112 



bifera, Camptosorus rhizophyllus, Pilea pumila, Aquilegia canadensis 

 and Hydrangea arborescens. Psedera quinquefolia hangs in long stream- 

 ers from the top of the cliff. On the edge of the cliff or on the talus be- 

 neath, where stream action is absent, occur Ulmus americana, Ostrya 

 virginica, Prunus serotina, Celastrus scandens and Vitis. Under the 

 cliff flourish such herbaceous plants as Sedum ternatum, Pilea pumila, 

 Impatiens, Equisetum arvense, Eupatorium perforatum, Ambrosia tri- 

 fida, Stellaria media, Galium, Carex and various mesophytic mosses. The 

 mesophytic condition is due largely to the constant shade. 



The vegetation becomes more and more mesophytic as the cliff be- 

 comes more overhanging. On account of the stability of the limestone, 

 this may continue until the cliff overhangs to a surprising extent, but 

 eventually overhanging portions of the cliff fall in large masses. This 

 process is aided by the presence of prominent cleavage planes in two 

 series at right angles to one another, but neither parallel to the edge of 

 the cliff. The breaking off of the large masses gives the cliff a jagged 

 appearance. The immediate result of the breaking off of a portion of 

 the cliff is a rejuvenation of the succession. The mesophytic vegetation 

 beneath the overhanging cliff is destroyed, both by being covered by the 

 fallen fragments and by exposui'e. Stream action on the base of the clifl^^ 

 is hindered or rendered impossible by the covering of the soft underlying 

 stratum. The stream is too weak to remove or wear away the fallen 

 fragments. The fallen portions of the cliff eventually become covered 

 with vegetation. The new, vertical faces of the cliff after a longer period 

 are clothed with plants. Soil and humus accumulate more readily than 

 before the interstices of the fragments, giving better conditions for 

 the growth of trees. With the increase of shade, more mesophytic condi- 

 tions prevail. 



Slowly the edge of the cliff and the fallen masses of rock are crum- 

 bled by action of the weather. The result is finally a gentle slope with 

 occasional remnants of the cliff projecting through the soil. The climax 

 mesophytic forest does not occur here, though conditions approaching it 

 are found at the lower end of the gorge. Tilia americana, Robinia 

 pseudo-acacia, Morius rubra and Fraxinus americana are the principal 

 trees, with an undergrowth of Sambucus canadensis and such herbs as 

 Galium, Poa pratense, and Sedum ternatum. 



On the whole, it would be difficult to find two rock-cliff successions 



