114 



GEOLOGY 



streams flow, generally unite to form wider and deeper ones (Fig. 73) . 

 These, in turn, join one another and become ravines, which are but 

 larger depressions of the same sort, and ravines lead to valleys, just 

 as gullies lead to ravines. Valleys, like streams, usually end at the 

 ocean or a lake; but in some cases, especially in arid regions, they 

 end on dry land. 



There is as a rule, some relation between the size of a valley and 

 the stream which follows it, though this relation is not one which 



Fig. 73. Slope with numerous gullies, the smaller ones joining the lar^-r 

 ones. Scott's Bluff, Neb. (U. S. Geol. Surv.) 



can be stated in mathematical terms. The large stream and the 

 large valley go together so often, however, that the combination 

 cannot be accidental. 



EROSIVE WORK OF RUNNING WATER 



Wherever water flows over the land, it wears the surface on 

 which it flows, and the faster it flows, the greater its power of 



