Ethnographic data on the production of rock art in far western North America - the historic hunter-gatherer cultures of California and the Great Basin - are reviewed and analyzed to identify widespread patterns in the origin and, in certain cases, symbolism of the late prehistoric/historical parietal art of this region. These data, collected in the first few decades of this century by a variety of ethnographers, suggest only two origins for the art: production by shamans; and production by initiates in ritual cults. In both instances, the artists were apparently depicting the culturally-conditioned visions or hallucinations they experienced during altered states of consciousness. The symbolism of two sites, Tulare-19 and Ventura-195, is considered in more detail to demonstrate how beliefs about the supernatural world, and the shaman's relationship to this realm, were graphically portrayed.