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Southern Illinoian Till Plain

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About the Southern Illinoian Till Plain

The Southern Illinoian Till Plain is a large region entirely contained within Illinois, and making up most of southern Illinois.

This region has a dissected landscape with gradual changes from north to south. Throughout the region, bedrock of sandstone, limestone, coal, and shale are covered with glacial till from the older, Illinoian glacial period, as well as loess, fine wind-blown deposits. This region escaped glaciation during the recent (Wisconsin) glacial period, and as such the till here is more leached of nutrients. Towards the south, the till becomes thinner, the bedrock is closer to the surface, and the landscape is steeper and more heavily dissected. Throughout this region, uplands have clayey or silty soils with poor internal drainage, however, these soils also dry out easily during prolonged dry periods. Soils here also tend to be acidic. Overall, conditions here are universally harsher for plant growth, making this area more poorly-suited to agriculture in particular.

The region has a humid subtropical climate with strong continental influences. Precipitation follows a weakly bimodal pattern, with a longer wet season peaking in May, followed by a drier late summer, and another, shorter wet season in November. There can be large swings of temperature, as well as unusually wet or dry spells, at any time of year.

In the early 19th century, this region was a mosaic of prairie and forest, with prairies covering about 40% of the land, and the rest of it a mix of forests and more open savannas and isolated groves of trees. The flat, poorly-drained uplands with clay soils supported an unusual mix of pin oak (Quercus palustris), post oak (Quercus stellata), swamp white oak (Quercus bicolor), and blackjack oak (Quercus marilandica). Dry slopes of valleys supported a more typical oak-hickory forest. Low ridges of glacial moraines supported mesic forests of northern red oak (Quercus rubra), American elm (Ulmus americana), American basswood (Tilia americana), and eastern black walnut (Juglans nigra).

The prairies and forests of this region have almost entirely been cleared for agriculture. The remaining forest cover is mostly limited to steeper slopes and flood-prone bottomlands along rivers. Cropland here produces soybean, corn, and wheat, and there is also significant livestock production. Due to poor natural drainage, nearly all the agriculture here is on land that has been tiled for drainage. Soybeans are more tolerant of both the drought and temporary flooding that is common in this region, and as such, this region has a greater portion of soybean production relative to surrounding regions. Some cultivated slopes here have experienced severe erosion. Most of this area is sparsely populated, but it includes some small cities, including Marion, Mount Vernon, and Effingham, and numerous smaller towns. Overall, this region has lower agricultural productivity and agriculture here has had a greater environmental impact than in some regions.

This region is bordered to the north by the Illinois/Indiana Prairies, which is better-suited to agriculture, except in the west where it is bordered to the north by the Western Dissected Illinoian Till Plain. To the east lies the Wabash River Bluffs and Low Hills, except along the major rivers where this region directly borders the Wabash-Ohio Bottomlands. At the very south of this region there is a small border with the Northern Shawnee Hills. The north of this region is bordered to the west by the River Hills. South of that there is a long border to the southwest with the Karstic Northern Ozarkian River Bluffs, and south of that there is a small border to the southwest with the Southern Ozarkian River Bluffs.

References

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.