EFFECT OF GROUND-WATER ON EOLIAN EROSION 271 



those beds became exposed at the surface, owing to the removal of the overlying 

 confining strata, their contained water escaped in such quantities as to have given 

 rise to a lake of considerable dimensions. 



A lake formed in such a manner would certainly put a sudden stop 

 to the downwearing effects of deflation. 



Mr. H. G. Lyons, 1 on the other hand, in an article written many 

 years before, states that during the erosion of the Nile Valley the 

 cutting-back of the escarpment separating the overlying limestones 

 from the underlying Nubian sandstone encroached upon the 

 southern limit of the oases, and let loose springs which greatly 

 increased the rate of erosion. This would appear to be a case 

 where the presence of ground-water facilitated erosion. 



Again, Mr. F. J. Bennett, 2 in 1908, seems to have the germ of 

 the same idea, when, discussing the solution-subsidence valleys 

 and swallow holes within the Hythe Beds area of West Mailing 

 and Maidstone, England, he suggests that the "upward hydro- 

 static action of water under pressure .... is a new contributing 

 factor in valley formation, and that this in conjunction with 

 subaerial stream erosion" formed the valleys and swallow holes 

 described. This, however, is an application to a region of normal 

 rainfall and is not strictly a propos. 



The idea advanced by Mr. Beadnell may be applicable to other 

 desert regions. It is, at any rate, worth considering, in connection 

 with Mr. Keyes's article, as a possible modus operandi of one 

 limiting effect of ground-water upon wind erosion. 



J "On the Stratigraphy and Physiography of the Lybian Desert of Egypt," 

 Quart. Jour. Geol. Soc, L (1894), 531-47. 



2 "Formation of Valleys in Porous Strata," Geog. Jour., XXXII (1908), 277-88. 



