1901] PAYSIOGRAPHIC ECOLOGY OF CHICAGO 157 
river series, other activities are at work in a young topography. 
The swamp series which has just been discussed is one illustra- 
tion. So also there are hills which are not due to erosive 
processes, but to other causes, notably in our region morainic 


"IG. 26.— Slope of limestone ledge at a island, showing mosses and higher 
plants aubaiie themselves in the crevice 
hills and sand hills. There are rock hills also which are. not 
connected with the present erosion cycle. All of these hill 
types have their peculiar vegetation features and must be dis- 
cussed apart from river activities, since they have an interesting 
history before they are attacked by stream erosion. 
We may speak first of rock hills, which in the vicinity of 
