LAKE BLUFF, RAVINE, AND RIVER VALLEY 263 



back and forth, depositing in some spots what it wears away in 

 others. The bottom of the ravine widens into a miniature flood 

 plain. The sides become less steep. Possibly in the lower 

 reaches the stream has cut down nearly to the level of the lake or 

 larger stream into which it flows and so runs sluggishly. It is 

 no longer a brawling brook, but a placid little creek. The 

 ravine has broadened into a miniature valley. All these stages 



FIG. 393. Rock ravine with vegetation on the sides, near the "Sag" 



are admirably seen along the north shore of the lake where the 

 clay bluffs abut upon the shore (Figs. 5 and 6). 



Since it is only the stream that is capable of rapid erosion in 

 rock, the rock ravine is prone to end at its upper end in a water 

 fall (Fig. 9). This fall gradually retreats as the stream eats its 

 way through the rock. In relatively soft rock the fall is likely 

 to be quite a high one, and the ravine ends abruptly. If the 

 rock is hard, a series of rapids may replace the fall, and the 

 ravine gradually becomes more and more shallow to its head. 



