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30 August 2014
Famous Utah rock art may be much recent than was thought

Since the original Barrier Canyon rock art panel, known as the Great Gallery, was first discovered by scientists in Utah's Canyonlands National Park (USA), experts have debated how old the images are, and what culture created them. Some archaeologists have theorized that the rock art may be as much as 4,000 to 7,000 years old. But new chemical analysis, combined with some other geological detective work, suggests it was painted much more recently, and may even be little more than 1,000 years old.
     "The painting of the Great Gallery occurred during a window between late Archaic time, around 1 CE, through the introduction of maize and the bow and arrow to Utah, and on to the peak of the Fremont culture ca.1100 CE," writes a team of archaeologists in a recent issue of the Proceedings of the National Academy of Sciences.
     Rock art is notoriously difficult to date, but paintings on rock, or pictographs, like those found at the Great Gallery, usually offer an advantage, because they have pigments that can be tested. But previous research at Barrier Canyon had found that the pigments contained no organic materials, and couldn't be radiocarbon dated. However, new technology - known as optically stimulated luminescence dating - has proven extremely useful in determining when mineral deposits that have been buried were last exposed to sunlight, and for how long.
     An international team of researchers, led by Utah State University archaeologist Joel Pederson, first set about setting a maximum age for the Great Gallery, by using luminescence to date the layers of sediment on the canyon floor. The team found two strata that revealed important geological events. The first was a thick layer of flood-driven sediment that filled the canyon high above the level of the Great Gallery, up until about 8,000 years ago. Over the following 5,000 years, most of this layer eroded away, eventually exposing the sandstone panel in the canyon for the first time. Therefore, the team writes, "the art is incontrovertibly younger than the top" of that layer. Then, their analysis showed, a second, newer layer of sediment was laid down between 3,000 and 800 years ago. This became the modern canyon floor. Taken together, these dates seem to disprove the oldest proposed dates for the gallery, the archaeologists say. "This reasoning alone makes an early Archaic (>5000 BCE) origin for the Great Gallery improbable, and any older hypotheses are ruled out," they write.
     But then there was the matter of how young could the artwork be. To find a minimum date, Pederson's team focused on that remnant of the Great Gallery that had collapsed in an ancient rockfall. The researchers tested quartz grains from the face of the fallen rock, as well as the sediment that the boulder landed on. Both of the luminescence dates returned the same date range: about 900 years old. Since the Great Gallery must have been painted before the rockfall, the scientists conclude, "these three convergent dates provide a very solid minimum age constraint of 1100 CE, the height of the Fremont culture."
     Finally, the luminescence technique provided one last data point that allowed the team to  determined that the face of the rockfall had been exposed to sunlight for at least 700 years before it collapsed, putting the approximate date of the artwork's creation in the 5th century. Judging conservatively, the team concludes that Canyonlands' Great Gallery was created between 900 and 2,000 years ago.
     This is the period, Pederson's team points out, when immigrant groups from the Four Corners region were thought to have first moved into the area north of the Colorado River, introduced local foragers to the game-changing practices of agriculture and village settlement patterns. Some experts suspect that this interaction gave rise to the Fremont culture, which in time developed its own distinctive, more geometrical rock art style. So rather than being the signature of a single, elusive group, the Barrier Canyon Style may be the artistic expression of multiple cultures as they mingled to form a larger and more enduring society, they say.

Edited from Western Digs (August 2014)

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