Natural History - Petroglyphs & Pictographs
For thousands of years, inhabitants of the Southwest have been creating rock art: designs etched into or painted onto caves, canyon walls, and boulders. What do these images and symbols mean? How can we use them to learn more about the history, cultures, and spiritual lives of the various people who created them? What are the limitations of this kind of study?
Rock Art is the term usually used to encompass both petroglyphs and pictographs. In this context, it means the petroglyphs and pictographs created by prehistoric inhabitants and their descendents. It is important to bear in mind in using this term that the producers of rock art may not have been created for sheer aesthetic pleasure. Rock art would not be used to describe present-day graffiti.
Petroglyphs are defined as images scratched, etched, or pecked into a rock surface.
Pictographs are images painted onto a rock surface.
Petroglyphs and pictographs are not like hieroglyphics, which are images used as a written language.
Techniques of Rock Art Creation
Petroglyphs are most typically created on surfaces such as cliff walls dark with desert varnish, because the contrast between the dark surface and the underlying lighter colored sandstone or basalt enhances the visibility of the design. The most common method of creating images is "pecking" a design into the rock. This could either be done directly, with an implement called a hammerstone, or with a hammerstone and a second implement that would have served as a chisel. Use of chisel and hammerstone led to more precise pecking, as it was possible to control to a greater degree the size and placement of the resultant peck. Occasionally, a preliminary outline would be pecked or scratched into the rock to aid in making the design more precise.
Additionally, petroglyphs have been created by scratching or incising designs into the rock. This method is most effective where the rock surface is particularly soft, perhaps lacking a heavy layer of varnish.
Pictographs are most typically found in sheltered caves and alcoves where the images are protected form elements. Preliminary to application of pigment, the sandstone was sometimes smoothed. Pigments seem largely to have been made from mineral substances, although it is impossible to ascertain definitely the composition of the paint on any pictograph without in the process damaging or destroying the pictograph itself. [Assumptions are based on inference from tested sites, historic and prehistoric, and from contemporary ethnographic studies.]
Colors most commonly used in pictographs are red, from hematite, or red iron oxide; white, most often from white clay; black, probably from charcoal; and orange, possibly form a mix of yellow limonite and red hematite. Green and blue, which appear less frequently, may have been created with ground malachite and azurite or turquoise, respectively. Mixtures of white clay with these other pigments may account for some pastel-hued pictographs.
These minerals would have been mixed with a vehicle and a binding agent of some kind to help them cling to the rock. According to more contemporary ethnographic sources, the vehicle and a binding agent are often combined in the form of organic substances such as saliva and various plant extracts.
Painting techniques would have varied. Some images seem to be "finger-painted" onto the rock. In other instances, brushes may have been created out of fibers like those of the yucca plant. Additionally, some images, such as the ubiquitous handprints, seem "stenciled" onto the rock, possibly by somehow spraying the paint. Finally, images are sometimes found drawn directly onto the rock, apparently with charcoal or a lump of pigment.
Dating Rock Art
It is very difficult to assign dates to rock art. In fact, given a petroglyph or pictograph panel in isolation, it is only possible (and then only sometimes) to assign dates to various elements relating one to another, i.e. "this image is older than that one."
In the case of petroglyphs, if within a panel some images are dark within the outline with desert varnish, while neighboring images are still the color of the underlying rock; the darkened figure is probably the older. How much older? Desert varnish is sporadic in its formation, so that is impossible to calculate. Where images are superimposed one on top of another, and the "top" image is the brighter of the two, it is possible to infer that the top is the later image.
Likewise, sometimes it is possible to assume that designs at the top of a cliff face may be older than those nearer the bottom, i.e., sand dunes may have been touching the top of the cliff 3,000 years ago but worn away gradually since then.
Historical context can provide a clue as to the age of rock art. For example, the atlatl (a type of spear thrower) was replaced by the bow and arrow around 200 C.E. Where atlatls appear, the picture is probably from a time when they were in use. Horses, on the other hand, were only in use on this continent after the arrival of the Spanish in 1520. Thus, pictures of horses must be historic.
Methods used to assign dates to ruins and artifacts don't work on rock art. Dendochronology, the study of wood rings to ascertain when a tree was cut down, only works when there is a cut-down tree (such as roof beams of a ruin) present. Unfortunately, the proximity of a datable ruin does not mean that the inhabitants of that ruin were the generators of the petroglyphs or pictographs. And carbon dating, so good with corn cobs and animal remains, does not work without some sort of organic matter. Petroglyphs, of course, contain no organic matter, and if the paint used in pictographs is organic, that can be ascertained only by destroying the pictograph.
Sometimes, stylistic similarities between an artifact and a petroglyph or pictograph can offer dating. Thus, striking similarities between an image on a pot and an image on a nearby wall may be due to a similar date of production. Then again, the artisan of the pot may have been inspired by the picture on the wall.
Stokes, William Michael & William Lee - Messages on Stone, Starstone Pub., 1980