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base of the cliff. After this erosion ceases, the plant succession is deter- 
mined for a time by the slope of the cliff. At first, only shelf plants can 
gain a foothold. As the slope is reduced, the number of species is increased. 
After a time, though, when a soil of considerable depth has formed, the 
succession becomes biogenic. The plants hold the soil, and the reduction 
of slope proceeds very slowly, if at all, particularly after grasses become 
prominent. The slope of the gorge wall where it is covered with a meso- 
phytic forest is but little gentler than that of the wall where only bushes 
occur. From the herb stage to the ultimate mesophytic forest, each plant 
stage prepares the way for the next in the succession by holding_the soil, 
accumulating humus and furnishing shade. 
The herb stage is succeeded by a bush stage. The most prominent 
species is Rhus canadensis, which often forms large colonies. Cornus 
paniculata and Salix longifolia are commonly associated with it. Rubus, 
Ribes, Rhus toxicodendron, Vitis vulpina, Crateegus, Psedera, Ptelea trifo- 
liata and others occur, together with a number of species characteristic 
of the preceding stage, such as Dipsacus sylvestris, Heracleum lanatum, 
etc. 
This shrub stage is probably very brief and pioneer trees soon appear. 
Two parallel tree stages appear. Considerable areas are found occupied 
by Ulmus americana, Celtis occidentalis and Cratwegus spp. In other sit- 
uations similar in all respects, Cersis canadensis, Robinia pseudo-acacia 
and Prunus americana dominate the vegetation. In both cases, the trees 
are accompanied by a large number of undergrowth herbs and shrubs, 
among them the following: 
Gleditsia triacauthos Heracleum Janatum 
Juglans nigra Daucus carota 
Cornus paniculata Taraxacum officinale 
Sambucus canadensis Aster spp. 
Rtibes cynosbati Verbasecum thapsus 
Vitis vulpina Nepeta cataria 
Psedera quinquefolia Poa compressa 
Menispermum canadense Solanum nigrum 
Dipsacus sylvestris 
Following these two parallel stages appears the ultimate stage of the 
region, the mesophytic forest. This stage occurs only on the east bluff of 
the gorge, immediately above Test’s Mills. That this forest is really 
