Weathering of Petroglyphs: Direct Assessment and Implications for Dating Methods

Article excerpt

Introduction

Few records of ancient human cultures evoke the sense of wonder and concern over their precarious survival than the remains of their `art', applied with pigments or etched into the rock of canyon cliffs and cave walls. But for all its potential value toward interpreting the past, rock art remains difficult to date. This is particularly the case with petroglyphs, or rock engravings, with little else but gouged clefts in the rock to provide information. Traditional dating methods using stylistic correlations are widely used, but only arriving at relative ages. More recently developed methods applying isotope decay, erosion rates and trace element chemistry have the potential to provide more accurate chronologies (reviewed in Bednarik 1996 and Dorn 2000), though debate over their application is still active. One such debate concerns the open-air engravings found in the Vale do Coa of Portugal (Rosenfeld & Smith 1997). The debate deepened with the discovery late in 1999 of new rock art buried under fluvial sediments at nearby Fariseu (J. Zilhao pers. comm.). Radiocarbon (Watchman 1995) and microerosion (Bednarik 1995a; 1995b) methods suggest that the petroglyphs are of recent (perhaps even historical) vintage. The reliability of the [sup.14]C dates has been questioned (Zilhao 1995; Dorn 1997; Welsh &Dorn 1997), and Watchman (1999) now recasts his original findings with more caution. Stylistic methods suggest that the petroglyphs are much older, perhaps Upper Palaeolithic in age (Zilhao 1995), establishing their prominence in the great collections of rock art from this time period. Phillips et al. (1997), using cosmogenic isotope analysis, determined that the engraved panels are of sufficient age to have been used by Palaeolithic artists. Stratigraphic [sup.14]C correlations with the new petroglyphs at Fariseu (Zilhao 1999) and tool markings (d'Errico et al. in press) may corroborate a Palaeolithic age.

Clearly, several unresolved issues inhibit reliable dating of petroglyphs. One core issue of background research relevant to petroglyph dating concerns rock weathering. There is a limited understanding of how weathering of the host rock impacts petroglyphs. Weathering affects not only the rock and petroglyph (Walderhaug & Walderhaug 1998), but many of the dating methods employed as well. Several studies (Walston & Dolanski 1976; Steinbring & Callaghan 1985; Campbell 1991; Sjoburg 1994; Laver & Wainwright 1995; Young & Wainwright 1995; Wang & Hua 1997; Walderhaug 1998; Waulderhaug & Walderhaug 1998; Russ et al. 1999) discuss the weathering of rock panels on which petroglyphs are engraved. Only Bednarik (1992; 1995a, 1995b) has made descriptions of `microerosion' of petroglyphs for the purposes of dating (erosion being an end product of weathering). The purpose of this paper is to expand upon these previous studies and specifically address the weathering of the petroglyph engraving itself.

Study sites and methods

This study surveyed four separate rock engraving locations, each with different lithology and age (TABLE 1). The Hieroglyph Canyon (FIGURE 1a) and Vale do Coa sites are `classic' petroglyphs engraved into dark coatings and surface weathering on exposed rock. The Alto Alentejo sites consist of symbolic engravings made on megalithic menhirs. The Central Park example is recent graffiti, a typical `heart with initials', carved into the Bethesda Terrace tunnel bridge (FIGURE 1b).

[Figure 1 ILLUSTRATION OMITTED]

TABLE 1. Rock hardness (R) data for Hieroglyph Canyon, Arizona; Evora and Monsaraz, Portugal; and Central Park, New York.

engraving        location(*)         ID          N   mean R

Hieroglyph Canyon, Pinal County, Arizona.
Pre-Columbian engravings into varnished rhyolite

`sheep 1'        inside        HGC-1a, 1b       26    57.5
                 outside       HGC-1c, 1d       26    61.3

`sheep 2'        inside        HGC-2a           12    43. …