Physiography

The Sierra Nevada is an asymmetrical range with its crest and high peaks decidedly toward the east. The peaks range from 11,000 to 14,000 feet (3,350 to 4,270 metres) abovesea level, withMount Whitney, at 14,494 feet (4,418 metres), the highest peak in thecoterminous United States. Summits in the northern portion are much lower, those north of Lake Tahoe reaching altitudes of only 7,000 to 9,000 feet.

Much of the rock isgranite or a near relative of granite. There are dividing bands of metamorphosed (heat- and pressure-altered) sedimentary rock—all that is left of a once extensive sedimentary basin—and some large areas ofextrusive rock, especially from Lake Tahoe northward; at the northern limit of the Sierras, these rocks merge with the volcanic rocks of the Cascades.

Geology

It has long been recognized that the Sierra Nevada is an upfaulted, tilted block of the Earth’s crust. A majorfault zone bounds the block on the east, and it was along this that the great mass that became the Sierra Nevada was uplifted and tilted westward. This explains the asymmetry of the range. As the block was uplifted the abrupt, east-facing escarpment was cut into by the erosive action of wind, rain, temperature change, frost, and ice, and a series of steep-gradient canyons developed. On its western flank, streams flow more gently down the geologic dip slope, creating massive alluvial fans thatencroach into the Central Valley of California. Though the massive uplift began many millions of years ago, much of it occurred in the past two million years. The present-day relief of 10,000 to 11,000 feet along the eastern slopes in the southern Sierra Nevada attests to the tremendous uplift.

Drainage andglaciation

The gentler west-facing slope has been dissected by a series of streams, much longer than those of the eastern slope. Such rivers as the Yuba, American, Mokelumne, Stanislaus, Merced, and Kern originate in deep valleys carved largely by glaciers into the predominant granite and some volcanics. All but the Kern drain either into theSacramento River in the Central Valley on the north or into theSan Joaquin on the south, their waters ultimately reaching thePacific Ocean through thecombined delta of these two rivers atSan Francisco Bay. Until the water was diverted for irrigation during the early 20th century, theKern River drained into the Buena Vista Lake basin, south of the San Joaquin River.

During thePleistocene Epoch (i.e., about 2,600,000 to 11,700 years ago), the river valleys were covered several times by great expanses of ice. Glacial climates developed and dissipated at least twice, and each time excessive snows built snow and ice fields and deep glaciers. The ice carved U-shaped valleys down to an elevation of about 5,000 feet on the western slopes. So much ice existed on the mountaintops that an ice cap was formed as the glacierscoalesced. This cap extended almost 200 miles from Lake Tahoe in the north to the southern high sierra near Mount Whitney.

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Extending from the cap were fingerlike valley glaciers, long on the more gentle western slopes but shorter on the sharply uplifted and steeper eastern face. The erosion caused by these glaciers is spectacular. It includes hugecirques (amphitheatre-shaped basins with precipitous walls), moraines (accumulations of rock debris at the formerglacier margins), and thousands of glacial lakes dotting the Alpine and subalpine landscape. Such striking and beautiful landforms are the focus ofYosemite National Park and the Lake Tahoe basin. Lake Tahoe is the largest and deepest Alpine lake in the world; it has a surface area of nearly 200 square miles and reaches a maximum depth of about 1,640 feet in its northwestern portion.