CYCLE OF EROSION 



then- arc upland plains or plateaus. The profile of the surface 

 about two adjacent valleys is represented in cross section by the 



: 



! 



1'iir. 54. Diagram to illustrate the leveling of the surface by valley erosion. 

 The profile represented at the top shows two young valleys, i and i, in an otherwise 

 flat surface. In time these valleys will develop the cross-sections represented by 

 j and 2, and later those represented by 3 and 3, 4 and 4, etc. The divide between 

 tlu-iii may finally reach 5, and the surface is then nearly flat. 



uppermost line in Fig. 54. As the valleys a and b are widened to 

 a' and b', the adjacent uplands are narrowed correspondingly. 

 When the valleys have attained the form represented by 3-3, the 

 intervening upland has been narrowed to a ridge, and the valley 

 flats have become wide. With continued erosion the ridge will be 

 lowered still more, and in time the surface will approach a plain. 

 In this condition it is known 

 as a peneplain. When the 

 ridges are obliterated the 

 peneplain passes into a base- 

 leveled plain. 



Tributaries are almost 

 sure to develop along each 

 main valley and their heads 

 ork back across the up- 

 lands between the main val- 



Fig. 55. Diagram showing tributaries in 

 an early stage of development. 





leys, dissecting them into secondary ridges (Fig. 55). Tributaries 

 develop on the tributaries, and these tertiary valleys dissect the 

 secondary ridges into ridges of a lower order. This process of 

 tributary development goes on until drainage lines of the fourth, 

 fifth, -sixth, and higher orders '. - ..:< ' tf;. ( f^ - v? ^ v.f ! -"> '%; ?'" 



are formed (Fig. 56). Since 

 the process of valley devel- 

 opment under such circum- 

 stances is also the process of ' 

 ridge dissection, a stagr is 

 presently reached where the 

 ridges are cut into such short 

 sections that they cease to 

 be ridges, and become hills 



Fig. 56. Diagrammatic rep 

 a surface much dissected by 

 ment of numerous tributaries. 



esentation of 

 the develop- 





