DEPOSITS OF THE LAKE CHICAGO BASIN 67 



6. This apparent land surface is covered by silt or peat beds, with an 

 occasional thin sand layer. Mollusks are abundant in places. These strata 

 are covered with a silty layer containing the plant Potamogeton, indicative 

 again of shallow water. The upper silt deposits are oxidized and contain 

 crayfish burrows, which are evidences of a land surface. The peat beds are 

 apparently local and were possibly laid down in kettle holes in the ground 

 moraine. These deposits vary from 15 to 52 inches in thickness. 



7. These deposits are everywhere capped by sandy-silt or peat deposits 

 changing to loam at the surface, averaging about 20 inches in thickness. The 

 lower part of the deposit contain the shells of shallow water mollusks. In the 

 Calumet-Sag channel the upper peat deposit is in places over 11 feet in thick- 

 ness. 



Standing out clearly in the foregoing summary are the following significant 

 facts: 



1. A low water stage early in the lake's history. 



2. A period of strong gravel formation indicating a rising water body. 



3. Two land and low water stages separated by water bodies of varying 

 depth. 



This interpretation, based on the north shore channel deposits, is expressed 

 in Plate XXX. 



The data set forth in the preceding pages are considered in chronological 

 order, the biological significance being given first place, in the chapter that 

 follows. 



