Yosemite Valley is one of the most beautiful and recognizable locations in the world. Such locations usually have an interesting geologic history, and Yosemite Valley is no exception.

Although the influence of man's activities in the last 100 years can seen on the valley floor, the steep, granitic rock walls, peaks, and waterfalls have existed in splendor for thousands of years.

How did the valley form? Yosemite Valley was not formed by a single geologic event.

From the Sierra Nevada, streams have flowed westward toward today's San Joaquin Valley, for over 100 million years. The streams eroded huge batholiths-volumes of intrusive igneous rock such as granite. The granite solidified from magma 80-150 million years ago. The streams also eroded large volumes of older, overlying metamorphic rock. The modern Merced River, the stream which is eroding the valley today, probably began carving a Yosemite Valley 3-4 million years ago. At this time, the highest Central Sierra Nevada peaks were 4000-5000' above sea level, unlike today's elevations of 12,000-14,000'. However, significant geologic and climatic activities were about to change the landscape.

Two to three million years ago, uplift and westward tilting of the Sierra Nevada occurred. This caused the pre-existing Merced River to gradually, then rapidly, erode a canyon. New or "youthful" canyons such as this Merced canyon are typically V-shaped and often quite deep. The Merced River canyon may have resembled today's lower Kern River canyon, just east of Bakersfield.

By 2 million years ago, North America's climate became a few degrees cooler and glaciers began to occur in the Sierra Nevada. During one of the major glacial periods, about 700,000 years ago, ice thickness in Yosemite Valley was up to 6000'. The Sierra Nevada was covered by an ice cap. Still, prominent features such as Half Dome and El Capitan were never completely covered by glacial ice. Massive, flowing glacial ice caused tremendous amounts of rock to be carved and transported toward El Portal. Rock churned against rock, leaving the U-shaped Yosemite Valley that we see today. U-shaped valleys are characteristic glacial features, as are the elevated "hanging" valleys and waterfalls which flow from them.

The ebb and flow of glaciers through Yosemite Valley occurred many times until 10,000 years ago, when the last major glacial period ended in North America.

The valley was actually deeper than today's valley, which is filled with lake bed and moraine sediments. Once the last valley glacier melted about 10,000 years ago, a large lake, Lake Yosemite, covered the valley floor. The lake was 9 miles long and a maximum of a few hundred feet deep. Ultimately, a combination of sediment deposition and erosion at the western end of the lake by the Merced River led to the disappearance of Lake Yosemite.

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