Thousands of prehistoric Indian rock art sites are scattered throughout the southwestern U.S. on cliffs, boulders, and cave walls. For the vast majority of sites, the meanings and functions of these drawings have been lost, so they have been of limited value as an archaeological resource. We have found that more than 100 of these sites record the sun's yearly cycle through interactions of sunlight on the rock art. These interactions are consistent throughout a vast range of the Southwest, marking the solstices and other dates with similar petroglyphs and similar sunlight interactions. This ancient tracking of the yearly cycle once existed at probably thousands of locations spread over the territories of several prehistoric cultures, and could become an important tool for studying these peoples. Slides and time-lapse movies will be used to illustrate the sites and the interactions.