The Mississippian Period is one of several broad categories (including Paleoindian, Archaic, and Woodland) that archaeologists use to subdivide the American Indian past of the Southeast and Midwest. Between AD 900 and about AD 1600, Mississippian people farmed maize extensively; lived in societies known as chiefdoms led by hereditary rulers; conducted long-distance trade in copper, marine shell, and other valuables; resided in towns, villages, and farmsteads; built monumental architecture in the form of earthen, flat-topped mounds; conducted warfare, often fortifying their towns with stockades; and shared religious and iconographic traditions.