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Friday, November 30, 2012 - Page updated at 07:00 p.m.

Grand Canyon may be much older than previously thought

By Joel Achenbach
The Washington Post

To stand on the South Rim and gaze into the Grand Canyon is to behold an awesome immensity of time. The serpentine Colorado River has relentlessly incised a 280-mile-long chasm that in some places stretches 18 miles wide and more than a mile deep. Visitors to Grand Canyon National Park will encounter an exhibit, "The Trail of Time," and learn that scientists believe the canyon is about 6 million years old, relatively young by geological standards.

Now a few contrarian scientists want to call time out. The canyon isn't 6 million years old, they say, but more like 70 million years old. If this order-of-magnitude challenge to the orthodoxy holds up, it would mean the Grand Canyon has been around since the days of T. rex.

"Our data detects a major canyon sitting there about 70 million years ago," said Rebecca Flowers, 36, a geologist at the University of Colorado and lead author of a paper published online Thursday in the journal Science. "We know it's going to be controversial."

About that she is correct. Her research, which reconstructs the ancient landscape using a technique called thermochronology, is being met with a cool reception from veteran geologists who study the Colorado Plateau.

"It is simply ludicrous," said Karl Karlstrom, 61, a professor of geology at the University of New Mexico who has made more than 50 river trips through the canyon — one with Flowers, when she chipped her samples off the canyon walls — and helped create the "Trail of Time" exhibit for the National Park Service.

"We can't put a canyon where they want to put it at the time they want to put it," said Richard Young, a geologist at SUNY-Geneseo who has been studying the Grand Canyon for four decades.

The Grand Canyon doesn't seem terribly mysterious at first glance. It's a gash in the landscape with a river at the bottom. The causality seems obvious. But Flowers and her fellow Old Canyon theorists say that what we see today in northern Arizona was originally carved, in large degree, by two rivers, neither of which was the Colorado River.

The western part of the canyon, they say, was largely incised about 70 million years ago by what has been dubbed the California River, which drained a mountain range to the west and flowed to the east, in the opposite direction from today's Colorado River. The eastern part of the canyon, they say, was created later, about 55 million years ago, by a different river.

Under the Old Canyon scenario, the Colorado River, which originates in the Rocky Mountains, about 6 million years ago took advantage of the pre-existing canyons and linked them in a fashion that creates the sinuous canyon of today.

The debate to some extent hinges on the semantic question of whether "an Ancient Grand Canyon" (as the Science paper calls it) is the same thing as the Grand Canyon of today. The Flowers paper says the depth of the ancient canyon was within a "few hundred" meters — roughly a thousand feet — of today's canyon.

Karlstrom warns that the Old Canyon theory threatens to confuse the park's 5 million annual visitors: "To them, it seems like dinosaurs might have lived with humans (like the Flintstones) and that geologists do not know if Grand Canyon was carved by the Colorado River or not (it was)," he wrote in an informal note in response to the new paper.

Flowers began advancing the Old Canyon scenario in 2008, and the idea has been championed by Brian Wernicke, a geologist at Caltech. "I see all the data as aligning very nicely for an Old Canyon model," Wernicke said.

There are many objections to the interpretation advanced by Flowers and Wernicke. The consensus estimate for the age of the Grand Canyon is based on multiple factors, including well-dated gravel deposits on the western mouth of the canyon where the river exits the Colorado Plateau and river sediments deposited into the Gulf of California.

Young believes the new Flowers research is recording the gradual recession of the cliff, not the carving of a deep canyon.

Joel Pederson, an associate professor of geology at Utah State, applauds the new paper, though he makes a semantic distinction when discussing the age of the Grand Canyon.

"They are looking at a really awesome precursor canyon that the Colorado River later in time took advantage of," Pederson said. "This new study really adds teeth to the realization that those paleocanyons, they were bigger and they were older than we thought they were."

But as for the age of Grand Canyon proper, Pederson is emphatic: "It is 6 million years old."

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