Climatic variability was the principal cause of cultural changes in the SSRB and the Northern Great Plains during the late prehistoric period. During the benign conditions associated with the Neo-Atlantic Climatic Episode (A.D. 900-1200), the SSRB and surrounding grasslands were relatively stable with regard to human occupation. During the same period, populations in the adjacent woodlands, particularly to the east and southeast of the SSRB, underwent an extended period of cultural change and expansion as a consequence of longterm improvement in climactic conditions. In regions surrounding the SSRB, the climatic deterioration associated with the Pacific Climatic Episode (A.D. 1200-1550) drove many woodland groups to the relative stability of the SSRB and northern Great Plains. Southeast of the SSRB, protracted desiccation prompted a region-wide abandonment of agriculture and a shift toward bison hunting. As conditions worsened during the Neo-Boreal Climatic Episode (A.D. 1550-1850), migrations to the SSRB from the woodlands to the east continued. Competition for resources, particularly bison, increased as groups originating south of the 49th parallel shifted their focus northward to the SSRB. Although longterm temperature decline and protracted drought undoubtedly reduced the biomass and available food supply in the SSRB, the impact of declining conditions in the regions surrounding it were much more severe. For more than 500 years after A.D. 1200, the SSRB and vicinity served as a relative refuge for groups experiencing climatically driven environmental stress. The dynamics of those migrations, undertaken during a period of protracted hardship, is essential to the understanding of both climatic adaptation and tribal occupation of the SSRB and the northern Great Plains generally.