Dating And Digitally Reconstructing Pictographs At The Hedley Rockshelter Site, British Columbia

Bryan C. Gordon, Curator Emeritus Research, Canadian Museum of Civilization

1. Introduction

Wall art (pictographs and petroglyphs) is extremely widespread temporally and geographically, dating from the Historic period to 70,000 years ago at Blombos Cave in South Africa (Henshilwood et al. 2002), 35,000 years in Fumane Cave near Verona, Italy (Broglio et al. 2006), and 32,000 in Chauvet Cave in southern France (Clottes 2003). Its most famous example is Magdalenian, dating back 20,000 years in Lascaux Cave in southwest France (Ruspoli 1987), and 14,000 years in Altamira in northern Spain (Beltran 1999). Other Old World art has a similar age range, while New World art verging on 8000 years old is poorly known. Wall art marks territory, hunting and astronomical events or drug, starvation or thirst-induced vision quests and shamanic trances, whose meaning is largely unprovable. Techniques for dating and clarifying wall art are described in this paper.

We are constantly amazed how beautiful ancient art can be, so it is fascinating to know when it was made, as its dating helps to track a culture’s spread over the land. Eight methods for dating rock art existed 40 years ago: stratigraphy, superposition, style, weathering, lichenometry, ethnohistory, prehistory and lab methods; none of these is reliable. Strata with similar datable mobiliary art under the wall art are rare, while superposed paintings only determine their sequence, not their date. Style and age may have no relationship, the first radiocarbon dates of Lascaux’s stylistically-dated paintings having been wrongly rejected. Furthermore, the bones of the animals depicted in its art are absent in its soil, in which reindeer bones overwhelmingly dominate. Lichen growth dating could begin any time after the art was made, while ethnohistoric records of the art may be used where Natives kept an oral tradition, but seldom exceed several centuries. Techniques for carbon dating of organic paint binders, also 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).

2. A New Method of Dating Wall Art

This method depends on co-occurring fallen pigment dust, droplets or rock fragments with AMS datable charcoal, plant, shell and bone fragments in a 20x50 cm rectangular soil scraping or peel below the rock art. Pecked petroglyph fragments also fall to the artist’s feet or cultural floor, but I will cover only chalk, paste or paint dust or droplets from pictographs. With the pictograph colour as standard, a quick scan of magnified digital photographs of each peel’s surface reveals fallen pigment among the soil grains. Where desired, follow-up electronic filtering can be programmed to reveal pigment only. We tested our method by chalking yellow and red ochre designs on plywood leaning over a sandbox. We electronically removed non-pigment colour with Photoshops’s Eyedropper. Freeware Paint.Net or GIMP does the same but can also automatically separate red, green and blue (RGB values). There is a need for precision because ochres and soil ferromagnesian compounds have similar colours. We also placed a pigment-coated neutral-gray paper strip directly on the peel as RGB standard. The dust from each successive pictograph fell on various colours of newly sprinkled 5mm-thick sand to a depth of 50mm. Tilting the camera vertically and photographing each new surface for later evaluation, we scraped away 5mm peels with a vertically held rectangular masonry trowel. It was impossible to scrape away to attain the original surface, but enough particles were found to confirm the method.

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