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Distant Storm Front, Yosemite Valley, California. By Ansel Adams
It seems well nigh unbelievable in these days of enlightenment that so eminent a scientist as the late Professor J. D. Whitney should have seen fit to deny the former existence of glaciers in the Yosemite Valley. Said he in his famous old Guide Book: “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. Nothing more unlike the real work of ice, as exhibited in the Alps, could be found. Besides, there is no reason to suppose, or at least no proof, that glaciers have ever occupied the valley or any portion of it . . .”
As a matter of fact, there are excellent reasons for believing that the Yosemite Valley was once invaded by ice, and the proofs of its glacial occupancy are abundant and indubitable. The wonder is that Whitney could have overlooked them. The very shape of the valley, trough-like, steep-sided, clean-cut; the great height of the hanging valleys from whose lips the thundering waterfalls pour; the giant stairway down which the Merced River tumbles in its descent from the Little Yosemite; these features are, on the face of them characteristically glacial, and impressively attest the great magnitude of the erosional work done by the ice. But perhaps the skeptical reader would prefer evidences of a more tangible sort, more immediately linkable with the intimate form and habits of glaciers, and demanding less from the imagination in the way of appraisal of the capacity of glaciers to erode, a subject on which even those best qualified to judge are by no means united.
Allow me to invite him to the floor of the Yosemite Valley, and, with our backs turned to the lofty hanging valleys and their eloquent cataracts, let us search for the less spectacular but more direct, and perhaps more convincing, proofs of ice work which there exist.
If we should set up a surveyor’s level in the meadows opposite the Sentinel Hotel and thence run down the valley, taking careful elevations on the way, we would find the altitude to remain essentially unchanged for miles. Indeed, as far as the El Capitan bridge there is no appreciable fall to the valley floor, and the Merced River meanders dreamily, in lazily swinging, sandy loops and curves. At the El Capitan bridge, however, there is an abrupt change. The stream awakens, as if refreshed from its nap in the valley, and with quickened pace, dashes over riffles and churns among boulders, tumbling lustily like a youthful mountain torrent. Its fall becomes rapid, fifty to one hundred feet per mile, whereas above the bridge, in a distance of six miles, it descends only about six feet.
Evidently, the El Capitan bridge marks a critical point in the course of the river and a dividing line in the valley itself. Broad and level above the bridge, below it the valley can scarcely be said to have any floor at all. Even the Bridalveil Meadows, which occupy the widest place, slant strongly toward the river, being a debris fan built by Bridalveil Creek. Farther down, the valley sides close in from either side and the river lies constrained in the bottom of a narrow V.
What may be the cause of this abrupt change of scene at the El Capitan bridge? No doubt many of the readers of the Bulletin have passed back and forth over that bridge, but probably few have taken careful notice of its peculiar location. The writer himself did not become aware of the significance of the site until after a sojourn of several months.
It was no mere whim that led Galen Clark to select that spot for a bridge. A strong ridge of boulders here lies athwart the floor of the valley, and it is across the gap in that ridge, worn through by the stream, that the bridge has been thrown.
South of the El Capitan bridge the grading of the wagon road has necessitated cutting away part of the ridge, but the huge boulders, of which it is largely composed, may be seen in the side of the cut. Climbing out of the road, one may follow the curving crest for a few hundred feet until it becomes lost in the coarse debris at the base of the Cathedral Rocks.
North of the river, the ridge runs west of the road, stretching across the valley for half a mile like a steep-sided, narrow-crested embankment. At first fully fifteen feet high, it gradually loses in height and prominence, and finally, toward the road forks, appears to die out altogether. However, it does not end here, but merely becomes buried under the toe of the huge debris slopes descending from the cliffs about Ribbon Fall.
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Sketch-map of Yosemite Valley. showing the extent of ancient Lake Yosemite
Were this peculiar ridge, unique in the configuration of the valley floor, situated in the open so that its form stood out conspicuously above the surrounding flat, no doubt from the first it would have attracted attention; its significance would have been looked into and now would be common knowledge. As it is, dense thickets of pine and cedar effectually mask the ridge; most passers-by are not aware of its existence, and even some of the scientists who have studied the valley in detail have missed the feature and thereby the key to the recent geological history of the entire valley floor.
The boulder ridge in question is a typical glacial moraine; no experienced glacialist would for a moment hesitate in identifying it as such. It is a terminal moraine, properly speaking,—that is, a debris ridge of the sort which glaciers commonly build up at their fronts. All glaciers, as is well known, carry a considerable amount of rock debris derived from the floor and sides of the valleys through which they advance, and this material, as the ice melts away, is released at the lower end. While the front of a glacier is inherently subject to frequent oscillations, some years melting back, at other times advancing, there are nevertheless occasional periods of relative constancy during which the front remains stationary, or very nearly so. It is then that this ice-freed debris accumulates in the form of an embankment or morainic ridge, as it is technically termed. When, moreover, the period of quiescence follows immediately upon one of advance and pronounced erosional activity, during which the glacier heavily loaded itself with debris, the moraine is likely to assume proportions that will enable it to endure as a topographic feature of some permanence.
This, in fact, is what occurred in the Yosemite Valley. When the ice front receded for the last time—there were several separate glacial epochs—it made a number of minor readvances, following one upon the other like so many gradually dying pulsations. Each of these readvances left a separate moraine, and accordingly a number of such ridges are found spaced at intervals across the valley floor. All of them are situated in the lower half of the valley, and the moraine at the El Capitan bridge, which may appropriately be called the El Capitan moraine, is the uppermost, the youngest of the series.
It is also the strongest, the most perfectly preserved of all. The other moraines today are represented only by truncated fragments, their major portions having been broken down and swept away by the swollen river. Around the broken end of one of these ridges, projecting from the extreme northwest corner of the Cathedral Rocks, the wagon road swings as it bends southward to the Bridalveil Fall.
The El Capitan moraine, it appears, not only escaped the partial demolishment that overtook its brethren, but, by virtue of its strength and peculiar situation, became a factor of importance in the post-glacial remodeling of the valley bottom. Stretching across the valley from wall to wall, like an unbroken dam, it ponded the waters behind it, and, as the ice melted back, transformed the upper Yosemite Valley into a lake.
This sheet of water—Lake Yosemite, it may aptly be called—like most lakes of a similar origin, was not destined to endure. No sooner had it come into existence than the Merced River, turbid with debris from the glaciers farther up, proceeded to build a delta at the upper end, and this delta, slowly but inexorably advancing, in time wholly extinguished the lake.
The manner in which the filling was accomplished one may today watch in Mirror Lake. Already reduced from a sheet of water more than a mile long, this little lake, famous for its reflections, is annually being diminished in area by an appreciable amount through the rapid forward growth of the delta of Tenaya Creek. Measurements of the delta front for a few consecutive years would afford a basis for an estimate of the length of time that the lake is likely to continue to delight the visitor with its beautiful reflections.
Although nothing now remains of ancient Lake Yosemite, its extent, nevertheless, is still easily ascertained. One need but follow the edge of the level meadows that now form the valley floor, in order to trace the former shore line. Evidently the lake occupied the entire extent of the valley, up to the cliffs that enclose its head; its length, therefore, must have been close to six miles.
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Nor was it a mere shallow pool. Its depth, there are reasons for believing, may have exceeded 500 feet. No actual measurements, such as might be obtained by borings, for instance, are available, it is true, and the figure mentioned cannot claim to be any more than a mere estimate. Yet it is not wholly without foundation, as a glance at the accompanying diagram has shown. That diagram represents one of a number of cross-sections of the valley, constructed by the writer with the accurate and abundant trigonometric data on which the detail map of the Yosemite Valley is based. Being free from vertical exaggeration, it affords a fair means for judging the probable depth of sediment now filling the valley. It is reasonable to assume that the Yosemite Valley, having been vigorously glaciated, possesses a somewhat concave rock floor, shaped like the bottom part of a U. Completing, tentatively, the missing part of the curve, therefore, one obtains an approximate measure of the depth of the extinct lake. In the cross-section published herewith, the curve has purposely been drawn quite flat, in order that the estimate of depth may not be accused of undue liberality. Yet, the depth indicated by the diagram is not far from 500 feet. Other cross-sections give closely accordant figures, those toward the head of the valley indicating still greater depths.
Is it to be inferred also, the question may here be asked, that the El Capitan moraine has a height of 500 feet? No, that ridge, in all probability, does not stand a hundred feet high above its base. A direct measurement of its height, unfortunately, cannot be had. The river has not yet cut the notch down to bed rock. At least so Galen Clark informed the writer. While still in charge of the valley, he had undertaken to enlarge the notch in order to lessen the danger from floods during the spring freshets. He had found only loose boulders, which he had removed with the aid of dynamite.
On its up-stream side the ridge is buried under lake deposits, and only the upper fifteen feet emerge. On its down-stream side it slopes down twenty-five to thirty feet, but there, too, its foot is covered by river gravels of unknown depth. Examination of other moraines in the Yosemite region, more especially those of the later ice invasions, to which the El Capitan moraine itself belongs, seems to indicate, however, that a height of one hundred feet is the maximum assignable. The majority of these ridges scarcely exceed fifty or sixty feet in height. Five-hundred-foot moraines are foreign to the region.
If the El Capitan moraine is not over one hundred feet high, how, then, shall we account for the great depth of Lake Yosemite, as indicated by the diagram? The answer is, by assuming the existence of a deep basin eroded in the rock floor of the valley by the ice. There is nothing violent in that assumption. Glaciers normally excavate extensive rock basins in the bottom of their valleys. The well-attested instances of such action are literally numberless. Lake basins are a familiar feature of all glaciated mountain regions, and in some cases—such as that of Lake Chelan—they occur on a truly stupendous scale, dwarfing Lake Yosemite into insignificance.1
Nor need one go outside the Yosemite region for examples. There is evidence of a lake basin on every tread of the stairwise descending branch canyons. The stair-like character of the floors of these canyons, it may be pointed out in passing, is a distinctly glacial trait, and the presence of lake basins hollowed out in the treads is only one of the concomitant features.
Thus the entire Little Yosemite Valley was once occupied by a lake. Filled with river gravels, like the main valley itself, it now presents the appearance of a gradeless flat of some three miles, above which only the crests of several curving terminal moraines emerge.
On the tread immediately above the Vernal Fall, again, is Emerald Pool, diminutive, yet as typical a glacial rock basin as one can find anywhere. Tenaya Canyon, it appears, once possessed four glacial lakes, situated at successively higher levels. All but the lowest, however, are now filled with sediment; Mirror Lake alone survives as a remnant of the largest lake.
After one has become familiar with all these lake basins in the branch canyons of the Yosemite Valley, and one has, moreover, gained an insight into their mode of origin, one can scarcely avoid reaching the conclusion that in the main valley, too, there is a deeply eroded rock basin, now covered by the silts of Lake Yosemite. The combined mass of the Tenaya and Merced glaciers here must have eroded with particular vigor. The very fact that each of these ice streams, by itself, was able to excavate rock basins of considerable extent and depth, leaves little doubt that united they achieved still larger erosional results. Besides, it has been noted that it is immediately below the confluence of glaciers that the ice usually attains the greatest power to excavate.
The El Capitan moraine, then, is not to be given sole credit for the creation of Lake Yosemite. That lake in all probability lay in a rock basin eroded by the ice, and the only function of the moraine dam was to raise the level of the waters, thus increasing their depth and extent.
In the meanwhile it should not be forgotten that the existence of the rock basin is purely inferential and is to be considered unproven until a series of borings along the whole length of the valley shall afford the necessary facts. 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.
That the Yosemite Valley has actually been occupied by glacial ice no one will venture to dispute; were all other ice signs in the valley rejected as untrustworthy, the El Capitan moraine alone would afford evidence sufficient and irrefutable. As to the extent to which the ancient glaciers have remodeled and excavated the valley, nothing, perhaps, would go further toward settling this vexed question than a series of direct measurements establishing beyond doubt the depth of former Lake Yosemite.2
Reprinted from Sierra Club Bulletin, January, 1913, pages 7-15.
1The writer’s attention has been called to what appears to be rock-in-place visible in the bed of the Merced in the upper part of the valley. The supposed great depth of sedimentary filling in the valley would thereby seem to be discredited. A visit to the spot in the fall of 1910, however, enabled the writer to satisfy himself that the outcrop of rock reported is in reality only an indurated bed of coarse river sand, irregularly gullied out by the current, and closely resembling solid granite. It is friable in the hand and is underlain by unconsolidated layers of sand and silt.
2The borings which Matthes hoped might eventually give conclusive answers to questions concerning the depth of Lake Yosemite, the fill of sediments in Yosemite Valley, and the depth and configuration of the bedrock floor beneath, have yet to be made. However, from another quarter, undreamed of in 1913 when Matthes published this essay, have come significant data relating to these questions, namely seismic soundings at many points on the valley floor. These soundings were made in 1935 and 1937, and the resultant technical report (“Seismic Explorations on the Floor of Yosemite Valley, California,” by Beno Gutenberg, John P. Buwalda, and Robert P. Sharp. Bulletin of the Geological Society of America, Volume 67 [August, 1956], pages 1051–1078) may be consulted by readers interested in the detailed interpretations drawn from the seismic data. The introduction says:
“For fully 50 years earth scientists have differed strongly as to the efficacy of glacial erosion. One discussion of considerable warmth centered on Yosemite Valley, which Muir stoutly maintained was largely the product of glacial sculpturing and which Whitney contended was a product of tectonic subsidence. Other workers thought that erosion by running water and moving ice had done the job, but it remained for Matthes to show that these two agents played essentially equal parts in creating Yosemite Valley. Matthes’ work is an outstanding attempt at a quantitative estimate of the magnitude of glacier excavation based on geological evidence.
“The seismograph shows that even Matthes, an enthusiastic glacial sculpturist, underestimated by a full order of magnitude the amount of glacial excavation on the bedrock floor of Yosemite Valley. Where he estimated 450 m of deepening by glaciers, at least another 550 m must be added. Where Matthes estimated the unconsolidated valley fill to be 90 m it is closer to 600 m. Thus, the steep granitic walls rising 900 m above the present valley bottom are more than 1500 m above the bedrock floor. The visitor sees but three-fifths the splendor and magnitude. . . .
“Yosemite Valley is thus an outstanding example of the efficacy of glacial excavation in over-deepening valleys and in creating large closed depressions. It is comparable in this respect to Lake Chelan, the Great Lakes, the Finger Lakes, and many fiords.” (References omitted from quotation.)
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