Are the images in the Great Gallery of Horseshoe Canyon much younger than previously thought?/Kurt Repanshek

Just how old is the artwork in the "Great Gallery" of Canyonlands National Park? It's a question that long has dogged archaeologists, and which recently produced a new, much more recent, time period assigned to it.

A decade ago, while I was researching a story on the Barrier Canyon rock art that is sprinkled through south-central Utah, Alan Watchman, a research fellow at Australian National University, said that radiocarbon analysis dates some of Great Gallery images to the Early Archaic period, from about 7430 B.C.to 5260 B.C. And archaeologist Phil Geib also thought the earliest of the images might date to the Archaic period. He noted that a figurine similar in style to Barrier Canyon rock art was recovered in a nearby cave in Utah above a layer of soil dating to around 7500 B.C. 

But now researchers at Utah State University offer a more recent date for the towering paintings of just 1,000-2,000 years ago.

"Our findings reveal these paintings were likely made between 1,000 to 2,000 years ago,"€ says lead author Joel Pederson, professor in Utah State University's Department of Geology. "The most accepted hypotheses pointed to the age of these paintings as 2,000 to 4,000 years old or perhaps even 7,000 to 8,000 years old. We show it is younger than expected."

Now, it's been suggested that nomadic bands of ancestral Puebloans, perhaps Fremont shamans, created the larger-than-life images in part by filling their mouths with red ocher-tinted paint and spraying it out with a mighty burst onto the sandstone. The 'œHoly Ghost' is the focal point of the Great Gallery, a vast mural some 300 feet long and featuring about 80 figures, located a five-hour drive southeast of Salt Lake City in Horseshoe Canyon. No one knows for sure what the images represent or why they were painted.

But that hasn't stopped researchers from trying to afix a certain date to their creation.

To understand how Professor Pederson and his colleagues reached their conclusion, it helps to know a little about geochronology and/or luminescence dating, which involves using the energy stored in rocks to date the age of the rocks.

"Luminescence takes advantage of how quartz, as a mineral, actually acts as a natural dosimeter. It actually stores electrical charges in its molecular structure as a function of how its getting dosed with radioactivity in its environment," the professor explained last week during a phone conversation. "So, quartz grains get buried in an outcrop, or bedrock. There's a little bit of uranium and a little bit of potassium that'€™s radioactive around them. And through time as that radiation is bombarding the quartz, it occasionally pops an electron into a crystal defect in the quartz, and then quartz accumulates this signal of these out-of-position electrons. And then we take that back to a laboratory and measure how much of that signal is there. That'€™s a function of time. So that'™s the typical way of using luminescence to measure how much radioactive dosing has accumulated in quartz, especially, but also the mineral feldspar."

"What we'€™re doing in traditional luminescence," continues Professor Pederson, trying to simplify the process, is that, "we don'€™t know when individual electrons are being trapped. Instead we just know the rate at which they'€™re being trapped. ... And then we know what the total amount is. We are measuring the total dose stored in those grains, and then we do a bunch of tests to those grains to figure out their sensitivity. And we also measure the radioactivity in the local environment. So, the short answer is we do the tests needed so that we understand the rate at which the dose is accumulating in the grains, and then we just measure the total amount accumulated. And once we know the total amount accumulated, and the rate at which it was accumulating, we just divide one by the other and it gives you the age. It'™s not simple but it'™s very, very widely used and established.'€

While this process pointed to a date of just 1,000 or 2,000 years ago for creation of the Great Gallery's images, what about the other dating processes that claimed a much older date?

When it comes to Watchman's dating, Professor Pederson said those dates were called into question by Watchman himself. There was concern that some of his samples might have been contaminated by groundwater that carried older carbon into the samples he tested, and one sample that might have contained a brush fiber was destroyed in a lab accident, said the Utah State professor.

"There was one particular result that he thought there didn't seem to have anything that we can tell wrong with it, and it gave a result of 900 AD," Professor Pederson added. "In the end, Alan Watchman could stand by only one result, and that was only tentative, and it ends up being a result that'€™s sort of in the window that we end up concluding."

As for Phil Geib's dating research, the professor pointed out that it was not tied to anything from the immediate area surrounding the Great Gallery.

"That was a compelling scholarly argument, that these figurines are in a stratigraphic context at 7 or 8000 B.C., and they look an awful lot like that rock art," said Professor Pederson. "I think in the end we've got these three different independent readings, and scientifically I would say that even though that'™s a good hypothesis (Geib's conclusion), it wasn't supported by any direct geochronology, and it appears that our results would rule that hypothesis out."

To the best of Professor Pederson's knowledge, his team's luminescence dating of the Great Gallery rock art marked the first time anywhere that the technique has been used to date rock art. While he hasn't tested any other Barrier Canyon images in the region to see if they produce the same age, he hopes to be able to do so in the future.