With stunning vistas of shear rock walls, cascading waterfalls, rounded granite domes and jagged spires, it’s no surprise that Yosemite is one of the most popular national parks in the U.S. While most visitors are drawn to Yosemite Valley, the park has numerous other wonders beyond the valley and even the park boundaries in the Sierra Nevada (=snowy mountain range in Spanish). Extending for about 650 km (400 miles) along the eastern side of California, and with a width around 110 km (70 miles), the Sierra encompasses nearly 25% of California’s land area. It is a recreational playground built by fire and carved by ice.
The story of Yosemite starts about 500 Ma (Ma=millions of years ago), when the western coastline of North America was in Nevada, and marine sediments were being deposited in shallow ocean water. We know about these ancient environments because their remnants have been preserved around the edges of the granitic rocks that intruded into them (see map below).
Starting in the early Mesozoic, around 200 Ma, the west coast of North America became an active plate boundary as an oceanic (Farallon) plate began to be subducted beneath the continental (North American) plate. As in the Cascade Range today, subduction caused rock to melt and magma to rise to create vast granitic plutons underground and a chain of volcanoes at Earth’s surface. Granite ages in the Sierra range from about 200–80 Ma, with the peak around 100 Ma, the average age of granitic rocks in Yosemite Valley.
Near the end of the Mesozoic Era, around 80 Ma, magmatism ceased due to changes in subduction geometry, and then resumed about 25 Ma, only to cease again as the plate boundary changed from subduction to transform. In the interim, uplift and erosion had removed the former volcanic chain and exposed the underlying granitic plutons at Earth’s surface by ~60 Ma. The lack of a high mountain range by ~30 Ma is demonstrated by lush vegetation and a vast megafauna in Nevada that could not have existed in the rain shadow created by the Sierra Nevada today. In addition, sediments and lava found west of the Sierra were able to flow from Nevada without the impediment of a large mountain range.
The current Sierra Nevada did not begin to form until ~5 Ma, when faults along the eastern edge of the Sierras became active. These faults are the western edge of the Basin and Range Province (see geologic map above) that is extending (pulling apart) the continent between the Sierra Nevada and the Wasatch Range in Utah.
As the Sierra was uplifted, streams cut valleys into the granitic bedrock. But another erosive agent was poised to make its mark on the mountains. About 2.8 Ma, Earth entered an Ice Age with alternating glacial and interglacial periods. The Sierra’s high elevation led to alpine glaciation, most recently during a peak about 18,000 years ago.
What is the evidence for glaciation in the Sierra? The following photos are examples of how geologists have been able to recreate the glacial history.
There are many other geologic stories to tell about the Sierra Nevada—for example, gold emplacement and mining, a watershed that encompasses 40% of California’s land area, active volcanism on the east side (Long Valley Caldera and Mono Craters)—that will have to wait for a future post!
Park history. Yosemite is one of the earliest parks. Yosemite Valley was first deeded to California by President Abraham Lincoln in 1864. Largely due to the activism of naturalist John Muir, President Theodore Roosevelt decided to preserve the area around the valley as federal land; it became the national park we know today in 1906, when California ceded the valley to become federal land. Yosemite is named after the Native American word (uzumate) for grizzly bear, a type of wildlife that is no longer found in California.