apparently anomalous position 

 of Holybourne Down can be accounted for by its being nearer to the 



