Showing posts with label Cookie Slide. Show all posts
Showing posts with label Cookie Slide. Show all posts

Monday, April 14, 2014

The Yosemite No One Sees in Summer...the Merced River Canyon

The drought in California is horrific. It is quite probably the worst drought in centuries, but we received a slight respite in the form of showers and snow during the last part of March and early April. It was a drop in the bucket towards relieving the huge water deficit that has built up in the last few years, but it gave a shot of energy to the seedlings of grasses and wildflowers. They perhaps should have sprouted and grown months ago, and they will be dried out in a few short weeks, but this week, the Sierra Nevada foothills were alive with color.
We were on our way to Yosemite Valley for our geology field studies course last Saturday, and I joked (only in half-jest) that the students had to "earn" the right to learn the geology of Yosemite Valley by first exploring the canyon of the Merced River. We had to prepare ourselves by undertaking a journey through the rocks that predate the granitic rocks that form the walls of the iconic valley. Our route took us through the foothill towns of Snelling and Hornitos, and then we drove up Highway 140 through Mariposa, over Midpines Summit and down into the deep gorge of the Merced River (originally the "Rio de Nuestra SeƱora de la Merced (River of Our Lady of Mercy)".
Highway 140 winds back and forth through a series of metamorphic terranes, bands of deformed and twisted rocks that traveled across the Pacific Ocean to be mashed into the western edge of the North American Continent. The rocks include slate, schist, marble, phyllite, quartzite and greenstone. These were the ancient rocks (between 600 and 200 million years old) that were intruded by the granitic rocks that eventually were exposed and eroded to form the dramatic cliffs of Yosemite Valley.

The Sierra Nevada is a huge westward tilted block. The slope has allowed the rivers draining the mountain range to carve deep gorges, some deeper than the Grand Canyon (Kings Canyon to the south is about 8,000 feet deep!). The depth of the Merced is more like 2,000 to 3,000 feet deep downstream of Yosemite, and in any other setting in most any other state, this would be enough for the establishment of national parks and the like. Instead, it is "just" the preliminary canyon one must pass through before arriving at Yosemite.

The carving of such deep canyons can have serious geologic consequences for society. The slopes of such canyons are inherently unstable, and mass wasting (landsliding) is a constant hazard. The building of roads and railways adds to the instability by undercutting the already steep slopes. Such is the case with Highway 140. CalTrans has struggled for years to prevent the collapse of a slope near the village of El Portal. In 2006, the slope flicked away the mitigation efforts and collapsed onto 600 feet of highway. The Ferguson Slide, as it is called, closed the highway for months and caused economic hardship for the surrounding communities. Two temporary bridges were constructed to cross the river and circumvent the huge slump. Decisions have yet to be reached regarding the permanent disposition of the highway.
The Ferguson Slide. 600 feet of Highway 140 is buried under the rocks.

As we drove deeper into the Merced River Canyon, the slopes seemed to come alive in orange and gold. The California Poppies have reached the zenith of their blooming cycle. In contrast to my previous gloom and doom post regarding fires, rejuvenation does indeed occur, as many of these slopes have been cleared of brush and chaparral by repeated wildfires, allowing for dense concentrations of wildflowers.

We passed El Portal, and then the Yosemite View Lodge. The name is not totally dishonest; the boundary of the national park does indeed lie a few yards east of the hotel complex, so the view of the granite slope beyond is indeed a view of Yosemite. But it is not yet the iconic valley with Half Dome and El Capitan and Yosemite Falls. It is pretty, though, and the Merced River makes a dramatic dash through the maze of gigantic fallen boulders.
 We arrived at the park entrance station, drove a few more miles, and made our last stop before Yosemite Valley. It was another site of mass wasting, but this time it was a rockfall, and it included granitic rock instead of metamorphic material. The so-called Cookie Slide took place in 1982, and involved around 100,000 cubic yards of material that bounced and fell down the slope onto Highway 140. Once again, the road was blocked for weeks.
 The size of some of the fallen blocks is astonishing. It's hard to see how a block like the one in the picture below held together during the downhill descent.
The rocks are part of an intrusive series called the El Capitan granite, which was intruded about 103 million years ago. A contemporaneous intrusion of dioritic rock lead to some co-mingling of magmas, including the enclave of darker rock in the granite (below).
I've always thought of granite as a particularly beautiful rock. The sparkling mass of quartz (glassy gray color), orthoclase feldspar (white grains), and biotite mica is pleasing to the eye, and tells an odd story: Yellowstone National Park, off in Wyoming, sits on top of a huge magma chamber that may one day explode again (but not tomorrow). In Yosemite, we are sitting within a former magma chamber, under the volcano. As such we are exploring the inner depths of an extensive magma system, brought to light by long periods of uplift and erosion.

In a coming post, we'll see how Yosemite looked on Saturday.

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...

Saturday, April 16, 2011

A Tale of Two Slides: Mass Wasting in the Yosemite Region

First-time visitors to Yosemite Valley who enter the park via Highway 140 from Mariposa are treated with views of a spectacular river canyon along with sights of some interesting ongoing geological processes. One of the odd places along the drive is a stoplight in the middle of the highway near Savage's Trading Post, and a narrow bridge that only allows one-way traffic. After a wait of several minutes, the light changes and traffic crosses the river, only to cross over another narrow bridge a short distance later. What's going on?
The reason becomes clear when one looks across the river. A vast pile of rock covers the old stretch of highway, and the slope above the debris looks not so stable, with big chunks of rock that seem to be taking aim at the river. This is the Ferguson Slide, one of the real headaches for engineers who designed the highway. The present mess dates to 2006, but the slope had been a problem for years. One of my former students who worked for CalTrans kept me posted about the "next thing" they were trying to keep the slope from collapsing onto the highway. It appears that the rock moved as a semi-coherent mass, termed a slump. I think of such slides as mimicking what I do when I hit a recliner at the end of a long day; I slide down into the chair until friction keeps me from moving further. Also, I am usually semi-coherent at that point.


Welcome to the process of mass wasting, the downhill movement of rocks and debris under the influence of gravity. Rivers get a lot of credit for eroding the landscape, but they only cover a very small percentage of the land. Mass wasting processes move material down slopes to where rivers can carry it away. Downcutting rivers steepen slopes, but gravity moves the rocks into the rivers.


The Ferguson Slide is just one example of a problem area along the river. Almost every time I drive the highway, boulders and cobbles from the overlying slopes are littered across the highway. The rocks are ancient volcanic and sedimentary rocks that have been metamorphosed and deformed by compressional forces in the crust related to the subduction zone that once existed in Central California. The rocks are cracked and fissured and are naturally unstable on the steep slopes of the deep canyon. There are few flat places on the valley floor near the river, so roadcuts contribute to the instability of the slopes.

The Ferguson Slide has been active for years, as the scarp at the top of the slide is thousands of years old. There were (and still are) fears that the slope could completely fail, and that a large mass could block the river forming a dam and lake that could inundate developments upstream, and block traffic for years.
A mass wasting event of a different sort is revealed along the highway shortly after entering the national park. In this area the metamorphic rock has been replaced by the more familiar granitic rock the typifies most bedrock exposures in the park. Huge boulders choke the riverbed, boulders far larger than the river could ever move. Glaciers can move such rocks, but glaciers haven't been in this particular area for more than 700,000 years. These rocks have no lichens or stains at all; they broke from the local cliffs much more recently. In 1982, to be exact.
The Cookie Slide covered Highway 140 for months, and broke the main sewer line that served Yosemite Valley (the river wasn't such a nice place to swim for awhile). Granite is a very solid rock, and can form cliffs thousands of feet high (the vast cliffs of Yosemite being an example). But on their way to being exposed at the earth's surface by erosion, the rocks expand from the pressure release, and form cracks and fractures called joints. The joints provide an avenue for water to move through the rock, and also provide a surface for slope failures. The rocks detached from the cliff, broke up into a chaotic mass and tumbled and slid down into the canyon.
Rock falls and slides are a common occurrence in Yosemite National Park and the surrounding region. These are just two examples, but many more can be found with a bit of research and observation. In my travels, I have heard numerous rocks falling, and witnessed two larger rockfalls (which were described in an earlier post here and here).