The Old Chicago Portage, 1673-1836
Bilek ET. 1967. The Old Chicago Portage, 1673-1836. Chicago, Illinois
About 25,000 years ago a huge glacier crept forward from Canada into the Chicago region. As this ice sheet advanced southward, it carried with it the rocks and soil it found in its path. Further north this glacier presented a solid block of ice across the continent. Later the glacier receded in stages, leaving its accumulation behind. Each time it paused in its recession great quantities of debris piled up at its edge forming big land ridges when the ice disappeared. Thus the Valparaiso Moraine, a rim of land paralleling the margin of Lake Michigan, was formed in the northeastern corner of Illinois. Water collected in the gorge hollowed out by the ice between the glacier and the moraine until reaching a level of sixty feet above the present level of Lake Michigan, A massive glacial lake was now born, Lake Chicago. Draining through a gap in the Valparaiso Moraine called the Chicago or Des Plaines outlet, the flow of water from Lake Chicago traveled down the Des Plaines Valley to the Illinois River Valley, The recession of the glacier and the lowering of the floor of the Chicago Outlet removed the barrier that withheld the flow of water previously Lake Chicago now receeded into three separate stages. The Glenwood stage where the water level vas fifty feet above the lake; the Calumet stage where the water was thirty-five feet above the lake; and the Tolleston stage which was twenty feet above the lake. All through these stages only the one outlet down the Des Plaines Valley existed, for the ice barrier still remained in the north.