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Physical Environment:
Geology

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In general, glaciers do not move continuously across the landscape. The ice margin may advance and retreat over a given area many times during an overall advance--a net movement forward. The same is true during the overall retreat; the ice margin may readvance multiple times during an overall meltback of the glacier. Advancing glaciers pick up debris left by previous glaciers, including sands and gravels, as well as large boulders. They grind across bedrock in areas not covered by glacial deposits, incorporating rocks stripped from the bedrock surface into their load as well. When the ice begins to melt, till -- the unsorted mixture of rock, sand, clays, and boulders picked up by the glacier -- is deposited. If the glacial ice is melting as fast as it is advancing, the margin of the ice sheet remains essentially in the same spot and deposits large amounts of material in the that area, forming an end moraine. Glacial debris can also be deposited by moving water within, on top of, or underneath a glacier. When this occurs, the materials are sorted, with the heavier rocks settling out of the flowing water sooner, while the finer sands and clays remain in suspension over a greater distance. Rocks, sands, and gravels can also be deposited as outwash -- material that is carried by meltwater streams and sorted and deposited both along the ice margin and some distance away from it. When meltwater from the glacier subsides, the wind picks up the lighter sands, silts and clays from the streambeds and carries them still further from the ice margin. The term aeolian is used to describe materials transported and deposited by the wind. Glacial debris can also be deposited in lakes via meltwater streams. When this occurs the material is well sorted, the heavier rocks, gravels, and sands are being deposited near the lake margin, and the finer clays and silts are deposited in the deeper water.

The tills in the Midewin area were deposited between about 18,000 and 14,000 years ago as the Lake Michigan Lobe (a lake of glacial ice) melted back out of Illinois. The Marseilles, Minooka, and Rockdale moraines and the Valparaiso morainic system, all important in the formation of the proglacial lakes that covered much of the Midewin site, were forming during this time period. Proglacial lakes form when the meltwater from a glacier is impounded between the ice margin and higher ground that lies in front of it -- an end moraine, for example. When they eventually drain, by either eroding a new or existing outlet through the moraine or overtopping the high ground, proglacial lakes leave behind lacustrine (lake) sediments and a lake plain as evidence of their existence.

Surface geology map
Surficial Geology of the Midewin Region
Midewin NTP outlined in Black
Excerpt from map by Willman and Lineback, 1970, Illinois State Geolgoical Survey (Willman, 1971).



This map shows the Rockdale Moraine (green, stippled, labled rd) which was deposited when the glacier was melting as fast as it was advancing. Ground moraine, till deposited as the glacier was moving, is labled as mmg (Minooka ground moraine - light green). The light purple color (lp) represents lake plain or floors of glacial lakes (such as Lake Wauponsee) with a thin layer of lake sediment in places that is underlain by glacial till. The dark purple areas labeled (S) are dolomite. The areas labeled (sl) are glacial sluiceways - areas where outlets of glacial lakes were cut into till and bedrock. Glacial sluiceways contain sand and gravel deposits. Areas labeled (hm) are also sand and gravel. The bright yellow color along the streams are deposits in floodplains mostly silt and sand with occasional gravel.


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http://exhibits.museum.state.il.us/exhibits/midewin/geology02.html, Last modified December 14th 2012, 10:15AM.