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Dating the Petroglyphs of the Mongolian Altai

The history of man and climate, written into the bedrock of Mongolia.
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The geologist crouches before the surface of a boulder, his finger hovering over an image of two ibex butting heads. “Where the horns converge, there is a beautiful symmetry to them,” he says. He indicates the half-circle curl of the horns; the wild goats’ legs are straight thin lines. Petroglyphs like this one have been here for many thousands of years, since Stone Age people first began gouging the rock. In the light of a Mongolian summer, the images seem to be suspended in air.

The glacial geologist, Aaron Putnam of the University of Maine, runs his hands along the greywacke sandstone, a common bedrock in this part of the Altai Mountains, on the shared border of Russia and China. With the rock’s original dark polish chipped away, the fresh stone makes a pointed contrast, helping the petroglyphs to stand out — a sort of picture frame.

The petroglyphs are difficult to date precisely. Over the past 15 years, Richard Kortum, a professor of philosophy and humanities from East Tennessee State University, has catalogued roughly 12,000 petroglyphs here, in Mongolia’s Altai Tavan Bogd National Park. He’s been determining their approximate age by correlating cultural clues with records of archaeology and paleoecology. “It’s like piecing together a complex jigsaw puzzle,” Kortum says.

The oldest images are jagged and include deer and stick-figure humans. There are also petroglyphs of bulls and yaks, but these have clean edges and are more anatomically correct. Kortum says they are likely from the Neolithic period. The majority of the rock art comes from the Bronze and Iron Ages: clear and precise images of chariots, and animals with beautifully curled antlers that come to a sharp edge.

Putnam’s here to study climate change and to develop a chronology of glacial retreat, taking samples from granite boulders that were once suspended in ice. Analyzing the petroglyphs is important for understanding the culture and movement of ancient people, but, when joined with a geologic examination, it can tell us a lot about the history of climate. Images of horses and camels, for example, indicate the domestication of animals and suggest an abundance of grassland in the area — which scientists can confirm by analyzing pollen that’s enclosed in sediment.

Back in Maine, Putnam will measure the accumulated cosmogenic radiation on the surface of granite boulders scattered in this area. These data points tell the story of a process that began the moment the rocks were first dropped out of glaciers and onto the landscape. From this information, Putnam can derive a date for the end of the Ice Age, which is his primary interest: A clear chronology of glacial recession can help us improve scientific models that predict how anthropogenic climate change will affect future societies. The dates also approximate when the first people could have lived here, but Putnam thinks there’s another technique to determine when the rock art was actually made.

He points to long white lines etched by massive chunks of glacial ice that existed here during the Ice Age, roughly 20,000 years ago. Back when the glaciers receded, the ice carved wide valleys in the hills, and these became pathways for the first people to enter the highlands. They took up sharp stones and pecked the smooth rind of glacially polished rocks — not knowing that, eventually, every detail would be scrutinized.

When it rains, a black mineral accumulates on the rocks — manganese oxide, sometimes called desert varnish. It builds up in layers, and Putnam thinks he can analyze them, like tree rings, to get an approximate date for when the images were actually made — not just when people could have first come. Kortum is not certain it will succeed, but he has said it would be “a revolutionary result” — a climatological revelation accidentally inscribed in the art of Mongolia’s earliest peoples.

This trip was funded by a fellowship from the Comer Family Foundation and Northwestern University’s Medill School of Journalism.