The century and a half from AD 1250 to AD 1400 saw massive shifts in the distributions of human populations on the Plains, the social groups that they lived in, and the interactions among these groups. These shifted again in the next 100 years, transforming the human landscape of the Plains in just a few generations. We see these changes more precisely in at least some areas because the radiocarbon calibration curve is smooth and straight for the 1400s (Figure 7.8). There seems especially to have been a major shift in settlement in the mid-1400s that corresponded with a dramatic drought (a “megadrought”) extending from the Dakotas into Texas (interestingly, tree-ring data suggest that the Canadian prairies were not affected by this drought, although the later 1400s were relatively dry there; Case and MacDonald 2003; Cook et al. 2007; Stahle et al. 2007). I consider this interval among farmers first and then turn to hunter-gatherers.