How do you find out the age of ancient rock art? And more importantly, how can you do that without disturbing the site? Meinrat and Tracey Andreae of the Max Planck Institute for Chemistry recently published the results of several measurements they took at rock art in Idaho, Wyoming and Montana. They showed that their non-destructive detection method was able to narrow down the age of several rock engravings, some of which were 12,000 years old.
With permission from the parks where the rock carvings were located, they visited several sites in the North of the Great Basin, an area home to Shoshone cultural history. The rock art in this region was made over the course of several centuries, so it’s not immediately obvious how old some of the engravings are – some might be a few centuries old, while others are over ten thousand years old. Often, rock engravings have also been worked on over a period of time, with some sections being much older than others.
Scientists can study the composition of a rock with chemical and geochemical methods, but some of these methods involve taking physical samples from the rocks. That would not be an option at such a culturally significant site. Just a few months ago, Caltech geoscientists were fined for having drilled samples at a petroglyph site in California in 2017.
But the method the Andreaes used didn’t require any drilling. They used a portable X-ray fluorescence device to measure the amount of manganese and iron on the surface of the rock. The composition of these chemicals in the outer layer of the rock changes over time at a constant rate, so even after a layer is scraped off when creating rock art, it gradually reforms again.
The researchers could see the difference between the amount of manganese and iron in the more recently formed layer on the carved sections of rock compared to the uncarved rock around it and used that to calculate how long ago the engraving was made. They confirmed their method by trying it on some rock art of which the age was already known, and by comparing it with archeological evidence from around the sites.
Based on their measurements, they were able to narrow down the age of rock carvings in the Northern part of the Great Basin.
“All of our analyses suggest that the earliest petroglyphs were created as early as the transition period from the Pleistocene to the Holocene, about 12,000 years ago, and were repeatedly revised by indigenous people over thousands of years until the recent past,” Meinrat Andreae told the Max Planck Institute.
It’s not a perfect method. It can be difficult to assess the exact age of an individual rock engraving, for example because weather conditions over the years might have affected the rock. But when measuring many different rock art images in the same location, this method is able to hone in on an approximate age of the rock carvings.
Andreae hopes that this technique can be used alongside existing archeological and historical methods to estimate the age of rock art. He says, “Our method provides a link between the natural and human sciences. It enables age estimates for a statistically relevant, large number of rock art elements – with modest effort and, above all, without the need for destructive sampling,”