First dating of Pilbara petroglyphs

The first attempts at scientific dating of petroglyphs in the Pilbara region of Western Australia are reported. Using microerosion analysis, a calibration curve has been established on the basis of a series of engraved historical dates, which is then used to estimate the ages of several petroglyphs. The results indicate that some of these are of Pleistocene antiquity. The consistency of the preliminary results of this project with observations made elsewhere in Australia and abroad is noted. THE DATING OF PETROGLYPHS Rock art occurs in two forms: as the results of additive processes, in the form of pictograms (paintings, stencils, drawings, beeswax figures); and as petroglyphs, i.e. the results of reductive processes (pounding, abrading, engraving, pecking, drilling). This division immediately determines the fundamentally different approaches to rock art dating, or rather, to estimating its antiquity. The substances added to the rock surface in the creation of pictograms provide the analyst with a variety of datable compounds that are of ages closely resembling the time the art was produced. For instance, paint residues may contain remains of organic binders or diluents (blood, saliva, orchid juice etc.) or pigments (charcoal, cochineal, berry juice etc.), as well as a great variety of incidental inclusions (brush fibres, pollen, bark fragments etc.) that contain radiocarbon of adequate quantities to permit accelerator mass spectrometry (Cole and Watchman 1992; but cf. Ridges et al. 2000). The finegrained mineral detritus removed in the production of petroglyphs is, however, not likely ever to be recovered for analysis (cf. recent attempts by G. Susino to do so). Therefore, in order to date petroglyphs directly we have to resort to analysing a physically related feature that either preor postdates the art (lichen, accretionary skin or crust, biogenic deposit etc.), providing a minimum or maximum age. In the language of Dunnell and Redhead (1988), such a procedure fails to focus on the 'target event', which is the creation of the petroglyph. Moreover, most of the methods used are seriously impaired by inherent uncertainties. For instance, dating bulk carbon isotope concentrations in accretionary crusts may be severely misleading, particularly if the crusts are open systems (Bednarik 1979, 1994a; Nelson 1993; Dorn 1997). Thus, the dating of petroglyphs remains one of the most intractable problems in archaeology. Indeed, of the methods proposed for petroglyph dating, only microerosion analysis (Bednarik 1992) seeks to determine Dunnell and Redhead's (1988) 'target event'. This paper presents the first application of this technique in Australia, and the first scientific age estimates for Pilbara rock art. Until the early 1980s, no inherently testable methodology had become available for the age estimation of rock art (Ward and Tuniz 2000). Previous dating claims had generally been based on non-scientific criteria, and in the case of petroglyphs it had been sought to determine the ages from their perceived iconographic content (i.e. what the art was thought to depict), style or technique; from spatial proximity to archaeological evidence (such as occupation debris) and by excavation (usually of purported occupation deposits); or from superimposition, patination and weathering. Iconographic interpretation, however, is not falsifiable in most instances, and stylistic constructs, while possibly comprising some valid elements, are not a sound basis for dating rock art, as has been demonstrated time and again in various continents (e.g. Macintosh 1977; Bahn and Lorblanchet 1993; Bednarik 1995a, b; Watchman 1995, 1996). Not only are such mental constructs of researchers untestable, they tend to reflect modem Western perception more than historically valid variables inherent in the rock art. They are also taphonomically naive in that they ignore that the surviving vestiges of any rock art are not random samples of living traditions (Bednarik 1994b), and they tend to be formed without adequate recourse to archaeometry. The technique of petroglyph execution, too, is rarely a reliable chronological marker, and should not be cited as a dating criterion without substantial other evidence. Proximity to occupation sites is almost irrelevant in the reliable dating of rock art, because rock art often occurs at 'focal' sites within a landscape, as do occupation debris, hence their co-occurrence is more likely to be related to this factor than to contemporaneity.