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Geologists Give A New Age To The Yosemite Valley

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Geologists obtained rock samples from an upland area on the northern rim of Yosemite Valley (blue arrow) and an exposed area at the base of Half Dome in Tenaya Canyon (red arrow). Analysis of these samples allowed them to estimate when Yosemite Valley was

Geologists obtained rock samples from an upland area on the northern rim of Yosemite Valley (blue arrow) and an exposed area at the base of Half Dome in Tenaya Canyon (red arrow). Analysis of these samples allowed them to estimate when Yosemite Valley was carved out from the native granite. They determined that glaciers and a river excavated the canyon by between 0.6 and 1.2 kilometers over the past 5 million years; yellow bars indicate the current canyon depth. The yellow star is the valley floor — sediment that fills the glacially carved valley as deep as 600 meters/Kurt Cuffey, UC Berkeley 1025

Yosemite National Park's famous valley might not be as old as previously thought.

If the geologists from the University of California, Berkeley, and the park are right, the Yosemite Valley is only about 5-10 million years old, not 50 million as long thought. The three -- Kurt Cuffey and David Shuster from the university and Greg Stock from Yosemite -- say the erosional forces that carved the valley came to bear no more than 10 million years ago, and probably only in the past 5 million years.

“Yosemite Valley is one of the most famous topographic features on the planet,” said Cuffey, a glaciologist and UC Berkeley professor of geography and of earth and planetary science. “And of course, if you go to Yosemite Park and read the signage, they will give you numbers for when it became a deep canyon. But up until this project, every single claim about how old this valley is, when it formed a deep canyon, was just based on assumptions and speculation.”

At the park, Stock notes that there's still a lot of disagreement over the forces involved in creation of the Sierra Range. 

“We know that the Sierra was a high mountain range 100 million years ago, when the granite was forming at depth. It was a chain of volcanoes that might have looked a bit like the Andes Mountains in South America,” Stock, the park's geologist, said in a press release from UC-Berkeley. “The question really is whether the elevation has just been coming down through erosion since that time or whether it came down some and then was uplifted again more recently. At this point, based on studies I’ve done for most of my career and supported by this study, I see a lot of evidence for recent uplift happening sometime in the last 5 million years.”

The scientists came up with a new age for the Yosemite Valley through an approach Shuster developed 15 years ago. According to the news release, the technigue, "called helium-4/helium-3 thermochronometry, reconstructs the temperature history of a sample of rock based on the spatial distribution of natural helium-4 in minerals, which is measured by comparison to an artificially-produced uniform distribution of helium-3. Because temperature increases with depth underground, the temperature history can tell when a rock was uncovered as the landscape eroded."

The expectation is that granite bedrock exposed on the broad uplands of the Sierra should show a long history of cool surface temperatures, since they’ve been exposed for tens of millions of years longer than bedrock more recently exposed on the floor of Tenaya Canyon, which feeds into Yosemite Valley from the northeast, the news release explained.

Though Sierra granite was first exposed about 50 million years ago, not much happened in the Yosemite region until 10 million years ago, when Yosemite was a shallow valley. Rivers started cutting a deep canyon (panel 1) beginning sometime between 10 to 4

Though Sierra granite was first exposed about 50 million years ago, not much happened in the Yosemite region until 10 million years ago, when Yosemite was a shallow valley. Rivers started cutting a deep canyon (panel 1) beginning sometime between 10 to 4 million years ago, though most likely 5 to 4 million years ago. Glaciers deepened and widened the canyon between 2 million years ago and 20,000 years ago, initially overtopping the current canyon rims (panel 2), though more recently only partially filling the valley (panel 3), allowing the higher rock formations (Half Dome, 3 Brothers, etc.) to be chiselled by rain and rockfall (panel 4). The glaciers carved a deep basin in the main part of Yosemite, which is now buried by sediment (panel 5), and widened the canyon walls. (Image credit: Eric Knight, courtesy of Yosemite National Park)

The geologists' experiments, conducted at the Berkeley Geochronology Center, "indicated that, while rock from the uplands has been close to the surface for about 50 million years, bedrock at the bottom of Tenaya Canyon has been exposed much more recently. The temperature history of the rock obtained from the bottom of Tenaya Canyon — from an exposed area of bedrock at the base of Half Dome — indicates that it was more than a kilometer underground 10 million years ago, and most likely only 5 million years ago. This means that a kilometer of rock was eroded away since that time."

“This upland surface that people are familiar with from parts of the Tioga Road and Tuolumne Meadows — that’s a very old landscape,” said Cuffey, who is the university's Martin Distinguished Chair in Ocean, Earth and Climate Science. “The question is: What about the deep canyon? Is that also very old, or is it relatively young? And what we found in our study, our big contribution, is that it’s fairly young. The best guess for the timing is in the last 3 to 4 million years, but maybe as far back as 10 million years for the start of the rapid incision.”

A 2022 view of Tenaya Canyon as seen from North Dome, with Half Dome on the right. Half Dome was sheared off as the canyon deepened over the last 5 million years. (Photo credit: Kurt Cuffey)

A 2022 view of Tenaya Canyon as seen from North Dome, with Half Dome on the right. Half Dome was sheared off as the canyon deepened over the last 5 million years./Kurt Cuffey

Stock said the conclusions will lead to a revision of how the park interprets the geology of the valley.

“The brief history of Yosemite Valley would be that there was some kind of valley in place for tens of millions of years — a river-carved canyon associated with the ancient Sierra Nevada. And then, in the last 5 million years or so, renewed uplift of the range through westward tilting caused rivers to steepen and deepen the canyons that they were in,” Stock said. “So, that probably carved out more of Yosemite Valley and may have started forming Tenaya Canyon. And then in the last 2 to 3 million years, as the climate cooled and glaciers came down through Tenaya Canyon and into Yosemite Valley, they further sculpted the rock, deepening those valleys. And in the case of Yosemite Valley, widening it out considerably. So, there’s some component of an old Yosemite Valley. But I think this recent work shows that much more of that topography is younger, rather than older.

"The timing of this new study is perfect in the sense that, over the next several years, we’re hoping to completely redo the Geology Hut displays at Glacier Point. I’m very excited to include these new results in those displays,” he said. “It’s a perfect place to tell that story, because there’s a view straight up Tenaya Canyon.”

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