212 ILLINOIS ACADEMY OF SCIENCE 



C. Topography. 



Both the pre-erosion and erosion types of topography 

 are represented in this region. The former embraces the 

 upland and its slope to the west, the lowland of the 

 Skokie, and the stable portion of the ravines; the latter 

 includes the eroding lake bluff, and the head and mouth 

 of the ravines. 



III. Plant Successions. 



Even the most casual survey of the region reveals how 

 much the grouping of certain plants into associations is 

 dependent upon the topography. The predominance of 

 secondary forest associations, and the present state of 

 progression or retrogression of these lowland or upland 

 forests has been occasioned by man's influence. The agen- 

 cies which have done most to bring about this condition are 

 cutting, grazing, burning and drainage. 



Ample illustrations of pioneer and subsequent associa- 

 tions of both hydrarch and xerarch successions are to be 

 found within a very narrow strip of territory extending 

 from Lake Michigan on the east to the Skokie Stream on 

 the west. The hydrarch succession has its best expression 

 in the Skokie region and is well, but less completely rep- 

 resented in the depressions of the upland; the xerarch 

 succession was represented in the upland, the lake bluff, 

 and the ravine. 



A. The Hydrarch Succession. 

 1. The Lowland-Skokie. 



The topography of the lowland consists of a series of 

 slight elevations and depressions. In its natural condi- 

 tion the lowest part of this area was an undrained swamp 

 of considerable size. By means of an artificial drainage 

 system, man has found it possible to reclaim large tracts 

 of this land for agricultural purposes. 



a. The unforested area. — The most poorly drained 

 land of the Skokie is unforested. Here are to be found 

 what Sherff calls the reed swamp, the meadow swamp, 

 and the shrub associations. More than two-thirds of the 

 western half of New Trier 1 occupies this area. (See un- 



