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Southern Illinoian Till Plain

Ecoregion in IL

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The Southern Illinoian Till Plain ecoregion is a partly dissected till plain that was once covered by forests and prairies. Broad flats, rolling hills, and subdued moraines are common. Towards the south, the terrain becomes more hilly as glacial till thins and bedrock approaches the surface (Nelson, 1978, p. 42; Schwegman, 1973, p. 21). Overall, dissection is greater, moraines are less prominent, and forests are naturally more extensive than in the Illinois/Indiana Prairies (54a). The Southern Illinoian Till Plain ecoregion is physiographically different from the Western Dissected Illinoian Till Plain (72i). The underlying Paleozoic sandstone, limestone, coal, and shale is mantled by loess and Illinoian-age glacial till; younger Wisconsinan-age till is absent, in contrast to Ecoregion (54) where it is extensive (Lineback, 1979; Kolata, 2005). Upland soils in the Southern Illinoian Till Plain (72j) are clayey or silty, derived from loess and till, and have poor internal drainage. Impervious fragipans or claypans are common and characteristic. Soils are droughty during dry periods, excessively wet during the spring, and acidic (Natural Resources Conservation Service, various dates; Nelson, 1978, p. 105; Schwegman, 1973, p. 21). Claypans are much more common, and natural drainage is poorer, than in the Western Dissected Illinoian Till Plain (72i). Overall soil quality and productivity are lower than in the Central Corn Belt Plains (54) (Mausel, 1970, p. 135). In the early 19th century, about 40% of the well-drained uplands in The Southern Illinoian Till Plain ecoregion were covered by prairies. The remaining uplands were covered by scattered trees, groves, and forests (Schwegman, 1973, p. 22). Groves containing pin oak, post oak, swamp white oak, and blackjack oak are native to nearly level, poorly drained uplands with clay-rich soils. Oak – hickory forests occurred on relatively dry valley slopes. Mesic forests containing red oak, elm, basswood, and walnut dominated low morainal ridges (Vestal, 1931, p. 214-215). Today, nearly all of the original prairies in The Southern Illinoian Till Plain ecoregion, and most of its original forests (especially in the south) have been converted to agriculture. Soybeans, corn, and wheat are the primary crops, and livestock farming is important. Forests are now largely confined to side slopes and river bottoms that are unsuitable for farming. Natural soil wetness conditions in The Southern Illinoian Till Plain ecoregion are generally unfavorable for crops, and, therefore, nearly all of the ecoregion’s flat and nearly level uplands have been tiled to improve drainage. Soybeans are better adapted to drought than corn, and can mature even when spring planting is delayed by field wetness; as a result, soybean acreage exceeds that of corn in the Southern Illinoian Till Plain (72j) (Nelson, 1978, p. 105; National Agricultural Statistics Service, 1997). Sheet erosion can be severe on cultivated slopes. The northern crayfish frog, eastern fence lizard, ground skink, and broadheaded skink are common in the Southern Illinoian Till Plain (72j), but are rare in the Illinois/Indiana Prairies (54a) (Illinois Natural History Survey, on-line resource). Sanctuaries providing nesting habitat for the greater prairie chicken, listed as endangered in Illinois, have been established in The Southern Illinoian Till Plain ecoregion (Illinois Department of Natural Resources, on-line resource “b”; Schwegman, 1973, p. 22).

— EPA

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