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Western Dissected Illinoian Till Plain

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About the Western Dissected Illinoian Till Plain

The Western Dissected Illinoian Till Plain is a region in Western Illinois, consisting of two discontinuous pieces: a smaller piece located east of the Illinois river, and a larger section to the northwest between the Illinois and Mississippi rivers.

This region is a well-dissected plain with broad regions interspersed with fairly steep slopes and ravnies leading down into floodplains of larger streams and rivers. It was glaciated in earlier glacial periods, but escaped glaciation during the most recent (Wisconsin) glacial period. The bedrock here is a mix of limestone, sandstone, shale, and coal, and atop this lies highly-leached glacial till and fine, wind-blown loess deposits. Soils here tend to be well-drained, acidic, and poor in organic matter. There are a small portion of richer soils derived from thick loess deposits, which formed under prairies.

In the early 19th century, this region was covered with a mosaic of prairie and forest. Slopes supported oak-hickory forests, whereas moister (mesic) sites supported white oak (Quercus alba), northern red oak (Quercus rubra), and American basswood (Tilia americana). Prairie was mostly found on the flat, upland regions between valleys. There were also small areas of marsh and wet prairie, but less than in areas to the northeast.

The prairies here, which covered the flattest ground and nearly all the areas with the best soil for farming, have been almost entirely cleared for agriculture. Some areas of forest have also been cleared, but more forest remains, mostly on the steeper slopes. Some slopes, however, are cultivated, leading to severe soil erosion. Because this area is naturally better-drained, this region has less tiling and artificial drainage than in areas to the east and southeast. This region mostly produces corn and soybeans. The small city of Jacksonville, IL is located here, but besides this most of the region is sparsely populated, although there are numerous small towns.

This region is bordered to the east by the Illinois/Indiana Prairies, which tends to have richer, less leached soils, and to the south and southeast by the Southern Illinoian Till Plain, a region with finer-texture soils that are both more poorly-drained and more drought-prone. This region is surrounded to the west, southwest, and north by the River Hills.

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1. Woods, A.J., Omernik, J.M., Pederson, C.L., Moran, B.C. "Level III and Level IV Ecoregions of Illinois", U.S. Environmental Protection Agency, Washington, DC (2006) Web.