Sunday, April 14, 2013

A Day for Dam Big Rocks

Some of my students have a penchant for misusing a certain expletive when they want to describe something related to reservoirs. I occasionally take advantage of oral presentations by referring to "dam engineers" in reference to some of the big mistakes made in reservoir engineering (i.e. Vaiont Reservoir in Italy and St. Francis Dam in Southern California). So I can't resist a bit more today.

I was out in the field again today, taking my students to Yosemite National Park, looking at some dam big rocks (DBRs). The first two DBRs were in the Merced River gorge near Elephant Rock. They are but two pieces of a very large rock fall that took place in 1982. The material in the slide mass (termed the Cookie Slide) totaled around 100,000 cubic meters. The slide closed highway 140 for months, and permanently closed the old Coulterville Road that once existed on the upper slope. I guess I could call these DBRs because if the rock mass had been any larger, it would have dammed the Merced River.
Just the same, I found myself wondering how such giant boulders could have survived the fall down the steep slope without breaking into small pieces.

Speaking of DBRs not breaking up, take a look at Leaning Tower near Bridalveil Fall in the picture below. The rock actually leans more than 200 feet over the valley floor. What the heck is holding up that dam rock?
And finally there is the biggest dam rock of all, El Capitan. That's a heck of a big blank wall of granite, nearly 3,000 feet of sheer rock. Can I make a case that these are DBRs too? Sure: when glaciers were scouring out Yosemite Valley, primarily during the Pre-Tahoe (Sherwin glaciation) about 800,000-1,000,000 years ago, the ice couldn't make much headway downward through hard ridge of granite on the valley floor between El Capitan and the Cathedral Rocks. This ridge, only about 300 feet beneath the valley floor, prevented the glaciers from cutting the valley any deeper, and thus served as a sort of dam.
Oh, but it was a pretty day for learning some geology in one of the most beautiful places on the planet. More later...
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