158 PROF. PRESTWICH ON 'IRE AGE, FORMATION, AND 



their edges now form long escarpments, which are still receding in 

 the direction of the dip of the strata, and therefore at right angles 

 to the slope of the scarp " *. 



That a plane surface of nneqiial resistance should suffer unequal 

 wear from the effects of rain and weathering is not to be contested, 

 but on the hypothesis that the escarpment of the North and South 

 Downs is due to ordinary slow action of this description, we should 

 assuredly expect to find in the valleys below them the harder and 

 indestructible debris of the removed strata, such as the flints in the 

 Chalk arid those in the overlying Eed Clay, and the pebbles of the 

 Tertiary strata. 



Sir A. Ramsay felt the difficulty, for he observes that " the absence 

 of flints over nearly the whole of the Wealden area, excepting near 

 the Downs, is explained by this hypothesis, for the original marine 

 denudation had removed all the Chalk, except near the margin (see 

 fig. 73), long hefore the rivers had begun simultaneously to scoop out 

 the valleys of the interior^ and to cut the transverse valleys across the 

 North and South Downs " f. In the section referred to the Chalk 

 is shown to extend no farther than the edge of the Lower-Green- 

 sand escarpment, a distance of four miles from the Chalk escarp- 

 ment. But is not this limited range based on the very assumption 

 of a fact which has to be proved ? 



Taking the range of the Chalk from Crossness in the centre of 

 the Thames Yalley, where its thickness is known, to the edge of 

 the Chalk escarpment at Otford, a distance of 14 miles, we find it 

 diminished from 650 ft. to 450 ft., a total reduction of 200 ft., or 

 of 14^ ft. per mile. At this rate the Chalk should have extended 

 31 miles beyond the escarpment, or, taking only the Chalk-with- 

 flints, some miles (16 ?) less. 



Within this area, if the Chalk had been worn back by ordinary 

 subaerial agencies alone, we ought to find some evidences at the 

 foot of the hills of the wreck of the Chalk with its massive layers 

 of flints, of the pebble-beds of the Tertiary strata, and of the bed of 

 Eed Clay-with-flints (both of which latter may have extended farther 

 than the Chalk-with-flints), in the manner represented by Ramsay 

 in fig. 70, p. 336, of his work above referred to ; but there is no bed 

 nor any talus of that description. Mr. Topley, however, is of opinion 

 that " we cannot expect to find any direct evidence that the escarp- 

 ments have been formed and worn back by subaerial agencies," but 

 considers that the whole features are such as can be readily explained 

 by subaerial denudation, whilst all ether agencies are inadequate to 

 account for the work donej. 



Nevertheless, if the hypothesis is to be accepted, some such direct 

 evidence ought to be forthcoming, even if we assign a more re- 

 stricted range southward to the Chalk and confine it to the limits 

 assigned by Ramsay : or, at all events, the drift in the valleys 

 within those limits should be in accordance with that hypothesis. 

 On this point the sections in the Yalley of the Darent offer a 



^ ' Physical Geology and Geography of Great Britain,' 5th edit. p. 351. 



t Op. ^cit. p. 344. 



+ Mem. Geol. Survey, ' Geology of the Weald,' p. 300. 



