THE WORK OF STREAMS 



161 



cutting is impossible. The point of maximum wear is now 

 transferred to the brink of the waterfall at C. Wear at this 

 point presently destroys the verticality of the slope, leaving 

 a steep descent down which the water flows instead of fall- 



FIG. 159. Diagram showing falls 

 with beds dipping upstream. 



FIG. 160. Diagram of a water- 

 fall developed on vertical beds. 



ing. The waterfall has been succeeded by rapids. Finally, 



the top of the hard layer is eroded to the level of the graded 



channel, and the rapids disappear (at D, Fig. 158). (What 



would be the effect of an elevation 



of the valley ?) The same sequence 



of events would occur if the beds 



dipped upstream (Fig. 159), but 



the waterfall would retreat a 



shorter distance and would, other 



things equal, be shorter lived. 



Waterfalls may develop also where 



the beds are vertical (Fig. 160). 



(How would the history of such 



waterfalls differ from the history 



of those noted before?) Again, 



waterfalls may develop on beds 



dipping gently, but not steeply, 



downstream. (May there be rapids where the beds dip 



steeply downstream?) 



Where there are eddies in streams, and they are particu- 

 larly common at the bases of waterfalls and in rapids, 

 stones may be whirled round and round, wearing in the bed 

 cylindrical, well-like depressions, called potholes (Fig. 161). 



B. & B. GEOL. 10 



FIG. 161. Pothole in bed of 

 stream in Smoky Mountains, 

 near Hot Springs, North 

 Carolina. (Trowbridge.) 



