Geology of Illinois
The geology of Illinois includes extensive deposits of marine sedimentary rocks from the Palaeozoic, as well as relatively minor contributions from the Mesozoic and Cenozoic.
[1] Precambrian rocks of Illinois are highly faulted; tectonic extension and related thermal subsidence have led to the formation of two major sedimentary basins.
[3] For much of the Palaeozoic, Illinois was located much further south than today, instead being near the equator; it was also underwater for much of this time, forming a shallow continental sea.
Simon Sandstone, the most basal of which are interpreted as being braided river deposits, while the remainder of the formation seems to represent marine tidal environments.
Ordovician features in Illinois include the now-buried Glasford Structure in Peoria County, a crater caused by a meteorite impact roughly 455 million years ago.
[10][6] Mesozoic rocks are overall poorly exposed in Illinois; those present are Cretaceous in age and only seen in extreme southern parts of the state.
They are largely terrestrial sands and gravels, though one marine unit, the Owl Creek Formation, indicates that the Western Interior Seaway covered parts of the state at one point in time.
The Mississippi River, fed by ice-sheet melt and water from glacial lakes, cut a deep valley as it flowed through the region.
This event occurred when the dam of a glacial lake located in what is now the Lower Peninsula of Michigan failed catastrophically, leading to a massive influx of water down the channel of the modern Kankakee River.
The Rock Creek Canyon is home to one such waterfall, which is eroding upstream at a rate of 3 inches (7.5 cm) per year[12][13] The most obvious glacial feature in Illinois is Lake Michigan, the basin of which was carved out by glaciers.
Located to the Southwest the Bloomington Moraine is a large, very flat plain; this area is in fact the lakebed of Glacial Lake Pontiac, which drained about 17,000 years ago.
Apple River Canyon State Park shows some of this dramatic topography in the eponymous canyon; the tributaries of the Apple River enter the main channel pointing upstream, indicating a reversal in flow direction due to the advance of the Illinoian glacier.
