How Is Prehistoric Rock Art Dated?

There is no generally accepted, precise method of dating rock art, but some techniques can give an approximation.
In the American Southwest, most petroglyphs are on cliffs or rocks covered with "desert varnish," a thin layer of dark material consisting of dead bacteria combined with iron or manganese salts. As soon as an image is incised, the varnish begins to form again in the lines, which become "repatinated" until they resemble the original patina: thus, the darker the lines, the older the glyph.
Rock art images near ruins may be dated through carbon-dating analysis of organic building materials such as roof beams or remnant food.
Rock art styles have been identified which indicate the image was made during a particular culture, which evidence proves occupied a given area in history.
Often an image will indicate a precise time after which it was applied. In the Southwest, the bow and arrow replaced the atlatl spear thrower about 500 years ago. Images of horses (sometimes with Spanish riders ) are post-Conquest, after the 16th century.

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