Physical Environment:
Geology

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About 16,000 (Hansel and Johnson, 1992) years ago, the large complex of moraines known as the Valparaiso morainic system were forming and the ice margin was at the Valparaiso moraine. Glacial meltwater poured down the Illinois River valley through an outlet in the Marseilles moraine. The meltwater flow was relatively great and there may already have been proglacial lakes in the area. Between 16,200 and 15,400 years ago (Hajic, 1990; Johnson and Hansel, 1992), meltwater from lobes of glacial ice to the east (the Saginaw and Erie lobes in Michigan and Ohio) flowed across southern Michigan and northwestern Indiana into the Kankakee River Valley.

The Kankakee Flood and Glacial Lake Wauponsee

The outlet through the Marsielles moraine was inadequate to accommodate this pulse of additional meltwater (Ekblaw, et al. 1925). Water backed up behind the Marseilles moraine, flooding across the uplands and creating several large proglacial lakes, including Lake Wauponsee which covered a large part of the Midewin site. Lake Wauponsee reached a maximum depth of 100 feet, but persisted for a relatively short period of time before draining into the Illinois River valley (Willman, 1971). Large segments of the Minooka moraine were washed away, giving it a truncated appearance on the map. The Rockdale moraine was also mostly covered by water during this episode, with the exception of a few high points, which remained as islands in the proglacial lake (Willman, 1971). The Rockdale moraine is a prominent feature at the Midewin site.

Rockdale Moraine, Midewin NTP
Rockdale Moraine
Midewin NTP


As the floodwaters spilled through (and eroded) the gaps in the Marseilles moraine and into the Illinois River valley, the lake levels were rapidly lowered. At that point, the floodwaters down the Kankakee were very intense, moving boulder sized pieces of dolomitic bedrock and depositing them along the valley. The sand deposited at the Midewin site has a direct and patterned effect on the prairie vegetation. It was probably during this time that areas seen as bedrock on the glacial geology map along the Kankakee and near the confluence of the Kankakee and Des Plaines were exposed by the churning floodwaters. It is in these areas, where the bedrock is within about 15 inches of the surface, that dolomite prairie occurs on the Midewin site.


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