The Prairie-Plains is the broad central region of North America, from the northern Mississippi Valley–western Great Lakes to the Rockies, in which open grasslands predominate. Grasslands, the basic ecology of the Prairie-Plains culture area, thus extend from central Canada through Texas, and from Alberta, Wyoming, and Colorado to Wisconsin, Indiana, and Missouri. Developing Woodland societies in the East began to influence the Prairie-Plains region in the first millennium b.c., when corner-notched atlatl points and knives somewhat larger than the usual Plains Archaic types came into fashion, and thick-walled pottery was made in the Midwest. Trade with Mississippians is shown by shells and an occasional Mississippian pot, but the essential independence of early Plains agriculturists is indicated by their pottery, an amalgam of Woodland and Mississippian ceramic traits manifested in several regional variants. Groups of Northern Prairie peoples in western Minnesota fissioned several times, and bands were consequently siphoned off to the west, where they concentrated more fully on bison hunting.