THE WORK OF STREAMS 



157 



15. How many cycles of erosion appear to be shown by Figure 152 ? 

 What is the evidence of each cycle ? Is the evidence strengthened 



FIG. 152. Profile across Dunning and Tussey mountains, Pennsylvania. 

 Length of section nearly nine miles. Vertical scale about two and one half 

 times the horizontal. (U.S. Geol. Surv.) 



by the fact that the beds which underlie the region are tilted? 

 Reasons? In what stage of the present cycle is the region? 



FEATURES DUE TO SPECIAL STRUCTURES AND UNEQUAL 

 HARDNESS 



Rapids and falls. A rapid is a place in a stream where 

 the current has exceptional velocity (Fig. 153), while a fall 

 is a place where the water drops (Figs. 154 and 155). Rapids 



FIG. 153. Chandlar Rapids in Chandlar River, Alaska. 

 U.S. Geol. Surv.) 



(Schrader, 



and falls develop in various ways, the more important of 

 which may be noted. If a stream formed by the overflow 

 of a lake (p. 142) were to flow over a vertical cliff, a fall 

 would result. If a main valley were deepened by a glacier, 



