However, there have been many changes in what is now Illinois, compared to what it was that long ago. Imagine going back to that time. The climate has just begun to warm up, after experiencing a period when glaciers entered the state.
Ice sheets are still as close as southern Canada.
The Great Lakes are beginning to form as the ice continues to melt. The Illinois River is a relatively new river, especially in the northern part of the state. The lower part of the Illinois River has taken the place of the ancient Mississippi River, which moved farther west during the glacial advance. Because these changes were occurring so quickly, some scientists believe many living things could not adapt. They had to move to other areas, or cease to exist around this time.