Vol. 63.~] ORIGIN OF CERTAIN CANON-LIKE VALLEYS. 507 



the Wash, the escarpment is broken up and the Eenward slope is 

 more gradual ; north-east of Dunstable its elevation suddenly falls 

 something like 200 feet, its face being out of line, as if eaten 

 back by erosion. While, however, the surface of this region has 

 been disturbed by the overriding ice, it has been protected from the 

 denudation which has affected the non-glaciated area, the erosion of 

 the one representing, I believe, the glaciation of the other. 



It seems to me probable that the denudation by which the one 

 part of the escarpment is distinguished from the other took place 

 during the Pleistocene Epoch, 1 under meteorological conditions very 

 different from those which now obtain in Great Britain. 



At present the abnormal valley -erosion and the accumulation of 

 gravel contemporaneous with it, and characteristic, I think, of the 

 Pleistocene Epoch, not only in this country but abroad, have to 

 a large extent ceased. 2 During the recent period such phenomena 

 appear to have been on a comparatively insignificant scale. 3 Much 

 of the area in question, once furrowed by torrents of swiftly- 

 running water, is now dry, and even where streams exist in the 

 lower parts of the valleys they are of an unimportant character. 



It seems to me possible that the conditions to which such 

 phenomena were due may not only have been coeval with the 

 existence of the ice-sheets, but have been also caused by them. 

 The extension of an immense and permanent body of ice, during 

 summer and winter alike, so far south as lat. 50° N. in Europe, 

 and 30° N. in North America, must have set up meteorological 

 disturbances of a far-reaching character. This subject, however, 

 cannot be pursued further iu the present paper. 



X. The Chalky Boulder-Clay and the Chalk-Escarpment. 



Prof. J". W. Gregory, in the paper before mentioned, has adopted 

 the view that the Chalk- escarpment was in existence before the 

 advance of the ice, the hills rising above the level of the latter. 



The ice-sheet split, he thinks, into two parts, the one keeping to 

 the south of the Chalk-ridge as far as St. Albans, Hendon, and 

 Einchley, the other flowing along the foot of the escarpment past 

 Hitchin and Dunstable, whence it trended to the north-west towards 

 Leckhampstead. 4 He believes, however, that the plateau-gravels 

 are older than the Boulder-Clay, a prolonged period of denudation 

 separating them ; correlating the former (other than the so-called 



1 The obsequent valleys now filled with Drift at Hitchin and Newport, 

 referred to on p. 494, represent a pre-Glacial denudation of the Chalk- 

 escarpment, and (with other facts) attest the pre-Glacial age of the latter. 



2 I use the term Pleistocene to include, not only beds originating during 

 the great extension of the ice-sheets, but some, formerly described as post- 

 Glacial, which were deposited while they were gradually disappearing. 



3 In some parts of East Anglia there has been but little erosion since Glacial 

 times. 



4 ' Natural Science ' vol. v (1894) p. 103. 



