Yosemite
What Glaciers Do, Erosion and Yosemite
When your tour guide, Dr. Alley, was much younger (the year he graduated from high school, 1976), he traveled with his sister Sharon and cousin Chuck on a camping tour of the great national parks of the American West (in Chuck’s car, a 1962 Ford Galaxy 500). At Yosemite, they hiked the 10-mile round trip to Glacier Point, climbing almost 1 km (0.6 miles) in elevation. The trail switch-backs up the granite cliffs, opening increasingly spectacular panoramas across the great valley of the Merced River. The view from Glacier Point, across the side of Half Dome and the thundering Vernal and Nevada Falls, is truly spectacular. It was here that John Muir helped convince President Theodore Roosevelt of the need for a National Park Service to care for the National Parks, which were protected by law but not by rangers for some decades after the parks were established.
The hikers were a bit disheartened by the crowd at Glacier Point—the view is also accessible by Glacier Point Road. While they sat and lunched, a tour bus pulled in. Most of the passengers headed for the gift shop, but three settled at a picnic table while a fourth strolled over to the railing to see the scenery for a few moments before joining the others at the picnic table. One of the three asked, “Anything out there?” To which the ‘energetic’ fourth replied “Nah, just a bunch of rocks. Let’s go check out the gift shop.” It must be a sad person indeed who would not walk 50 feet to see the glory of Yosemite.
To anyone with open eyes, Yosemite Valley—the “Incomparable Valley”—is well worth inspection. It is carved into the granites and similar rocks of the high Sierra Nevada of California. Once, this granite was magma (melted rock below the surface), far beneath an earlier mountain range. The magma may have fed subduction-zone volcanoes much like those of the Cascades, which continue to the north of the Sierra. However, stratovolcanoes along this part of California have died as the East Pacific Rise spreading center ran into the trench along the west coast, forming the San Andreas Fault but ending subduction, as you learned earlier in the course. Such a fate eventually awaits the Cascades volcanoes, some millions of years in the future.
The Sierra Nevada was raised above Death Valley and the rest of the Great Basin by motion across great faults. Earthquakes that continue to occur, and breaks in recent sediments caused by earthquake faults, show that the mountain range is still being lifted above the still-dropping Great Basin.
The tough granite of the Sierra Nevada is more resistant to weathering and erosion than most rocks, however, granite does eventually break down, and some streams have managed to exploit weaknesses and cut deep channels through the range. These streams include the Tuolumne River, which carved the mighty Hetch Hetchy Valley, now dammed so that a valley that rivals Yosemite is lost underwater. The Merced River, which runs through Yosemite Valley, also cut into the range.
The stage was then set for the ice ages. Glaciers gathered on the high peaks, flowed into the valleys, and began to change the landscape. Later in the course, we will briefly discuss why the climate changed naturally to bring the ice ages, and why knowledge of these natural changes fully confirms our scientific understanding that human fossil-fuel burning is warming the climate today. For now, we’ll look at what a glacier is, what it does, and how we know glaciers were much more widespread during the ice ages than they are today.