
Igor] PHYVSIOGRAPHIC ECOLOGY OF CHICAGO IS 
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, repre- 
senting three beach lines of Lake Chicago, as the glacial exten- 
sion of Lake Michigan has been called. The upper and oldest 
a ar 



Fic. 34.— Portion of an ancient beach line (Calumet beach) at Summit, showing 
the characteristic oak vegetation, in this case chiefly bur oaks (Quercus macrocarpa). 
of these ridges has been termed the Glenwood beach, the inter- 
mediate ridge the Calumet beach, and the lower and younger 
ridge the Tolleston beach. The geographic relations of these 
beaches is well discussed by Leverett and also by Salisbury 
and Alden,?7 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 propor- 
tions between these trees varies strikingly, though the bur or 
6 OP. cit. 55-85. °7 Op. cit. 31-51. 
