Glenwood Canyon only 3-4 million years old |

Glenwood Canyon only 3-4 million years old

Dennis Webb
GSPI News Editor

The same volcanic dating that has helped to indicate large-scale geological collapse locally has helped nail down the timing of the creation of Glenwood Canyon.

Vince Matthews of the Colorado Geological Survey said researchers used to think much of the canyon-cutting and mountain-building that occurred in Colorado took place tens of millions of years ago.

But recent findings show that many of these features are perhaps only 3 million to 4 million years old – fairly young, geologically speaking.

The many peaks exceeding 14,000 feet in Colorado support this thinking. Older peaks would be less likely to be so tall.

“Erosion attacks things pretty quickly,” said Matthews.

An ancestral Colorado River once ran south of today’s route through Glenwood Canyon, perhaps flowing in the Cottonwood Pass area.

Geologist Bob Kirkham believes the river eroded a wide valley sometime after about 20 million years ago.

Volcanic activity diverted the river to its present course about 7.8 million years ago, when it started carving the 2,700-foot-deep inner gorge of the canyon. Kirkham estimates the downcutting accelerated in the last 3 million years, when more than half of the entire canyon was created.

Volcanoes helped establish Glenwood Canyon’s age because lava flowed out onto the river bottom at various times. River rock deposits establish the river’s different levels, and dating of volcanic flows associated with these deposits helps create a timeline for the downward cutting of the canyon.

“It wasn’t until we found these volcanic rocks that were in fortuitous positions, and then we dated those rocks, could we say with confidence when the river did its incision,” said Kirkham.

Garry Zabel, a geology professor at Colorado Mountain College in Glenwood Springs, has long believed the canyon was largely created in the last 2 million to 3 million years, as ice ages sent glacial meltwaters rushing downstream.

Various experts have had various views about the canyon’s age. The recent work of Kirkham and others has produced “better, tighter more accurate information” about the rate of downcutting in the canyon, he said.

Matthews noted that the timing of the carving of Glenwood Canyon fits with the latest thinking of the age of the Grand Canyon, “which is nice because it’s the same river” that cut both gorges.

The latest consensus is that the Grand Canyon was created in the last 3 million to 4 million years ago, he said. Previously, it was thought the canyon might have been 10 million to 20 million years old.

For Kirkham, it’s been exciting to be involved in work that has been so instrumental in establishing the age of Glenwood Canyon and in establishing the concept of widespread collapse in the surrounding region.

Recently retired from the CGS, he looks back at his work helping identify the collapse as a capstone to his time with the agency.

“It was a real significant part of my career. I really enjoyed working on it,” he said.

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