160 



PHYSICAL GEOLOGY 



increasingly steep. Finally, a vertical cliff is developed at A, 

 over which the stream falls (Fig. 158). The rapids have 

 been replaced by a waterfall. The falling water wears the 

 soft rock faster than the hard rock which overlies it, so that 

 projecting ledges of the hard rock are formed. From time 





FIGS. 156, 157, 158. Diagrams to illustrate the development and extinc- 

 tion of a waterfall. 



to time pieces fall down from these unsupported ledges, and 

 by this process of undercutting (sapping) the waterfall 

 retreats upstream. Downstream from the waterfall the 

 river develops a slope upon which it is at grade. It is evi- 

 dent from Figure 158 that as the waterfall retreats upstream 

 the bottom of the hard fall-making layer approaches the 

 level of the graded channel. Finally, the two meet (at B), 

 and the waterfall ceases to retreat because further under- 



