500 



Valleys and Dislocations. 



in some of the valleys, these faults when formed were, 

 as far as regards the present surface, thousands of feet 

 deep in the earth. All they could do might have been 



FIG. 98. 



a Present surface of the ground. 



The dotted lines show the continuation of an anticlinal curve 

 broken by a fault/. 



The dotted lines above the surf ace a a represent a certain amount 

 of strata removed by denudation. 



to establish lines of weakness along which subsequent 

 denudation may have excavated valleys. 



The real explanation of such cases as those of the 

 Wye and the Usk is this. At some period, now un- 

 certain, the beds of the Old Eed Sandstone, well 

 seen in the escarpment of the Beacons of Brecon, a-, 

 and the Caermarthen Fans, once spread much farther 

 westward, forming a great plain, b b (fig. 99), the result 

 of earlier denudations. This plain sloped gently east- 

 ward, and the dotted line shows the general state of 

 old outcrops of the strata. The river then ran over 

 ground perhaps even higher than the tops of the hills 

 of the present escarpment, and by degrees it cut itself a 



