Byway Story - Geology

The Illinois River Valley as it looks today is the result of continual and often dramatic natural changes to the landscape.

Starved Rock view of Illinois RiverSome of the processes that shaped this river and its valley are, literally, as old as the hills. Sandstone and limestone bluffs recall an era, millions of years past, when an ancient sea overspread this region. The Mississippi River once flowed through this valley, before glaciers shifted its course to the west. South of Hennepin, the Illinois River follows the Mississippi's ancient channel. A more recent glacial event sculpted the upper reaches of the Illinois River. Seventeen thousand years ago, glacial melt waters burst through a rock-earth dam holding back a massive lake, unleashing the Kankakee Torrent, which carved the river valley all the way to Hennepin.

As the glaciers retreated, lichens and plants reinhabited the barren landscape, many of which sprouted from seeds left behind in the glacial deposits. In the 12,000 years since then, rich topsoils accumulated, and complex and varied communities of plants and animals-from lush bottomland hardwood forests and riparian floodplains to tallgrass prairies and woodland spring seeps-have established and flourished here.

The Illinois River Road sites below to enable you to experience how geological transformations have shaped the Illinois River Valley and how, since the end of the Ice Age, dynamic natural communities have become established in the Illinois River Valley.

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    Fondulac Park District

    Boggio’s Orchard

    Buffalo Rock State Park

    Dickson Mounds Museum

    Donnelley DePue State Fish & Wildlife Area & Complex