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Calloway's <i>One Vast Winter Count</i> describes the Mississippian corn chiefdoms. These corn chiefdoms developed between 900 and 1200 when a global warming trend dramatically increased rainfall in the Mississippi River Valley. During this time the city of Cahokia was founded and became the largest in North America. Calloway describes Cahokia as planned city which built enormous burial mounds. While the social structure of the Cahokia is somewhat opaque, archeologists have described it is a well organized and hierarchal society. It is highly probable that numerous tribal groups were organized into Cahokia social structure. Additionally, Cahokia does not resemble any of the tribes found by European colonists four hundred years later. By the fourteenth century, Cahokia went into a steep decline and was abandoned. Eventually the population of this city was dispersed to smaller surrounding villages and the social structure and hierarchy of Cahokia disappeared. It is not clear what if any tribal identity the Cahokians possessed.
West describes how the introduction of the horse to the Cheyenne and other tribes encouraged them to reject their old lifestyle and relocate to the Great Plains to hunt buffalo. The Sioux relocated from the Great Lakes to Missouri Valley to take advantage of the ubiquitous buffalo. The Arapahoes were originally farmers from Minnesota, but the horse convinced them to move south and west towards the Black Hills.<ref>West at 66-67</ref> These dramatic moves caused hostilities between tribes and encouraged intermarriage with new groups. DuVal describes how even Africans slaves were also freely incorporated into Indian tribes.<ref>DuVal at 81-82.</ref>