84 



GEOLOGY. 



of an erosion cycle is sufficiently long for even very slow changes to 

 reach great magnitude. The sea-level, therefore, can hardly be accepted 

 as the absolute base-level, unless (1) the absolute base-level is a variable, 

 and unless (2) the absolute base-level be a surface below which rivers 

 may cut to the extent of at least 250 feet. 



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

 limits the down-cutting of running water; for only very large streams 

 cut much below its level. Other barriers, such as lakes, and the out- 

 crops of hard rock in a stream's bed, have a comparable, though more 

 temporary, effect on the development of valley plains above. Plains 

 thus developed have been called temporary hase-levels. They differ 

 from other graded plains in being controlled primarily by a barrier 

 below, rather than by conditions which exist above. 



Since river valleys have a beginning and pass through various stages 

 of development before the country they drain is base-leveled, it is impor- 

 tant 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. 



■piG, 67.— A gully developed by a single shower. (Blackwelder.) 



The cycle begins with the beginning of valley development, 

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

 fant valley is the gully or ravine (Figs. 67 and 6S). It has steep 



