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.
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.
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 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).
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.