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A non-invasive field triage strategy to assess rock art stability

Ronald I. Dorn1*, Niccole Villa Cerveny2, Steven J. Gordon3, and David S. Whitley4

1 Ronald I Dorn, Department of Geography, PO Box 8710104, Arizona State University, Tempe AZ 85287-0104, USA. E-mail:

2 Cultural Sciences Department, Mesa Community College, 7110 East McKellips

Road, Mesa, Arizona 85282

3 Department of Economics and Geography, United States Air Force Academy, Colorado Springs, CO 80840

4 W&S Consulants, 447 Third Street, Fillmore CA 93015

*Correspondence to: Ronald I Dorn, Department of Geography, PO Box 8710104, Arizona State University, Tempe AZ 85287-0104, USA. E-mail:

Abstract

Existing strategies to characterize the stability of stone require more time, expertise and cost than can be given to surveys of the thousands of rock art sites potentially in danger. In order to identify those petroglyph and pictograph panels most susceptible to erosion, we propose a field-friendly index including various elements of existing strategies. This Rock Art Stability Index (RASI) has five categories: “Setting the Stage”; “Preparing for Future Detachment”; “Loss of Stone Incremementally”; ”Loss of Stone in Chunks”; and ”Rock Coatings and Deposits". Initial testing reveals that training of individuals with no prior background in weathering can be conducted within a two-day period and yield reasonable results. RASI's use as a tool of cultural resource sustainability includes the use of a Geographic Information System to store, display and analyze rock art.

Keywords: archaeology; conservation; field methods; geomorphology; petroglyph; pictograph; stone; weathering

Introduction

That archaeological sites worldwide are imperiled is well understood. Cultural resource management (CRM) has developed as a professional specialization and career path in response to this fact and, overall, CRM has made great strides in slowing the loss of cultural resources in the USA. Almost all archaeologists, applied and academic, are quick to accept disciplinary responsibility for the well-being of the archaeological record. For example, phrases such as “Saving the Past for the Future” are widely used by both CRM and academic archaeologists as sound-bites to illustrate aspects of the relevance and goals of the profession.

While portable surface remains and subsurface artifacts remain in danger, perhaps the greatest risk to the richness of the archaeological record comes from the daily loss of rock art. There can be little doubt that anthropogenic factors and natural erosion continue to result in the destruction of countless numbers of motifs engraved or painted on rock surfaces (ICOMOS, 2000; Bertilsson, 2002; J.PaulGettyTrust, 2003; Varner, 2003; Keyser et al., 2005).

Many USA academic archaeologists make the claim that ignoring rock art in training their students is based on solid reasoning, that little can be achieved through its study. Commonplace are questions such as: what are the tools I can use to study this cultural resource? Why is rock art important? What can be learned that will advance insight into culture? An explosion of rock art research over the past two decades has answered these challenges (Clottes, 1997; Lewis-Williams, 2001; Whitley, 2001; Whitley and Keyser, 2003; Whitley, 2005), except for the question asked of weathering researchers: how do we even identify rock art sites, let alone rock art panels, that are endangered and require immediate study?

Although rock weathering researchers have dozens of strategies to measure weathering, no single easily-learned approach could be used by archaeologists to conduct a "triage" of the thousands of rock art panels in the western United States. The problem of identifying endangered rock art is beyond the ability of rock weathering specialists to solve by ourselves, even if everyone dropped extant research agendas and concentrated collective field time. The effort will require hundreds of thousands of hours, likely by enthusiastic volunteers wanting to "save the rock art".

Rock weathering specialists are desperately needed, however, to design a quick strategy usable by public volunteers. Thus, we present here a Rock Art Stability Index (RASI) as a first-step towards developing and implementing an evaluative strategy. Use of any triage RASI would have to require minimal training (e.g., a week-end workshop), but otherwise no specialized expertise in rock weathering or geomorphology.

In suggesting a new index, we first review existing strategies of analyzing the stability of stone, concluding that extant methods do not address the need. The middle section of the paper outlines our proposal for a field classification system that can be used by non-specialists, and how to turn a classification of weathering forms into an index that has meaning for cultural resource managers. We then explain the replicability of this methodology with individuals who had no prior experience in the examination of rock art panels. If future research validates our overall strategy or its inevitable methodological refinements, a RASI would need to be linked to a GIS capable of being used by weathering researchers and cultural resource managers.

No matter how the reader reacts to our particular list of weathering forms and RASI to create an understandable index, we believe it is critical to lay this problem at the feet of weathering researchers as soon as possible. Every year we delay in helping cultural resource managers will see the loss more priceless cultural knowledge — information that could have been saved by identifying endangered rock art panels.

Existing Strategies Cannot Triage Thousands of Panels

Rock weathering as applied to sustainability of stone cultural resources interfaces a wide variety of disciplines (Figure 1). Analyzing potential future instability requires clear and replicable methods of classifying weathering as it would relate to future erosion. Of the ways to understand just chemical weathering, the literature a decade ago contained more than fifty methods (Dorn, 1995), and at least three dozen more have been added since. Weathering research on rock art has utilized a wide variety of methods (Dolanski, 1978; Campbell, 1991; Benito et al., 1993; Pineda et al., 1997; Van Grieken et al., 1998; Pope, 2000; Fitzner, 2002; Pope et al., 2002; Fitzner et al., 2004; Hoerle and Salomon, 2004; Tratebas et al., 2004; Barnett et al., 2005; Hoerle, 2005; Wasklewicz et al., 2005; Hoerle, 2006). Consider that one research group's methods (Fitzner, 2002) include over twenty field strategies and over forty laboratory procedures. Rock weathering researchers have at their disposal a closet-full of approaches to analyze the decay of stone.

The variety of technical methods under consideration (Table I) includes technical and expensive geochemical and geophysical analyses that yield strategies appropriate for engineering purposes. Geomorphological conceptualizations to understand natural patterns of weathering tend to be more field-friendly, and include elements that need to be included in an analysis of rock art panels (Turkington, 2005; Turkington and Paradise, 2005). No single strategy in Table I, however, would work for rock art specialists trying characterize the thousands of rock art panels endangered in the western USA alone.

The most appropriate extant strategies to analyze decay in rock art panels rest in the stone conservation literature (Table I) (Ashurst and Dimes, 1990; Winkler, 1994; Smith and Warke, 1995; Price, 1996; Fitzner, 2002; Siegesmund et al., 2002; Warke et al., 2003). Despite eminently suitable methods appropriate for use in building stones, there are some serious incompatibilities with analyzing rock art panels.

Panels on such surfaces as joint faces exist in a framework where the host rocks are usually heavily weathered prior to engraving or painting (Battiau_Queney, 1996; Ehlen, 2005). Building surface analysis strategies were simply not designed to be used where the rock had an extensive history of complicated paleoweathering (Fitzner and Heinrichs, 2002; Moropoulou et al., 2003a; Moropoulou et al., 2003b; Pininska and Attia, 2003; Salvadori et al., 2003; Striegel et al., 2003; Warke et al., 2003; Fitzner et al., 2004; Vicini et al., 2004; Smith et al., 2005; Turkington et al., 2005). Rock art panels do not only suffer from destruction of the surface of stones like buildings, but often from entirely rotted rock.

Despite tremendous potential for fertile exchanges between stone decay researchers and natural weathering researchers, a polarization extends to different academic training, different conferences, different publication venues, different feelings about publishing proprietary conservation insights and different foci (Smith et al., 2005). This offset of viewpoints is seen clearly when papers from same field site derive from different paradigms (Fitzner and Heinrichs, 2002; Paradise, 2005). A systemic example of this divide comes from the treatment of rock coatings.

Building classification schemes for rock coatings (Fitzner, 2002; Fitzner and Heinrichs, 2002; Fitzner et al., 2004) are simply unworkable for natural coatings (Dorn, 1998). For example, "black varnish" in the building literature can be mix of iron oxide and carbonaceous matter (Thomachot and Jeannette, 2004), fungi (Diakumaku et al., 1995) or other materials (Moropoulou et al., 2003b). In contrast, black coatings in the petroglyph world is a mixture of manganese and iron hydoxides cementing clay minerals to panel faces (Dorn, 2001). Beyond terminology offsets, in the rock art world most rock coatings are generally positive in their effect — an attitude opposite to views posited in the building literature. In the rock art context, rock coatings often stabilize by creating a case hardening effect (Conca and Rossman, 1982; Dorn, 1998; Viles and Goudie, 2004; Turkington and Paradise, 2005). Even though lichens and other lithobionts weather the underlying material (Stretch and Viles, 2002; Gordon and Dorn, 2005), they are not simply erosional (Viles and Pentacost, 1994; Viles, 1995) but often exist as a "caprock" to protect the crumbly material underneath (Souza-Egipsy et al., 2004). No building environment analysis scheme treats rock coatings as a protective agent. This is not meant as a critique. The focus in building conservation research rests in understanding urban pollution mitigation (Sharma and Gupta, 1993; Urzì et al., 1993; Young, 1996; Van Grieken et al., 1998; Striegel et al., 2003; Smith et al., 2005), not on the interaction of rock coatings with panel erosion (Tratebas et al., 2004).

The most critical offset rests is funding requirements. Working with building stones requires a fiscal base well beyond the rock art site manager. A case in point is the Bangudae petroglyph site in Ulsan, Korea (Fitzner et al., 2004). The damage diagnosis at Bangudae is an ideal to shoot for in future rock art conservation studies, but the funding required to undertake that level of analysis is simply not available. The thousands of rock art sites in the western United States alone requires a far cheaper and less-technical triage strategy that can be employed by individuals without prior training in rock weathering.

A Field Classification System Usable by Non-Specialists

We propose a field-based panel erosion classification system that can be utilized by site managers and their project assistants (Table II). The terminology represents a compromise between minimizing jargon and articulating to the weathering literature. Additional explanation with exemplars exists in a supplementary on-line Atlas of Petroglyph Weathering Forms used in the Rock Art Stability Index (RASI) (Dorn et al., 2006) (Reviewer Note: This atlas exists at a stable Internet site of: http:alliance.la.asu.edu/rockart/stabilityindex/RASIAtlas.html) This Atlas of Petroglyph Weathering Forms is used at field sites by downloading the entire atlas accessible by any browser.

Three-dozen weathering forms are classified in our RASI under these five broad categories that are organized to facilitate training of volunteers and also to align with the weathering literature as much as possible:

Setting the Stage: Fissures and Rock Weaknesses

This section of the index addresses inherent weaknesses in the art substrate — the bedrock. The idea is for the indexer to first examine the rock art panel from a distance of tens of meters and look at the patterns of jointing and bedding (Figure 2). (Review Note: we submit both B&W images for journal printing and color figures for the InterScience site.) The first three rows in the index worksheet (Table II) address the natural occurrence of structural weaknesses in the parent rock that could produce differential weathering and erosion. Visible fissures present in the rock may result from such processes as jointing, calcrete wedging, and fractures parallel to bedding planes. The last row, rock hardness, is an obvious element to be measured. Simple Moh's hardness tests take place on the freshest rock on a hidden back section of a panel, but never on any visible section of the panel.

Preparing for Detachment

Between the visible fissure and the obvious detachment of stone rests a host of factors that could set processes in motion that generate loss of stone. This section focuses on factors that could readily erode a panel surface, issues that would lead an indexer to predict future spalling (Figure 3). First taught examples would be flakes (fingernail-millimetre scale spall) or scales (thicker than flaking) of rock not yet eroded. Thus, the second section of the RASI scoresheet addresses the progression of weathering where indexers address the presence of portions of the art panel that could spall/erode in the foreseeable future, either from weathering along weaknesses (scaling, flaking), organic activity (roots), spalling due to fissuresols (Villa et al., 1995), undercutting, weathering rind development, or other processes.

Loss of Stone Incrementally

The obvious erosion of a rock art surface most often occurs in smaller (millimetre-thick, centimetre-thick) pieces (Figure 4). Indexers go through a lengthy list of forms that indicate erosion has already taken place. Small but constant loss of surface may occur as a result of abrasion from sediment transported by water, aveolization, disintegration in crumbles and/or granules, flaking of pieces, scaling of pieces, lithobiont pitting and release, rounding of edges, differential loss (such as around nodules) splintering, and other gradual loss.

Loss of Stone by Breaking-off Chunks

Decimetre-thick and larger pieces do dislodge from rock art panels instead of the generally slow, steady loss of material in the previous section. Calling these large fragments 'chunks' successfully allows indexers to discriminate these larger spalls (Figure 5). The index accounts for loss of large pieces from human activity, wedging from fissuresols, fire spalling, undercutting of the surface, roots, earthquakes, and other sources.

Rock Coatings and Deposits

Thus far, the indexer addresses issues that would lead to a higher RASI index score, meaning that the given art panel is experiencing greater decay and in need of preservation. Phenomena like fissures, weathering rinds, roots, and tafoni all lead to loss of the panel surface and therefore the art that is upon the surface. Rock coatings, in contrast, may alternately preserve the art by stabilizing the surface and protecting the art. Similarly, the presence of case hardening processes actually stabilize the surface and are therefore given a negative value in the index to represent their stabilizing role in RASI. Yet, human rock coatings like chalk and graffiti degrade the surface and the art itself, and natural deposits like salt efflorescence lead to surface loss and spalling (Figure 5). Thus, only four components of rock coatings and deposits are scored by the indexer and included in RASI.