330 Historical Rock Slides in Yosemite

October 8, 2008 7:46:16 PM PDT
The U.S. Geological Survey reports some 330 Historical Rock Falls in the Yosemite Valley since 1857.Yesterday and todays will no doubt be added to that list. But predicting the next one says one Fresno State Earth Science Professor is just not possible at least not yet.

Less than a decade ago rocks from the same granite face of Glacier Point came raining down on the Curry Village area one person was killed ... Others were injured.

Three years earlier, in 1996, the Happy Isles area of Curry Village was hit also causing one death and numerous injuries. It was this slide that sent the park service to the U.S.Geological Survey for information and advice.

Fresno State Earth Sciences Steve Lewis said, "The Park Service asked the Survey to come up and sort of do a rock fall hazard assessment."

The result suggests it would be safer for humans and facilities to be farther from the cliff walls and rocky piles called tallas ... The evidence of previous such rock falls. Granite contracts and expands with the change in temperature and when water freezes and thaws in the cliff's many fissures.

Steve Lewis, Fresno State Earth Sciences: That thermal expansion and contraction can also contribute to the growth of these cracks that eventually allows the pieces to break loose and come off the mountain top."

The resulting rock fall is inevitable. It's been going on for millions of years. So why not just move 'us and our stuff' to the center of the valley? The catch 22 is there's a river running through it.

Dr. Lewis said, "As you go further and further from the cliff faces away from rock fall debris you get closer and closer to the Merced River and those developed facilities will become increasingly at risk of floods."

And yes, Dr. Lewis does think last weekend's rain and chilly overnight temperatures in Yosemite probably contributed to this week's rock falls. But since the condition of the cliffs as it over time is unpredictable ... It's not possible to say when it will give way ... Only that does.

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