112 J. E. CRIBBS 



carried. Since the vertical cutting is more rapid than the lateral, 

 small streams of this sort soon become flanked on either side by 

 steep slopes; the steepness being dependent upon the nature of 

 the soil and the position of the rock bed beneath. 



When ravines are formed in the rock which underlies a shal- 

 low soil, they are characterized by steep or almost vertical slopes; 

 but when formed in the glacial clays, the slopes, steep at first, 

 are much more rapidly eroded as the stream increases in length, 

 and volume of current. Thus, during the later stages of any 

 stream's history there is a series of topographical features al- 

 ways present, each one of which, because of the different physical 

 conditions accompanying it, displays a floral composition of it's 

 own. The clay and rocky ravines will be found on the outer and 

 higher margins of the basin, and are continuous below first with 

 the broad ravine, then with the narrow valley, and finally with 

 the broad open valley where the slopes are of low gradient and 

 the stream has built an alluvial floodplain of the material carried 

 down from the higher levels. 



There is a close correlation of the plant associations to the 

 physical factors involved in these habitats; and the variation of 

 certain of these factors, as for instance the position of the water 

 table, or the exposure to direct sunlight, etc., is responsible for 

 accompanying changes in the vegetation. 



The Rocky Ravine 



Rocky ravines develop where there is an outcropping of under- 

 lying rock, or where it approaches close to the surface. They 

 are commonly found on side slopes of broad valleys where the 

 action of erosion by the main stream has exposed the rock strata, 

 and small tributaries entering, have cut their way through. 



Such ravines at first are but a few feet in width, and because 

 of the highly resistant character of the rock, the walls are 

 usually quite vertical. By the process of erosion they are 

 slowly broken down and the material resulting from the weather- 

 ing collects at their bases forming small talus slopes upon which 

 most of the vegetation develops. The narrowness of such de- 



