CYCLE OF EROSION 69 



advam-ing years, the slope of a graded valley normally decreases. 

 The same principles apply to graded surfaces outside of valleys. 



When a stream has brought the bottom of its valley to grade, 

 it may be said to be at the level of base-level if the gradient is low; but 

 a narrow valley flat at this level is not a base-level. This term, 

 in the sense of a base-leveled plain, is applied to extensive areas 

 only. Any extensive area* degraded by running water to essential 

 flatness is a base-level. Under later conditions of erosion, even with- 

 out uplift, a base-leveled surface may be reduced (slightly) to a 

 lower base-level. There is no sharp distinction between a base- 

 level and an extensive graded surface of low gradient, if the latter 

 was reduced by running water. 



The ocean may be looked upon as a barrier which in a general 



Fig. 58. A shallow river valley in a plain. Cerro Gordo Co., la. Contrast 

 with Fig. 59. (Calvin.) 



way limits the down-cutting of running water. Other barriers, 

 such as lakes, and outcrops of hard rock in a stream's bed, 

 have a comparable, though more local and temporary, effect on the 

 development of valley plains above themselves. Plains thus de- 

 veloped have been called temporary base-levels. 



Stages in a cycle of erosion. Since river valleys have a begin- 

 ning and pass through various stages of development before the 

 country they drain is base-leveled, it is convenient to recognize 

 their various stages of advancement. Nor is this difficult. An 

 old valley and a young one have different characteristics, and the 

 one would no more be mistaken for the other by those who have 

 learned to interpret them, than the face of an aged man would be 

 mistaken for that of a child. 



Youth. The cycle begins with the beginning of valley develop- 

 lent, and at that stage drainage is in its infancy. The type of the 



