THE YOSEMITE NATIONAL PARK.
The foregoing description of the Sierra Nevada will serve to give the reader some idea of the region in which the Yosemite National Park is situated. That reservation lies near the middle of the range, on its long western slope, extending from about the 4,000-foot level up to the Sierra crest. It embraces the great forest belt containing the sequoias and other large trees, the upper zones of sparser vegetation, and the picturesque alpine zone that reaches from timber line to snow banks and frost-shattered peaks. The lowest valleys reach down to 3,500 feet; the highest summits rise more than 13,000 feet above sea level.
A glance at the accompanying map (fig. 10) will show that the park includes the upper courses and headwaters of two of time great master streams of the Sierra Nevadathe Merced and Tuolumne Rivers. The Merced, occupying with its drainage basin the southern half of the tract, gives the park its most admired feature, the Yosemite Valley; the Tuolumne, embracing with its tributaries the northern half, produces the somewhat smaller yet scarcely less remarkable Hetch Hetchy Valley. In form each of these two canyon valleys may be likened to a short, stout tree trunk, forking upward into two or more heavy branches, which again ramify into smaller and smaller ones, like the spreading limbs and twigs of a well-proportioned elm tree. The heads of most of these branches are on the crest of the Sierra, many of them in low gaps or passes that provide avenues for travel across the range. Especially well known are the Tioga and Mono Passes, through which trails lead down the eastern slope of the Sierra to Mono Lake and its interesting volcanic setting.
Between the Yosemite and Hetch Hetchy Valleys. which, sunk deep in the Sierra flank, lie roughly parallel to each other, is a 10-mile stretch of forested upland, embossed with rounded hills and traversed by shallow vales. The traveler through this upland is impressed chiefly with the subdued character of the relief and the general absence of strongly accentuated sculptural features; nor does he find anywhere so much as a hint of the proximity of a chasm until he emerges upon the brink of the trough of the Merced or the Tuolumne Rivers.
As the tourist ascends to higher altitudes he perceives a gradual change come over the landscape. He observes not only that the timber becomes sparser and shorter in stature, but that the topography assumes a distinctly novel aspect. As he enters the so-called "High Sierra" he notes that the regular alternation of ridge and valley, spur and ravine, which is characteristic of the lower slopes, makes way for a less orderly, less obviously systematic arrangement of topographic forms. Irregularly grouped knobs and domes of granite fill the landscape; the streams are capricious in course; the valleys descend with irregular gradients; picturesque ponds and lakelets appear in odd places, many of them without outlet; the soil is scanty and over large tracts is entirely wanting; the bare rock of the mountain swells shines white through the thinning trees, which, with their roots wedged in fissures, are stunted for lack of nourishment.
The farther up one goes the more accentuated becomes this peculiar topographic facies. until, as the timber line is reached, it dominates the entire landscape. The nearly bare peaks and mountains, hewn of solid granite, present a most varied and spectacular array of sculptural forms, ranging from smoothly rounded domes to pinnacled, castellated crags and spires. Among them wander the streams, in parts of their courses lost in valleys disproportionate to their size. in other parts following no defined valleys or channels of their own; here meandering in broad, swampy meadows, there rushing wildly down over rocky slopes or cliffs.
The larger valleys and canyons, moreover , assume a character entirely in harmony with these strange features. Steep sided and flat floored, they impress one more as smoothly gouged channels of huge proportions than as valleys slowly gullied out by streams. Depressed below the general level of the upland, they also seem overdeepened with respect to their lesser tributaries, which have a strangely disjointed appearance, hanging, so to speak, at some height above the canyon floors, to which they send their waters down in cascades and waterfalls.
Nor do the main canyons themselves descend by even, steady gradients, but stairwise. by alternating steps and flats. The length of the flat treads varies considerably, from a few hundred yards to several miles, and the steps range in height from scores to hundreds or even thousands of feet. Many of the treads, moreover, are hollowed and inclose lake basins.
Strangest of all are the valley heads. They are not ordinary narrow, steep-sided, deeply incised ravines or gulches, but they open out, as a rule, into broadly rounded, bowl-shaped hollows, scooped out in the solid rock, and in the bottom of each of these amphitheaters usually lies ensconced a small lake or tarn in a smooth rock basin of mysterious molding.
As one surveys this assemblage of unwonted features he can not but intuitively feel that they were not produced by the ordinary processes of weathering and stream erosion, by which the familiar hill-and-valley landscapes are known to have been evolved. Running water, judging from the general arrangement of the valley systems, undoubtedly has played an important part in sculpturing this land, but its work appears to have been interfered with and disordered by another agent, which operated on a somewhat different plan. And this, indeed, is what has happened. The mysterious remodeling agent was none other than the ice of the glacial epochs. After the valley systems of the High Sierra had been established by the waters running off the slanting Sierra block, they were invaded by great ice tongues or glaciers that extended down from the summit regions. and through the erosion of these glaciers they were in various ways remodeled and changed.
As is well known, large portions of the North American Continent and of Europe, in times geologically not remote, were covered by extensive sheets of ice. Over the low-lying regions of moderate relief these ice sheets spread broadly and in great thicknessprobably thousands of feetmuch as they spread to-day in Greenland. But in the rugged mountains of the far West , as in the European Alps, the ice was more local, originating on the crests of the ranges and descending thence down the valleys and canyons in the form of individual streams or glaciers. This was the condition in the Sierra Nevada. The summit regions of that range, except a few of the highest wind-swept peaks and crests, were buried in snow, which, in the valley heads, reached depths of 1,000 to 2,000 feet.
Snow masses so enormous, it scarcely need be said, do not remain stationary and inert, as do snow banks only a few feet deep. Compacted to the consistency of granular ice their very weight suffices to overcome their rigidity and to bring on a slow, flowing motion which has been likened to that of a highly viscous substance, such as tar. The ice masses that occupied the upper valleys of the Sierra Nevada therefore moved forward like broad, sluggish streams, each in a separate valley, and thus formed a system of glaciers corresponding to the system of valleys previously carved by the rivers. The super abundant snow supply in the crestal regions caused the glaciers to lengthen downward to lower and lower levels, and finally to protrude far beyond the zone of glacial climate into regions where the snows of winter entirely melted away during the warmer seasons, the conditions resembling those presented to-day by the glaciers of the Canadian Rocky Mountains and the Alps of Switzerland. On the east side on the Sierra Nevada, indeed, the ice tongues reached down to the level of the desert, but on the west side they terminated at some distance above the range foot, withering away under the hot sun.
It is obvious that a moving ice mass 1,000 feet deep or more exerts considerable force on the sides and bottom of the valley it occupies. Even though its lower layers are retarded by friction their forward movement, combined with the great weight of the overlying masses, is sufficient to scour the valley of all loose débris and to dislodge fresh blocks from the rock bed as fast as these are loosened up by frost and the solvent action of percolating waters. Glaciers therefore tend to clean out their valleys and to widen and deepen them. By their lengthwise motion they plane away the spurs and ravines that corrugate the valley sides; they undercut and truncate the valley slopes so as to transform the original constricted V-shaped cross-section of the valley to a broadly open U-shaped one. They widen sharp, angular turns in the valley course to broadly sweeping bends, substituting a succession of smooth-flowing curves for the zigzag alignment that is characteristic of stream erosion. The net effect is to give each valley the appearance of a simple, steep-sided, U-shaped trough. Lyell Canyon is a particularly excellent example of this trough type. (See fig. 7.)
The amount of excavational work that glaciers are capable of performing, however, depends so largely upon their thickness that the major ice streams deepen the main canyons much faster than their weaker tributaries can excavate the side valleys. The smaller glaciers, in other words, can not keep pace with the larger ones, and, as a consequence, after the ice has disappeared, the mouths of the smaller valleys are found "hanging" at some height above the floors of the larger ones. The strangely overdeepened aspect of the main canyons of the High Sierra, with respect to the upland vales, is thus a direct result of the glacial occupancy of the region.
A word further about the manner in which glaciers deepen their channels. Without going into the details of the process, it may be stated that, in general, the tendency of glaciers is to accentuate the unevennesses of the longitudinal valley profile. For mechanical reasons the ice masses erode most vigorously at the foot of the steeper stretches where the valley profile is concave, while in the flatter stretches, and especially where the valley profile is convex, they accomplish least. The net effect, therefore, is to replace the original stream-worn profile by a pronouncedly stairlike one, possessing alternating steps and treads. At the foot of each declivity, moreover, the descending ice tends to hollow out a lake basin in the tread below. Thus the step-wise descent of time Sierra canyons and the frequent occurrence of lakelets on their flat reaches are seen to be characteristically glacial traits.
It is in time extreme valley heads, however, that glaciers have the fullest play for their distinctive mode of sculpturing, for here the ice masses are powerfully seconded in their erosional work by the rapid diurnal changes in temperature characteristic of high altitudes, which effectually shatter the cliffs, and by frequent frosts which, congealing the water held in crevices, disrupt the rock by their distending force. Under the attacks of these auxiliary agents the cliffs crumble rapidly and recede, so that the valley heads become enlarged into bowl-shaped amphitheaters, or "cirques," as they are technically termed. (See fig. 6.) An examination of the administrative map of the Yosemite National Park1 will show how prevalent such cirques are along the Sierra crest. These horseshoe-shaped hollows occupy a large part of the summit region, and, as is obvious from the courses of the contour lines on that map, have eaten into the peaks and ridges and developed at their expense.
No one who has penetrated the highest portions of the Yosemite Park can have failed to be impressed with the clean-cut, well-preserved appearance of its rock-hewn forms, as compared with the canyons and other erosional features farther down the Sierra slope. It seems as if it were only yesterday that the ice had left them. Fresh and unweathered, like new quarries, are the cirque walls, while smooth, glassy "glacier polish," the result of long-continued grinding and "sandpapering" by the débris-laden ice, still shines upon their bare rock floors. In the canyons below such polish may also be found in places, but there it has a more aged look. The luster has grown dim and much of the smooth enamel has already flaked off from the weathered, porous rock.
The reason for this notable difference in freshness of the ice marks in the summit regions and farther down is twofold. In the first place, it should be understood that what is popularly referred to as "the glacial period" really consisted of a succession of glacial epochs, separated from one another by intervals of milder climate. Each glacial epoch brought on an extension of the ice. The glaciers gradually lengthened from the summit downward until they reached a maximum limit, and then as gradually melted back to the cirques that gave them birth. In duration each epoch probably amounted to several thousand years, but so slow was the advance of the glaciers, and so slow the retreat of their fronts, that although the cirques and upper canyons were ice-filled for long periods, the lower canyons were invaded for only a fraction of the time. In the second place, it is but quite recently that the last glacial epoch came to a close. Indeed, in one sense it has not ended yet, for on time Sierra crest a few small ice bodies still hold their own. The uppermost cirques, there is good reason for believing, have only just been released from the dominion of the ice, but the lower canyons have been ice free for a considerable lapse of time and subject to normal weathering and to stream erosion.
The gradual change in the character of the upland between the canyons, which one notices in ascending from lower to higher levels, is similarly explained. In the lower portions of the Yosemite Park the ice has existed for only brief periods and at long intervals; its influence on the topography has been slight, and in the long stretch of time since the final retreat of the ice has become partly obliterated by ordinary erosional processes and by the advent of vegetation. But higher up the glaciation has been prolonged and intense, and has persisted until a relatively late day. Its characteristic effects have therefore become strongly accentuated, and are still well preserved.
Such, then, in brief, is the explanation of the singular physiognomy of the High Sierra. Much of its charm and grandeur is inherited from the former reign of icethat mysterious chain of events in the earth's recent geologic history that has wrought so effectively for the scenic endowment of several of our finest national parks.
Last Updated: 02-Apr-2007