—— 
ee eee 

Igor] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 95 
surface wash the slopes become more and more gentle. Meso- 
phytic vegetation comes in at the foot of the bluff and creeps 
up as the slopes decrease. Finally the xerophytes are driven 
from their last stronghold, the top of the slope, and the meso- 
phytes have come to stay, at least until the river returns and 

7-—Ravine in the Niagara limestone at Lockport, showing the beginnings 
FIc, 
of a flood plain. 
enters upon another stage of cliff erosion. The growth of a 
ravine intoa valley with xerophytic bluffs is rapid, when expressed 
in terms of geology, but far less rapid when expressed in terms 
of vegetation. A ravine in the vigor of youth may develop so 
slowly that forest trees may grow to a considerable size with- 
out any perceptible change in the erectness of their trunks. 
Thus in figs. 2 and 5 it will be seen that most of the trees stand 
approximately vertical. But the activity of the erosive forces, 

