138 



GEOLOGY. 



be to aggrade the channel at that point and so to prevent the develop- 

 ment of the fall. Falls occur only on streams which have relatively 



FiG. 118. — The Upper Yosemite Falls. 



high gradients. This means that the streams which have falls are 

 well above base-level, and streams well above base-level are young. 

 Falls therefore are a mark of topographic youth. 



