VI. THE RIVER CLIFF ASSOCIATIONS 



With the exception of a few poorly developed limestone clififs along 

 the Des Plaines River and its tributaries there are no rock cliffs in the 

 immediate Chicago region. High limestone cliffs are to be seen on the 

 Mississippi at Savanna, 111., while at Starved Rock Park are cliffs con- 

 sisting of rather easily eroded sandstone which permits the rapid develop- 

 ment of mosses and seed plants but is unfavorable to the growth of 

 lichens. Among the mosses several species of Polytrichum (e.g., P. 

 juniperinum and P. piliferum) are prominent. In even slightly protected 

 situations a layer of sandy soil soon accumulates, hence it is not sur- 

 prising that upon level and gently sloped areas a vegetation similar to 

 the oak associations of the sand dunes (q.v.) soon becomes established. 

 Many of the plants in the following list are quite as characteristic of 

 river banks of clay as of the rock cliffs. 



Trees and Shrubs 

 Pinus Strohus L., white pine. 



Thuja occidentalis L., arbor vitae. , 



*Juniperus virginiana L., red cedar. 



Populus tremuloides Michx., American aspen. 



" grandidentata Mich., large-toothed aspen. 

 *Ostrya virginiana K. Koch,, hop-hornbeam. 



Car pinus caroliniana Walt., water-beech. 



Rihes Cynosbati L., prickly gooseberry. 

 *Physocarpus opulifolius Maxim., nine-bark. 



Crataegus tomentosa L., red haw. 



" spp., haws or thorn apples. 



Amelanchier canadensis (L.) Medic, service-berry. 



Prunus virginiana L., chokecherry. 



Pyrus arbutifolia L., chokeberry. 



Pielea trifoliata L., hop tree. 



Rhus glabra L., smooth sumach. 



* " typhina L., rough sumach. 



" Toxicodendron L., poison-ivy. 

 Ilex verticillata (L.) Gray, northern holly. 

 Celastrus scandens L., climbing bittersweet. 

 Vaccinium pennsylvanicum Lam., blueberry. 

 " canadense Kalm., blueberry. 



*Lonicera Sullivantii Gray, Sullivant's honeysuckle. 



* Viburnum prunifolium L., plum-leaved haw. 



Herbaceous Plants 



"^ Selaginella rupestris (L.) Spring. 

 Poly podium vulgar e L., polypody fern. 

 Pellaea atropurpurea Link., purple cliff-brake. 

 Pteris aquilina L., common brake. 



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