286 THE BEAUTIES OF NATURE CHAP. 
near Cirencester, and cuts through the escarp- 
ment of the Chalk between Wallingford and 
Reading. The cutting through the Chalk has 
evidently been effected by the river itself. 
But this could not have happened under 
existing conditions. We must remember, 
however, that the Chalk escarpment is gradu- 
ally moving eastwards. The Chalk escarp- 
ments indeed are everywhere, though of 
course only slowly, crumbling away.  Be- 
tween Farnham and Guildford the Chalk is 
reduced to a narrow ridge known as the 
Hog’s Back. In the same way no doubt the 
area of the Chalk formerly extended much 
further west than it does at present, and, in- 
deed, there can be little doubt, somewhat 
further west than the source of the Thames, 
almost to the valley of the Severn. At that 
time the Thames took its origin in a Chalk 
spring. Gradually, however, the Chalk was 
worn away by the action of weather, and 
especially of ram. The river maintained its 
course while gradually excavating, and sink- 
ing deeper and deeper into, the Chalk. At 
present the river meets the Chalk escarpment 
