The region destined to be covered by metropolitan Chicago took natural form following the retreat of the North American ice cap 10,000 years ago. Meltwaters from the glacier's Lake Michigan Lobe, pent up for a time behind morainic ridges deposited at the ice sheet's margins, formed glacial Lake Chicago and drained southwestward, scouring what is today the lower Des Plaines valley. As ice receded and water drained away, Lake Michigan remained behind, contained within its modern shoreline. The area straddles what turned out to be a permanent low-lying continental drainage divide between the basins of the Great Lakes and the Mississippi River. The numerous lakes and marshes of the region represent the retreating glacier's messy legacy. By the early nineteenth century a tall-grass prairie environment covered much of the area, with thin strips of forest colonizing sandy beach ridges and shallow valley bluffs.

Author: Michael P. Conzen
Source: Newberry Library