Chicago, Illinois Geology

I was born in Libertyville, Illinois, which is a small suburb 30 miles north of Chicago, Illinois. This area is part of the Great Plains of North America. Half of the plains, which is approx 1064 square miles, is a monotinuous plain, largely lake bottom. In the Chicago region, erosion of old drift by water, ice was common, about 20,000 years ago and earlier glacial deposits survived beneath the drift locally. The land is result of sediment over the years

http://all-geo.org/highlyallochthonous/2010/12/the-flat-of-the-land/