GEOLOGIC HISTORY OF THE YOSEMITE VALLEY
By FRANÇOIS E. MATTHES
The problem of the origin of the Yosemite Valley inherently demands a solution in quantitative terms. Its essence is, To what extent is the valley a product of glacial action, to what extent a product of stream erosion? The principal result of the investigations upon which this report is based is the determination within narrow limits of the preglacial depth of the Yosemite Valley and of other facts concerning its preglacial development which permit fairly definite estimates of the proportionate shares of work performed by stream and by glacier. The investigations comprise a detailed survey of the glacial and geomorphologic features of the Yosemite region and an equally intensive study of its rock formations, supplemented by reconnaissance work of both kinds in adjoining parts of the Sierra Nevada. The petrologic studies were made by Frank C. Calkins; the glacial and geomorphologic survey by François E. Matthes.
Detailed mapping of the morainal system of the ancient Yosemite Glacier has served not only to determine the farthest limits reached by that glacier but to throw new light on the significance of the hanging valleys of the Yosemite region. It is reasonably certain that the glacier never extended more than about a mile beyond the site of El Portal. The hanging side valleys of the Merced Canyon below El Portal, therefore, hang not because of any glacial deepening suffered by that canyon. The explanation is offered that they hang because their streamlets have been unable to trench as rapidly as the Merced River since the rejuvenation of the Merced by the last uptilting of the Sierra Nevada. The streamlets were handicapped not only by their comparatively small volume but also by the fact that their courses trend northwestward and southeastward, substantially at right angles to the direction of the tilting, and therefore have remained essentially unsteepened, whereas the Merced's course trends southwestward, directly down the slope of the Sierra block, and therefore has been appreciably steepened.
Projection of the longitudinal profiles of these hanging valleys forward to the axis of the Merced Canyon shows that they are closely accordant in height. Their profiles indicate a series of points on a former profile of the Merced with respect to which the side streams had graded their courses prior to the last uplift. This old profile can be extended upward into the glaciated part of the Merced Canyon above El Portal and even into the profoundly glaciated Yosemite Valley, accordant points being furnished by a number of hanging side valleys (due allowance being made for glacial erosion suffered by those valleys). However, not all the hanging valleys of the Yosemite region are accordant with this set. Several of them, including the upland valley of Yosemite Creek, constitute a separate set indicating another old profile of the Merced at a level 600 to 1,000 feet higher than the first. Others, including the hanging gulch of lower Bridalveil Creek, point to an old profile of the Merced about 1,200 feet lower than the first. There are thus three distinct sets of hanging valleys produced in three cycles of stream erosion. The valleys of the upper set, like those of the middle set, were left hanging as a result of rapid trenching by the Merced induced by an uplift of the range, there having been two such uplifts. Only the valleys of the lower set hang because of glacial deepening and widening of the Yosemite Valley, the cycle in which they were cut having been interrupted by the advent of the Pleistocene glaciers. They consequently indicate the preglacial depth of the Yosemite Valley. That depth, measured from the brow of El Capitan, was about 2,400 feet; measured from the rim at Glacier Point it was about 2,000 feet.
During that remote cycle of which the hanging valleys of the upper set and the undulating Yosemite upland are representative the Yosemite Valley itself was broad and shallow, past mature in form. That early stage in its development, accordingly, is called the broad-valley stage. The deeper hanging valleys of the middle set were graded with respect to a deeper Yosemite of submature form which must have had the aspect of a mountain valley. That stage in its development is therefore called the mountain-valley stage. The short, steep hanging valleys of the lower set and certain topographic features associated with them show that during the third cycle of erosion the Yosemite was a roughly V-shaped canyon with a narrow inner gorge. This stage of the Yosemite, which immediately preceded the glacial epoch, is therefore called the canyon stage.
Correlation of the Yosemite upland with the upland of the Table Mountain district, between the canyons of the Tuolumne and Stanislaus Rivers, shows that it is in all probability a feature of late Miocene age; for fossil remains of plants and animals found in the lava-entombed stream channels on the upland near Table Mountain appear to be of late Miocene age, according to determinations made by Dr. Ralph W. Chancy and Dr. Chester Stock. Uplift of the Sierra Nevada at the end of the Miocene epoch initiated the next cycle, during which the mountain-valley stage was evolved. That cycle lasted presumably through most if not all of the Pliocene epoch. The canyon stage was produced in all probability wholly during the Quaternary period.
The excellent preservation of the hanging valleys of the upper set, in spite of their great age, is explained by the exceedingly resistant nature of the massive granite that underlies them. The valleys of the middle set were carved in prevailingly jointed rocks that were less resistant to stream erosion, and the gulches of the lower set were carved in closely fractured rocks in which the streams eroded with relative ease. The important part played by massive granite in the preservation of the upland valleys and the upland itself is most convincingly demonstrated near Wawona, in the valley of the South Fork of the Merced, which may be termed a half-yosemite. The north side of this valley, which is carved from prevailingly massive granite, has sheer cliffs, that reach up to a lofty upland, the analog of the Yosemite upland; and from the hanging valley of Chilnualna Creek, on this upland, leaps a waterfall similar to the falls that leap from the hanging side valleys of the Yosemite. The south side, which is carved from prevailingly jointed rocks, has sloping forms, and the side streams there descend to the level of the master stream without making any falls.
In addition to the three sets of hanging valleys and the three old profiles of the Merced to which they point, there are at hand enough other topographic data to permit an approximate reconstruction of the configuration of the Yosemite Valley at each stage. Three bird's-eye views are shown, all drawn from the same point of view, in which the development of the significant features of the Yosemite Valley can be traced from stage to stage. A fourth bird's-eye view affords a direct comparison of the canyon stage, immediately preceding glaciation, with the U-trough stage at the end of the glacial epoch.
The gradients of the two higher profiles of the Merced, further, furnish data from which the amplitude of each of the two great uplifts of the Sierra Nevada can be calculated roughly. The uplift at the end of the Miocene epoch added about 3,000 feet to the height of the range; the uplift at the end of the Pliocene epoch added fully 6,000 feet more. Mount Lyell, which now stands about 13,090 feet above the sea, therefore stood at about 7,000 feet during the Pliocene epoch and about 4,000 feet during the Miocene epoch.
Of the earlier geomorphologic history of the Yosemite region a glimpse is afforded in the explanation of the origin of the southwesterly course of the Merced River and the arrangement of the lesser tributaries at right angles to it. The Merced established its course conformably to the southwesterly slant of the Sierra region, presumably early in the Tertiary period, when there still existed remnants of a system of northwestward-trending mountain ridges of Appalachian type, which had been formed at the end of the Jurassic period by the folding of sedimentary and volcanic strata of Paleozoic and Mesozoic age. As it grew headward the Merced probably captured the drainage from the longitudinal valley troughs between these ridges. Below El Portal, on the lower slope of the Sierra Nevada, where the folded strata still remain in a broad belt, the lesser tributaries of the Merced are for the most part adjusted to the northwesterly strike of the beds. In the Yosemite region and the adjoining parts of the High Sierra, from which the folded strata are now stripped away, so that the granitic rocks are broadly exposed, the northwesterly and southeasterly trends of many of the streams are largely an inheritance by superposition from the drainage system of the now vanished older mountain system. It seems entirely probable that the northwesterly trend of the Clark Range, the Cathedral Range, and certain stretches of the main crest of the Sierra Nevada is likewise inherited from that ancient mountain system.
The mapping of the morainal system of the Yosemite Glacier has, further, led to the recognition of three stages of glaciation. During the last or Wisconsin stage, which is recorded by well-preserved, sharp-crested moraines, the Yosemite Glacier advanced only as far as the Bridalveil Meadow. During the preceding El Portal stage, which is recorded by relatively obscure, partly demolished moraines, the glacier reached as far as a point about a mile below El Portal, in the lower Merced Canyon. Remnants of a valley train of outwash material can be traced for a distance of 30 miles farther down the canyon. Of the still more remote Glacier Point stage evidence is found only in erratic boulders that lie scattered at levels about 200 feet above the highest lateral moraines of the El Portal stage, notably near the west base of Sentinel Dome, 700 feet above Glacier Point, and on the broad divide east of Mount Starr King.
On both sides of the Little Yosemite the younger lateral moraines culminate in two parallel crests of about equal height, yet separated by a broad depression. The interval of time between the deposition of the outer and inner crests appears to have been much shorter than an interglacial stage, hence it is concluded that the Wisconsin stage was characterized by two glacial maxima. On the other hand, the interval of time between the Wisconsin stage and the El Portal stage must have been very long, for whereas the massive granite on the sides of the Little Yosemite retains over large areas the polish imparted to it by the Wisconsin ice, it has lost all the polish that was imparted to it by the El Portal ice and in addition has disintegrated and been stripped by rain wash to a depth of several feet. The most reliable measures of the stripping are furnished by three dikes of slow-weathering aplite that project like little walls from the summit of Moraine Dome. Since the ice of the El Portal stage passed over the dome these dikes have come to stand out, by reason of the stripping of the surrounding granite, with heights of 7, 8, and 12 feet. It is estimated, accordingly, that a period at least ten times and perhaps twenty times as long as the post glacial interval has elapsed since the El Portal stage. That stage is therefore perhaps to be correlated with either the Illinoian or the Kansan stage of the continental glaciation.
That the El Portal stage was in turn separated from the Glacier Point stage by a long interval of time is inferred from the fact that whereas the oldest lateral moraines of the El Portal stage still persist as continuous bodies, even on steep slopes, nothing is now left of the moraines of the Glacier Point stage save a few boulders of exceptionally durable quartzite and siliceous granite, even on nearly level surfaces where the conditions are particularly favorable for the preservation of moraines. The Glacier Point stage is therefore held to be comparable in age with the Kansan or possibly with the Nebraskan stage of the continental glaciation.
The discussion of the glacial history of the Yosemite region is supplemented by a map of the valley on a scale of 1:24,000 on which all the moraines and the more significant erratic boulders are shown in detail, also by a map on a scale of 1:125,000 on which are delineated the entire Yosemite Glacier from its sources on Mount Lyell and in the Tuolumne Basin down to its terminus near El Portal and all the lesser ice bodies that lay within the area tributary to the Yosemite Valley. A brief description is given of each glacier as it appeared in the earlier and in the later ice stages. It is shown that Half Dome at no time was overtopped by the ice; also that the Yosemite Glacier at no time received a tributary ice stream from the Illilouette Valley, as has been commonly assumed. Instead, a lobe of the Merced Glacier pushed up into the Illilouette Valley. Imprisoned between that lobe and the Illilouette Glacier lay a temporary lake, whose extent is indicated by deposits of sand and gravel.
The depth of glacial excavation in the Yosemite Valley is revealed in a longitudinal section affording a direct comparison of the preglacial and postglacial bottom profiles. It increases gradually from 500 feet at the lower end of the valley to a maximum of 1,500 feet opposite Glacier Point; thence it decreases abruptly to a minimum of 250 feet at the mouth of the Little Yosemite. The glacial widening is shown in a series of cross sections in each of which the preglacial form is superimposed upon the postglacial form. It exceeds the glacial deepening at all points and ranges from a maximum of 1,800 feet on each side in the upper half of the valley to a minimum of 500 feet on each side at the head of the lower Merced Gorge. These marked variations are explained by the selective action of the glacier in rocks of widely varying structure. Where the rocks were thoroughly jointed the glacier excavated effectively by quarrying; where the rocks were too massive to be quarried the glacier only ground and polished. The capacious U form of the Yosemite Valley is therefore a product of wholesale quarrying in an area of well-jointed rocks. The gorges above and below the valley, on the other hand, have remained narrow because glacial abrasion has been able to effect but slight changes in their prevailingly massive rocks.
Highly significant in this connection is the fact that the Yosemite Valley lies in an area where a plexus of local intrusions composed mostly of graniodiorite, diorite, and gabbro, all well-jointed rocks, breaks the continuity of the vast bodies of siliceous granite, generally massive in habit, that make up the central part of the batholith of the Sierra Nevada. This plexus did not of itself give rise to the formation of the Yosemite, but it happened to lie in the path of the Merced River, which was superimposed upon it.
The stepwise mode of ascent of the floors of the Yosemite, the Little Yosemite, the upper Merced Canyon, and Tenaya Canyon is a characteristic result of glacial action. However, the edges and risers of the steps are composed invariably of massive rock not susceptible of being quarried. They were therefore not migrant features that receded rapidly headward during the process of glaciation, in the manner implied by certain hypotheses that have been advanced in explanation of the development of glacial stairways. They were essentially fixed features definitely related to the structure of the rock. The canyon steps, accordingly, are conceived to have been produced by selective glacial quarrying. Each tread is essentially a basin quarried out in jointed rock; each edge is essentially a residual obstruction of unquarriable rock, smoothed on the upstream side by abrasion, steepened on the downstream side by the removal of jointed rock. Glacial excavation proceeds with greatest vigor at the head of each tread, because there the ice exerts the greatest force in consequence of its plunge from the step above and accumulates to greatest thickness.
This explains how the steeply rising preglacial floor of the Yosemite Valley was replaced by a nearly level, basined rock floor, and why the depth of excavation is three times as great at the head of the valley as at its lower end. The ice descended into the head of the valley not merely by way of the giant stairway from whose steps the Vernal and Nevada Falls now leap, but during the culminating phases of glaciation it also plunged from the lofty platform at the southwest base of Half Dome in the form of a mighty glacial cataract. The deep, walled-in heads of the Little Yosemite and Tenaya Canyon similarly were excavated mainly by great cataracts of ice.
Structure control also has determined the level of each step. The high level of the Little Yosemite was determined by the height of the body of massive granite that forms the upper step of the giant stairway. The absence of a step at the mouth of Tenaya Canyon, on the other hand, is explained by the fact that glacial excavation there was facilitated by the presence of a belt of fractured rock.
The detailed sculpture of the walls of the Yosemite Valley is likewise a function of the structure of its rocks, the actions of the weathering processes having been sharply controlled by local variations in the jointing. Vertical master joints have determined the profile and orientation of most of the great cliff faces, including the sheer precipices over which the waterfalls leap. Northeasterly and northwesterly master joints account for much of the faceted sculpture. Easterly master joints have controlled the trend of the great precipice of the upper Yosemite Fall and of the famous cliff at Glacier Point. Oblique joint planes dominate the sculpture of the Three Brothers and of many lesser spurs. Prevailingly sparse jointing in the more siliceous rocks explains the predominance of massive rock forms. Narrow zones of intense fracturing, on the other hand, have given rise to deep recesses, even in places where no drainage descends or formerly descended from the uplands. All the notches, gulches, and alcoves in the vicinity of the waterfalls at the mouths of hanging valleys and on the steps of the giant stairway are carved along fracture zones. Only a few have been produced in the manner explained by Branner, by torrents that flowed along the margins of the glaciers.
The domes of the Yosemite region have been evolved from giant monoliths by long-continued exfoliation due to expansion of the granite, presumably in consequence of relief from load by denudation. The irregularities of their curvature still betray to some extent the trend of the master fractures that originally bounded the monoliths. Half Dome is exceptional in that its sheer northwest side has been exposed only recently by glacial plucking and therefore still retains the plane form which it has inherited from a sheeted structure with northeasterly trend. Exfoliation here and in certain other localities is producing essentially plane sheets. On cliffs ground concave by the glaciers, notably on the step above the Vernal Fall, it produces concave shells.
Examination of the debris piles at the bases of the cliffs dispels the belief of some of the earlier observers that 90 per cent of the material was precipitated by a single great postglacial earthquake. There is evidence that in addition to many small rock falls there have occurred several great rock avalanches, and that the intervals between those avalanches were of sufficient length to permit forests to grow up repeatedly on the talus slopes. Earthquake action appears to be indicated most definitely by far-flung hummocky masses of debris that contrast with the sloping taluses and that must have been precipitated from the cliff fronts in their entirety.
The greatest postglacial change in the appearance of the Yosemite region was brought about by the tilling of the glacial-lake basins with stream-borne sediment. The level sandy floors of the Yosemite and the Little Yosemite and the successive treads of Tenaya Canyon all replace glacial lakes. The floor of the Yosemite Valley does not, however, indicate the exact level at which the water of ancient Lake Yosemite stood. It is a flood plain of the Merced River cut about 15 feet below the old lake level, which is indicated by terraces. Mirror Lake is not a remnant of a glacial lake but was impounded by great rock avalanches that fell from the cliffs at the mouth of Tenaya Canyon, presumably as the result of an earthquake, some time after the glacial epoch.
An attempt is made in this volume to set forth these facts and interpretations in language intelligible to the general reader as well as to the scientist. The Yosemite Valley is treated not by itself but in its setting, as an erosional feature of the Sierra Nevada that came into being and was evolved by successive stages in consequence of certain epochal events in the orogenic history and in the glaciation of the range.
In the appendix the nature and significance of the remarkable complex of igneous intrusions into which the Yosemite Valley is hewn are outlined by Frank C. Calkins. A geologic map of the Yosemite region, the upper Merced Basin, and the upper Tuolumne Basin shows the complex in its relations to the vast intrusive masses that occupy the surrounding parts of the Sierra Nevada. The rocks described range from nearly white alaskite to nearly black hornblende gabbro, yet a strong family resemblance is visible in all. Two distinct series of intrusions are recognizedthe biotite granite series of the Yosemite Valley and the Tuolumne intrusive seriesand in addition there are several kinds of rock not definitely assignable to either of these series.
Last Updated: 28-Nov-2006