The Sierra de San Borja is the northernmost outpost of the Great Mural rock art of the central Baja California mountains. In explorations in the 1970s, author Harry W. Crosby found six major sites of Great Mural rock art southeast of the San Borja mission. Several others have since been discovered. Great mural rock Art consists of prehistoric paintings of humans and other animals, often larger than life-size, on the walls and ceilings of natural rock shelters.