At the threshold of Far West Texas, the Lower Pecos Canyonlands contain some of North America’s most remarkable rock art. Here, where the Pecos and Devils rivers join the Rio Grande, on cave walls, prehistoric hunter-gatherers painted more than 350 rock-art panels of a distinctive style. Their scale is remarkable – one spans more than 300 feet. But what’s most arresting is the intricacy of the imagery – of human-like figures with headdresses and spear-throwers, of plants, animals and geometric forms.
The panels were long assumed to be disparate images. But in 2016, Carolyn Boyd, founder of the Shumla Archaeological Research & Education Center, showed that at least one was a cohesive composition, a mural. And, she argued, its meaning could be interpreted.
Now, Shumla Center researchers have released new findings about this “Pecos River Style” imagery. They’ve used cutting-edge science to date the murals, with results that baffle the mind.
Dr. Karen Steelman is Shumla’s science director. She conducted the rock-art dating at her lab in tiny Comstock, Texas.
“And my background is unusual,” Steelman said, “in that I’m a chemist, not an archeologist. It’s like when you’re on the airplane and someone asks you what you do – I have to give a whole paragraph to try to explain it.”
Developed in the 1940s, radiocarbon dating revolutionized archeology, rewriting the timelines of ancient societies. But it requires a sizable sample of charcoal or other organic material. And the paints used in the Lower Pecos were made not from charcoal but from ocher and other earthy materials. For decades, such pictographs remained un-dateable.
Then, in 1990, Texas A&M chemist Marvin Rowe showed that rock art could be dated with a technique called plasma oxidation. A decade later, Steelman, as a chemistry PhD student with a passion for art and culture, learned the technique from Rowe. In the years since, she’s become a leading expert.
Minerals give these pictographs their color. But to create lasting paints, ancient creators mixed those minerals with organic material – probably deer bone marrow, and the soapy compounds in yucca roots. With an instrument she built, Steelman creates an oxygen plasma that reacts with these organic “binders.”
“So we're taking the binders that are in the paint samples,” Steelman said, “and we're putting it in a vacuum chamber and we're putting in pure oxygen and we're running electricity through it. And it glows.”
It’s a painstaking process, but it yields carbon that can be dated.
Then, Steelman employs a second technique. The cave walls here are covered with a crust of calcium oxalate. Secreted by bacteria or carried in on mist, this “biomineral” accrues over time, and it too can be dated.
Steelman dated oxalate beneath rock art and on top of it – like an Oreo cookie, she said, with the painted imagery as the creamy center.
“So by dating the Oreo cookie,” she said, “we were able to show that the plasma oxidation dating of the organic material was correct, because we got maximum and minimum ages that agreed. It's really the first extensive study where both methods have been used.”
Steelman ultimately dated 57 samples, from 12 murals. The results were published last month in the journal, ScienceAdvances.
Pecos River Style imagery was created from about 3500 BCE to 900 CE, or more than 4,000 years. Across time, its creators followed consistent rules: In each mural in the study, all the black paint was applied first, followed by red, then yellow and white.
The foraging peoples who created these murals preserved a distinctive style and complex iconography even as other aspects of their lives changed. It’s a stunning continuity, and a reminder of the depth of our region’s human story.
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