Rock art, consisting of pecked, scratched, or painted images, on rock surfaces comprises one of the most important archaeological artifacts of the Colorado Plateau region (Cole, 2009). They provide glimpses into the life, behaviors and customs of the prehistoric peoples who created them. Rock art is widely distributed geographically and is composed of a broad sweep of themes relating to essentially everything of social significance: hunting, religion, ritual, agriculture, warfare, birth, water, migration, and much more. An important step in relating these images to other physical artifacts that will advance our understanding of these people would be a reliable and non-destructive way of dating the petroglyphs.
There are presently several techniques used to date petroglyphs which can be broadly categorized as direct and indirect. For a review of rock art dating methods see Chapter 5 in Francis and Loendorf (2002). Relative dates can be discerned through superimposition and through relative levels of re-patination although Dorn (2007) warns against this practice due to micro-environmental and other effects. Themes and styles can also provide approximate dates principally through association with dateable artifacts of similar theme located nearby. Direct physical dating of rock art has been attempted using a variety of methods including atomic mass spectroscopy (AMS) measurements of 14C, Particle-induced X-ray Excitation (PIXE), analysis of (K+Ca)/Ti cation ratios, and studies of micro-laminations in the desert varnish (Dorn, 1981,1989, 1994, 1998a-b, 2007). There are some clear successes with these approaches, but some cautionary tales as well (Beck, 1998). One drawback to these methods is the requirement for a physical sample to be extracted from the desert varnish which, though usually small, damages the artifact.
fluorescence (XRF) experiments to measure the level of elemental manganese accumulation in petroglyphs of the Coso region which, if the manganese level can be correlated to known dates, could provide a means to estimate rock art age. The method relies on the peculiar fact that desert varnish, the dark blue-black pigment that accumulates on rock surfaces throughout the desert southwest, is unusually rich in manganese while the underlying base rock generally contains very little of it. Dorn (1981) found that the manganese accumulates through a metallogenic microbial process. Since the underlying rock is poor in manganese, the manganese must arrive on the rock surface as dust from elsewhere which is then concentrated by the metallogenic microbes. However, this process is poorly understood and other processes that transport Mn may be involved. The operatingassumption of Lytle, et al. is that when a glyph is pecked onto a rock surface, the patina is removed and the manganese accumulation process begins anew. Over time the Mn accumulation darkens the glyph eventually returning it to the same color as the background desert varnish. As a biological process, growth rates are highly variable, depending on micro environmental conditions, moisture, dust, surfaceorientation and texture, over/under hang, etc. In addition the pecked surface of petroglyphs are more susceptible to erosion and spalling further complicating the picture.
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X-ray Fluorescence Measurements of Manganese in Petroglyphs and Graffiti in the Bluff
