CHAPTER VII 



THE DUNES AND THEIR PLANTS 



O REGION about Chicago manifests such 

 rapid changes in physiographic features 

 as the dune region, and nowhere are 

 the effects of these changes more ap- 

 parent on the distribution of plants and 

 animals. It is, therefore, a very favor- 

 able region for an initial study of plant 

 and animal societies. 

 It extends from Gary east along the shore of the lake and 

 well up the eastern shore. The area is from a quarter of a mile 

 to several miles in width and with occasional interruptions where 

 cla}^ bluffs come to the shore it is a hundred miles or so long. 

 In general it consists of a succession of sandy ridges that are 

 roughly parallel to the lake. These vary in height; the crests 

 of the tallest stand 200 feet above the level of the lake. Those 

 nearest the shore are quite barren, but those farther back are 

 increasingly covered with trees and associated shrubs and 

 herbs. Between these steep-sided hills are valleys occupied 

 often with long narrow ponds or with marshes and swales (see 

 chap. ix). 



The effective storm winds in the Chicago region come out 

 of the northwest. These pile up the waves that erode the shores 

 and heap the sand upon the beaches. After a heavy storm a 

 newly formed sand bar ofi'shore is a common sight (Fig. 68). 

 Since the shores of Lake Michigan lie north and south, the eff'ect 

 of these furious winds is to tear down the lateral shores by wave 

 action and drive the debris by currents gradually southward. 

 Ultimately this debris will be pulverized by the waves, deposited 

 in bars, and thrown up in sandy beaches on the east side and the 



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