THE SIERRA NEVADA.
The general lay and configuration of the tract inclosed by the Yosemite National Park can he best understood by taking a broad, preliminary view of the Sierra Nevada as a whole. That range, which is one of the greatest mountain ranges of the western United States, runs lengthwise through the State of California, roughly paralleling the Nevada border. More than 300 miles long and about 80 miles wide, it constitutes a huge rampart separating the desert basins on the east from the Great Valley of California on the west. Rising abruptly from the desert by a steep face or escarpment several thousand feet high, it descends to the low level of the Great Valley by a long and gentle sweep. It is thus pronouncedly unsymmetrical in cross section; its crest line, instead of occupying an axial position, runs along the top of its steep eastern face.
This one-sided configuration the Sierra Nevada acquired not through erosion of any kind, but through upheaval of the crust of the earth. One should picture the range to himself as a huge, elongated block of the earth's crust, strongly tilted to the west and bounded on the east by a great line of fractures. It did not rise as an isolated block; rather, it was originally part of a vast. dome produced by the broad upwarping of the entire region now known as the Great Basin of Nevada and Utah. This dome caved in irregularly, some portions settling lower than others, thus giving rise to the alternating deserts and mountain ranges characteristic of the Great Basin. The Sierra Nevada, which constituted the westernmost margin of the dome, remained standing in its elevated position, but the blocks immediately to the east sank down to relatively low levels, leaving a high and precipitous front to mark the line of fractures.
The crest of the Sierra Nevada stands at altitudes ranging from 10,000 to 14,000 feet, culminating in Mount Whitney (14,501 feet), the highest summit in the United States. The deserts skirting its eastern base stand at elevations ranging from 3,500 to over 6,000 feet. The greatest drop is in the vicinity of Owens Lake, where the eastern scarp descends abruptly from 11,000 to 3,569 feet.
Although on the whole fairly regular in outline, the Sierra block should not be conceived as having plane sides and smooth, straight edges. Nature does not fashion her forms with mathematical exactness on so large a scale. The great fracture or series of fractures bounding the block on the east is full of irregularities, and in many places even consists of several separate, roughly parallel, lines placed in "echelon."
The surface of the block that now forms the western slope was originally diversified by hills and valleys, themselves the worn-down remnants of an earlier mountain system, and since the uplift of the range has been scarred and furrowed by new stream-cut gorges and valleys. Indeed, the present aspect of the Sierra Nevada is to be regarded as the product of a long and complicated series of events. Not even the broader outlines were formed at one bound. The fractures on the east side are of various dates. Some of them occurred as early as the end of the Cretaceous period; others were added at different times throughout the succeeding Tertiary period. It was at the end of Tertiary time, however, that the greatest inanifestation of nmuntain building took place, and that sharp, renewed uplifts, accompanied by a general outburst of volcanic eruptions and lava flows, gave the block its present strong westward tilt.
The greatly increased inclination thus acquired naturally gave fresh impetus to the waters running down the western slope, and as a consequence these waters eroded with greater vigor than before, intrenching themselves in deep canyons and valleys. A great series of nearly parallel valleys was created, spaced at fairly regular intervals, most of them transverse to the axis of the range. Each of these valleys heads on the Sierra crest and sends forth a powerful river to the Great Valley of California. Of these streams the most important are the Feather, Yuba, Bear, American, Mokelumne, Stanislaus, Tuolumne, Merced, San Joaquin, King, and Kern Rivers.
Throughout Quaternary time, down to the present day, this vigorous stream erosion has continued, etching the surface of the Sierra block with a network of ramifying branch valleys, between which residual portions of the old surface have remained standing in the form of mountains and ridges. During the glacial epochs, moreover, the advent of glaciers on the higher regions has served to intensify the erosion and to hasten the dismantling of the Sierra block.
The rock débris resulting from all this destructive work was little by little carried down by the rivers and deposited in the Great Valley at the foot of the range. The enormous quantities of material thus brought down filled the valley to the depth of several thousand feet, so that while the Sierra block was being denuded in its upper portions it was being cloaked with waste on its lower slopes. The present foothills, therefore, should not be taken as the real base of the range, for that lies buried under great thicknesses of sediment.
CLIMATIC ZONES AND VEGETATION.
The great height of the Sierra Nevada above the country surrounding it, together with its position close to the Pacific coast, have endowed the range with rather unnusual climatic conditions. The prevailing winds in this part of the country blow in from the Pacific Ocean, laden with moisture. Forced to ascend to great altitudes in passing over a range of mountains, they gradually cool and discharge their content of water in the form of rain. The Coast Ranges are naturally the first thus to abstract some of this moisture; but, being of moderate elevation only, they are able to catch but a small percentage of it. The rest is carried farther eastward, to the Sierra Nevada, where it descends in copious rains and snow on the upper slope and crest. Thus the upper Sierra is well watered, while the low-lying Great Valley to the west is parched and in need of irrigation many months in the year. Furthermore, the intense concentration of rain and snow on the crest of the Sierra leaves the westerly air currents passing over it to continue eastward fairly wrung dry of their moisture, so that the land east of the range is now a barren desert. Even the east face of the range partakes in some measure of this aridity, for the transition in climate from the crest of the range to its eastern base is amazingly abruptas abrupt, in fact, as the escarpment itself. In a few miles cue descends from the frigid air of perpetual snow fields to the burning heat and glare of deserts.
As the climate of the steep east face of the Sierra Nevada is characterized by sharp and extreme contrasts, so that of the long west slope is marked by notably gentle gradations. On this slope several more or less distinct climatic zones lie parallel to one another, each recognizable by a characteristic belt of vegetation. The foothills for a thousand feet or more present essentially the same aspect as the great plain from which they rise. Verdant only a few brief weeks in spring, their sun-baked slopes remain most of the year of a uniform straw color, relieved only here and there by scattered groups of olive-colored live oaks. Farther imp the slope these trees become more numerous, and low, chaparral-like brush appears. Occasional forlorn-looking specimens of so-called Digger pine (Pinus Sabiniana), with long, bluish needles, mingle with the oaks. Still farther up the brush becomes progressively denser and taller; the rigid, red-stemmed Manzanita grows to astonishing dimensions, 12 to 15 feet in height; oak and pine congregate in groves and become more luxuriant, until, at last, near the 3.000-foot level, the first yellow pines are reached. Here the true Sierra forest begins; the landscape loses its thirsty parched look and displays a brilliant green foliage, bespeaking refreshing moisture. With every step upward the forest waxes in height and in majesty; the trees grow larger and more perfect. With the yellow pine (Pinus ponderosa) stands the incense cedar (Libocedrus decurrens), the Douglas spruce (Pseutdotsuga Douglasii), and the most stately of all Sierra trees, the sugar pine (Pinus Lambertiana). All these attain remarkable heights, the forest in places lifting its spires 200 to 300 feet above the ground.
In this belt of maximum forestation, evidently thriving in a temperate and particularly propitious climate, grow also the giant trees of California, the Sequoias (Sequoia gigantea). Interspersed with the other huge trees in groves of a few dozen to several hundred, they represent the straggling survivors of a species that is threatened with extinction. In the central portions of the range they occur at levels near 6,000 feet, and it will be noted that the Mariposa, Tuolumne, and Merced groves all stand at this elevation.
From the 7,000-foot line upward climatic conditions become less favorable to forest growth. The precipitation is mostly in the form of snow and the winters are long and severe. The forest assumes a less robust and less impressive aspect; the great yellow and sugar pines gradually make way for smaller and hardier species, more especially for firs and the so-called tamarack pine (Pinus Murrayana). Near the 8,000-foot level the sturdy mountain pine (Pinus monticola) makes its appearance and the forest thins out and seeks the sheltered slopes and basins. The increasing rigor of the winter climate becomes painfully manifest. Near the 9,000-foot level, at last, the forest gradually comes to an end. Only a few particularly resistant alpine varieties hold out, evidently against great odds. Weighted down by the heavy snows of winter, they form low-crouching, gnarled, and compact thickets, many of them eventopped by the gales, as if trimmed by a careful gardener. Beyond these fighting outposts, constituting what is termed the "timber line," only small alpine bushes and flowering plants manage to exist, and the crags are mostly bare of soil and vegetation.
Thus time extent and character of the forest cover of the Sierra slope are everywhere exactly adjusted to the conditions imposed by the climate.
As for the snow line, no distinct line of perpetual snow is traceable through the Sierra upland. The total annual snowfall is heavy, it is true, aggregating many feet and giving the range in winter that dazzling white appearance that led the Spanish settlers on the hot plains below to name it the Sierra Nevadathe Snowy Rangeyet most of this snow cover vanishes in summer under the fierce rays of the sun. Only in sheltered nooks where the wind-driven snow accumulates in eddies do banks and fields of snow of considerable thickness lie perennially from one winter to the next. In the lee of several of the main peaks there are even sonme large bodies of old, granular ice, which may properly be classed as glaciers. Particularly noteworthy are those on Mounts Lyell, Dana, and Conness. Situated at elevations from 11,500 feet up, on northeasterly slopes, in the lee of the westerly winds, and protected from the midday sun, these ice bodies represent the only surviving remnants of the huge glacial blanket that once covered the upper Sierra. Most of them lie on the east side of the main crest, and therefore just outside the limits of the Yosemite National Park, but they are nevertheless within easy reach of the tourist.
The peculiar climate of different parts of the Sierra Nevada and the extent and character of the forest cover are thus the natural consequences of the elevation and the geographic position of the range, and in all their features and gradations they are intimately related to its conformation.
Last Updated: 02-Apr-2007