Popular Science Monthly/Volume 85/July 1914/Genesis and Revelations of the Yosemite Valley
|GENESIS AND REVELATIONS OF THE YOSEMITE VALLEY|
By HAROLD FRENCH
THE marvelous structure and sculpture of the Yosemite Valley kindles the imagination of every visitor to this great natural wonder and causes scientist and layman alike to evince the most intense interest in the origin of this mile-deep trough amid the granite waves of the High Sierra. Each pilgrim to this Mountain-Mecca endeavors to satisfy his mind as to the causes of its carving. Every one who wanders there wonders and guesses at its genesis; but the revelations of its geomorphogeny are far from being satisfactory or complete. Scientists have come and savants have gone, but few have agreed in their conclusions. The scenic grandeur of the Yosemite has sunk deep in the souls of poets and painters, artists and literateurs; and to the most practical of men, engineers and miners, do its unique features equally appeal. Awe-inspiring evidences of colossal dynamic agencies, such as the undermining and subsidence of vast areas, or the tremendous upheaval of sky-piercing peaks and ridges, the quarrying and ground sluicing of Brobdignagian blocks of granite, are all of deep significance and extreme interest to the mind of the miner. Over all this weird wonderland broods the spell of an enigmatic Sphinx. To this day, the Yosemite is, of a verity, the Valley of Mystery.
General Geological Features
Before discussing the conflicting theories conjectured about the origin of the Yosemite Valley, it will be proper to present the salient features of its surroundings. Trite, but essential to clearer understanding, is the statement that the valley is approximately eight miles long and nearly a mile in extreme width and depth. Its floor averages 3,960 feet above sea level. At first glance, it will impress the miner as being a great open cut quarried through blocks of more or less resistant granite. And to many it will appear to be a great basin, the bottom of which had sunk to unfathomed depths. Whatever forces may have quarried this great open cut—if open cut it may be truly called—the accumulation of the tailings down stream from this titanic denudation is conspicuous by its absence. Therefore, the secret of the transportation of these billions of tons of tailings is one of the mysteries of the geological history of the Yosemite yet to be unfolded.Looking up the valley from its lower portal, two striking differences in the structure of its walls are seen in bold contrast to each other.
El Capitan, 3,100 Ft. Sheer. Most striking illustration of massive, resistant granite.
As massive monoliths of smooth, sand-papered granite, appear El Capitan, Sentinel Rock and Dome, Glacier Point and the Half and North Domes. These burnished, jutting promontories attract far more attention than do the hollowed-out recesses of the tributary canyons. And yet, it is more among the shadows of these deep-furrowed clefts that the geologist must look for the most convincing evidences of the actual sculpturing of the Yosemite Valley. The domes stand out more as mighty monuments signalizing their resistance to erosion, rather than as facetted and modeled masses of rock. But between these promontories are zones of fissured strata showing the shearing and sawing of corrasive forces. The pinnacled Cathedral Spires and Rocks, the wrinkles of the Three Graces and the creases of Three Brothers most distinctly illustrate the differentiation of zones of more friable, granitic materials. Beneath one's feet is the remarkable floor of the valley, whose gradient is but little more than a foot of fall to the mile. Far above the rim of the main gorge of the Merced are dozens of "hanging valleys" cut off abruptly by the transverse trend of the precipitous walls of the Yosemite Basin. Above and beyond rise an ascending series of polished domes and U-shaped troughs culminating in the serrated crest of the High Sierra. Its eastern slope affords a most striking contrast to the gentler gradient towards the Pacific. The sunrise-fronting spurs of the Sierra plunge abruptly at a high angle down to the Mono plain seven thousand feet below. Alternating with steep escarpments, are deep-carved canons descending giant staircases, whose hollowed treads are frequently filled with azure lakelets. Beyond, over the drab desert, arise an array of dead "fire-mountains," recording an important chapter in the history of the High Sierra.
According to Professor Joseph Le Conte, this mighty range was born out of the ocean during the Jurassic period, the strata bulging, mashing and crumpling as it yielded to horizontal pressure. Its first physical appearance in the poetic diction of John Muir was as "one vast wave of stone in which a thousand mountains, domes, canons and ridges lay concealed." Geologists agree that the original crest of the Snowy Range was in the vicinity of the Yosemite Valley, but, at the end of the Tertiary and the beginning of the Quaternary periods, the Sierra block was tilted upward by volcanic upheavals which burst forth all along its eastern border. The Sierras were pitched en masse in a steep slope toward the west, while a great fault system produced the precipitous escarpment towering above the desert. Consequently, the crest of the range was transferred to its eastern rim. Throughout the Quaternary period, a newer system of rivers, accelerated by the increased inclination of their watersheds, cut their beds deeper and deeper along the lines of least resistance. Then followed an "over-deepening" of these stream courses by corrasive forces of far greater potentiality than the agency of running water.
out." The commonplace processes of work-a-day life psychologically bore their imprint in their deductions. Miners could only see gigantic excavations. The metallurgist would detect an apparent analogy in the swelling of these domes to the bubbling and solidification of molten metal. Engineers were most impressed by the evidences of stresses, strains and the rupture of weaker materials.
One of the most ingenious interpretations of the subsidence theory was advanced most ingenuously by one of the gentler sex. To newcomers in the Yosemite I was explaining the "cataclysmic hypothesis," still popular with those who prefer the more spectacular speculations. Suddenly, she exclaimed: "Did you ever bake a cake?" I answered proudly in the affirmative. "Well, did you ever spoil a cake?" Again, but with less pride, I admitted that such had been my experience. "Then you will know," she continued, "how it is that heat and the gas formed by baking-powder make the batter rise. Up it swells. Then a crust forms. If all goes well, the cake becomes crisp and compact. But, if some one jars it, while it is rising, down it drops in the middle. And so I guess the Yosemite Valley must have dropped in just such a way." This homely homology from an expert in culinary science was unanswerable. Her mind was made up. Even the most uncompromising glacialist could only have caused her to concede that the ice sheet was but a frosting spread over the surface of her hypothetical cake.
During the summers of 1863-4, Professor Josiah Dwight Whitney, chief of the California Geological Survey, conducted a most thorough reconnoissance of the Yosemite and High Sierra region. In his first report, published in 1865, he advanced his theory that the Yosemite Valley had been formed by the subsidence of a limited area during the processes of upheaval of the Sierra. This supposed sinking of its floor he attributed to the fracturing of its strata in a series of cross fractures and faults traversing each other, generally at right angles. The pioneer geologist declared that "this great cataclysm may have taken place at a time when the granitic mass was in a semiplastic condition below, although, quite consolidated at the surface and for some distance down." But Professor Whitney must not be misunderstood as maintaining that the gorge of the Yosemite is one gigantic fissure. Later, in 1870, in his celebrated "Yosemite Guide-book," he demonstrates that "the valley is too wide to have been formed by a fissure." Had such been the case its opposite walls would have corresponded in most details, instead of differing as notably as they do. Rather did he regard its fracturing as having been the resultant of a chaotic complication of dislocations. Believing that its supports had been withdrawn during the convulsive movements of the plastic strata, he assumed in his homely phrase that "the bottom of the Yosemite dropped out." When pressed for further evidence to support his theory, he could show no conclusive, concrete indications of such complex Assuring and faulting, but reasoned in rebuttal, that pressure from the mile high rock-masses above had so united yielding materials, subsequently transported by erosion, that all the traces of fracture had been lost.
Professor Whitney was most positive in his idea that denudation had but little affected the formation of the walls of the valley or its domes. Had there been a considerable amount of erosion, its evidence would have been piled up in masses of debris along the course of the Merced River. "No ordinary denudation moulded the domes," he declared. Instead, he insisted that the Sierran domes of granite were fashioned in a series of concentric layers while the igneous rocks were cooling.
Associated with Professor Whitney was young Clarence King, who later won fame as the chief of the United States Geological Survey. King was the first to trace the courses of glaciers down into the Yosemite Valley. He called attention to the beautifully-polished surfaces gleaming above the floor of the valley and pointed out four distinct moraines, one between Half Dome and Washington Column, a medial moraine between Tenaya Creek and the Merced River, a third, lingering in the gorge of the latter stream above the Happy Isles, while a fourth forms an imposing barrier below the narrows where the Cathedral Rocks approach El Capitan. Clarence King did not claim that ice filled the Yosemite Valley, but he declared its maximum depth to have been about 1,000 feet.
When John Muir first explored the Sierras, some forty-five years ago, he became imbued with the belief that the carving of the Yosemite had been effected almost entirely by ice. His earliest contributions to the literature of science and of the Sierras was a series of papers in which he endeavored to show how a vast sheet of ice, forty or fifty miles in width, cut across the crest of the range, quarrying for thousands of feet down through more friable formations; or enveloping and sweeping over the harder masses of granite, leaving striated and polished domes in the wake of the congealed flood. Muir attached great significance to the "hanging valleys," cut off abruptly by precipices, two and three thousand feet sheer. To this glacialist, their only logical explanation was that a great plow of ice, shod with sharp abrasives, had furrowed the main valley below to untold depth. All the wealth of scenic wonders for which the Yosemite region is so famous, he contended had been chiseled by grinding glaciers. Tracing the tributaries of the San Joaquin, Merced and Tuolumne Rivers to their fountains of perpetual snow, he discovered, during the seventies, no less than sixty-five surviving glaciers still busy at their lapidary labors.Professor Whitney at first credited the discoveries of Clarence King, referring in his report of 1865 to the fact that "King and Gardner obtained ample evidence of the former existence of a glacier in the Yosemite Valley." But, five years later he reversed his decision, declaring,
North Dome of Concentric Granite Formation 3,600 Feet Above Merced River.
that "there is no reason to suppose, or at least no proof, that glaciers have ever occupied the valley or any portion of it. ... A more absurd theory was never advanced than that by which it was sought to ascribe to glaciers the sawing out of these vertical walls and the rounding of the domes." At the same time, Professor Whitney was thoroughly convinced that the Tuolumne watershed to the north had been entirely filled by a gigantic glacier which he agreed had hewn out the Hetch Hetchy Valley, a miniature Yosemite situated twenty miles northwesterly and three hundred feet lower in altitude.
An able ally who came to the aid of Professor Whitney was the noted glacialist, Israel C. Russell. Detailed by the United States Geological Survey to study the eastern slope of the High Sierra and particularly the basin of Mono Lake, he devoted several summers during the early eighties to extended observations of the adjacent region. Although he mapped the ramifications of a vast system of Quaternary glaciers, flowing down from the cirques of the Snowy Range, he adopted in toto the theory first advanced by Professor Whitney that dislocations of the cross-fractured granite, comprising the former bulk of the Yosemite Basin, had caused the orographic block to subside to an unknown depth, forming an appalling abyss.
"Those who seek to account for the formation of the Yosemite," Professor Russell wrote, "by glacial erosion should be required to point out the moraines deposited by the ice streams that are supposed to have done the work." It is evident that he did not seriously consider the four reputed moraines on the floor of the valley which King first discovered. "The glaciers of this region," he continued, "were so recent that all the coarse debris resulting from their action yet remains in the position where it was left when the ice melted." In reply to this reasoning, John Muir insisted that sufficient time had elapsed since the recession of the glacier from the Yosemite to permit the disintegration of most of the detritus and its transportation to the lower levels by the Merced River. And Mr. Muir has strengthened his argument by demonstrating the rapidity with which aqueous and aerial agencies transform loose granite into sand. The extreme depth of the alluvium in the San Joaquin Valley far below is undeniable evidence of the decomposition of thousands of cubic miles of igneous rock, crushed by the mills of the glaciers and conveyed by the ground-sluices of the streams. A general conclusion reached by Professor Russell was, in his words, that
Still, the relative importance to be attached to each of the several agencies at work in this wonderland remains to be estimated accurately by the geological engineer.
Le Conte and Later Investigators
Bridal Veil Falls; and Typical Hanging Valley. The Three Graces in the Background.
he notes, "two very distinct kinds of structure in the granite of this region ... which determine all the forms about this wonderful valley, ... the concentric structure and a rude, columnar structure, or perpendicular cleavage." By these differentiations he traced the sculpture of every ragged spire or rounded dome. Later, he confirmed his observations by demonstrating the effect of dynamic forces upon the separate formations, showing the processes of sculpturing, whether by quarrying along the least resistant lines of perpendicular Assuring, or the exfoliation of concentric layers of onion-like granite. Coming in close association with John Muir, he accepted his theories of glacial erosion. Following the ancient trough of the grand Tuolumne glacier, he traced its tangential branch which overflowed southward and poured its powerful volume down the deep-gashed gorge of Tenaya Creek into the Yosemite Valley. Professor Le Conte estimated that the main Tuolumne glacier ranged in thickness from a thousand feet in the upper meadows of the modern stream to half a mile in the brimming basin of the Hetch Hetchy. Its length he measured as having been fully forty miles. In the upper canon of the main Merced, Le Conte and Muir studied the striations of grinding ice recorded on every outcropping surface, particularly in the "hanging valley" of "Little Yosemite." Together with the Tenaya glacier, the Merced River of ice mingled at the upper end of the Yosemite Valley, proper, to form a grand glacier which Le Conte believed was the most potent factor in the carving of the U-shaped trough of the Yosemite. And, despite the doubts of Whitney and Russell, Le Conte pointed as prima facie proof the remaining medial moraine at the base of Half Dome near the junction of the two reputed rivers of ice.Several Geological Survey parties have been sent to solve the riddle of this Sphinx of the Sierras, but still no explanation of the origin of the Yosemite has won the general recognition of geologists or reconciled their differences of opinion. Mr. Francois E. Matthes, of the Survey, in his recent monograph, "The Origin of the Yosemite and Hetch Hetchy Valleys," approaches the problem along the following unbiased lines of logical observation and deduction. Like Le Conte, he lays great stress upon the phases of erosion in the more fissured zones. He accepts the general theory of the uptilting of the Sierra block during the late Tertiary period by volcanic levers, but gives little credence to the subsidence dictum of Whitney. As an ardent advocate of the glacial theory, he traces the apparent agency of ice in the evidence of over-deepened rock-basins and quarried canons like the steep gorge of Tenaya Creek. This striking instance is a graphic illustration of the excavation of a yielding mass of perpendicular cleavage set between walls of massive and adamantine granite. In brief, he argues that the Yosemite and Hetch Hetchy Valleys have been developed by an early system of rapidly-eroding streams; then "greatly deepened and enlarged by repeated ice invasions which modelled in the rough. The finer fashioned details of diversified sculpturing he attributes to more recent aqueous agencies and fracturing by frost and aerial forces. Far the most convincing evidence of the former glaciation of the Yosemite Valley he finds in the apparent scooping out of the rock-basin of the Yosemite to the depth of approximately 500 feet. This is confirmed, he contends, by the absence of sills of bed-rock, such as are usually seen outcropping among ordinary watercourses. Another important factor in the leveling of the floor of the valley he sees in the signs of several advances and recessions of terminal moraines across the bottom of this basin, in which process they were arranged in local ridges at frequent intervals. The most conspicuous terminal moraine is that which still
remains below the base of El Capitan, and which he believes dammed the downflowing waters and formed a lake six miles in length. Subsequently, sediments filled Lake Yosemite until they made the entrancingly beautiful meadows of to-day. A continuation of this process is now beheld in the outgrowing delta of Tenaya Creek which gradually is encroaching upon Mirror Lake.
Messrs. Matthes and Calkins of the Geological Survey have intimated to the writer that the forthcoming report of their latest reconnoissance will endeavor to prove that the Yosemite Valley was produced almost entirely by various forces of erosion as revealed by their investigation of the influence of corrasive forces upon the fissured zones of granite. They are now looking for such concrete evidence.
Conclusions Based on Concrete Facts
In this summary of the several theories of the genesis of the Yosemite, the general drift of their method of presentation has been to lead to a few conclusions based on concrete facts. But, perhaps, the first and most logical conclusion is that the evidence is still far from being complete. Local geologists, however, have much faith that some new discoveries are about to be made public by the government investigators. The absence of rock sills outcropping along the rim of the valley has been explained as being due to their overburden of detritus." And yet there are reasons for believing that the solid bottom of the Yosemite did actually subside during the period of upheavals and dislocations. The enigmatic formation of the domes suggests the expansion of plastic masses of igneous rocks. If these waves of stone once swelled to such stupendous heights, there must have been intervening troughs. And so, the original sinking away of the valley floor along the lines of cross-fracture still is obvious to many competent observers. The present-day frequency with which great joint-blocks of granite give way in weathering, or from earthquake shocks, and fall to talus still suggest the subsidence hypothesis of Whitney.
A compromise, composite theory may now be advanced. Consideration of all the evidences of the structure and sculpture of the Yosemite region will justify the premise that during the upheaval of the molten granitic magma, domes swelled in rounded masses to much their present form, cooling in concentric layers, which were later exfoliated by aqueous and glacial denudation and aerial forces. Between these domes, the trough of the stone wave, faulted and cross-fractured along cleavage planes, formed the original floor of the Yosemite Valley. Subsequently, the tilting of the Sierra range caused the carving of the usual form of V-shaped valleys. Then, during the Quaternary period, down plowed the glaciers scooping out the bed-rock, sapping and scoring the walls of the valley, and leaving their moraines as mementoes to remind mankind of their ancient estate and power. But these moraines are either so wasted or considerable that much doubt is still entertained as to their extent. If erosive forces carved out the mile deep Yosemite, the question of the transportation of the detritus is still to be raised. The comparatively lightly-covered bed of the Merced River below the valley does not indicate the accumulation of any immense amount of detritus. Only by extremely fine comminution of boulders to silt could this immense mass be almost completely carried away. There is every evidence that a glacier reached the floor of the Yosemite Valley, but the extent of its quarrying is still unknown. Whether by direct grooving, or by undermining the overhanging escarpments, it unmistakably transformed the canyon's V into a broad,
U-shaped valley. As the glacier receded, torrential streams cut into the glacial moraines, changing the vicinity of the upper reaches of the Merced into more angular surfaces, with the ultimate purpose of eliminating U and substituting V.
After all the theories have been advanced, their truth or fallacy will remain a matter of conjecture until the underlying facts are brought to light. Buried far below the flowery meadows and boulder overburden lies the unseen, pristine bed rock. Graven on tablets of stone in indelible characters is the key to the cryptic cipher of this Sierran Sphinx. The mining geologist is more practical than academic. He would prospect the floor of the valley to unearth that hidden key. Therefore, he would advise the sinking of shafts down through the sediments, ascertaining their character and striking the sunken sill. Following the rim of the latter for a short distance, the miner-geologist would soon determine in finality the genesis of the Yosemite Valley. If this deep basin was once shattered by a cataclysm, causing the falling away of an almost bottomless abyss, the zones of its marginal fracturing could be definitely discovered. Or, if overdeepening of the bed rock by the abrasive-shod plow of a glacier be the true cause of its carving, the striated strata and the morainal drift will tell the tale. Stream-erosion will also be revealed to its exact extent. Therefore, it would seem that no appropriation for the researches of the Geological Survey in this region can be expended to better advantage than by prospecting in miner-like fashion for the hidden truth of the making of this great natural wonder. That this practical idea meets with the approval of the Geological Survey party is shown by a recent expression by Mr. Francois E. Matthes in the Sierra Club Bulletin. "It is to be hoped that some day such borings may be undertaken; they would not merely serve to solve a problem of great local interest, but would contribute much-desired data regarding the still-challenged eroding efficiency of glaciers."
The Yosemite at the Exposition
One of the most interesting features of the federal government's participation at the Panama-Pacific International Exposition will be the presentation of a large relief model of the Yosemite National Park. Cartographers of the United States Geological Survey are putting the finishing touches on this miniature of the Yosemite Valley and its surroundings. They will show its stupendous steeps and glaciated basins as vividly as though viewed from an aeroplane. Every visitor to the Exposition who may be interested in geological science will find this replica replete with fascinating details carefully set forth.
Buffalonians laid special stress upon the nearness of Niagara Falls to the Pan-American Exposition. Likewise, Californians take great pride in their unique wonderland, the Yosemite National Park. It can now be reached by train and auto stage in a dozen hours from the Exposition City. The Department of the Interior is spending large sums in putting this great public playground in order for the enjoyment of probably 100,000 visitors who will view its glories in 1915. Few there are who have not dreamed of making a pilgrimage to Nature's most awe-inspiring shrine at least once in their lives. Tourists to the Exposition at San Francisco will have the double opportunity of seeing all of its varied attractions and at the same time include the Yosemite in their itineraries. With notably reduced rates to the Pacific Coast throughout 1915, they will have the chance of a life time to see the Yosemite and other scenic wonders of the west under the best possible traveling and tarrying conditions. The riddle of this Sphinx of the Sierras is still far from being solved; therefore, geologists will find an unlimited field for special investigations and study following in the footsteps of John Muir and Joseph Le Conte.