The Terrain of Illinois
Only the southernmost tip of Illinois, and the far northwest and western edges of the state still have pre-glacial terrain. It consists of forested hills.
The terrain of the rest of Illinois was altered by the four glaciers that flowed and stayed over Illinois during the Ice Ages. The glaciers brought rocks and other debris to Illinois from Wisconsin and Canada and deposited them when they melted. Some of the lighter deposits were blowed by winds to cover even more of the state.
The map to the right illustrates the depth of loess blown across the state. Notice that the deepest deposits are along riverbeds.
The terrain of Illinois was written about and speculated upon by French explorers and English settlers.