Yosemite National Park

Perhaps the most recognizable rock formation in the world, Half Dome in Yosemite National Park is a granite dome that rises almost 5,000 ft in the air and 8,900 ft above sea level. George Anderson was the first to reach the summit in 1875, despite an 1865 report declaring that it was "perfectly inaccessible, being probably the only one of the prominent points about the Yosemite which has never been, and never will be, trodden by human foot". Its distinct shape has lead scientists to believe that the other half has either eroded away or was simply never as well formed as the eastern slope. Half Dome rock is the sheerest cliff in North America with an astonishing 93% grade.

Not only is Yosemite National Park the most famous region of the Sierra Nevada, but with 3.7 million visitors per year, it has become one of the most iconic places on Earth. The uplifting of the Sierra Nevada caused two major changes to the landscape: streams became much steeper, which resulted in deeper and narrower canyons; and approximately 1 million years ago, glaciers that were formed in the higher alpine meadows began to move down the river valleys. 

Glaciers are formed when snowfall is exceeded by snowmelt over a number of years. The accumulation zone is the area in which the snow is undergoing a slow process of being transformed into ice. The area wehere the melting occurs is the ablation zone. The line between the two parts is known as the firn line ("firn" means "old snow" in German). If a glacier is in equilibrium, one third of the glacier will be ablation zone and two thirds will be accumulation zones. Even when a glacier is retreating, it will still flow forward, downhill, because they are so massive that the stress of gravity is greater than the force holding together the particles of water. At a depth of about 120 ft, internal deformation of the glacier begins to occer. Meltwater can also aid the movement of glaciers over rock surfaces, this is how they are able to abrade when carrying debris. 
Image courtesy of: http://www.indiana.edu/~sierra/papers/2008/gray_clip_image002.jpg

This diagram is depicting a rock formation known as roche mountenee (sheep rock), which is formed by glacial movement. Yosemite has many examples of this formation, including Lembert Dome in the Toulumne Meadows (shown in the image following this caption), which was given its shape by the Tioga glacier. A rouche mountonee is formed when a sufficiently large glacier reaches a hill. Instead of going around it, the glacier goes over it, leaving behind a smooth slope upstream and a steep slope downstream from where the boulders have been plucked by the departing ice.
Image courtesy of: http://www.indiana.edu/~sierra/papers/2008/gray_clip_image008.jpg

Lembert Dome in the Toulumne Meadows. For an explanation of how this rock formation got its shape, see the previous diagram and its caption.

It is estimated that the ice thickness in Yosemite Valley may have reached up to 4,000 ft during the early glacial episode. The downslope movement of these ice masses are what carved many of the landforms in Yosemite, including its scenic U-shaped valleys, jagged peaks, rounded domes, waterfalls, and moraines. Further evidence of  these ice masses can be seen in the park's abundance of glacially-polished granite. There have been at least four major glaciations that have occurred in the Sierra Nevada, locally called the Sherwin (also called the pre-Tahoe), Tahoe, Tenaya, and Tioga. The largest of the four were the Sherwin glaciers, which filled Yosemite and other canyons in the area. The retreating of these glaciers has often left recessional moraines that impounded lakes such as Lake Yosemite, which is a shallow,  5.5 mile-long lake that periodically covered much of the floor of Yosemite Valley.

One of Yosemite National Park's many majestic waterfalls.

It terms of the geological composition of the park, nearly every landform is cut from the granitic rock of the Sierra Nevada Batholith (as described in detail in my previous past, a batholith is a large mass of igneous rock that formed deep below the Earth's surface). Only about 5% of the landforms in Yosemite are metamorphosed (completely changed in form or nature) sedimentary and volcanic rock. Because these rocks once acted as a roof to the underlying granitic rock, they are often referred to as "roof pendants".

A breathtaking shot of Bridalveil Fall, one of the most prominent waterfalls in the Yosemite Valley. The waterfall is 617 ft in height and flows year round. The Ahwahneechee tribe believed that Bridalveil Fall was home to a vengeful spirit named Pohono which guarded the entrance to the valley, and that those who were leaving the valley mustn't gaze directly into the waterfall lest they be cursed. They also held the belief that inhaling the mist of Bridalveil Fall would improve an individual's chance of getting married. Many hanging valleys (tributary valleys with the floor at a higher relief than the main channel into which it flows) were left behind by the glaciers that carved Yosemite Valley. All of the waterways that feed these falls have carved the hanging valleys into steep cascades, with the exception of Bridalveil Fall. This particular waterfall still enters into the valley from the edge of a precipice, although that edge has moved back into an alcove from the original edge of the valley. The primary source of Bridalveil Fall is Ostrander Lake, which is approximately 9.9 miles to the south of Yosemite Valley.

Works Cited

Allen, Casey. Professor Allen’s Geog 1202 Syllabus. University of Colorado Denver, 2013. Web. 13 February 2013.

Guyton, Bill. Glaciers of California: Modern Glaciers, Ice Age Glaciers, Origin of Yosemite Valley, and a Glacier Tour in the Sierra Nevada: Vol. 59. California Natural History Guides. Berkeley: University of California Press; 1998. Retrieved 7 March 2013.

Hill, Mary. Geology of the Sierra Nevada: Vol. 80. California Natural History Guides. Berkeley: University of California Press; 2006. Retrieved 7 March 2013.

NPS. Half Dome Day Hike. National Park Service, 3 March 2013. Web. 7 March 2013.

NPS. Yosemite National Park. National Park Service, 11 June 2012. Web. 7 March 2013.

World of Earth and Science. “Batholith”. Encyclopedia.com, 2003. Web. 7 March 2013.

One last look back before we leave Yosemite.