Ross: The Dunesland Heritage of Illinois 19 



reworking of the shoreline by the waters of the lake and the wind, 

 and finally to intermittent changes in lake levels. 



RECOLONIZATION AFTER THE GLACIERS 



What of the life of the Dunesland area during the period of the 

 Ice Age? The ice sheets that shoved or carried boulders and pebbles 

 from the north also overrode the life of the land. The area that is 

 now the Dunesland was once covered by ice a mile thick, a mass eight 

 and one-half times as high as Chicago's Board of Trade Building, 

 including the statue on its top. All life beneath this sheet of ice was 

 obliterated. Everything now living on the areas once covered by the 

 ice has spread into them from other areas. 



When we consider that practically the northern half of the North 

 American continent was covered with a blanket of ice extending from 

 coast to coast, the question naturally arises: What happened to the 

 Arctic and Boreal forms of life that today are restricted to areas far 

 north of Illinois, fig. 3? Fossil remains, especially pollen from peat 

 bogs and other deposits, indicate that the northern, cold-adapted 

 kinds of life moved south ahead of the ice front. The ice front moved 

 probably only a few miles a year, and most species, with ranges 

 several hundred miles across, could move with the changing climate 

 ahead of the glaciers. 



It seems possible that when the glaciers were at their maximum 

 southward extension truly Arctic conditions prevailed for only a 

 short distance in front of them, perhaps only a few miles in front 

 of the ice itself. It is certain that a large proportion of the present 

 northern species were able to persist somewhere if not everywhere 

 along this band of cold conditions. Other Arctic and subarctic species 

 persisted in the large unglaciated portion of Alaska. As the edges of 

 the ice sheets melted back, the various aggregations of living things 

 spread northward from the southern part of the continent and east- 

 ward from Alaska. 



Any Dunesland sand beaches formed soon after the front margin 

 of the last glacier retreated northward would probably have been 

 populated by some of the Arctic poppies and the dwarf willows that 

 today characterize the tundra far to the north. At a later time, when 

 the surrounding countryside was probably predominantly spruce, fir, 

 and pine forest, any new sand ridges and beaches in the Dunesland 

 area would have been colonized by plants and animals typical of the 

 successional stages of the northern coniferous forest. 



Many small colonies of animals, especially insects, indicate that 

 at some time during the last Ice Age climates that we might describe 



