EROSIVE WORK 



59 



lead to ravines. Valleys, like streams, usually end at the ocean or 

 a lake; but in arid regions many of them 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 

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





Fig. 41. Slope with numerous gullies, the smaller ones joining the larger ones. 

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



large valley go together so commonly, however, that the combina- 

 tion cannot be accidental. 



EROSIVE WORK OF RUNNING WATER 



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

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

 \\rar. The rate of flow is determined chiefly by (i) the gradient 

 (slope), (2) the amount and especially the depth of water, and (3) 

 the amount of sediment (load) it is carrying. The steeper the gradi- 

 ent, the deeper the water, and the less its load, the faster it flows. 

 When it flows off in a sheet, as on a smooth surface, the depth of the 

 water is slight, the flow not very swift (unless the slope is very steep), 

 and the wear correspondingly slight. Such wear is sometimes 

 called sheet erosion. 



