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 rain. 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 



