Ill 



The shrub stage is succeeded by a xerophytic tree stage, correspon(i- 

 ing probably to the usual oak-hickory stage. Ulmus americana is the 

 pioneer tree. With it occur Celtis occidentalis, Crataegus, Robinia 

 pseudo-acacia, Cercis canadenses, Prunus americana, Gleditschia tria- 

 canthos, Juglans nigra and Sambucus canadensis. 



The pioneer tree association gradually merges into the ultimate 

 stage of the region, the mesophytic forest. Mesophytism is indicated by 

 the following species: Fagus grandifolia, Acer saccharum, Coprinus car- 

 oliniana, Ostrya virginica, Asimina triloba, Impatiens pallida, I. biflora, 

 Viola cucullata, Galium spp. 



For a more complete account of the succession in the Whitewater 

 Gorge, see a paper by the writer in the proceedings of the Indiana Acad- 

 my of Science for 1910. 



The rock exposed in the gorge at Elkhorn Falls is Niagara limestone. 

 The falls are occasioned by the presence underneath the hard Niagara 

 limestone of a softer layer, which is probably Hudson River limestone. 

 Below the falls is a gorge about one-half mile in length and 150 to 350 

 feet in width. On the walls of this gorge, various stages in plant succes- 

 sion may be observed. 



In general, the earliest stages in the succession are to be found 

 nearest the falls, though they may be found wherever a rejuvenescence 

 of the succession has occurred. The pioneer association consists almost 

 entirely of lichens, a large, gray, leathery species of Umbillicaria being 

 the most prominent. This lichen covers the rock in all exposed situa- 

 tions, sometimes growing to a diameter of three inches. 



The lichen association is followed by another, made up of a small 

 black moss, probably a species of Grimmia, and such seed plants as Poa 

 compressa, Nepeta cataria, Verbascum thapsus. Aster and Solidago. 



These are succeeded, after further weathering of the rock and the 

 accumulation of humus in the widening cracks, by an association dom- 

 inated by Hydrangea arborescens and Aquilegia canadensis. These may 

 be accompanied by Psedera quinquefolia. 



The falls overhang a distance of 10 to 20 feet, on account of the 

 weathering away of the softer lower stratum. For the same reason, 

 the cliff soon becomes overhanging. This condition is more marked where 

 stream action is prominent. Under these overhanging cliffs a very mes- 

 ophytic association develops. Here occur Conocephalus, Cystopteris bul- 



