Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

THE SIERRA NEVADA

GENERAL CHARACTER AND DIMENSIONS

The Sierra Nevada is a single unbroken mountain range, yet it is comparable in magnitude to an entire mountain system. As is manifest from the relief map (pl. 1), it occupies slightly less area than all the Coast Ranges of California together. Incredible though it may seem, the Sierra Nevada is nearly as extensive as the French, Swiss, and Italian Alps combined. It is a mammoth range, the greatest in the United States, exclusive of Alaska. It measures 430 miles in length and from 40 to 80 mies in breadth.

In height also the Sierra Nevada outrivals all the other mountain ranges in the United States. Not only is Mount Whitney (14,496 feet) the highest summit in this country, but the range as a whole stands higher above its immediate base than any other. The Rocky Mountains, many of whose summits rise above 14,000 feet, stand only 9,000 feet above the Great Plains, which attain altitudes of about 5,000 feet at the foothills; but the Sierra Nevada stands not less than 11,000 feet above Owens Valley, at its east base, and 14,000 feet above the Great Valley of California, at its west base.

In general form the Sierra Nevada may be likened to a gigantic ocean wave rolling landward from the west. It rises from the hollow of the Great Valley in a long, gentle slope, culminates in a resplendent row of snowy summits, and with its precipitous front seems to threaten the deserts to the east. At its north end, where it joins the less imposing Cascade Range, the wave form is indistinct, but southward it gains in height and sharpness, the climax being reached near Mount Whitney. Still farther south the range declines and, curving toward the west, merges with the Coast Ranges near the Tehachapi (te-hatch'a-py) Pass. So strongly asymmetric is the Sierra Nevada that its crest line is for the most part within a few miles of its east base but 30 to 70 miles from its west base.

The peaks on the main crest gain in altitude from about 7,000 feet at the north end to 9,000 feet in the neighborhood of Lake Tahoe (tah'ho), 12,000 and 13,000 feet in the Yosemite National Park, and more than 14,000 feet in the Mount Whitney region. Beyond this they decline progressively to altitudes of 12,000, 9,000, and 6,000 feet.

The east front of the Sierra Nevada is among the greatest mountain escarpments in the world. (See pl. 5, C.) The northernmost part is the least impressive, being only a few thousand feet high and split into minor ridges that branch off in northerly directions from the northwestward-trending body of the range. But toward the south the front gains in height and continuity. East of the Yosemite region it has a height of fully 6,000 feet above the basin of Mono Lake; opposite Bishop, at the head of Owens Valley, it measures 10,000 feet; and in the neighborhood of Mount Whitney nearly 11,000 feet, or about 2 miles.

The Sierra Nevada, because of its continuity, height, and ruggedness, presents a formidable obstacle to east-west travel. The few available gaps or notches in its crest are neither deep nor easily accessible. The Donner Pass, through which the Southern Pacific Railroad is laid and which was formerly the main gateway for the California-bound wagon trains, has an altitude of about 7,000 feet. The Tioga Pass, familiar to motorists as the picturesque notch on the route from the Yosemite to Mono Lake, has an altitude of 9,941 feet. It is rarely free of snow before July. Thence southward to the Walker Pass there is a stretch of 180 miles without a single pass available for vehicular traffic—only steep and laborious pack trails which climb to altitudes over 11,000 feet, and even over 12,000 feet.

The streams that drain the Sierra Nevada are arranged largely at right angles to its axis, and as the divide between the westward and eastward flowing streams coincides essentially with the crest line, the drainage plan as a whole is as strongly asymmetric as the form of the range itself. The gentle west slope is traversed by a series of long rivers; the steep east front by small, short streams. Both the west slope and the east front are profoundly and intricately sculptured. Each master stream lies in a canyon several thousand feet deep. The Tuolumne and Kern Rivers, notably, have canyons between 4,000 and 5,000 feet in depth; the Kings River and its main branches have canyons between 6,000 and 8,000 feet in depth.

As the main canyons lie mostly at right angles to the crest line, one would expect the major corrugations of the range to have a similar trend; but this is not true everywhere. In many parts of the range the more prominent ridges trend northwestward, roughly parallel to the main crest. Subsidiary crests of this kind, 2,000 to 4,000 feet high, occur above the Yosemite region and throughout the south half of the range. To them is largely due the extreme ruggedness of its upper portion—that alpine region which has become known as the High Sierra.

RELATIONS TO ADJOINING LOWLANDS

There could scarcely be a more complete antithesis between contiguous features of the earth's surface than there is between the Sierra Nevada and the lowlands that adjoin it on each side—the Great Valley of California and the Great Basin of Nevada and Utah. As compared with the lofty range the Great Valley of California is an almost featureless plain. More than one-third of its total area—nearly 3,000 square miles—is less than 100 feet above the level of the sea. Upon this plain, which is on the whole arid, owing to the prolonged heat and drought of its summers, issue the numerous snow-fed rivers that descend from the west flank of the range. It is the life-giving waters of these rivers that have made possible the development of the great farming and fruit-growing industries for which the valley is famous.

The Great Basin, which begins at the east foot of the Sierra Nevada, is a lowland of a wholly different type—a vast province of sagebrush plains interspersed with sharp-crested mountain ranges. Though low as a whole compared with the Sierra and the other highlands that inclose it, its plains lie mostly at altitudes between 3,000 and 6,000 feet, and its ranges attain 10,000 to 13,000 feet. The outstanding features of the Great Basin, however, are the saline lakes and salt-incrusted desert basins in which nearly all its streams terminate, the water evaporating into the thirsty air and leaving its salt content behind. Great Salt Lake, in Utah, is the largest and best known of these briny desert lakes, but there are many lesser ones. Several lie close to the Sierra Nevada and receive the waters from its east front. Among them are Mono Lake, which is directly east of the Yosemite region, and Owens Lake, which is southeast of Mount Whitney.

CLIMATIC CONDITIONS AND LIFE ZONES

To one who stands on the crest of the Sierra Nevada and views, on the one hand, the well-watered and largely forest-clad body of the range, and, on the other hand, the vast desert wastes that stretch to the east of it, the contrast between the conditions prevailing in these two regions is impressive. Why, he may ask, is this mountain country so well favored by nature and the lowland to the east so neglected and desolate? The reason is that the Sierra Nevada, like every other mountain range of great height and extent, is itself a "climate maker." In large measure it is the author of its own weather conditions and controls those of the regions that are situated to the leeward of it. Lying parallel to the Pacific coast, it forms a barrier over which the vapor-laden winds that blow in from the ocean must rise. As they are forced up to high levels, they are chilled and discharge their condensed water vapor. Most of this falls in the winter, in the form of snow, the summers being remarkably dry.

The exceeding abundance of the winter snows in the Sierra Nevada is not generally known. Actually they exceed those in any other part of the United States, save the Olympic Mountains and the northern Cascade Range. According to the records of the United States Weather Bureau the annual snowfall at stations on the Southern Pacific Railroad, at altitudes between 6,000 and 7,000 feet, aggregates 30 to 40 feet in depth. In some winters it reaches a total of 60 feet. As much as 20 feet of snow has been reported in a single month, and often there is 10 or 12 feet of snow on the ground at one time. This heavy precipitation on the Sierra Nevada explains the general barrenness of the Great Basin. The air currents are fairly wrung dry as they pass over the range.

Even the Sierra Nevada itself is not equally favored with moisture in all its parts. The bulk of the snow and rain falls on its west slope between the 4,000-foot and 9,000-foot levels. The foothills and lower slope partake in large measure of the semiarid conditions that prevail in the Great Valley; and the higher peaks and crests also are relatively dry, the air currents having discharged most of their vapor content before reaching those heights. The High Sierra, it is true, retains its white garb much longer in spring and summer than the middle slope, but that is due primarily to the lingering cold, which retards the melting. As for the east front of the range, naturally it also is arid as compared with the west slope.

The unequal distribution of the snow and rain on the Sierra Nevada, together with the wide range in temperature from the torrid foothills to the wintry crest, give rise to several distinct climatic belts or zones, each of which has its characteristic forms of vegetation and animal life. These zones are broadest and most distinct on the west flank of the range.

The semiarid foothill belt, hot and dry in the summer but rainy in the winter, corresponds to what biologists term the upper Sonoran life zone. Its vegetation consists characteristically of thin grass, bushy chaparral, and scattered groups of live oaks and digger pines. Between altitudes of 3,000 and 4,000 feet it merges with the great forest belt or transition life zone, which has a more genial climate in the summer and receives considerable snow in the winter. In the midst of its stately forests of yellow pine, sugar pine, incense cedar, Douglas fir, and white fir stand scattered groves of giant sequoias, or big trees. (See pl. 6, B.) The Yosemite National Park contains three of these groves—the Mariposa, near its southern boundary, and the Merced and Tuolumne, near its western boundary.

PLATE 6.—A (top), CHARACTERISTIC STORM-BENT TREE AT TIMBER LINE. A white-bark pine growing prostrate along the ground, in the direction of least resistance with respect to the prevailing winds. B (bottom), GIANT SEQUOIAS IN MARIPOSA GROVE. The sequoias stand in groves surrounded by pines, firs, and cedars, about halfway up on the western flank of the Sierra Nevada. Photograph by J. T. Boysen.

The Yosemite Valley itself, being at an altitude of 4,000 feet, is situated in the forest belt, but owing to the fact that the cliffs on its north side are daily heated by the sun, whereas the cliffs on its south side remain mostly shaded and cool, it is in a sense the meeting place of many different species, including chaparral from the foothill belt and trees from the surrounding uplands.

At altitudes ranging from 6,000 to 7,000 feet begins the Canadian life zone, which is characterized mainly by stands of lodgepole pine, Jeffrey pine, and red fir. It is sought in summer for its exceptionally delightful climate but shunned in winter because of its frigidity. Toward the 9,000-foot level the silver pine and mountain hemlock make their appearance, and the Hudsonian life zone sets in. This is the picturesque timber-line belt, in which only the hardiest species of trees can thrive. At its extreme upper limit, between 10,000 and 11,000 feet, the white-bark pine occurs in curiously storm-twisted, recumbent, or wholly prostrate forms. (See pl. 6, A.)

Above the timber line, in the alpine life zone, the mountain sides and peaks rise essentially bare of vegetation. Here, from altitudes of 11,000 feet upward, the precipitation, even in summer, consists largely of snow or hail, and the temperature in the shade seldom rises much above the freezing point. Snowdrifts abound until far into midsummer, and perennial bodies of old, hard snow and even a few small glaciers linger in the shaded, steep-walled recesses among the higher peaks. (See pls. 14 and 28, B.)