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Illinois State Geological Survey

GeoNews Online

Glacial diversion of the Mississippi River

About 20,000 years ago, the Wisconsinan glacier intercepted the southeast- flowing Mississippi River (near the site of Peoria today) and dammed it up in a glacial lake. When the lake filled to overflowing, the waters broke out and carved a new course for the river along its present valley from Rock Island, Illinois, to St. Louis, Missouri. Near the village of Lomax in Illinois, a shallow wetland was flooded by glacial meltwater, which created a slackwater basin collecting layers of silt, clay, and organic debris. Today, a 70-foot-high exposure in a bluff near Lomax shows ample sedimentological and biological evidence for this diversion, says B. Brandon Curry, glacial geologist. Dr. Curry presented his findings at the North-Central Section Meeting of the Geological Society of America in Madison, Wisconsin, May 1-2, 1997. The glacier--river collision, according to four radiocarbon ages for spruce twigs and mosses unearthed at Lomax, occurred 20,350 years ago (give or take 100 years).

Though near the ice margin, this region was much like the southern Canadian prairies of today. "I found minute fossil ostracodes in the clay beds of the ancient lake at Lomax," says Dr. Curry, adding that the descendants of these creatures (sand grain in size) thrive today in the lakes of southern Manitoba and Saskatchewan. "Knowing the climate and ecology where ostracodes live now, we can infer the paleohydrologic conditions, such as the water chemistry and depth, of lakes in western Illinois back in the Wisconsinan. The land next to the glacial ice was probably not wet and forested, as people might expect, but more like a prairie or tundra with trees clustered around a few lakes."

The fossil wetland and lake record at Lomax runs from about 20,350 to 17,250 years ago, when the lake disappeared under layers of sandy alluvium. Dust storms raging from about 16,500 to 12,000 years ago buried Lomax under 50 feet of silt and sand.

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GeoNews / July 1997 / Mail comments to ISGS Computer Applications Unit: