The Rocky Mountains are traversible by horse in all sections. In Montana, the passes are only 5,000 to 7,000 feet in elevation, and the Indians crossed over the same routes followed today by highways. The Wyoming Rockies similarly presented no serious problem to nomadic peoples. Although the Yellowstone Park area posed some difficulty for travel, a more southerly route led over South Pass at a gentle gradient only 7,550 feet in altitude. That the mountains were no great challenge is illustrated by the fact that Indians traveling from Green River to Wind River often preferred to take more direct routes through passes over 10,000 feet high rather than to follow the more circuitous trail over South Pass. Farther south, the Colorado Rockies and their passes are far more lofty, but even these high ranges did not totally prevent travel. None of these areas, of course, could be traversed in the winter. When the snow left the high country in late spring and early summer, however, the mountains were not only avenues of travel, but they were also hunting grounds. None of the buffalo hunters relied completely on that animal for food, and the high mountain parks abounded in elk, bighorn sheep, moose, and deer. Streams were fished, berries were gathered along their banks, and roots were dug in the surrounding hills. For the tribes living on their flanks, the mountains afforded an important source of subsistence. If the Rocky Mountains did not effectively isolate Plains societies from those of the Basin-Plateau, differences in environment certainly did. Plains economy was based upon two animals, the horse and the buffalo, both of which depended on sufficient grass for forage. The horse diffused northward along both sides of the mountains, and the richest herds were probably found among the Plateau tribes and not in the Plains (cf. Ewers, 1955, p. 28). The greatest herds of buffalo were found east of the Continental Divide, however, although smaller and more scattered herds roamed the regions of higher elevation and rainfall, immediately west of the Rockies, until about 1840. Thus, although the standard image of "Plains culture" is derived from the short-grass country east of the Divide, the tribes living west of the mountains were also mounted and also had access to buffalo. Their varying involvement in this subsistence pursuit and its associated technology, combined with the diffusion of other items of culture, resulted in what Kroeber has called "a late Plains overlay" of culture in the area (Kroeber, 1939, p. 52).