THE LIFE OF GLACIAL LAKE CHICAGO 107 



In the northwestern part of Cook County, wells penetrate a black soil 

 below till at a depth exceeding 100 feet. 72 This is probably post-Hlinoian or 

 Sangamon in age. Near Elgin a lower till is encountered at 67 to 70 feet. In 

 other places near Elgin the lower till occurs at 165 feet. Near Arlington 

 Heights a black soil was observed beneath blue till at a depth of 70-75 feet, 

 under which there was another till sheet. 



In Township 41, Range 10 east, a well section shows the following strata: 78 



Yellow till 10-15 feet 



Blue till 125 " 



Black soil 4 " 



Sandy till 50 " 



Gravel with water 2 " 



A section of the Joliet Mound, near the city of Joliet, presents interesting 

 data. 



Surface coating of silty clay 1-4 feet 



Coarse gravel and cobble 10-12 " 



Sandy gravel of medium coarseness cemented in places 25-30 " 



Fine sand or loam 4 " 



Blue pebbleless clay, laminated, calcareous 8-10 " 



Boulder bed, containing clay balls and a sandy clay matrix, extending to level of 

 river on east side of mound, but underlaid at slight depth by limestone at west 



side 5-20 " 



The boulder bed is believed to be the result of interglacial erosion of a till 

 sheet. 74 



Ulinoian drift is known to underlie Wisconsin till at a depth of 160 feet at 

 Barrington, Lake County. 75 Two miles east of Beecher, Will County, an old 

 soil lies beneath 50 feet of glacial gravel. 76 East of Summit, Cook County, 

 an older till underlies the Wisconsin till. 77 No specific evidences of life have 

 yet been recorded from these old deposits. 



M. SUMMARY 



The detailed studies discussed in the previous pages may be summarized 

 as follows: 



1. During the late Wisconsin ice invasion all life which occupied the Great 

 Lakes region and the northern part of the United States, as well as nearly all 

 of British America, was destroyed or driven southward. As the ice sheet 

 retreated, the biota followed as closely as climatic conditions would permit. 



73 Op. dt., p. 586. 



7< Leverett, op. cit., p. 377. 



75 Op. cit., p. 581. 



71 Op. cit., p. 651. 



77 Op. cit., p. 407. 



