Wow, this is so cool: this map depicts the Chicago area at the end of the last ice age, around 14,000 years ago. (I knew about Lake Chicago - the predecessor of Lake Michigan - and was aware of its general environs, but had never seen an actual map.) The shaded portions are the land areas that existed back then, while the dotted lines show the modern-day Chicago River and shoreline of Lake Michigan. What is now the city of Blue Island was indeed an island back then, and even today with the waters having receded, it's an unusually elevated area in an otherwise flat landscape. Same thing for Mount Forest Island, which is between Lemont and Willow Springs; I ride right past there every day on my train.
What's really interesting to me about Mount Forest Island is the two outlets that are shown: the Sag Outlet, the low-lying ground where the Cal-Sag Channel was dug during the early 1900s; and the Desplaines Outlet, which is the current course of the Des Plaines River. As Lake Chicago receded, the Des Plaines formed into a river, bending to the north near McCook Station and roughly following the course of the old shoreline shown in the map.
I love these things!
Posted by: Paul Lamb at Jan 25, 2014 2:47:50 AM