Driving across central Illinois, one encounters one of the flattest landscapes in the eastern U.S.; it is a classic till plain. Plowed flat by the Pleistocene glaciers, this region was also covered by a thick veneer of till (pulverized rock and organic debris) as the ice sheets retreated toward Canada. In concert, meltwater streams began to mold the plain, draining toward the Ohio and Mississippi Rivers; since they were eroding soft, loosely compacted deposits, these streams meandered, creating broad, shallow valleys that give little relief to the flat plain.
Before white settlers arrived in the Midwest, this vast till plain was covered by tallgrass prairie. Periodic drought, wildfires, high winds and the grazing of massive bison herds maintained the prairie and woodlands were limited to the stream valleys. Today, trees also rise along fence lines and throughout the numerous towns and cities that dot the landscape; the tallgrass of the Prairie State, while protected or reestablished in some areas, has given way to the American Cornbelt.
Illinois lies at the heart of North America's till plain, a geophysical province that stretches from central Ohio to the eastern Dakotas and northward into Canada. The Continental Glaciers molded the landscape of the Midwest but, more importantly, produced the thick, rich soil that feeds much of the world's population.
