Jay Stravers

Jay Stravers Northern Illinois University

Jay Stravers, an instructor with Northern Illinois University's Department of Geology and Environmental Sciences, likened a glacier to a blob of motor oil spilled on a cold sidewalk.The spilled oil does not flow in a straight line but oozes in all directions.The same happens with glaciers.

"It has to flow out from a central point of thickness," Stravers said.

See GLACIERS, page 6
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The common image of an ice sheet a mile thick covering Illinois is inaccurate, Stravers said.

The most recent glaciation occurred from the east, Stravers said, and in Illinois, the sheet was no more than about 500 feet thick.

The thickest ice was in central Canada, where it did reach a thickness of a mile or more.

Here in Illinois, the thin glacial margins, chock full of rocks and soil, began to melt as the summers warmed to perhaps 40 or 50 degrees.

"That would be enough to melt quite a bit of ice," Stravers said.

Meltwater then raged down between two moraines, or ridges - the St. Charles Moraine to the west and the Minooka Moraine to the east - left from previous glacial periods.

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Just as the glacier pushed south and west into this area from Wisconsin and Michigan, it melted from west to east, Stravers said.

Many gravel pits in northern Illinois are evidence of the burden of rock scraped up by the ice as it inched across the landscape, pushing it ahead like a broom.

The mound at Johnson's Mound Forest Preserve in Blackberry Township was formed when a hole in the ice filled with debris from streams flowing across the surface of the glacier.

The Chain O' Lakes in Lake and McHenry counties are remnants of the melting glaciers into which the river channel disappears, only to pick up again on the south side of the chain.

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Stravers said giant mammals probably roamed the glacial margins, where there were forests, tall grass prairies and expansive wetlands.

"In terms of habitat, it would have been one of the most diverse in the entire world," he said.

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