The natural landscape of Illinois can be divided into 14 Natural Divisions, based on topography, glacial history, bedrock, soils, and distribution of plants and animals. The prairies of Illinois were by no means a homogeneous stand of grasses and forbs, and in the comprehensive natural community classification adopted for Illinois, six main subclasses of prairie including:
Further divisions are made based on soil moisture classes yielding 23 prairie types in Illinois.
The largest original prairie type was the Grand Prairie (black soil prairie) of central Illinois, with flat landscapes, deep loess soil, and poor natural drainage resulting in wet conditions during part of the year. This kind of prairie is the rarest today because the soil is so productive for agricultural crops.
Along the shores of Lake Michigan and the Illinois, Kankakee, and Mississippi rivers, are extensive sand deposits, often forming dunes or ridges and swales, and several kinds of sand prairies can be found in such areas.
Hill prairies are found on dry, southwest-facing, loess-covered hill tops above bluffs overlooking floodplains of rivers, especially the Illinois and Mississippi rivers.
In northeastern Illinois some distinctive prairie vegetation can be found in very wet alkaline fens and marl flats.
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