64 



THE PLANT SOCIETIES OF 



There are a number of interesting sand hills and ridges at some 

 distance from the lake. Some of these are fifteen miles from the 

 present lake shore, while others are found at various intervals nearer 

 and nearer the lake. It has been found that these can be grouped for 

 the most part into three series, representing three beach lines of Lake 

 Chicago, as the glacial extension of Lake Michigan has been called. 



Fig. .'^8. — Portion of an ancient beacli (Glenwood beach) near Thornton. The trees here 

 are chiefly black oaks (^Quercus coccinea iinctoria); the beach is higher, and the trees more 

 luxuriant than usual. 



The upper and oldest of these ridges has been termed the Glenwood 

 beach, the intermediate ridge the Calumet beach, and the lower and 

 younger ridge the Tolleston beach? The geographical relations of 

 these beaches is. well discussed by Leverett, and also by Salisbury and 

 Alden, and nothing need be said here except as to the vegetation. In 

 general these ridges and hills have a xerophytic forest flora, dominated 

 by the bur, black, and white oaks [Quercus macrocarpa, Q. coccinea 

 tinctoria, Q. alba). The proportions between these trees varies strik- 

 ingly, though the bur or black oak is usually the chief character tree. 

 No satisfactory reason can yet be given for these variations, though 

 the bur oak appears to be more abundant on the lower and less 

 drained ridges, while the black oak is inore abundant on the higher 



