80 



GEOLOGY, 



A CYCLE OF EROSION. ITS STAGES. 



From what has preceded it is clear that the topography of a region 

 undergoing erosion will change greatly from time to time. The first 

 effect of erosion is to roughen the surface by cutting out valleys, leaving 

 ridges and hills. The final effect is to make it smooth again by cutting 

 the ridges and hills down to the level of the valleys. 



The base-level of erosion has already been defined ; but the mode 

 of its development may now be illustrated in the Ught of the preceding 

 discussion. Suppose a land surface affected by a series of parallel young 

 valleys without tributaries (Fig. 63). Between them there is a series 



Fig. 63. — Diagram showing three parallel valleys in a land surface. 



of upland plateaus. The profile of the surface between two adjacent 

 valleys is represented in section by the uppermost line in Fig. 64. As 





.H.. 



**4 



--.J 



v' 



1- 





,^' 



>-' 



'7< 





"^^;^ 



*•**, 



t'-— 



y 



J- 



is 



Fig. 64. — Diagram to illustrate the lowering of the surface by valley erosion. The 

 successive cross profiles of the valleys are represented by the lines 1-1, 1-1', 2-2, 

 2-2', etc. 



the valleys are widened from 1-1 and I'-l', to 2-2 and 2'-2', the inter- 

 vening plateau is correspondingly narrowed. When the valleys have 

 attained the form represented by 3-3 and 3'-3', the intervening upland 

 has been narrowed to a ridge, a, and the valley flats have become Avide. 

 With continued erosion the ridge will be lowered (to 6 and below), and 



