59 8 SCIENCE PROGRESS 



On referring to the note in Plate I it is obvious that the cherts 

 and ferruginous sandstone fragments peculiar to this greensand 

 formation were washed on to the chalk plateau from an 

 originally higher level, though now the greensand is well below 

 it. 1 The significance of this appears to have escaped notice. 

 The friable nature of chalk is well known, but its resistance 

 to erosion when protected by turf or humus is not so apparent. 



Before discussing this question of the chalk erosion, I would 

 call attention to one feature of the section which strongly sup- 

 ports the interpretation here put forward, and furnishes a 

 most striking proof of its general correctness, in that it so 

 graphically acounts for the absence of flints, except along 

 the original margins of the chalk, which rested on the flanks 

 of the loftier wealden range. The limit of flint dispersion from 

 the north escarpment is really less in the main than shown in 

 this Plate. 



How does the nature of chalk lend itself to the idea of its 

 resistance to sub-aerial erosion ? 



In the chalk formation the question of erosion bears a 

 distinct and remarkable character. Chalk is absorbent, and 

 can contain its own weight of water. Consequently there are 

 no elevated surface streams, and nothing to act as a constant 

 carrying or denuding agent of surface material. 



Apart from direct or flank attack by the sea and by rivers, 

 chalk is mainly removed in solution by subterranean streams, 

 hence its peculiar resistance to surface weathering and its ' dry ' 

 valleys. 



The turf covering the chalk enables it to drink in water 

 like a sponge, and thus to last for ages without reduction of 

 general level. Rain, on falling, sinks down immediately until 

 it meets with the impermeable stratum of the lower chalk, 

 and then seeks a more horizontal exit, emerging as a spring 

 from that level to wear back the escarpment and form coombs, 

 or following the dip of the chalk to the sea, until it finds an 

 outlet ; thus initiating (and probably entirely forming) the 

 ' dry ' valleys, by carrying off lime in solution and thus 

 causing the subsidence of the overlying chalk. 



Mr. Topley, though he made the following observation, 



1 With the exception of Hindhead and Leith Hill, which are higher than the 

 chalk plateaus. It is noteworthy that there are no flints to the south of Leith 

 Hill, and that the contiguous chalk escarpment here has an unusually greater dip. 



