18 Illinois Natural History Survey Circular 49 



cation that at the height of each glacial period the climate was cooler 

 than it is today and that during the times when the glaciers were not 

 present the climate was warmer than it is today. Great divergence 

 of opinion exists as to how much cooler or how much warmer these 

 climates were. Special differences of opinion exist as to how far 

 south of the glaciers their cooling effects extended. 



FORMATION OF THE DUNESLAND 



There is no evidence that either Lake Michigan or the other four 

 of the Great Lakes existed before the Ice Age. When ice moves, it 

 flows somewhat like water, especially in that it moves into and along 

 low places in the terrain. The thrusts of the great ice lobes presum- 

 ably gouged out the basins of the Great Lakes. It can be inferred 

 that some of these lake basins coincided with the valleys of streams 

 along which the glaciers moved in their southward advance. After 

 the southern parts of the glaciers melted away, these depressions 

 filled with water and formed the Great Lakes. In the present basin 

 of Lake Michigan and extending farther south was a much more 

 extensive lake that geologists call Lake Chicago. Eventually, the 

 level of Lake Chicago went down, leaving certain peculiarities of soil 

 and terrain as evidence of its former presence. The flat, sandy area 

 centering around Evergreen Park is part of the old basin of Lake 

 Chicago, as are the series of sandy beach dunes in the neighborhood 

 of Homewood and South Chicago. 



Neither Lake Michigan nor its shores are static features; both 

 have movement and change. Storm-driven waves eat away the sand 

 and gravel along part of the shore; other waves may transport the 

 sand and gravel and deposit them along the shoreline in other places. 

 The Dunesland is one of these latter places. 



The sand and gravel deposits along the lake shore form a series 

 of beaches and bars caused primarily by changes in lake level com- 

 bined with deposits of storm-driven sand. Formation of the high 

 sand dunes along the southeastern shore of the lake, for example 

 those at the Indiana Dunes State Park near Tremont, is due almost 

 entirely to wind, which blows the sand over the beach and deposits 

 it in ever-increasing windrows along the shore. The prevailing winds 

 that blow across Lake Michigan are from the northwest, west, and 

 southwest; hence, the dunes occur principally on the southeastern, 

 eastern, and northeastern shores of the lake. Only low, narrow ridges 

 and small dunes have been formed on the western shore, as at Illinois 

 Beach State Park, fig. 3. The peculiar topography of the Dunesland 

 thus was due first to the formation of Lake Michigan, then to the 



