John V asks “ How do you tell how old rock art is?”
Rock art is produced in a variety of ways. Pictographs are images formed through applying pigment (painting and stenciling) while Petroglyphs are made by manipulating the surface of natural rock by pecking, grinding, or carving.
Dating rock art can be quite difficult. Changes in decorative styles and motifs are often used to develop “relative chronologies” (placing styles or sites in chronological order without being able to assign precise numerical dates). Oral and written histories and the insights of Indigenous cultural experts provide valuable information to help contextualize the images. Considerations of the superpositioning of images and comparisons with well-dated categories of material culture can also help support and pin down relative chronologies. The depiction of images of known arrivals or disappearances of an animal or technology can help provide a more accurate chronological frame for images. For example, in the region where I work in India, the domestic horse was introduced after 1000 BCE, allowing us to date images of people riding horses after that time.
“Absolute dating techniques” can sometimes be used on rock art. Radiocarbon (C14) dating is the most common technique and is used for dating charcoal and paints containing organics. With refinements in AMS (Accelerator Mass Spectrometry) C14 dating, samples of only a few milligrams can be dated, making it feasible to directly date samples without damaging the art. Archaeologists have also attempted AMS C14 dating of micro-organics in the crusts or “varnishes” (also called desert varnish) that sometimes form over exposed images. Another radiometric technique, Uranium-Thorium dating has been used to date calcium carbonate coatings that form in limestone caves.
Archaeologists are working to develop absolute techniques for dating petroglyphs: attempting to quantify rates of erosion of rock surfaces, rates of accumulations and of chemical transformations of “desert varnish,” among other approaches. While specialists debate their accuracy and await future refinements of some of these approaches, we can certainly expect more and better techniques in years to come.
Prepared by Carla Sinopoli, Director, Maxwell Museum