PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 145 



too great, but the instability of the soil together with its shallow- 

 ness forbid such a type; and instead there occurs a vegetation 

 rather low, more or less open, and in some respects not unUke 

 that occurring in the deciduous forest stage of the swamp series. 



The composition here is usually as follows: The chief tree 

 representatives: Betula lenta, Acer ruhrum, Tsuga, and Primus 

 mrginiana. The following members when represented occur 

 upon the upper portions of such slopes: Quercus alba, Quercus 

 velutina, Amalanchier canadensis, Primus pennsylvanica, Popu- 

 lus tremuloides, Populus grandidentata, Rhus typhina and 

 Quercus coccinea. The herbaceous and shrubby species most 

 typical of these steep, moist slopes include Aspidium spinulosum, 

 Osmunda cinnamomea, Silene virginiana, Lonicera americana, 

 Lonicera Sullivantii, Lonicera dioica, Smilacina racemosa, Con- 

 ocephalus conicus, Pellia epiphylla, Aster junceus, Solidago 

 Drummondii, Ruhus idceus, Agrimonia gryposepala, Cornus 

 stolonifera, Gaylussacia, and Vaccinium. 



Flood Plains 



Flood plains are most extensively developed by rivers during 

 their late history when they flow through broad winding valleys. 

 The gradient of the stream is then low and the valley slopes long, 

 low, and sweeping. They appear much earlier in the history 

 of the river, but it is in the broad river valleys that the most 

 characteristic plains appear. Their topography is practically 

 level and the soil chiefly alluvial in nature. When the river has 

 too low a gradient, and does not cut a channel of considerable 

 depth, the plain is submerged during periods of high water. 

 Under these conditions the water table is high, and the area is 

 broken up by the presence of loops and old channels, which re- 

 tain water and give the whole region a low swampy aspect. 

 Very commonlj^, however, there is a channel eroded through the 

 plain to sufficient depth to retain the water during ordinary 

 stages; so that it is only at rare intervals of flood that the upper 

 portion of the plain is submerged. The deepening of the river's 

 bed insures better drainage of the plain, and together with the 



