Saturday, August 13, 2011

Vagabonding across the 39th Parallel: Crossing the Sierra Nevada

Vagabonding across the 39th Parallel is an informal exploration of the geology of an interesting slice of the American West that I followed in July. One of our rules on the trip was to seek out new routes we hadn't seen before. When leaving our home base in the Central Valley, that's not so easy to do. There are only a few paved crossings of the Sierra Nevada, and we've followed most all of them. So we found ourselves on the shortest route, through Yosemite National Park.

A trip to Yosemite National Park to many people means a visit to Yosemite Valley, the most heavily populated seven square miles in a park that covers more than a thousand.  If one is on a tour, or has limited time, they will obviously want to see the main attraction, but there is much more to the park. There are the paved highways to Wawona, Glacier Point, Hetch Hetchy, and Tuolumne Meadows, and there are the hiking routes along the John Muir Trail to the east and south, and the vast untrammeled wilderness of north Yosemite and the Grand Canyon of the Tuolumne River.  We were crossing the Sierra Nevada, so we were following Highway 120 over Tioga Pass.
From Crane Flat (the turnoff to Yosemite Valley), the road to Tioga Pass climbs from 6,000 feet to just short of 10,000 feet. The first miles are in a thick forest of red fir, but as one crosses into the drainage of the Tuolumne River, granitic rock starts to appear in large outcrops. If one is unfamiliar with the process of exfoliation, this road is a great place to learn.

When the weight of overlying rock is removed by erosion, the underlying granitic rock expands outward, parallel to the surface, and fractures into exfoliation sheets. The process tends to remove corners and edges first, resulting in a dome-like shape. An example can be seen in the picture above.
Roadcuts expose the exfoliation sheets very nicely. They are a few feet thick, and frost-wedging does a nice job of breaking the slabs loose and sending them downslope. Unless the roadbuilders do it first...

The road winds through the upper drainage of Yosemite Creek (which ends in Yosemite Valley at Yosemite Falls). The highest peaks, around Mt. Hoffman (see the picture at the top of the page), are close to 11,000 feet high. The view from Hoffman (a moderate hike with a small bit of rock scrambling at the summit) is spectacular. A photojournal of a hike we took there in 2001 can be seen here.
The road climbs to 8,000 feet and more and more of the high country becomes visible. Clouds Rest is at the upper end of Yosemite Valley, and is one of the largest exposures of barren granite to be seen anywhere. The parallel depressions running down the slopes are avalanche chutes. The constant sliding snow prevents trees and soil from accumulating. Most people stop at Olmsted Point to look at Clouds Rest and Half Dome, but we stopped at an unmarked point that framed Clouds Rest nicely instead.
 Tenaya Lake is one of the most beautiful glacially carved lake to be found anywhere. The origin is a bit unusual however, and ties into why Yosemite Valley, which is downstream, is so deep. The Merced River, which flows through Yosemite Valley, has a relatively small drainage basin, so the glaciers that abraded the canyon during the last 2 million years should not have carved such a deep basin. The tributary of Tenaya Creek has an even smaller drainage, so it should not have contributed very much ice.

The Tuolumne River basin is much larger, and in fact had the longest glacier system in the Sierra Nevada during the ice ages. The Grand Canyon of the Tuolumne is actually several thousand feet deeper than Yosemite Valley (although it generally lacks the sheer cliffs that make Yosemite so exceptional). The ice was so thick coming out of Tuolumne Meadows that some of the ice spilled over a low divide and flowed down Tenaya Creek towards Yosemite Valley, adding considerably to the carving power of the ice down-canyon. Beautiful Tenaya Lake was also the beneficiary of the extra ice.

The lake is a nice place for discovering glacial polish, striations, grooves, and erratics. It is also a popular picnic spot (this sentence is an understatement; Tenaya is one of the most popular Yosemite destinations outside the valley).
A few more turns and we arrived in Tuolumne Meadows. One of the first features to be seen is Pothole Dome (below). It is a nice example of a roche moutonnée, a glacially shaped rock exposure that is smooth on one side from the abrading of debris-laden ice, and steep on the other, due to the plucking action of the ice.
Tuolumne Meadows is a huge subalpine meadow complex that provides nice views of the high country of Yosemite. It is a major trailhead for trips in all directions. It is one of the few places where the Yosemite visitor can see some rocks that are not granitic in origin. The high peaks on the Sierra Crest include metamorphic rocks that predate the granite intrusions that dominate most of the park.
There will be much to say about Tuolumne Meadows at another time. We were actually trying to cross the Sierra Nevada on this trip. We reached the summit of Tioga Pass (9,945 feet), and made a brief stop at Dana Lake on the other side (below).

Tioga Pass is a col, a U-shaped pass formed by glacial erosion. Because cols have a flat floor, they are most often utilized as transportation corridors where people need to cross mountain ranges. Tioga has been used this way for many centuries. Crossing the summit of Tioga, the asymmetry of the Sierra Nevada is immediately apparent. The mountains have the overall shape of a tilted block of rock, with a gentle western slope, and steep eastern side. We had been climbing gradually for the last 100 miles or so, but we dropped into the town of Lee Vining in less than 12.
The east side of the Sierra Nevada is also the rain shadow side of the mountains. Moisture from Pacific storms falls mostly on the western slopes of the mountains, and the east side is thus fairly arid. Just the same, a fairly substantial system of glaciers flowed down Lee Vining Canyon, leaving some excellent erosional features. Below is the deep U-shaped valley left behind by the glaciers (the scar on the left side of the canyon is the highway) .
Looming high above the road is an excellent example of a hanging valley. A small tributary glacier could not carve as deep as the main trunk glacier, so the valley floor remains high above. Hanging valleys often have waterfalls spilling over the edge.
From a different angle a little farther down the road, there is a nice view up the hanging valley to the cirque valley, the bowl shaped valley that was the source area for the glacier. The cliffs were formed by the plucking action of the glacier as it pulled away from the rock wall.
The road has to get down the hill somehow, so they built it across the massive talus slope on the north wall of the canyon. Talus is composed of the boulders that break loose from the cliffs and fall and bounce down the slopes. I bet they don't have too many engineering/maintenance problems on that section...
We had successfully crossed the Sierra Nevada! My relatives in the 1840's couldn't do it and got stuck in the snow and some of them ate each other. We did it in 6 hours, including extended photo stops. Such are the advances of technology. We found the last spot available in a corner of the RV park in Lee Vining and set up for the night. RV Parks are not always the best spot for a tent camper, but the view was very nice, even if we tenters were a bit crowded together. We got dinner and went to look at Mono Lake. That's the subject of the next post.
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