THE WEALD 599 



bringing him very near to the truth, failed to see its signifi- 

 cance. The quotation is as follows 1 : 



" But there is another point beside the relative hardness 

 of the rocks. We find that the escarpments are composed of 

 porous rocks, whilst the plains at their feet are generally 

 formed of impervious rocks. Therefore a great part of the 

 rain, which falls on the top, or dip-slope, of the rock forming 

 the escarpments soaks into it, and has little or no mechanical 

 denuding effect. But a part of this water issues as springs 

 at the base of the escarpment, where the impervious bed occurs, 

 and thus tends to wear back the face." 



Again, on p. 265 in explanation of the coombs : 



11 At the foot of the escarpment there are frequently springs, 

 some of them of considerable power. These do not break 

 out at the prominent points of the escarpment, but in the 

 receding curves. Where these curves indent the escarpment 

 deeply they are called coombs. As a rule, the deeper and 

 more strongly marked the coomb, the stronger the spring 

 which it contains." 



In the face of these clear statements, which indicate that 

 the springs form the coombs, it is difficult to understand why 

 the ' dry ' valleys are considered not to be yet explained, 

 nor why there is " hesitation " in accepting the idea of subter- 

 ranean erosion. 2 (See Appendix, Underground Erosion.) 



The subterranean passage of water is well known in lime- 

 stone areas, where it causes sink-holes and caverns — though 

 not infrequently in chalk districts a modified form of these 

 also occurs — but with chalk it acts somewhat differently, and 

 owing to less cohesion in the mass, brings about a gradual 

 subsidence in the neighbourhood of the underground flow 

 whereby the chalk assumes that rounded appearance of its 

 ridges and the curves of its valleys which are so marked a 

 feature of chalk contours. 8 



Were it otherwise, the valleys would not be dry as the 

 upland valleys universally are, nor would they form these 



1 Topley's Geology of the Weald, p. 274. 



' In the Lewes-Falmer Valley there is striking evidence that the upper chalk, 

 with its flint strata, which now forms the top-soil, has been let down intact hundreds 

 of feet as a gradual subsidence caused by the subterranean erosion of the chalk 

 by the formerly invisible stream of the Winterbourne, and by its still hidden 

 tributaries. 



3 The V-shaped valleys are assisted in their formation by faults. 



