SUMMARY AND CONCLUSIONS
The geologic history of the Yosemite Valley, so far as present knowledge permits, may be epitomized as follows:
Probably early in the Tertiary period the area now occupied by the Sierra Nevada first acquired an appreciable slant to the southwest, and the Merced River came into existence as a southwestward-flowing master stream. For long periods thereafter the region, though affected by sundry gentle upwarpings, remained of moderate altitude, and the valley of the Merced was shallow and sloped gently to the sea, which then occupied most of the Great Valley of California. During that stage in the Miocene epoch of which definite indications still remain visible in the features of the Yosemite upland the river, by sluggishly winding from side to side, gave its valley a broad, level floor. That part of the valley which was destined to become the Yosemite was flanked by rolling hills averaging less than 1,000 feet in height. The crown of El Capitan was one of those hills. It rose from the valley in gentle slopes to a height of about 900 feet. Half Dome and Clouds Rest, however, stood well above the general level of the hills, being less slowly reduced by erosion because of the exceedingly massive nature of their rocks, and the region about the headwaters of the Merced was diversified by parallel mountain ranges 1,000 to 3,000 feet in heightremnants of an earlier mountain system. The climate was mild and rainy, and the land was covered by luxuriant vegetation.
In the later part of the Miocene epoch there set in a series of uplifts that raised the eastern border of the Sierra region several thousand feet and steepened its western slope. Fault fractures were formed along certain parts of the eastern border, and the country beyond subsided in part or remained relatively low, and thus the Sierra came to stand out as a tilted mountain block. The flow of the Merced River having been greatly accelerated by the tilting, that stream intrenched itself rapidly in the floor of its old broad valley and carved a narrow inner gorge. During the ensuing interval of stability, which lasted through all of the Pliocene epoch, this gorge developed gradually into a rugged mountain valley more than 1,000 feet in depth.
The tributary streams were not accelerated by the tilting as much as the Merced. A number of these streams, having courses inherited from the earlier mountain system and trending at right angles to the master stream and therefore at right angles to the direction of the tilting, were not steepened at all and flowed no faster than before. Their valleys, consequently, remained hanging untrenched above the gorge of the master stream. Later, when the master stream had cut its gorge down to a low gradient and was trenching with less vigor, most of the tributary streams caught up with itall but a few in the Yosemite region whose valleys were underlain by exceptionally resistant, massive granite and which succeeded only in notching the mouths of their hanging valleys with short, steep gulches. Accordingly, by the end of the Tertiary period the Yosemite had become a mountain valley about 1,500 feet deep flanked by billowy uplands from whose hanging valleys the waters cascaded down through steep gulches. Yosemite Creek made a cascade fully 600 feet in height.
The beginning of the Quaternary period was marked by a second series of uplifts, which were greater even than the first series. The eastern edge of the Sierra block was raised about 6,000 feet more and gained its present great altitude. At the same time Owens Valley and the other lowlands to the east were depressed, or remained low, and thus the great eastward-facing escarpment of the range was formed. The course of the Merced having again been steepened, that stream was accelerated to torrential speed. Its cutting power was enhanced, moreover, by spring freshets caused by the melting of snow that now fell in abundance on the crest of the range in winter, and so the river intrenched itself with greater energy than before. It produced a new inner gorge 1,500 feet in depththe gorge in which it still flows from the lower end of the Yosemite Valley to the foothills.
As happened after the first uplift, so after the second, most of the tributary streams, especially those flowing at right angles to the direction of the tilting, were unable to trench as rapidly as the master stream. Even those which had previously succeeded in cutting their valleys down to the level of the Merced, among which were Illilouette Creek and the two forks of Indian Creek, were now unable to keep step with its vigorous cutting. And so the Yosemite acquired a second, lower set of hanging valleys and a second set of cascades.
Several streams, however, among which was Bridalveil Creek, being specially favored by local jointing in the rock, were able to carve short gulches that reached all the way down to the floor of the chasm. These gulches still afford some indication of the depth to which the Yosemite had been cut prior to the ice age. They show that it had a depth of 2,400 feet measured from the brow of El Capitan, and 2,000 feet below the rim at Glacier Point. It was a profound and rather narrow, roughly V-shaped canyon, with a three-story profile telling of three successive stages of cutting. Its sides were deeply gashed by ravines, and through these the waters cascaded boisterously from the hanging valleys. Yosemite Creek now made a cascade 1,900 feet in height.
During the glacial epoch of the Quaternary period the Yosemite chasm was invaded at least three times by a trunk glacier, formed at its head by the junction of two lesser glaciers that came from the High Sierraone through the upper Merced Canyon, the other through Tenaya Canyon. The earlier of these ice invasions reached to the vicinity of El Portal and filled the Yosemite to the brinks; but the last ice invasion was of relatively small extent, reaching only to the Bridalveil Meadow and filling the Yosemite to about one-third of its depth.
By these successive glaciations, but mainly by the great and prolonged earlier ones, the Yosemite was transformed from a strongly winding V-shaped river canyon to a slightly sinuous U-shaped glacier trough. It was so greatly widened and deepened that the inner gorge was wholly wiped out, and even the mountain valley of the Pliocene epoch was largely destroyed. The craggy canyon sides were cut back to sheer cliffs, and the broken cascades were replaced by high leaping falls. What is more, the gulch of Bridalveil Creek was cut off by a 600-foot precipice, and so the Bridalveil Fall was created.
Throughout the length of the valley, save at the portal between El Capitan and the Cathedral Rocks, which are largely massive, the glacier quarried the coarsely jointed granitic rocks with exceptional efficiency. At the head of the valley it produced the largest results, for there, during the periods of maximum glaciation, the ice plunged into the valley from the top of the head wall in a glacial Niagara of tremendous power. Thus the depth of glacial excavation was increased up the valley from 500 feet at the lower end to 1,500 feet at the head, and the bottom of the valley, which before the advent of the glaciers had a steep slope, was scooped out in the form of a basin. When the glacier finally melted away this rock basin, deepened in addition by a moraine dam, held a lake 5-1/2 miles longancient Lake Yosemite.
Below the valley the prevailingly massive structure of the granite prevented glacial quarrying on any large scale, and as a consequence the valley there contracts abruptly and the inner gorge remains preserved.
In the Merced Canyon between the main valley and the Little Yosemite, the Merced Glacier, with the power derived from its 2,000-foot descent, hewed out the steps of the giant stairway, and in the Little Yosemite itself it scooped out a shallow rock basin analogous to that of Lake Yosemite. In Tenaya Canyon the ice produced a flight of imperfect steps with basined treads culminating in the 600-foot step from which the Tenaya Cascade now descends. In Tenaya Canyon, however, the structure of the granite facilitated quarrying vertically downward but impeded widening, whereas in the Little Yosemite the structure facilitated lateral quarrying but impeded deepening. As a consequence Tenaya Canyon is deep but narrow, and the Little Yosemite is wide but shallow and remains suspended high above the main valley.
Since the ice age the basin of Lake Yosemite and the lesser basins in the Little Yosemite and Tenaya Canyon have been filled by forward-growing deltas of gravel and sand deposited by the streams. The cliffs have been dismantled in places, and the débris that has fallen from them has accumulated in piles at their feet. Huge avalanches of rock, some thrown down probably by earthquakes, have dammed Tenaya Canyon at its mouth, thereby impounding Mirror Lake; others have fallen from the face of El Capitan, and still others have produced the Rock Slides, in the lower part of the valley, thereby creating slopes over which man has been able to build roads leading out to the uplands.
To the unusual and varied structure of its granitic rocks the Yosemite owes its highly distinctive cliff sculpture. The sparsely jointed granites have given rise to high, smooth walls; the more closely jointed rocks have been sculptured into angular, faceted forms, in which the controlling influence of master joints trending in different directions is evident. Narrow zones of intensely shattered rock, particularly vulnerable to the weathering processes, have been etched out here and there, giving rise to variously shaped recesses, clefts, and gulches, sharply incised into the walls. And the large bodies of wholly undivided, massive granite, by the progressive casting off of concentric shells from their surfaces, have been reduced to smoothly rounded domes.
The waterfalls of the Yosemite region, being intimately associated with the cliffs, also reflect the influences of the rock structure. The high leaping falls are associated with cliff faces determined by vertical or steeply inclined master joints; the broken cascades are associated with the hackled surfaces of rock masses traversed by numerous fractures; and the gliding cascades and spreading "aprons" are associated with large bodies of massive granite whose smooth surfaces are due in part to exfoliation, in part to glacial grinding.
The story of the evolution of the Yosemite Valley, then, is a story of several chaptersof successive periods of valley and canyon cutting by the Merced River, induced by successive uplifts of the Sierra Nevada; of vigorous glaciation, several times repeated, during the ice age, the quarrying action of the ice being controlled and guided locally by the varying structure of the granitic rocks and giving rise to exceptionally bold, clean-cut sculptural effects; and finally of a period of dismantling, resulting in greater detail and intricacy of sculpture. The production of slopes of rock waste, and the formation of level valley floors.
Last Updated: 28-Nov-2006