LAND WATERS STREAMS 



125 



The first effect of erosion by running water is to roughen the sur- 

 face 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 valley bottoms. 



Base-level, peneplain, grade. A base-level of erosion has 

 already been defined; but the mode of its development may now 



V 



^. 



Fig. 89. Diagram to illustrate the leveling of the surface by valley erosion. 

 The profile represented at the top shows two young valleys, 1 and 1, in 

 an otherwise flat surface. In time these valleys will develop the cross- 

 sections represented by 2 and 2, and later those represented by 3 and 3, 

 4 and 4, etc. The divide between them may finally reaches 5, and the 

 surface is then nearly flat. 



be illustrated in the light of the preceding discussion. Suppose a 

 land surface affected by a series of parallel young valleys without 

 tributaries (Fig. 89). Between them there is a series of upland 

 plains or plateaus. The profile of the surface between two adjacent 

 valleys is represented in section by the uppermost line in Fig. 89. 

 As the valley? are widened from 1 and 1 to 2 and 2, the inter- 



Fig. 90. Diagram showing tributaries in an early stage of development. 



vening upland is correspondingly narrowed. When the valleys 

 have attained the form represented by 3 and 3, the intervening 

 upland has been narrowed to a ridge, and the valley flats have be- 

 come wide. With continued erosion the ridge will be lowered still 

 more, and in time the surface will approach a plain. In this con- 

 dition it is known as a peneplain. The ridges will be obliterated 

 finally, and a base-leveled plain results. 



