The Mississippi River traverses from Quincy (bottom center) to its confluence with the Illinois River near Grafton (upper center) in this south-southeast-looking, low-oblique photograph. North of their junction is dark, forested Pere Marquette State Park, the largest state park in Illinois. This part of the Illinois River was the main course of the Mississippi River until a continental glacier that covered most of Illinois more than 20 000 years ago pushed the river further west into its present course. The Missouri River is visible in the upper right portion of the photograph.