PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 

 WITH SPECIAL REFERENCE TO PHYSIO- 

 GRAPHIC RELATIONSHIP. II 



J. E. CRIBBS 



Grove City College, Grove City, Pennsylvania 



THE RIVER SERIES 



The vegetation associated with physiographical features 

 developed by rivers is in some respects similar to that just 

 described for the valley. There is the young topography of the 

 river, when the eroding or sandy shores are succeeded almost 

 immediately by steep cliffs; and at the other extreme there is 

 the old topography which is characterized by a broad floodplain 

 and low sweeping valley slopes. The stages between these are 

 transitional and are closely comparable to the smaller valley type. 



Bluffs 



River bluffs may be best studied along the Allegheny River, 

 for throughout much of its course it is included by abrupt slopes 

 which rise from either side to a height of 200 to 500 feet. At 

 most points on the river there is considerable talus formed at the 

 base of the slopes, and the rocks which outcrop here are thus 

 covered in part by loose rocky soil which affords an excellent 

 place for plant development. However, when the outcropping 

 rocks form vertical cliffs, there is presented a situation, which, 

 because of the extreme exposure to desiccating influences, limits 

 the vegetation to a few crevice plants. Aquilegia canadensis, 

 Verhascmn, Clematis virginiana, Ruellia ciliosa, Rhus toxico- 

 dendron, Poa compressa, etc., may appear in this position. 



In striking contrast to the xerophytism of the rock bluffs 

 is the talus below them. This is formed by the accumulation of 

 weathered materials from the cliffs above and hence is practically 



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