Did a glacier once cover Illinois?

Yes, much of Illinois was covered by a glacier during the Pleistocene Ice Age. The glacier advanced and retreated several times, leaving behind a landscape that is characterized by rolling hills, moraines, and outwash plains. The last glacier to cover Illinois, the Wisconsin Glacier, reached its maximum extent about 18,000 years ago. It covered all of northern Illinois and much of central Illinois. The glacier retreated about 12,000 years ago, leaving behind the landscape that we see today.