Glaciers

An early glaciation 1.2 million years ago filled Yosemite Valley to the brim and excavated the valley. This glacier advanced down the deep, narrow canyons created by the rivers. In places such as Yosemite Valley, glacial ice traveling through was thousands of feet deep. Half Dome projected 900 feet above the ice, but many peaks to the north were engulfed.

The grinding, gouging action of the heavy river of ice eroded the canyons and valleys and widened and deepened them into U-shaped troughs. A system of joints and cracks in the granite allowed the glacier to erode out great blocks of granite at vulnerable points and carry them away.

In other areas the glacier merely scraped, buffed, and polished the granite surface. Later glaciations did not fill the valley as much as this early glacier and did little to further modify the valley, leaving spires such as the Lost Arrow and Sentinel Rock. These formations would have been destroyed if glaciers had filled the valley to its rim. The spires were formed by weathering processes over the last million years, long after the end of the extensive glaciation that filled the valley to its rim.

The last period of glaciation in Yosemite Valley, called the Tioga Glaciation, began 30,000 years ago and ended about 10,000 years ago. The glacier's terminal moraine (rock and rubble deposited in front of the glacier) dammed the valley near the narrow western end, and the glacier's subsequent melting created ancient Lake Yosemite. This moraine can still be seen extending across the valley as a broad hill between El Capitan and Bridalveil Fall. This was probably the last of many Lake Yosemites that formed following periods of glaciation.

Eventually, enough sediment accumulated in Lake Yosemite to fill it completely. The process of succession describes how such lakes eventually become wetlands, then meadows, then forests. This process continues today.