Archaeological research in the Upper Mississippi River valley between 43°N and the St. Croix River confluence (44.7°N) has produced a series of large Oneota villages from which corn <i>(Zea mays)</i> has been recovered. Radiocarbon determinations indicate that the sites in the northern reach of this study area date from 1010 to 1440. These sites are characterized by large village areas and associated mound groups. Unlike villages in the northern region, sites in the LaCrosse area are more extensive, and burial mound complexes are absent. The southern villages have carbon 14 dates ranging from 1030 to 1520. Based on the differences in community plans and artifact assemblages, it is suggested that the northern Oneota groups are not ancestors of the neighboring LaCrosse variant. Historical documents for Europe indicate that at similar latitudes, the climate began to deteriorate after 1300. With the onset of cooler summers characteristic of the Neo-Boreal climatic episode, corn agriculture became unreliable in the northern portion of the Mississippi basin after 1400. In response to the unfavorable climatic conditions, large semi-permanent villages were abandoned and an outward migration began. The peak cold at ca. 1550 (the Little Ice Age ) caused the collapse of agriculture in the LaCrosse region as well. Historical documents, archaeological evidence, and palynological data indicate that the climatic recovery did not ensue in this region for more than two centuries. After 1750 climatic conditions were again favorable for the cultivation of aboriginal corn.