Geologic History of Illinois
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One
key to the rich biodiversity of present-day Illinois lies in its varied geologic
past. Illinois has always had a relatively low relief. Paleozoic Illinoi was
characterized by level terrain with flat underlying rocks that made up the Illinois
Basin. Although drainage systems account for most landforms in the state, the
southernmost portion of Illinois has a terrain formed, in part, by faults and
uplifts. Glaciers were also very significant in the development of Illinois
landscapes. Having been covered repeatedly throughout the Pleistocene (a period
spanning 2 million to 8000 b.p.), glaciers have altered the face of the state by filling
pre-extisting valleys, eroding rises, forming new streams, depositing moraines.
Moraines are gently rising ridges marking the leading edges of past glaciers. Even
though southern Illinois is said to be "unglaciated," glacial meltwaters had a
profound impact on the present topography. The beautiful and biologically diverse
Shawnee Hills with its blufftops and canyons are a product of the sculpting hand
of Preglacial landforms Illinois' topography probably has been relatively flat through much of its recent geologic history. Before glaciation, the low-relief landscape was due to the generally flat-lying Paleozoic (greater than 245 million years old) rocks that fill the Illinois Basin. Structural uplift, folding, and faulting were responsible for some landforms, especially in the southern part of the state. The major landforms in central Illinois were largely the result of well-developed drainage systems Glaciation has been one of the most significant geologic processes to shape the landscape in much of the northern United States. Glaciers repeatedly covered Illinois throughout the Pleistocene Epoch (approximately 2,000,000 to 8,000 years ago). Glacial erosion and deposition has changed the landscape of Illinois by filling preglacial river valleys, changing the course of rivers, and creating new landforms. Some of the most prominent landforms created by glacial deposition in Illinois are end moraines. End moraines are broad, low-relief ridges that mark the advances and readvances of the glaciers. The morphology of the moraines is variable and can be dependent on the length of time that the glacial margin occupied a given position on the landscape, the amount of sediment carried by the glacier, and/or the age of the moraine. Pre-Illinoian glaciation These early glacial advances, which took place approximately 2,000,000 to 500,000 years ago, were responsible for reducing the relief of Illinois' already flat topography both by erosion and deposition. Many preglacial bedrock valleys were partially filled with thick deposits of outwash sand and gravel. The extent of the pre-Illinoian glaciation is not exactly known as most of these deposits were subsequently buried and/or eroded by the later Illinois glacial advance. Illinois glacial episode The Illinois glacial advance, which took place approximately 200,000 to 120,000 years before present, created new landforms found in much of western and southern Illinois. During this advance, the maximum reach of the ice in Illinois marked the southernmost extension of continental glaciation in North America. At one time, ice of this glacial episode covered nearly 90% of the state and today its deposits comprise approximately 55% of the surficial deposits in Illinois. The Illinois glacial advances left behind mostly low-relief, rolling ground moraine. End moraines may have formed, but have lost their prominence due to subsequent erosion, particularly mass wasting under paraglacial/periglacial conditions, and the deposition of a blanket of wind-blown silt (loess) across most of the landscape during the Wisconsin glacial episode. Drainage networks developed in the ground moraine are responsible for most of the modern topography. The most recent glaciation in Illinois was during the latter part of the Wisconsin glacial episode between about 25,000 and 14,000 radiocarbon years before present. At one time, ice of this glacial episode covered 33% of the state's surface, primarily in the northeastern quarter of Illinois. This glacial episode produced many landforms, including end moraines, ground moraine, and flat lake beds. The most visible of these landforms are the end moraines.
Blake Schneider, Leah Schwendeman, Rebecca Rice, Sarah Bourdo, Zach Roberg
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