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Geological Survey Professional Paper 160
Geologic History of the Yosemite Valley |
INTRODUCTION
"The incomparable valley" the Yosemite has been called by those who admire and love it—and rightly so, for where can be found another valley of such distinctive beauty, such impressive grandeur, such captivating charm? Yet hardly less appropriate, from another point of view, would be the title "valley of mystery," for surely no other valley has aroused more curiosity among scientists or laymen or given rise to more speculation and discussion as to the secret of its origin. So extraordinary is the appearance of the Yosemite Valley, with its sheer, monumental cliffs and massive, rounded domes, its lofty, swaying waterfalls and verdant, parklike floor, that it seems in a class by itself, created in some special, unusual way, apart from all other valleys and canyons. No person of intelligence, it is safe to say, ever beheld the Yosemite Valley who did not instantly wonder by what strange process it was formed and through what fortuitous circumstances it became endowed with such surpassing beauty and loveliness.
Not unnaturally the layman's inclination has ever been to appeal to a dramatic, violent cause, to a cataclysm of some kind; for those not versed in the earth sciences find it difficult to believe that so unusual a chasm could have been fashioned by slow everyday processes. Of varied and imaginative conjectures as to the Yosemite's mode of origin, consequently, there has been no lack. Speculation began on the very day, in 1851, when the valley was first entered by white men. Seated around their camp fire on the bank of the Merced, in view of the stupendous cliffs of El Capitan, the rough pioneers composing the historic Mariposa Battalion hazarded the first crude guesses.
Even among scientists there have been more than one to whom the remarkable configuration of the Yosemite Valley has seemed inexplicable save as the result of some paroxysm of nature. It happened, indeed, that the first geologist of note who was in a position to speak authoritatively on the subject, Prof. Josiah D. Whitney, believed in a catastrophic cause, and his belief naturally did much to confirm the prevailing trend of thought. Whitney, as State geologist of California, in 1863 instituted the first scientific exploration of the Sierra Nevada, the great mountain range in which the Yosemite is situated, and as a result of his observations was led to conclude that the valley had been roughly hewn into its present proportions by a dislocation of the earth's crust involving the engulfment of a great block, or group of blocks.1 In his own homely but expressive phrase, he conceived the bottom of the valley to have dropped out, owing to some convulsive movements within the earth, associated, presumably, with the upheaval of the Sierra Nevada. This hypothesis, having been set forth by Whitney not only in his official report but also in his popular Yosemite guidebook,2 which was issued with the approval of the State legislature, became widely accepted as authoritative; and, being simple, plausible, and sensational, it did not fail to make a strong appeal to the popular imagination.
1Whitney, J. D., Geological survey of California; Geology, vol. 1, pp. 421-422, 1865.
2Whitney, J. D., The Yosemite guidebook, pp. 84-87, 1870.
The underlying idea of Whitney's hypothesis was, it should be added in all fairness, not so absurd as some of his opponents have intimated, for there are many well-authenticated instances of valleys that have been created by the subsidence of blocks of the earth's crust. Several such depressions are associated with the Sierra Nevada—notably Owens Valley and the basin occupied by Lake Tahoe. However, Whitney's hypothesis rested on no tangible evidence, and nothing has been found thus far to substantiate it.
Another scientist who believed in a catastrophic origin of the Yosemite was Clarence King, who in the early sixties was one of Whitney's younger assistants on the survey of the Sierra Nevada and who later became chief of the United States Geological Survey of the Fortieth Parallel and, finally, Director of the United States Geological Survey. King 3 conceived the Yosemite to be essentially a "yawning crack" due to the rupturing, or "splitting asunder," of the earth. He was the first to discover the unmistakable evidence of the passage of a glacier through the valley, but though he saw that that glacier had been at least 1,000 feet thick, he ascribed to it only slight influence in the modeling of the valley, for, like the other members of Whitney's staff, he believed glaciers to have inherently small excavating power.
3King, Clarence, Mountaineering in the Sierra Nevada, 4th ed., pp. 7, 134, Boston, 1874.
Not long after the pioneer explorations of Whitney and King; however, it became manifest to scientists that the Yosemite Valley, far from being the product of a violent paroxysm of nature, had been evolved little by little, in the course of long periods of time, through the slow "eroding," literally "gnawing" action of streams and glaciers, seconded by the destructive work of rain, frost, and other atmospheric agents.
Prof. William P. Blake, of the University of Arizona, in 1866, was the first to express the belief that the Yosemite like most other valleys and canyons on this earth, is a feature wrought by erosional processes. He regarded it primarily as a stream-worn canyon in whose final shaping a glacier had played a part, but, like other geologists of that time, he was inclined to attribute the remodeling of the chasm to torrents flowing under the glacier rather than to the glacier itself.4 Unfortunately he presented this tentative interpretation, as well as the remarkably accurate analysis which he made on a later visit,5 to scientific audiences alone, and thus the general public has remained unacquainted with them.
4Blake, W. P., Sur l'action des auciens glaciers dans la Sierra Nevada de Californie, et sur l'origine de la vallée de Yo-semite: Compt. Rend., vol. 65, pp. 179-181, 1867.
5Blake, W. P., Glacial erosion and the origin of the Yosemite Valley: Am. Inst. Min. Eng. Trans., vol. 29, pp. 823-835, 1900.
It was John Muir,6 the keen student and ardent lover of nature, who first saw clearly that the glaciers themselves had done most of the excavating. Although not a geologist by profession, he was quick to perceive the manifold evidences of powerful glacial action in and about the Yosemite. Fascinated by the story which he read in them, he devoted most of his spare time during his sojourn in the valley in 1869 and the years following to tracing the pathways of the ancient glaciers, and thus the conviction grew upon him that the Yosemite and indeed all the great canyons of the Sierra Nevada are primarily channels gouged out by ice streams. Later, through his charming writings, he disseminated his ideas of glaciation far and wide, thereby uprooting in the minds of many people the primitive belief in a catastrophic origin of the valley.
6For a partial list of Muir's writings on the Yosemite Valley, see bibliography, pp. 130-131. A complete bibliography of Muir's writings was published in Sierra. Club Bull., vol. 10, pp. 41-54, 1916.
Joseph Le Conte,7 of the University of California, upon visiting the Yosemite in 1871, was deeply impressed by Muir's interpretations. He also recognized the valley to be an ancient glacier channel, but he differed from Muir in that he attributed a considerable share of the excavation to stream action prior to the coming of the glacier.
7Le Conte, Joseph, A journal of ramblings through the high Sierra of California by the University excursion party, San Francisco, Calif., 1875 (republished as Ramblings through the nigh Sierra: Sierra Club Bull., vol. 3, pp. 1-107, 1900).
Of the controversy that arose between Muir and Whitney little need here be said. Whitney tenaciously clung to his "dropped-block hypothesis" and even denied outright that the Yosemite had ever been invaded by the ice,8 although he had previously published the indubitable evidence, reported by Clarence King, of the former presence of a glacier in the valley.9 Muir, on his part, went too far in his claims for glacial erosion. Dominated by the belief that the Sierra Nevada had been mantled with ice "from summit to base" and to a depth sufficient to bury practically all its features,10 he maintained that the Yosemite and, indeed, all the great canyons of the range, thousands of feet in depth, had been gouged out entirely by the glaciers. These canyons, he asserted, owe not only their general forms but their very "trends and geographical positions" to glacial action.11 The average depth to which the Sierra had been stripped of rock by the ice he estimated at considerably over a mile.12
8Whitney, J. D., The Yosemite guidebook, pp; 83-84, 1870.
9Whitney, J. D., Geological survey of California; Geology, vol. 1, p. 422, 1865.
10Muir, John, Studies in the Sierra—IV, Glacial denudation: Sierra Club Bull., vol. 10, p. 316, 1918 (reprinted from Overland Monthly, vol. 13, pp. 174-184, 1874).
11Idem, p. 308.
12Idem, p. 318.
To many persons, naturally, these statements seemed extravagant. Geologists, especially, were slow to accept them, because of the evident lack of substantiating proof. It is to be borne in mind, also, that at the time when Muir first set forth his views of wholesale glaciation the ice age was still a new and rather vague concept. Little was known of the magnitude which the glaciers and ice sheets have attained in different parts of the earth, nor of the extent to which they have changed the configuration of the lands. The ability of glaciers to excavate deep canyons in hard rock was still regarded as a matter to be demonstrated.
How widely the best informed men of Muir's time differed in their estimates of the excavational work that has been accomplished by the ice in the Yosemite Valley is illustrated by the fact that Prof. Israel C. Russell,13 of the University of Michigan, in 1889, after having studied the extensive débris ridges, or moraines, which the glaciers descending from the east flank of the Sierra Nevada have built at the mouths of their canyons, declared that in his opinion the absence of similar bulky moraines at the foot of the Yosemite Valley argued strongly against the supposition that that valley had been excavated mainly by glaciers. Though himself a noted student of glaciers, Russell therefore reverted to Whitney's hypothesis for an explanation.
13Russell, I. C. Quaternary history of Mono Valley, Calif.: U. S. Geol. Survey Eighth Ann. Rept., pt. 1, p. 350, 1849.
Again, Henry W. Turner, of the United States Geological Survey, upon extending his investigations in the Sierra Nevada southward to the Yosemite Valley in 1899, found it so similar in many respects to certain stream-worn canyons observed by him that he felt convinced that its features could be explained as products largely of stream erosion and weathering processes, facilitated by the jointed structure of the granite. The glaciers, in his opinion, had done little more than clear the valley of loose débris.14 Turner made the first attempt to determine the farthest limits reached by the Yosemite Glacier and tentatively located these limits in the vicinity of El Portal, only 9 miles below the valley. The relatively modest dimensions of the ice stream thus indicated, together with the small amount of débris it had left behind in the form of moraines, strengthened his conviction that it had had but slight erosive power. On the other hand, he rightly insisted that the upheaval of the Sierra Nevada in preglacial time must have greatly accelerated the flow of the Merced River and caused that stream to intrench itself deeply.
14Turner, H. W., The Pleistocene geology of the south-central Sierra Nevada, with especial reference to the origin of Yosemite Valley: California Acad. Sci. Proc., 3d ser., vol. 1, pp. 319-350, 1990.
Turner's interpretation of the Yosemite as primarily a stream-cut canyon was, of course, challenged at once by the apostles of glacial erosion, notably by Henry Gannett, then chief geographer of the United States Geological Survey. Gannett, as a result of his studies on Lake Chelan, in the Cascade Range, had come to regard "hanging" side valleys as characteristic accessory features of deeply glaciated canyons, and contended that the height of such valleys affords a rough measure of the depth of glacial excavation in the main canyon. The Yosemite, he pointed out, has hanging side valleys of great height (the upland valleys from whose mouths the waterfalls pour into the chasm), and he therefore pronounced it to be "quite an ordinary and necessary product of glacial erosion."15
15Gannett, Henry, Origin of Yosemite Valley: Nat. Geog. Mag., vol. 12, pp. 86-87, 1901.
Prof. John C. Branner, of Leland Stanford Junior University, on the other hand, stood by Turner, and on the strength of his cursory observations concluded that in the Yosemite Valley "the wearing done by the ice was trivial as compared with the wearing done by the glacial streams."16
16Branner, J. C., A topographic feature of the hanging valleys of the Yosemite, Jour. Geology, vol. 11, p. 551, 1903.
Several other scientists have since advanced tentative hypotheses in explanation of the Yosemite's origin, each based, however, on only a brief examination. All of them, significantly, assign a large share of the excavational work to glacial action, but they differ considerably in their estimates of that share. E. C. Andrews, of the Department of Mines, New South Wales, interpreted the Yosemite as being essentially a gorge created by the powerful erosive action of a plunging ice cascade, or ice fall, in the Yosemite Glacier, that receded gradually headward, just as the Niagara Gorge has been created by the erosive action of the slowly receding Niagara Falls. The cross cliff over which the Yosemite Glacier plunged he estimated to have been "at least 2,000 feet in height."17
17Andrews, E. C., An excursion to the Yosemite, California, or studies in the formation of alpine cirques, "steps," and valley "treads": Roy. Soc. New South Wales Jour, and Proc., vol. 44, p. 312, 1900.
Prof. Douglas W. Johnson, of Columbia University, following Gannett's idea out to its logical conclusion, undertook to deduce the depth of glacial excavation in the Yosemite Chasm from the heights and gradients of its hanging side valleys. Reconciling as well as he could the discordant results which he obtained by this method, he arrived at a measure of "2,200 to 2,500 feet at least."18
18Johnson, D. W., Hanging valleys of the Yosemite: Am. Geog. Soc. Bull., vol. 43, p. 831, 1911.
Prof. Andrew C. Lawson, of the University of California, believed the Yosemite to have been elaborated by glaciation from "a profound erosional gorge."19 From the insight into the preglacial history of canyon cutting in the Sierra Nevada previously gained through his studies in the Kern River Basin, it was manifest to him that the Yosemite, like the other great canyons of the Sierra, must have had great depth prior to the arrival of the glaciers. He believed it to have been further deepened and also widened by glacial action, but the work done by the ice, in his estimation, was "probably not great," as the morainal débris deposited below the Yosemite is of scant volume.
19Lawson, A. C., Geology of Yosemite National Park, in Handbook of Yosemite National Park, compiled by Ansel E. Hall, p. 120, 1921.
The wide discordance in these different estimates of the amount of excavational work accomplished by the glaciers and the preglacial streams, respectively, was due chiefly to the lack of reliable information as to the extent and magnitude of the ancient Yosemite Glacier and its tributaries, to the haziness of ideas that prevailed regarding the preglacial history of the Yosemite region, and to fundamental differences in conception as to the eroding capacity of glaciers and the precise manner in which glaciers do their erosional work. It Will be readily understood, therefore, why in 1913 the United States Geological Survey at once responded to the popular demand, voiced by the Sierra Club, for further and more definite information regarding the geologic history of the Yosemite Valley and instituted a systematic and intensive investigation covering the entire Yosemite region and the neighboring parts of the High Sierra. There was additional warrant for such an investigation in the fact that since the end of the nineteenth century there had come to maturity a new branch of geologic science—geomorphology, or physiography, as it is also termed—which deals specifically with the origin and development of the surface features of the earth and within whose scope a problem such as that of the Yosemite Valley largely belongs. Furthermore, a new detailed topographic map of the valley that afforded an excellent base for the proposed studies had recently been completed.
The privilege of carrying out the new investigations fell to Frank C. Calkins and the author of this paper. Mr. Calkins's task was to study the characteristics, significance, and relationships of the different kinds of rock that occur in the Yosemite region, and the writer was charged with deciphering its glacial history and studying the evolution of its sculptural features. For several years the two investigators carried on their work conjointly throughout the Yosemite region and the adjoining areas in the High Sierra. Then each made supplementary reconnaissances, Mr. Calkins carrying a geologic section across the range from the mouth of the Merced Canyon up through the Yosemite region and the writer extending his glacial and geomorphologic studies northward to the regions drained by the Tuolumne and Stanislaus Rivers and southward over the drainage basins of the San Joaquin, Kings, and Kaweah Rivers.
As a result there is now at hand a considerable body of new and definitely correlated data, upon which, with more confidence than has been possible heretofore, the story of the origin and evolution of the Yosemite Valley may be built. In the first place, the farthest limits reached by the ancient Yosemite Glacier and all its tributaries and neighboring ice streams have now been traced out and definitely mapped. Much has been learned also of the sequence of advances and retreats of the ancient glaciers, and some perspective has been gained upon the spans of time involved in those fluctuations. Thus it is now established beyond reasonable doubt that the Yosemite region—indeed, the entire Sierra Nevada—was glaciated at least three times during the ice age, having gone through alternating glacial and interglacial stages, like the central parts of the continent. Further, three important chapters of the Yosemite's preglacial history now stand revealed, and the successive stages in the valley's development recorded in those chapters are definitely linked with successive epochs in the growth of the Sierra Nevada. The geologic age of the earliest of those epochs, moreover, has been tentatively determined from fossil remains found in other parts of the range. Again, the depth to which the Yosemite was excavated at each stage in its history has been ascertained within narrow limits, and so the crucial question as to how much of the work was done by the streams and how much by the glaciers can now be answered fairly definitely. Last of all, the reason for the distinctive form of the Yosemite Valley and for its exceptional wealth of sculptural features has been found in the selective manner in which the glaciers and the other agents of erosion have acted upon the irregularly and unusually structured granitic rocks.
In the pages that follow the purpose has been to set forth the story of the Yosemite Valley in language simple enough to be understood by one having no geologic training, yet in sufficient fullness to leave no doubt in the critical reader's mind as to the foundation of observed facts or as to the processes of reasoning whereby the conclusions are reached. Each geologic concept is briefly explained, and ponderous scientific terms are avoided as far as practicable. Discussions of technical points regarding which there has been diversity of opinion are purposely made brief, in order not to weary the reader nor to rekindle a spirit of controversy.
For the guidance of those who may desire to acquaint themselves with the older hypotheses and compare their merits more fully than has here been attempted, a bibliography of all the scientific literature bearing on the geologic history of the Yosemite region is added at the end of the volume.
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