Rock art analysis has received a bad rap in archaeology over the years, but in the last decade or so, some advances have been made to begin changing that. It’s easy to see why many archaeologists might have a hard time with rock art in general: rock art is near impossible to accurately date and its artistic nature makes interpretation very subjective. Is the image a symbol for an idea or concept that is consistent from site to site and even cross-culturally, or is it merely the artistic expression of the individual who created it? Does the scene depicted in a rock art panel represent a real event, a myth or story, or is it just the daily musings of an artistically inclined hunter-gatherer?
Despite past marginalization of rock art analysis, the glyphs and images painted, etched and carved are artifacts of the past. They’re material remains of an ancient culture and, in some cases, are all that remain to speak for that culture. Luckily, new techniques and standards are being developed to overcome the marginalization of rock art and look at it with an objective and scientific eye.
Among the chief concerns of archaeologists in examining a rock art site is dating. When were the images on the rock created? Where they created at once? Were there earlier registers with later ones added in stages? Was maintenance done over the millennia on one or more registers? If one register was maintained by re-applying pigment, and not others, why is this so?
There are a few different relative dating techniques that can apply to rock art: examining how one motif overlays another; dating intrusions like water stains; dating artifacts found in the vicinity; and so on. But these only give relative dates to each other and no absolute date from which to begin. There are, however, a few techniques that are being developed that can absolutely date rock art.
Rock Varnish Dating
Microbes on the rock surface capture fine particles of dust that build up laminations in a micro-stratagraphic sequence. From this, several methods can be used to derive an age of the varnish and, thus, the earliest possible age for the glyph. The first involves measuring the cation-ion ratio. The chemistry of the rock varnish is examined at the microscopic level to determine the rate at which major trace elements like potassium and calcium are leached out compared with other elements like titanium. While not the most precise of dating methods, CR dating does compare very well to dating a site based projectile point and ceramic styles.
A second technique involving rock varnish is VML (varnish miscrolamination). This technique looks at rock varnish layers in a way similar to dendrochronology in that it depends on the consistent application of varnish layers over time with exception to significant climatic interruptions. And, it is these climatic interruptions that are depended upon to create visible markers in the layered sequence since they result in micro-stratigraphic layers that are rich or poor in manganese for wet and dry conditions respectively. There appears to be good use for such techniques in samples with ages over 10,000 years.
A third method involves accelerated mass spectrometry (AMS) radio-carbon dating of organic matter that can become trapped in small crevices and cracks. Like each of the rock varnish techniques, AMS depends on the fact that the varnishing of the rock surface occur after the glyph has been etched. The AMS method can also be used on pictographs, however, if the pictograph was painted with a charcoal-based paint or one created with an organic binder like blood or saliva, assuming that he organic compounds can be extracted. Pictographs can also be dated using optically stimulated luminescence (OSL) of wasp or swallow nests since the period in which the quartz granules contained in the nest matrix have been deprived of sunlight, all the while building up luminescence, can be determined. Such a technique doesn’t tell when the pictograph was painted, of course, but it can tell at least how long it’s been present.
The subject of rock art analysis is one that is fascinating and it’s a topic that I, for one, intend to follow more closely. One of my primary interests in archaeology is that of ancient religions and beliefs and, for the pre-literate, pre-pottery societies artistic representation on rock surfaces is among the only material remains these societies have left behind. Indeed, being pre-literate in no way implies that these people did not have stories to tell and a desire to share them with subsequent generations. If one assumes that these people were aware that oral traditions naturally suffer from accident omission, forgetfulness, and exaggeration, wouldn’t it follow that they would want some sort of framework from which subsequent story-tellers can flesh out the tale? Tales of origins, tales of tricksters, tales that answer the why and how questions that burn in all of us. Are we so different today with our blogs and our YouTube and countless other means of recording our histories and the things that we find significant in our attempts to answer the questions of why and how?
The following is a bibliography that I hope others may find useful. Much of the above was derived from it, but I’ll include a few sources I have yet to read but are on my list.
Dorn, Ronald I; Jull, A.J.T.; Donahue, D.J..; Linick, T.W.; Toolin, L.J. (1989). Accelerator mass spectrometry radiocarbon dating of rock varnish. Geological Society of America Bulletin, 101, 1363-1372.
Dorn, Ronald I.; Whitley, D.S. (1983) Cation-ratio dating of petroglyphs from the Western United States, North America. Nature, 302, 816-818.
____ (1984). Chronometric and Relative Age Determination of Petroglyphs in the Western United States. Annals of the Association of American Geographers, 74, 308-322.
Liu, Tanzhuo; Dorn, R.I. (1996). Understanding spatial variability in environmental changes in drylands with rock varnish microlaminations. Annals of the Association of American Geographers, 86, 187-212.
Liu, Tanzhuo; Broecker, W.S. (2000). How fast does rock varnish grow? Geology, 28, 183-186.
Liu, Tanzhuo; Broecker, W.S.; Bell, J.W.; Mandeville, C.W. (2000). Terminal Pleistocene wet event recorded in rock varnish from the Las Vegas Valley, southern Nevada. Paleogeography, Paleoclimatology, Paleoecology, 161, 423-433.
Valledas, H.; et al (1992). Direct Radiocarbon Dates for the Prehistoric Paintings at the Altamira, El Castillo and Niaux Caves. Nature, 357, 68-70.
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