A period of time, c 400-1000 AD, in the American Midwest, when populations spread west to the eastern slopes of the Rockies and were in contact with eastward-moving Puebloan people. A favorable agricultural period was indicated by the marked increase in village size and in population density. Areas along major streams were occupied by various interrelated cultural groups collectively known as the Plains Mississippian cultures. Part of this complex was connected to the developing Mississippi complexes to the east by diffusion and, to some degree, by a migration of such groups as the Omaha and Ponca from the St. Louis area by about 1000 AD. It follows the Middle Woodland era but lacks the elaborate Hopewellian artifacts and structures.