As it turns out, there is a historic ridge running through Oak Park, Illinois, and many other towns around the Chicago area, which formed the shoreline around Lake Chicago, which was formed by glacial runoff.
This ridgeline is also a boundary between other Continental Divides that drain water into the St. Laurence River via the Great lakes, or south to the Mississippi River (see map — see more about the Eastern Continental Divide here). It is also so low, compared to the surrounding ground, that when the Des Plaines river would flood, it would spill over this ridge into an entirely different drainage system — ultimately ending up in the St. Laurence River (via something called “Mud Lake”, then to the Chicago River, then to Lake Michigan).
For context, this is all connected to the Chicago portage, and the idea behind reversing the flow of the Chicago River, and the building of the Illinois and Michigan Canal, and later the Chicago Sanitary and Ship Canal, in 1900. More on the latter:
Construction of the Ship and Sanitary Canal was the largest earth-moving operation that had been undertaken in North America up to that time. It was also notable for training a generation of engineers, many of whom later worked on the Panama Canal.
Then click over to the Wiki page for the historical geology of the Great Lakes, and be sure to click on the link for the Niagara Escarpment for some cool pics.
I never knew all this when I lived in Oak Park for 14 years! My oldest son used to play baseball sometimes in Taylor Park, which is shown at Division St. right on the Continental Divide.
It’s funny how much influence water has on our history, isn’t it?