Spatial Analysis Tools for Archaeologists


Dating Pictographs And Petroglyphs Through Their Fallen Pigment Or Stone Dust In Underlying Archaeological Floors:
A New Method

Bryan C. Gordon, Canadian Museum of Civilization (bryan.gordon@civilization.ca)


Introduction


Bednarik (1997:147) says the earliest known art and related acts provide the base for human symboling, language and cognition, suggesting an origin in several hundred thousand year-old Lower Palaeolithic hominids. The earliest known rock art (pictographs and petroglyphs) is in central India, Sudan and the Korannaberg of South Africa, while the earliest in Europe is at La Ferrassie in southwest France. There are thousands of Pleistocene rock art sites in Australia, while the oldest American art is in Argentina (Bednarik, pers.comm. 2009). Wall art marks territory, hunting and astronomical events or drug, starvation or thirst-induced vision quests and shamanic trances, whose meaning is often tentative in the absence of historic documentation. Proposed future research for dating and clarifying wall art are described on the following page.

We are constantly amazed how beautiful and puzzling ancient art can be, so it is fascinating to know when it was made. Dating helps to ease our fascination, but it also helps us track the spread of a culture or art form across the land. Eight supposed indicators for dating rock art existed 40 years ago: stratigraphy, superposition, style, weathering, lichenometry, ethnohistory, prehistory and some lab methods. None were reliable. In the underlying soil, motifs on carbon-datable ivory, bone or wood portable art rarely resemble those on the wall. Superimposed paintings only determine their sequence, not their date. Style and age may have no relationship, and elegance of design may deteriorate as new art is introduced. Lichen growth begins any time after the art, while ethnohistoric records of the art seldom exceed several centuries. Techniques for carbon dating of organic paint binders, costly and often problematic, include accelerator mass spectrometry (AMS), spectroscopy, amino-acid analysis, scattered electron microscopy (SEM) and chromatography. AMS is the most promising, but binders may include dead carbon brought up as sap from the soil (e.g, oxalic acid in cactus). These techniques are expensive, lab-oriented, time-consuming and too complex for most archaeologists.


Previous Research


(1)  Direct AMS pigment dating papers include Loendorf (1990, 1992), who dated a pigment-covered smoothing rock in a stratified level. Other papers include Mazel and Watchman (1997) on Natal Drackensberg paintings, Prous (1999) on Brazilian rock art, Watchman and Cole (1993) on plant fiber binders in northern Australian rock paintings and Chaffee, Loendorf, Hyman and Rowe (1994) on a Pryor Mountain pictograph in Montana. CR (cation ratio) dating (Loendorf 1990) does not work.

(2)  Historic dating papers include Klassen et al.'s (2000) account of Piegan elder Bird Rattle's petroglyphs at Writing-On-Stone in Alberta; and Stoffle, Loendorf, Austin, Halmos and Bulletts' (2000) accounts of the Ghost Dance pictographs in the Grand Canyon.

(3)  Papers of post-dating rock art above undisturbed strata include Francis et al. (1993:713) in the Bighorn Basin of Wyoming and Montana. They AMS-dated rock art penetrating the soil surface to a level yielding an uncalibrated age of 1920±140 years. An early investigator was Mulvaney (1969:176) who found a fallen engraved slab in an upper archaeological level at Devon Downs in South Australia’s Lower Murray Valley. In the Jinmium rockshelter of northern Australia (See 5), Roberts et al. (1998:358) found pecked cupules at 97cm subsurface and an engraved sandstone fragment at 100cm.

(4)  Dating papers using fallen visible spilled paint and painted chips include Cole (2000) at Laura, Australia; Fiedel et al. (1996), who dealt with hundreds of lumps and actual drops of red ochre, and Haynes et al. (1997:1952) who dated an Amazonian Paleoindian Site, stating in their Technical Comments that dating was based on abundant paint spatters of the same chemical composition as the art. Roosevelt et al. (1996:378) also found hundreds of lumps of red ochre, as well as two small pigmented wall spalls in Caverna da Pedra Pintada. Thackeray et al. (1981:64-67) found engraved stones in sealed radiocarbon-dated, 10,000 year-old archaeological levels at Wonderwerk Cave in the northern Cape Province of South Africa.

(5)  Minimum-age dating of an underlying stratum. Fallen petroglyph or pictograph fragment dating was done by Susino (1999) in Australia’s Mutawintji National Park and near Sydney on cultural microdebitage (quartz grains), establishing their rock engraving periods. At the Jinmium rock shelter in northern Australia, Roberts et al. (1998:358-362) questioned 50-75,000 year-old thermoluminescence dates on quartz grains associated with buried circular engravings (pecked cupules) and thermoluminescence ages of 116–176 kyr on the underlying cultural deposits. They found 22,000 year optical ages for the grains, with some grains being recently buried, concluding that Jinmium is younger than 10,000 years, with the deposit base at 22,000 years.

(6)  Macintosh (1965), Wakankar (1983), Combier (1984), Linares (1988) and Chippindale and Tacon (1998) indirectly dated wall art by colour-matching dated soil and art pigments. They were unconvincing because pigments are amorphous and change colour with exposure, blending, distance to camera, wetness, lighting, binders (and their decomposition) and source. Pigments fallen to the soil may be protected from sunlight, but soil pH, water and other minerals can alter their colour. Macintosh (1965:92-93) relied on Munsell and eyeball colour estimates to evaluate visible ochre-bearing sediments in the Southern Shelter on the southeast slopes of Mt. Manning, 72 km north of Sydney, Australia. Excavating in 3 cm (1st test) and 1 cm (2nd test) levels, he found a dark red stratum at 4-6.75 cm subsurface, collecting 41 g of hearth charcoal at 3.7-4 cm subsurface and 50 g of hearth charcoal at 27-28 cm. These dated AD 1806a/1830b and 1369, respectively (GX-0069a,b & 70), with high sigmas of 120-155 years. His light red ochre soil had a Munsell value of 2/R 5/7, while a worn ochre nodule was 5R 4/8. His results on abundant ochreous sediment and conventional carbon dating were good for their time, while the later authors also relied on visual comparisons.

(7)  Accurate dating from northern Australian beeswax figures was done by Brandl (1968), Chaloupka (1993) and Welch (1995). They are the most amenable for dating because they were made from fresh wax which is very stable.

(8)  Accretionary deposit dating. Wall art under carbonate, silica and oxalate skins or crusts may assist in estimating age, but we really do not know what is being dated.

(9)  Indirectly dating wall art by matching dated soil and art pigments using physico-chemical methods. In reality, iron oxides and hydroxides are highly susceptible to chemical alterations that inevitably involve changes in reflective properties (colour), and it is naive even to assume that either the pigment on the wall or the pigment in the ground would retain the original colour. Both are likely to be modified differently, and will probably not match, bearing in mind the metastable character of all iron salts (which form the majority of pigments used in rock art). Can naturally occurring pigmented compounds in soil be separated from culturally occurring art pigments? Red and yellow ochre are common iron oxides or hydroxides used in pictographs.


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