CHAP. VIII BIVERS AND LAKES 285 



instance, the Wye, the Mole, the Darenth, 

 the Medway, and the Stour; and on the 

 south the Arun, the Addur, the Ouse, and 

 the Cuckmere. 



They do not run in faults or cracks, and 

 it is clear that they could not have excavated 

 their present valleys under circumstances 

 such as now exist. They carry us back in- 

 deed to a time when the Greensand and 

 Chalk were continued across the Weald in a 

 great dome, as shown by the dotted lines in. 

 Fig. 38. They then ran down the slope of 

 the dome, and as the Chalk and Greensand 

 gradually weathered back, a process still in 

 operation, the rivers deepened and deepened 

 their valleys, and thus were enabled to keep 

 their original course. 



Other evidence in support of this view 

 is afforded by the presence of gravel beds 

 in some places at the very top of the Chalk 

 escarpment — beds which were doubtless 

 deposited when, what is now the summit 

 of a hill, was part of a continuous slope. 



The course of the Thames offers us a some- 

 what similar instance. It rises on the Oolites 



