Paleontology and geology
During the Cretaceous, a shallow seaway extended from the Arctic Ocean to the Gulf of Mexico, through the central part of North America. Portions of the seaway covered western Illinois, but as the waters receded, they left an arm of the sea behind over southern Illinois, referred to as the Mississippi Embayment. Rocks deposited in the seaway and in the embayment are found in the western part of the state near Quincy, and to the south. Leaf fossils of willow and walnut have been found in the MacNairy Formation in southern Illinois.