DEPOSITS OF THE LAKE CHICAGO BASIN 65 



Sandy till 50 feet 



Gravel with water 2 " 



Total thickness 196 



It is believed that the till sheet beneath this old soil represents the Illinoian 

 glaciation and the old soil the Sangamon (and possibly the Peorian) interglacial 

 interval. It is probable that lake deposits similar to those recorded in the 

 preceding pages were formed during this interglacial stage, but the evidences 

 of such have been obscured or obliterated by the later Wisconsin glaciation. 



VI. Summary 



The data set forth in the pages of this chapter may be summarized as 

 follows : 



1. Above the boulder clay there is usually a layer of sand, varying in thick- 

 ness from the fraction of an inch to twelve inches. This varies in composition 

 from fine sand to coarse sand and gravel. 



2. Superimposed upon this sand layer is a bed of silty, carbonaceous, or 

 peaty material, varying in thickness from one to forty inches. This is some- 

 times interstratified with one or more beds of sand. This stratum contains 

 numerous fresh water shells belonging to shallow water species. Oak leaves 

 and spruce cones are associated with the shells. 



North of Devon Avenue, and extending as far as Wilmette, the equivalent 

 of this stratum is represented by a strongly oxidized covering of the boulder 

 clay, the upper ten to forty inches of the latter being filled with rootlets of the 

 vegetation which grew upon the oxidized surface. This old land surface has 

 been reported from many parts of Evanston. Leverett says, 11 "some of the 

 sewer ditches in Hyde Park, west of Grand Boulevard, have reached peat 

 deposits below sand, at a level a few feet above the lake." This deposit is 

 probably the same as the one found in the north shore channel and elsewhere. 

 In the Ogden ditch, near Austin Avenue, a peat deposit occurs below a Unio 

 bed. Near Oak Park and Ogden avenues a silt deposit is found, below sand, 

 which is oxidized. In the Calumet-Sag channel the boulder pavement may 

 represent this episode. , 



The geological character of these deposits, and especially their biological 

 contents, leads at once to the conclusion that they represent a low water stage 

 of glacial Lake Chicago, the waters of which were warm enough (cold tem- 

 perate) to support a varied molluscan fauna. Oak and spruce grew upon the 

 higher ground. The lake at this stage could not have been higher than about 

 ten feet above the present datum or 590 feet above tide. 



3. The silt and peaty deposits are covered by gravel and sand from 2 to 19 

 inches in thickness. This deposit varies in character from fine sand of a silty 



11 Pleistocene Features, p. 51. 



