250 GEORGE B. CRESSEY 



is a definite beach ridge, together with low dunes, just south of the 

 road. Analyses of sand along section lines through this third 

 belt of dunes showed an increase in the size of grain as compared 

 with the sand immediately to the north, and on later search shells 

 and pebbles were found. The evidence for a former strand line in 

 this position is strong. 



South of the third dune belt just referred to, especially in the 

 neighborhood of Johnsville, there is another pronounced beach line 

 banked with low dunes on the south. The beach here is well 

 marked and strewn with beach-worn pebbles. None of these 

 shore-line evidences were anticipated but were first suggested by 

 variations in the coarseness of the sand. Since the interruption 

 in the gradation of size was a rather regular phenomenon and 

 scarcely to be accounted for if the sand had been subjected to greater 

 wear, search was made for additional evidence of shore lines. In 

 connection with each of the three suspected beach lines, additional 

 criteria were found in the form of beach gravels or coarse sands, 

 and in the case of the two beaches most distant from Lake Michigan 

 the topographic evidence was strong. 



The history of the dunes therefore appears to be closely asso- 

 ciated with the development of Lake Michigan. With the north- 

 ward retreat of the Late Wisconsin ice sheet a lake was formed in 

 the Michigan basin, known as Lake Chicago. Since the present 

 northern outlet was blocked by the ice, the water level was higher 

 than now, the overflow being to the south through the Illinois 

 River, as is well known. With the lowering of this channel and 

 the uncovering of other outlets by the retreating ice front, there 

 followed successive levels of the water. These halts are known as 

 the Glenwood, Calumet, and Tolleston stages of Lake Chicago, the 

 level of the water above Lake Michigan being respectively 56, 32, 

 and 17 feet. In the absence of topographic maps or precise eleva- 

 tions in the area under consideration, determinations of the eleva- 

 tions of the old shore lines have not been made. Judging from the 

 gradient of the two streams which cross the dunes and by the posi- 

 tions of the beaches in adjacent areas, it is thought best to assign the 

 four belts of dunes to the various stages in the history of the lake, 

 including the present. On this basis, each dune belt in the complex 



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