The entire village of Oak Park lies on the shore of ancient Lake Chicago, which covered most of the city of Chicago during the last Ice Age, and was the forerunner to today's Lake Michigan. Ridgeland Avenue in eastern Oak Park marks the shoreline of the lake, and was once an actual ridge. As with the geographical setup of the Chicago River, which connects to the present day Lake Michigan just north of the city's Loop, the ancient Des Plaines river once emptied into glacial Lake Chicago, making prehistoric Oak Park a "Plains river Delta" system. One of North America's four continental divides runs through Oak Park. This divide, a slight rise running north–south through the village, separates the Saint Lawrence River watershed from the Mississippi River watershed, and is marked by one plaque on Lake Street at Forest Avenue and another in the northwest corner of Taylor Park.