Our friend, Roy Plotnick has a new article over on Medium.  This one is a summary of the changes to the natural geology of Chicago from the last ice age about 20,000 years ago until the present.  As always, it’s a good read!

About 20,000 years ago, where I am sitting in Chicago was covered by about a kilometer of ice. This was the height of the most recent advance of ice sheets in the Northern Hemisphere, which had begun some 80,000 years earlier. About 15,000 years ago the ice sheets began to retreat and within the next 5000 years they had totally disappeared. These great ice sheets left an indelible mark on the landscape, the most notable being Lake Michigan and the other Great Lakes, carved by the glaciers in the underlying bedrock. Paralleling the current shore of Lake Michigan to the south and west are linear hills, moraines, up to a hundred feet high, deposited by the glacier as it temporarily slowed in its retreat. And as the ice melted it produced an ancestor to Lake Michigan, Lake Chicago, whose northeastern shore was the retreating glacier and whose southwestern shores were the morainal hills. In one spot, to the southwest, a mile-wide river cut a channel in the moraine, through which flowed water not only from Lake Chicago but from the entire newborn Great Lakes.