PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 



117 



of the slopes, accompanied by slumping. This is especially 

 true in exposed situations, and the slopes there are practically 

 void of vegetation. 



The early stages of the clay ravine are seen to be xerophytic 

 in character. This is true of those which are exposed; but if 



Fig. 4. Undergrowth in the deciduous stage of the broad ravine, showing the 

 prolific development of ferns which is a characteristic feature. Osmunda 

 claytonia, and Polypodium vulgare are the forms most conspicuous above. 



such development takes place in protected areas, as for example 

 in a mesophytic forest, it may present an entirely different 

 aspect. The erosion here is much slower and is seldom suffi- 

 ciently rapid to completely remove the humus from the slopes; 

 although there is usually a distinct creep, resulting in a greater 



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