259 



race extended along the whole border of the lake, but with the ele- 

 vation of the water during the Nipissing stages the greater part of 

 the terrace was washed away except in the Chicago district and in 

 the area north of Waukegan. This interpretation, which signifies 

 that the ridges are of about equal age, is substantiated by observa- 

 tions upon the plant associations. Jennings, in his work on Presque 

 Isle (1909: 294-305), under "historical development," says that the 

 ridges were formed at different dates, and that a line of plant suc- 

 cessions could be traced which confirmed the physiographic inter- 

 pretation. In the Beach area, however, evidence goes to show that, 

 with the exception of the fringing dune from Zion City down to 

 Waukegan, the ridges were formed at one time. The fringing dune, 

 as it now exists, is undoubtedly a product of historic times. Since 

 the building of the piers to protect the harbor at Waukegan, con- 

 siderable sand has accumulated north of it, and the formation of a 

 new dunal ridge a little north of the pest-house is now (1910) be- 

 ginning to show. North of Zion City, particularly between Win- 

 throp Harbor and Kenosha, the shore-line is being washed away a 

 noticeable distance every year. These ridges are all oblique to the 

 present shore-line but they are parallel, or very nearly so, to the 

 shore-line that existed at the time of their formation, namely, the 

 Calumet ridge. The work of erosion, which bid fair to allow the 

 lake access to the Glenwood ridge south, as w^ell as north, of Kenosha, 

 has been to a considerable degree, checked by piers at Kenosha and 

 by breakwaters, behind which the lake is being artificially filled. 



CUMATfi 



As there are no weather bureau stations in the region having 

 records of long duration, the records of the stations at Milwaukee 

 and Chicago, situated at equal distances north and south of the area, 

 are used. It is fairly safe to assume that the records for this region 

 in very similar sort of country may be obtained by interpolating those 

 given. It is recognized that these data do not actually give the con- 

 ditions under which the plants live, but only a general indication of 

 the climate. The records are given in curves to facilitate in- 

 terpretation (PI. XL-XLII). As climatic factors do' not usually 

 have edaphic influence, they are of value only in determining the gen- 

 eral character of the vegetation that will occupy a given area. 



Edaphic Factors 



Far more important than the climatic factors in determining the 

 floristic composition within an area are the edaphic factors. Of 



