Modoc is an unincorporated community in Randolph County, Illinois, United States, located four miles southeast of Prairie du Rocher under the bluffs of the Mississippi River. The Modoc Rock Shelter, a U.S. National Historic Landmark, is located near Modoc and the Ste. Genevieve-Modoc Ferry across the Mississippi River connects Modoc and the surroundi…Modoc is an unincorporated community in Randolph County, Illinois, United States, located four miles southeast of Prairie du Rocher under the bluffs of the Mississippi River. The Modoc Rock Shelter, a U.S. National Historic Landmark, is located near Modoc and the Ste. Genevieve-Modoc Ferry across the Mississippi River connects Modoc and the surrounding communities with Ste. Genevieve, Missouri.