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Rock River Drift Plain

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About the Rock River Drift Plain

The Rock River Drift Plain, in some documents referred to as Rock River Hill Country, is a region in far northern Illinois and southern Wisconsin, extending roughly from the Rock River west to the beginning of the Driftless area.

This region consists mainly of till and outwash plains. The terrain is varied, ranging from nearly level (especially along the Sugar river) to hilly. There is a well-developed stream network; uplands are dissected, with numerous low ridges and many small valleys, and a few areas with steeper slopes and ravines. The bedrock here consists mostly of dolomite, with some limestone and shale, and occasional sandstone, but the bedrock is mostly buried under at least a few feet of glacial sediments. Soils are mostly formed on glacial till, outwash, or alluvium, although there are a few spots where the bedrock is exposed. The glacial deposits here are a bit older than that of surrounding areas, and as such, the drift here is thinner and highly weathered. Especially in the north of this region, there is a cap of silt-loam loess on the surface. The soils in this region thus vary considerably, from silt-loam through loam and to coarser sands. Although surface soils are more leached, the calcareous bedrock is shallow enough (usually 6 feet or less) to allow access by mature plant roots.

In the early 19th century, this region was covered with a mix of oak savanna (mostly white oak (Quercus alba), black oak (Quercus velutina) and bur oak (Quercus macrocarpa)), prairie, and some forest. Bottomland forest was concentrated along the Sugar river, and oak forests were found on upland sites sheltered from fire, mostly along small streams in the hillier terrain. Sugar maple (Acer saccharum) and American basswood (Tilia americana) were found along rockier streams where conditions were both moist and well-drained. Fire played an important role in maintaining the prairies and savannas throughout much of the region.

Nowadays, most of this region has been cleared and is utilized for agriculture. The region mainly produces forage and feed grain for use in the dairy industry, but there is some production of other grain as well. Most of this region is rural; the largest city is Janesville, WI, followed by Beloit, WI. Extensive bottomland forests are still found along the Sugar river, making a mostly-unbroken corridor along its length. There are also some areas of forest along steeper slopes. In the flatter areas, some small, fragmented forest also can be found as small woodlots on private property. There are significant protected areas along the Sugar river, and there is also a protected bog at Rockton Bog Nature Preserve in Illinois, and extensive upland forest at the Robert O Cook Memorial Arboretum in Janesville, WI, a forest that connects to the narrower forested bottomlands along the Rock river.

This region is bordered to the north by the Southeastern Wisconsin Savannah and Till Plain, to the east by the Kettle Moraines, a region with more recent glacial deposits, more lakes and marshes, and a less-developed stream network. The border to the west is with the Savanna Section of the Driftless area; this border is abrupt and is marked by the limit of glaciation. There are three borders to the south: the west of this region is bordered by the Rock River Hills, and the east is bordered by the flat, mostly-agricultural Illinois/Indiana Prairies; this border is more arbitrary and ill-defined. In the very far east there is also a tiny border to the southeast with the hillier Valparaiso-Wheaton Morainal Complex; this border is well-defined and marked by the presence of moraines.

Plant Lists & In-Region Search

We do not yet have data to generate plant lists for a region as fine-tuned as this one. However you can move up to the broader Southeastern Wisconsin Till Plains and generate lists for that region: native plants or all plants. Or search that region's plants here:


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.

2. Albert, Dennis A. "Regional landscape ecosystems of Michigan, Minnesota, and Wisconsin: a working map and classification.", General Technical Report NC-178, U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station, St. Paul, MN (1995) Web.