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Rock River Hills

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About the Rock River Hills

The Rock River Hills are a hilly region in northwestern Illinois, that is mostly dominated by agriculture.

Most of this region consists of rolling hills and undulating plains, but the southeast and northwest has some more rugged sites with steeper ravines and bluffs. The bedrock here is a mix of limestone, dolomite, and sandstone. There are some caves in the limestone and dolomite, but the karst topography is not as pronounced here as in some areas with limestone and dolomite. Glacial till here is thin, and there is also a significant layer of loess, fine-textured wind-blown sediments. Soils here are mostly derived from loess, but have some diversity, with some alluvial soils and others formed on glacial outwash, till, or residuum. The climate is humid and continental with hot summers and cold winters, and can experience extreme temperature swings; the climate becomes drier to the west.

In the early 19th century this region was a mosaic of prairie and forest. Uplands supported dry shortgrass prairies, and moister sites supporting mesic prairies of big bluestem (Andropogon gerardii) and indiangrass (Sorghastrum nutans). Ridge slopes and uplands protected from fire were forested. Cooler sites on bluffs and ravines supported yellow birch (Betula alleghaniensis) with an understory of canada yew (Taxus canadensis). Bottomlands supported floodplain forests. In upland areas, the plant communities here can differ substantially based on bedrock, with some species only occurring on sandstone substrates.

Nowadays, over half of this region is utilized for cropland, and there is also significant livestock farming. Much of the cropland is tiled for drainage. This region primarily produces corn, soybeans, and wheat. Forests are mostly limited to steep slopes and riparian areas. This area is mostly rural; Freeport, IL is the only city of appreciable size contained within this region, although there are numerous small towns. The larger city of Rockford, IL, is mostly located outside this region to the east, although a substantial portion of it is also in this region.

This region is bordered to the east by the flatter Illinois/Indiana Prairies, which has more intensive agriculture. To the north lies the Rock River Drift Plain. There is a smaller border to the south with the Sand Area. Most of this region borders the Upper Mississippi Alluvial Plain to the west, but in the north there is a small border to the west with the Blufflands and Coulees, and north of that, the Savanna Section.

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 Central Corn Belt 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.