One of the major problems presented by the study of the petroglyphs (carvings or inscriptions on rock) of ancient American Indian peoples is that of dating the art. Although the ability to absolutely date a particular site is rare, various methods provide a means of relative dating. Patination is one means of determining the relative ages of petroglyphs made at different times on the same cliff or boulder surface. Patina is a black or brown stain of hydrous iron and manganese oxides that forms on rock surfaces. When a petroglyph is made, the design is scratched through this layer, exposing the original color of the rock. On this exposed surface, a new layer of patina immediately begins to form. If two or more figures on a surface were made at different times, the more recent one will be lighter in color. Because patina varies with the composition of the rock and its exposure to the sun and rain, however, the degree of patination on a petroglyph alone is not an absolute guide for dating it.
The placement of rock art on a cliff face or within a rock shelter can also be an indication of age. In the river canyons of the Colorado Plateau during prehistoric times, for example, petroglyphs were sometimes made above the tops of sand dunes lying against the cliffs. The dunes were removed eventually by floods, and petroglyphs were made by subsequent canyon occupants in the scars left behind. Thus vertical stratigraphy is present today, with the oldest work on top. Similarly, ledges in rock shelters and along cliffs often enabled early artists to reach locations now inaccessible, and the work of later cultural groups, made after the ledges fell, is often present below.
Habitation debris also helps to date rock art. The art often occurs at habitation sites, which frequently can be placed within a given time frame by the pottery and other artifacts present. A useful means of ordering the chronology of certain rock art styles has been the comparison of rock art figures with those on the artifacts such as pottery, clay pipes, or wall plaster, whose age can be reliably determined by other means.