CHAP. VII RIVERS AND LAKES 985 
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 
