GLACIAL HISTORY OF THE YOSEMITE VALLEY
ANCIENT GLACIERS OF THE YOSEMITE REGION
EXPLANATION OF MAP OF ANCIENT GLACIERS
Although the ancient glaciers no longer exist in the Yosemite region it is possible to describe them in considerable detail, for the data furnished by the moraines, the erratic boulders, and the other evidences of glaciation are very full and mutually consistent. Not only are the position, shape, and dimensions of each individual ice stream definitely indicated, but also the slope and to some extent the configuration of its surface, and hence one can form a fairly clear conception of the character of these ice bodies and of their appearance in the landscape.
The map of the ancient glaciers (pl. 39) is the product of a systematic and comprehensive survey of the moraines and other glacial features. On it the glaciers of the Wisconsin stage, for which the data are fullest, are represented with the symbols commonly used on maps for existing glaciers. Those of the El Portal and Glacier Point stages, which are known in less detail, are shown merely in outline. As they were of very nearly the same extent no attempt has been made to differentiate between their outlines on the small scale of the map. Only the glaciers of the Yosemite region and those in the Tuolumne Basin which need to be considered in this study are shown. The glaciers in the basins drained by the San Joaquin River and the South Fork of the Merced River are purposely omitted in order to make more clear the limits of the area that contributed ice to the Yosemite region.
The fullness of detail with which the glaciers of the Wisconsin stage are portrayed deserves, perhaps, a word of explanation. The crevasse symbols are used not merely in a conventional manner but in part specifically to indicate crevasses that are definitely located and orientedsome spaced close together, some far apart, some crossing one another in an intricate network. The symbols for medial moraines likewise are employed in a specific rather than in a purely conventional way. They show such moraines to begin in definite places, to wind in curves down the backs of the glaciers, and to terminate in definite places. To what extent, the reader may ask, are these specific details based on ascertained facts and to what extent are they merely imagined?
The crevasses of glaciers are distributed and oriented in accordance with well-known laws. Transverse crevasses, extending approximately at right angles to a glacier's axis, are of rather general occurrence, being due to the tearing apart of the brittle upper layers of the ice as they ride on the more pliant, ductile lower layers. Naturally such crevasses are particularly numerous wherever a glacier descends over a step in its bed, its surface there being convexly curved and its upper layers stretched in longitudinal direction. They are least numerous at the base of each step, where the glacier's surface is concavely curved and the upper layers are under compression. It follows, then, that some of these crevasses are definitely associated with irregularities in the glacier's bed, forming and reforming constantly at the same critical spots, to heal up again in the quieter stretches below. They may be likened to the standing waves in the rapids of a swift river. Now the canyons of the Sierra Nevada still exhibit, almost unchanged, the steps and other obstructions on which the ice streams of the last glacial stage broke; hence the precise spots at which transverse crevasses were formed in those ice streams are known, and even the directions in which the crevasses extended are definitely indicated.
Another set of crevasses is associated with the mar gins of a glacier. They owe their existence to the fact that the middle part of a glacier moves forward faster than the marginal parts, which are retarded by friction. Such marginal crevasses, produced by torsional stresses in the ice, extend as a rule from the sides inward, describing arcs bowed upstream.
The nature and mode of occurrence of medial moraines have already been explained on page 58. Such moraines begin invariably at the points of confluence of ice streams and extend downstream as narrow, sinuous bands, parallel to the glacier's current. It is entirely in order, therefore, to show on the map one such medial moraine beginning at the point of confluence of each pair of joining glaciers. Those trunk glaciers which had several tributaries in their upper courses naturally bore a corresponding number of medial moraines. Normally medial moraines extend all the way down to the terminus of a glacier, merging there in the mantle of débris released by the melting of the ice; but where a glacier cascades tumultuously over abrupt steps in its path the medial moraines disappear, as a rule, the débris being entrapped by the crevasses.
The positions of the medial moraines can be determined on the earlier glaciers with just as great precision as on the later; but the same is not true for the crevasses, for the steps in the canyon floors over which the earlier glaciers cascaded are now either destroyed or greatly modified. It has been deemed best, therefore, to omit from the map any representation of the surface features of the earlier glaciers.
GLACIERS OF THE WISCONSIN STAGE
The glaciers of the Wisconsin stage will be described first, as they are most definitely known and were for the most part simple ice tongues confined to individual valleys. Having obtained a definite image of them, the reader will then be able to visualize also the more extensive glaciers and ice fields of the earlier stages, which coalesced over divides and in part moved in disregard of them.
Yosemite Glacier.During the Wisconsin stage of glaciation the Yosemite Valley was occupied by an ice stream of only moderate dimensions. This ice stream, which may be named the Yosemite Glacier, did not extend the full length of the valley but terminated within a mile of the lower end, just above the Bridalveil Meadow, as is attested by the moraines which it left there. Nor did it receive a single tributary ice stream from any of the hanging valleys on either side of the chasm. It was formed wholly by the confluence of the Merced Glacier and Tenaya Glacier, the two great ice streams that issued from the Little Yosemite Valley and Tenaya Canyon.
At the head of the valley, where these two ice streams joined each other, the Yosemite Glacier had a thickness of about 1,500 feet, but down the valley its thickness diminished by degrees, so that in the portal opposite the Cathedral Rocks it was only 400 to 500 feet. Throughout the greater part of its course the surface of the glacier sloped rather gently and evenly, the average rate being 100 feet to the mile, but at the upper end as well as at the terminus it sloped much more steeply.
A better conception of the depth to which the glacier filled the Yosemite Valley may be gained by reference to a few well-known landmarks. The ice surface reached almost up to the shoulder above the Royal Arches; it was about level with the rock platform above the lower Yosemite Fall and lay just below Columbia Rock. On the south side of the valley Glacier Point towered fully 2,000 feet above the glacier; Union Point rose about 1,000 feet above it, and even the Cathedral Spires stood entirely clear, the ice reaching barely up to the base of the lower spire. The Bridalveil Fall, then as now, leaped from its 600-foot precipice, but its lower portion probably disappeared in an abyss which the waters had melted in the side of the glacier.
The Yosemite Glacier must have presented the appearance of a fairly clean ice streammuch cleaner than most of its contemporaries in the Cascade Range or the Rocky Mountains, much cleaner also than the present glaciers on Mount Rainier, for the granitic rocks of the Yosemite region and the adjoining High Sierra, being not only hard but tough and prevailingly massive, yielded a much smaller proportion of débris to the passing ice streams than the relatively well-jointed and less coherent rocks in the other regions mentioned. Nevertheless the Yosemite Glacier was not a dazzling white "mer de glace" throughout. Its margins were littered with large blocks and smaller fragments of granite, and along its middle extended a medial moraine which resulted from the junction of the Merced and Tenaya Glaciers.
Doubtless this medial moraine was one of the glacier's striking features. Beginning at the head of the valley as a narrow dirt band, it broadened gradually, at the same time assuming the form of a low, hummocky ridge on the back of the glacier. Medial moraines commonly stand out as low ridges on rapidly melting glaciers, owing to the fact that the débris, where thick enough, absorbs a large share of the sun's heat and thus retards the melting of the ice beneath it. Such medial ridges are therefore composed not wholly of débris, but of dirty ice mantled with débris. Excellent examples of prominent medial ridges are to be seen on several of the glaciers on Mount Rainier. The Nisqually and Kautz Glaciers, notably, have medial ridges from 10 to 25 feet in height.
On the margins of the Yosemite Glacier the débris probably had a similar though less pronounced protective effect, thus keeping the surface slightly raised. The glacier is to be pictured, accordingly, as having had two broad lanes of clean ice parted by a dirt-covered medial ridge and flanked by inconspicuous dirt-covered ridges at the margins. The clean lanes became narrower toward the glacier's terminus and ultimately vanished, as the entire surface became mantled with débris.
The ice front itself doubtless varied in character as it oscillated back and forth. During periods of advance it was abrupt and wall-like; during periods of recession it sloped at a moderate angle, and when stationary for any length of time it probably was not clearly distinguishable from the moraine in process of being piled up before it. As the glacier finally melted back toward the head of the valley, its front, being immersed in the water of Lake Yosemite, was kept sheer by the breaking off of ice masses that floated about as small icebergs.
Merced Glacier.The glacier which occupied the upper Merced Canyon and the Little Yosemite, and which may be called the Merced Glacier, had its sources in an ice field about 70 square miles in extent, that occupied all of the upper Merced Basin. The north half of this Merced Ice Field consisted of a series of parallel ice rivers parted by the long, attenuated spurs of the Cathedral Range. The south half, on the other hand, presented an almost unbroken expanse of crevassed ice that sloped down in sweeping curves from the Clark Range, yet it too was made up of many parallel ice streams which, though coalescing with one another, possessed each a distinct current and followed each a separate channel. Through the middle of the ice field, completely filling the upper Merced Canyon, flowed the great ice current that, farther down, became the Merced Glacier.
Besides being fed by the snows that accumulated in the cirques on the Cathedral and Clark Ranges, the Merced Ice Field also received large contributions by overflow from the great Tuolumne Ice Field, which lay to the north. This fact, which is plainly attested by the directions of the glacial striae in several gaps in the Cathedral Range, was recognized as far back as 1863 by the exploring parties of the Geological Survey of California under Whitney. Accordingly, the broad ice stream that filled the twin valleys of Emeric Creek and Fletcher Creek consisted largely of ice that had been forced over Tuolumne Pass; and the glaciers in the basin of Echo Creek received reinforcements through the gap north of Rafferty Peak and through the saddle west of Unicorn Peak. A strong current of ice also came through Cathedral Pass and swelled the glacier in the valley of the Cathedral Fork. At the climax of the Wisconsin stage these tributary ice streams attained thicknesses of 500 to 1,000 feet, as is shown by the height of the glacier polish and the positions of the lateral moraines; but the Merced Ice Field itself was almost twice as thick at the center. At Washburn Lake the ice had a thickness of fully 1,800 feet, and thence it increased to 2,200 feet opposite the south spur of Sunrise Mountain, where the Merced Glacier issued from the ice field. This glacier, about 2 miles wide, not only filled the entire gorge of the Merced but overflowed the broad rock benches on both sides to a height of 1,400 feet.
At the head of the Little Yosemite, which was capacious enough to hold the entire volume of ice, the Merced Glacier contracted to a width of only 1 mile. It filled the valley, however, literally to the brim, and even pushed a short lobe through the saddle northeast of Moraine Dome, as is shown by the moraine loops. (See pl. 29.) In the broad lower half of the Little Yosemite the Merced Glacier deployed gradually to a width of 1-1/2 miles, at the same time diminishing in thickness from 1,700 feet opposite Moraine Dome to 900 feet opposite Liberty Cap. Consequently it failed to overtop Liberty Cap, leaving its summit standing out like a small round island. The Merced Glacier, however, overrode Mount Broderick by 300 feet, breaking on its dome-shaped top in radially diverging crevasses. Half Dome stood wholly above the ice, not even its base on the south side being touched.
From the mouth of the Little Yosemite the ice stream tumbled in magnificent chaos down the giant stairway, whose great steps are now marked by the Nevada and Vernal Falls, and it also plunged over the sheer front of Mount Broderick and through the gaps on each side. Broken into fantastic blades and pinnacles (seracs) the cascading portion of the glacier must have presented the appearance of a tumultuous cataract frozen into immobility. By contrast the even, compact ice mass that lay below in the Yosemite Valley must have seemed like a deep, tranquil pool.
The head of the ice cascade is still indicated by the highest lateral moraine on the south side of the Little Yosemite. Just west of the fork of the Mono Meadow Trail and the Glacier Point Trail this moraine begins to descend steeply along the edge of the precipice. A short distance farther west, where the ice surface broke, it disappears.
Tenaya Glacier.The Tenaya Glacier had its sources far beyond the limits of the drainage basin of Tenaya Creek and drew a large part of its substance from the Tuolumne Ice Field, one of the largest in the entire High Sierra. The gently sloping surface of this vast mass of ice extended dazzling white, scarcely sullied by medial moraines, from Mount Lyell and the adjoining peaks of the Cathedral Range northward to Kuna Crest and Koip Crest, to Mount Dana and Ragged Peak, and westward to Tuolumne Peak and Mount Hoffmann. It had an area of 140 square miles and attained over the Tuolumne Meadows a thickness of about 2,200 feet. Northward and westward it connected with the numerous lesser glaciers of the upper Tuolumne Basin, the aggregate area being close to 500 square miles.
The main outflow from this vast ice field went by the Tuolumne Glacier, one of the longest and mightiest trunk glaciers in the Sierra Nevada. It followed the Grand Canyon of the Tuolumne River, filling that chasm, which is 4,500 feet deep, to overflowing, as is impressively attested by the lateral moraines at Harden Lake and those on the uplands bordering the Hetch Hetchy Valley. So superabundant, however, were the snows that accumulated in the ice field that they could not all be drained off by the Tuolumne Glacier, and as a consequence overflow took place in several directions through notches and gaps in the confining mountain ranges. Small ice streams escaped, as already stated, southwestward through the gaps in the Cathedral Range and reinforced the feeders of the Merced Ice Field. A broad ice stream poured southeastward over Donohue Pass and joined the glaciers in the Rush Creek Basin; and relatively large ice streams flowed eastward through Parker Pass and Mono Pass and northward through Tioga Pass, descending through the precipitous canyons of the eastern Sierra front and spreading out in the basin of Mono Lake.
The largest diversion of ice, however, took place over the hilly divide east of Tuolumne Peak. There an ice sheet 4-1/2 miles broad split off and passed over into the Tenaya Basin. The highest knobs on the divide were but small obstacles in the path of this ice sheet; they were submerged during the culminating phases to depths of more than 1,400 feet. Fairview Dome, the highest of the granite knobs, standing 1,200 feet above the Tuolumne Meadows, was overtopped fully 800 feet.
Not all of the diverted ice, however, went to feed the Tenaya Glacier. Although much of it gravitated directly into the head of Tenaya Canyon, a considerable proportion deployed southwestward along the base of Mount Hoffmann, and this ice lost most of its substance by melting as it spread out. At Snow Flat its thickness was reduced to 400 feet, and a mile farther southwest it was too thin to override any more knobs and ridges and was deflected to the south by the long ridge that extends southward to Mount Watkins.
The descent of the main ice current at the head of Tenaya Canyon was marked, of course, by crevasse-torn ice cascades, but these cascades probably did not compare in spectacular grandeur with those of the Merced Glacier, for when glacial conditions approached their climax the head of Tenaya Canyon was almost filled with ice so that the entering glaciers had but a few hundred feet to fall. After the ice flood began to wane the entering glaciers made higher plunges but had less volume.
In Tenaya Canyon itself the ice formed a narrow; compact glacier of great thickness that probably wasted but slowly, as it lay mostly in the shade of the lofty ridge of Clouds Rest and had a small exposed surface proportionately to its bulk. The narrowness of the Tenaya Glacier is strikingly revealed on the glacier map (pl. 39) and stands in marked contrast to the breadth of the Merced Glacier. In only a few places was the Tenaya Glacier as much as a mile wide; through most of its length it was only three-quarters of a mile wide, and in one place, opposite the blunted nose of Mount Watkins, it contracted to half a mile.
However, the Tenaya Glacier and the Merced Glacier are not to be compared on the basis of their width alone. It is necessary also to take cognizance of their thickness. As is evident from the cross section in Figure 23 the Tenaya Glacier was by far the thicker of the two. Opposite Half Dome it was 1,800 feet thick, whereas the Merced Glacier was only 1,000 feet. Their cross sections consequently differed but little in area; as measured on the diagram they stood to each other in the ratio of about 7 to 8. When it is considered, further, that opposite Half Dome the Tenaya Glacier had a more rapid fall and therefore greater velocity of flow than the Merced Glacier, it will be seen that the daily discharge of the two ice streams was closely balanced.
The cross section in Figure 23 reveals also the marked disparity in level between the two glaciers. Opposite Half Dome the surface of the Merced Glacier was at an altitude of 7,100 feet; the surface of the Tenaya Glacier, on the other hand, was at only 5,900 feet. The Merced Glacier descended to the Yosemite Valley by a tumultuous ice cascade, but the Tenaya Glacier sloped down rather evenly and joined the Yosemite Glacier without break.
The highest level reached by the ice of the Wisconsin stage is not so readily determined in Tenaya Canyon as in the Little Yosemite, for moraines are almost wholly absent from the steep, smooth walls of Tenaya Canyon. Fortunately, however, there are sculptural features that serve to indicate roughly the level attained by the ice in the canyon. As may be seen in Plate 40, B, several short spurs project from the sloping rock façade of Clouds Rest. These are all distinctly truncated, their lower parts having been pared away by the glacier. Their slopes above the points of truncation are rough as a result of long-continued weathering and give footing to bushes and trees, but the walls below are sheer and smooth and practically bare. The upper limit of glacial action may thus be inferred approximately from the change in the sculpture of the cliffs. A line drawn through the most definite points connects well with the moraines in the upper Tenaya Basin on the one hand and with the level determined for the ice in the Yosemite Valley on the other hand. The absence of a lateral moraine across the mouth of the hanging valley of Snow Creek corroborates the essential correctness of the line as drawn. Evidently the Tenaya Glacier did not reach quite up to the level of that hanging valley. By inference, it must have lain fully 800 feet below the base of the sheer front of Half Dome.
Snow Creek Glacier.57The southerly slopes of Mount Hoffmann, which were exposed to the heat of the sun and also to the sweep of the southwesterly gales, naturally accumulated but little snow. Nevertheless, when glacial conditions reached a climax, these slopes bore ice fields of moderate thickness, which, uniting at the base of the mountain with the broad lobe of Tuolumne ice that advanced from the direction of Snow Flat, formed a small glacier in the valley of Snow Creek. This, the Snow Creek Glacier, was but 1 mile long and ended fully 2 miles north of the brink of Tenaya Canyon.
Hoffmann Glacier.On the steep north side of Mount Hoffmann the conditions were particularly favorable for the development of glaciers. There, in the lee and shade of four lofty westward-trending crests, the snows accumulated to great thickness and formed several cirque glaciers that united into one massive ice stream more than 1,000 feet thickthe Hoffmann Glacier. Reinforced by tributaries from Tuolumne Peak and from the group of mountains north of Mount Hoffmann this ice stream turned westward and then southward, down the valley of Yosemite Creek. It filled that valley completely, some of the ice spilling through gaps in the western divide.
At the point where the Tioga Road crosses the valley of Yosemite Creek the Hoffmann Glacier attained its greatest widthabout 3-1/2 milesand a thickness of 1,300 feet; but so great were the losses which it suffered by melting on its broad surface that from this point southward the glacier dwindled rapidly in width and thickness, coming to an end just before reaching the brink of the Yosemite Valley. (See pl. 29.) Clearly it is useless to look for glacier polish on the platforms and slopes of bare rock above the Yosemite Falls or on Yosemite Point; they were never touched by the ice of the last glacial stage. Nor are the fantastic rock tables that lie near Yosemite Point ice-borne erratics; they are remnants of a sheet of rock from 6 to 8 feet in thickness that has disintegrated in place and has been stripped away almost entirely since the time of the earlier glaciers.
The long ridge that hems in the upland valley, of Yosemite Creek on the west was high enough to generate small glaciers of its own. Five such ice bodies carved cirques into its eastern slope, but none of them were long enough to connect with the Hoffmann Glacier. On the west side of the ridge the snows evidently were more plentiful and gave rise to glaciers several miles in length. These extended down into the basin of Cascade Creek, but none of them reached the Yosemite Valley. The longest ended about 2 miles above the bridge on the Big Oak Flat Road.
Glaciers of the Illilouette Basin.Contrary to the belief of previous students of the Yosemite region, the Illilouette Basin furnished no tributary to the Yosemite Glacier but contained only ice tongues of moderate size that were confined to its upper half. The longest did not reach within 5 miles of the mouth of the valley. This paucity of ice in the Illilouette Basin is explained by the fact that but little snow accumulated on the sun-heated and wind-swept southwest flank of the Clark Range, and that Buena Vista Crest and Horse Ridge, which encircle the head of the basin, were too low to generate glaciers of any considerable magnitude.
The largest ice streamthe Illilouette Glacier, properly so calledissued from the capacious amphitheaters northwest and southwest of Merced Peak. Reinforced by lesser ice streams from the vicinity of Merced Pass, it attained a length of about 7 miles. A remarkable series of lateral moraines, one of the finest in the Yosemite region, attests the numerous oscillations in volume and extent that marked the final waning of the Illilouette Glacier. They were laid down mostly along the right-hand margin of the glacier, which was abundantly supplied with débris from the masses of fractured granite in the vicinity of Merced, Ottoway, and Red Peaks.
From the cirques west of Red Peak issued another ice stream worthy of note. It extended down the valley of Red Creek and, being also heavily laden with débris, built up marvelously high and regular morainal embankments. It attained a maximum length of about 2 miles, but it failed to connect with the Illilouette Glacier. Immediately west of the Illilouette was another glacier of nearly the same length and even slightly greater breadth, which may be named the Buena Vista Glacier, as it was made up of a number of confluent ice streams that originated in the lee of the long line of cliffs on Buena Vista Crest. Still farther west lay some smaller glaciers that came from Horse Ridge.
Glaciers of the Bridalveil Basin.The upland valleys of Bridalveil Creek and its tributaries were largely devoid of ice during the last stage of glaciation. Only at the extreme heads of these valleys existed a few small ice tongues. All of the region now traversed by the automobile road that leads to Glacier Point was free from ice, and the beautiful natural meadows that are skirted by that road probably were already in existence.
Of the small ice tongues at the head of the basin the easternmost and largest is of peculiar interest, as it scooped out the basin of Ostrander Lake and built with the slabs and blocks of granite which it quarried from the north flank of Horse Ridge and from the lake basin the surprisingly large and rough moraines over which one travels with difficulty to reach the lake. In few other places in the Yosemite region are the effects of glacial quarrying and transportation more vividly exemplified on a small, compact scale than at Ostrander Lake.
Lowest level of glacier generation.In the vicinity of Ostrander Lake, furthermore, the student of glacial phenomena will find valuable indications of the lowest level at which glaciers were formed in this part of the Sierra Nevada during glacial time. Short glaciers, manifestly, give more precise information as to that level than long glaciers, as they protrude but little below the region of snow accumulation. Indeed, the shorter a glacier is the more closely does it indicate the level of glacier generation. Now it happens that on the northeast slopes of the low mountains north of Ostrander Lake there clung two tiny glaciers that scarcely projected beyond the bottoms of their cirques. These glaciers, it may be concluded, indicate accurately the lowest level at which, in favorably situated spots, snow could accumulate to sufficient thickness to form a true glacier. That level was slightly above an altitude of 8,000 feet.
Observations of a like nature made in other parts of the Sierra Nevada have yielded closely accordant data, and as a result the level of glacier generation can now be traced for a considerable distance over the range. Toward the north, naturally, it descended gradually; toward the south, on the other hand, it rose to higher and higher altitudes.
The definite determination of the lowest level at which glaciers were formed in the Yosemite region helps to settle some much mooted questions. Thus it is now clear that no glaciers could have been formed locally on the walls of the Yosemite chasm, as was thought by some of the earlier observers. Muir, notably, supposed a small glacier to have lain at the foot of the great eastward-facing cliffs west of the upper Yosemite Fall; but the altitudes there were 1,500 to 2,000 feet too low for the generation of a glacier even under the most favorable conditions. Again, it is commonly supposed that the northwest side of Half Dome owes its sheerness to the sapping action of a small glacier that lay in the shade and the lee of the great rock monument; but it is now evident that such can not be true, as the altitudes at the base of the cliff are 800 to 1,600 feet too low. Neither was it possible for a small glacier to be formed in the shady recesses of the south wall of the Yosemite Valley above Artist Point, for the altitudes there are nearly 2,000 feet too low.
GLACIERS OF EARLIER STAGES
Great extent and thickness.The aspect of the Yosemite region in those earlier stages of the Ice Age when the glaciers attained their greatest spread is most readily pictured to the mind by imagining all the glaciers of the last or Wisconsin stage, just described, considerably enlarged. The Yosemite Glacier and its principal branches had, of course, the same general arrangement as in the Wisconsin stages for they followed the same canyons and valleys, but all were much longer and broader and thicker, and in many places they spilled over the flanking divides. Moreover, the ridges and knobs on the Yosemite upland generated glaciers of considerable magnitude, and as a consequence large areas of upland which in the Wisconsin stage remained bare were in the earlier stages mantled by extensive ice fields.
The greater length and greater volume of the main glaciers in those earlier stages might be ascribed to a proportionately greater piling up of snow on the crest of the range, yet that is not the true explanation. Strange though it may seem, the ice fields in the Merced and Tuolumne Basins had but slightly greater thickness than they had in the Wisconsin stage, and the cirques on the Clark Range, the Cathedral Range, and the main divide were filled to scarcely any greater depth.
This remarkable fact is brought out clearly in Figure 22, in which the longitudinal profiles of the Yosemite and Merced Glaciers in the earlier stages are superimposed upon the profiles of those glaciers in the Wisconsin stage. The profiles are based on a systematic survey of moraines and other features that indicate the highest level reached by the ice in each stage. In the lower part of the Yosemite Valley, it is to be noted, the earlier ice was 2,700 feet thick at the place where the later ice came to an end. In the upper part of the valley the two ice surfaces were about 2,000 feet apart in altitude, and thence upward the difference between them diminished rapidly. Opposite Liberty Cap it was only 1,000 feet, and at the head of the Little Yosemite it was only 500 feet. At Washburn Lake the earlier ice stood only 370 feet higher than the later, and farther up toward the Sierra crest the difference became so small that in the cirque at the immediate base of Mount Lyell it dwindled to a negligible quantity. It likewise dwindled in the other cirques at the head of the Merced Basin. In the Tuolumne Basin and, indeed, throughout the High Sierra, the earlier and later ice levels similarly coincided in the summit cirques.
From these facts it would appear, then, that the summit cirques of the Sierra Nevada received annually little if any more snow in the earlier stages of glaciation than they received in the Wisconsin stage. Why, then, it may be asked, did the glaciers of the earlier stages grow to so much greater length and bulk than did the glaciers of the Wisconsin stage? Primarily, it is believed, because the earlier stages were each much longer than the Wisconsinin each of them the snow was permitted to accumulate for a longer period of time. It is probable that the earlier stages were marked also by somewhat greater severity of climate, so that the zone of snow accumulation in each of them was extended to a lower level, but as to this the evidence is not clear. In any event the great expanse of snow and ice must itself have become a secondary factor that served to intensify the wintry conditions and to increase the precipitation of snow by its chilling effect on the moisture-laden winds that swept up over the west flank of the range.
Upper limit of glaciation in the High Sierra. Because in the High Sierra the ice reached so nearly the same level in the earlier and in the later stages, the upper limit of glaciation, which for convenience may be called the ice line, is there much more definitely marked in the landscape than lower down on the flanks of the range. And because the work of the glaciers in the High Sierra was largely erosional and only incidentally depositional, the ice line there is defined chiefly by sculptural features and only locally by moraines.
The ice line is particularly prominent in that part of the High Sierra of which Cathedral Peak is the outstanding feature. The mountains in that area have massive, rounded forms (pl. 40, A), the projecting angularities having been smoothed away by the ice currents that swept over and around them in passing from the Tuolumne Basin over, into the Tenaya and Merced Basins. Of this fact glacier polish, striae, and grooves afford abundant evidence. But surmounting these rounded, ice-smoothed mountains here and there are craggy pinnacles and splintered crests. These delicate superstructures doubtless have always stood above the ice, exposed to the atmosphere, like the "nunataks" that project above the ice cap in Greenland. Had they been overwhelmed by the ice forever so brief an interval, surely they would have lost their angular forms and been largely demolished. Noteworthy examples of these frail superstructures are the spires of Cathedral Peak (pl. 12, A) and the horn of Unicorn Peak, but even more impressive, because giving more definite indication of the ice line at their base, are the clustered pinnacles of the Echo Peaks and the attenuated crest of the Cockscomb (pl. 40, A). Very clearly marked also is the ice line at the bases of those lesser and more readily accessible cockscombs southwest of Cathedral Pass, of which the best known bears the slender shaft named Columbia Finger.
In the vicinity of Mount Lyell and on the main divide the ice line is distinct in only a few places, as the mountains there are so high that they were not extensively overswept by the ice. They were, in fact, the main centers of dispersal from which the ice streams radiated in different directions. On these lofty mountains, nevertheless, there are abundant indications of an upper limit above which the ice never rose. The more extensive summit tracts of these mountains are clearly unglaciated. Their gently sloping or undulating surfaces are littered with angular blocks, large and small, which have been loosened by intense frost action but which have never been moved by gravitating glaciers. Nor are there any other evidences of glaciation such as polish, striae, or moraines. Doubtless these summit tracts have at no time borne anything more than temporary drifts and fields of snow, the violent gales that sweep over them having kept them partly bare.
It follows that these summit tracts are remnants of the preglacial landscape that have remained but little changed in configuration. Most of them have the appearance of elevated, uneven table-lands ending in sharp edges above the cliffs of the glacial cirques and canyons that bite deeply into the mountain sides. Some of them are so regularly scalloped by the inset arcs of cirques as to suggest, as Willard D. Johnson has aptly put it, "the irregular remnants of a sheet of dough, on the biscuit board, after the biscuit tin has done its work."58 Unglaciated summit tracts of this kind exist notably on Parsons Peak and on the mountains east of Vogelsang Lake. Smaller tracts remain on Blacktop Peak, and Kuna Peak, and on some of the higher knobs of Kuna Crest. (See pl. 41, A.) The largest and most impressive remnants of preglacial topography are on Mount Dana and Mount Gibbs.
Merced Glacier.It follows from what has been stated about the nearly exact coincidence of the ice levels in the High Sierra that the Merced ice field must have presented substantially the same appearance in the earlier stages of glaciation as it did in the Wisconsin stage. This is true especially of its upper portion, but at its lower end the earlier ice was appreciably thicker than the later. It overrode the south spur of Sunrise Mountain, which in the last ice stage stood out as a prominent landmark, scraped off the pinnacles, and rounded that spur to its present smooth contours. Only a slender strip of land on the back of Sunrise Mountain remained bare of ice. Clouds Rest, too, was isolated, the ice of the Merced Glacier coalescing with the ice of the Tenaya Glacier across the saddles to the east and west of the crest. A small ice field of local origin lay even in the upland vale southeast of the Pinnacles on Clouds Rest.
Half Dome was deeply immersed in the ice, but its crown was not overwhelmed. The roundness of its crown has been ascribed to the abrading action of overriding ice masses, Half Dome being pointed to by many as a "roche moutonnée"59 of gigantic proportions; but the fact is that Half Dome, like the other domes of the Yosemite region, owes its roundness to long-continued exfoliationthe casting off of successive concentric shells of rockas explained more fully on page 115.
The exact height to which the ice rose on the sides of Half Dome is not indicated by any discernible line, for the marks of glaciation have been long since destroyed by weathering; but the ice level can be ascertained within narrow limits from the highest moraines on the mountains to the north, south, east, and west of Half Dome. These moraines afford sufficient data for the construction of a contour map of the ice surface, and a map so drawn shows that the ice did not reach within 500 feet of the level of the dome's summit. At the upstream side of the dome the ice banked up to an altitude of about 8,350 feet, the subsidiary dome from which the ascent of Half Dome is usually made being left just barely emergent. At the southwest or downstream end of the dome the ice surface declined to an altitude of about 8,200 feet.
South of the Little Yosemite the Merced Glacier deployed broadly over the Starr King Meadows and, when it reached its culminating stage, overtopped the divide east of Mount Starr King, thus coalescing with the ice in the Illilouette Basin. Up the Illilouette Valley, moreover, the glacier thrust a massive lobe that eventually met and united with the Illilouette Ice Field, and thus the Starr King group was completely surrounded by ice, the two highest summits only remaining uncovered, like two smooth, egg-shaped nunataks.
Illilouette Ice Field.The conditions that prevailed in the Illilouette Basin during this stage of maximum glaciation differed vastly from those of the Wisconsin stage. The glaciers generated on the Clark Range and on the crests at the head of the basin coalesced with the lobe of the Merced Glacier that blocked the mouth of the Illilouette Valley to produce an ice field of great magnitude and thickness. The obstructing lobe caused the level of this ice field to rise until at last the ice overflowed through the saddle southwest of Mono Meadow, passing westward with a thickness of about 200 feet and joining the ice field that lay in the Bridalveil Basin.
After the climax of glacial conditions had passed them Illilouette Ice Field again subsided below the level of the saddle and ceased to have any outflow. Nor did it need any, for the local glaciers, on the one hand, and the lobe of the Merced Glacier, on the other, melted back in opposite directions, leaving a space in which the waters collected to form a temporary, fluctuating lakeancient Lake Illilouette. As the water accumulated, it found an outlet presumably over the surface of the ice that lay close-packed in the gorge of the Merced below.
During a later phase, when the obstructing lobe had melted away, the Merced Glacier itself remained high enough to pond the water at the mouth of the Illilouette Valley. In the shallow lake thus created a bed of stream-borne gravel and sand was laid down. This deposit has since been trenched by Illilouette Creek, but much of it remains in the form of low terraces. (See pl. 29.) The terrace on the east side of the stream still extends directly out to the brink of the Illilouette Gorge. Further examinations may show that this gravel bed was formed as late as the Wisconsin stage of glaciation.
Bridalveil Ice Field.The basin of Bridalveil Creek contained a broad expanse of snow and ice. But the thickness of that ice field was only moderate, perhaps 500 feet at the most, and the outflow to the Yosemite Glacier was small. The obscureness of the morainal deposits in the basin renders it difficult to ascertain the exact relations which the Bridalveil Ice Field bore to the trunk glacier. It is entirely probable that the ice field was short lived, its existence being limited to that period when the Illilouette Ice Field overflowed through the saddle near Mono Meadow; for it seems unlikely that the low crests encircling the head of the basin were able of themselves to generate glaciers of sufficient volume to cover the entire area.
Tenaya Glacier.The diversion of ice from the Tuolumne Basin to the Tenaya Basin took place on a much larger scale in the earlier stages of glaciation than in the last. On the divide between the two basins the ice attained a thickness of fully 1,700 feet. It rose high enough to overtop the crest that connects Tuolumne Peak with Mount Hoffmann, thus isolating both summits. It overrode Polly Dome to a depth of 600 feet and stood 2,300 feet thick in the basin of Tenaya Lake. Tenaya Canyon itself was so completely filled that there was no cascade over the cliffs at its head.
The entire upland north of Tenaya Canyon was overmantled, largely by Tuolumne ice but in some measure also by ice from the southerly slopes of Mount Hoffmann. That mountain itself stood above the ice, clad in resplendent white. Only the narrow rock partitions and craggy chimneys that diversify its broad back remained silhouetted in black against the brilliant snow. The valley of Snow Creek was filled to a depth of 1,500 feet, the ice closing over both Mount Watkins and Indian Ridge, so that there was no distinct Snow Creek Glacier to be seen. The extreme summit of Indian Rock, however, probably remained emergent like a small island. Basket Dome lay under 700 feet of ice; North Dome under 600 feet.
South of Mount Hoffmann, in what is now known as Porcupine Flat, the westward-expanding Tuolumne ice met southeastward-flowing currents from the Hoffmann Glacier. The composite ice mass moved southward and invaded the basin of Indian Creek, but there its flow was checked by the powerful Yosemite Glacier, and it remained almost stagnant, like back water in a side slough of a great river in flood.
The highest level attained by the Tenaya Glacier itself is shown in the cross profile in Figure 23, which is based on all the information available from the altitudes of moraines. The surface of the Tenaya Glacier was slightly lower than that of the Merced Glacier, and hence it may be inferred that in the broad saddle between Clouds Rest and Half Dome the ice currents moved in northwesterly directions from the Little Yosemite toward Tenaya Canyon.
Hoffmann Glacier.The Hoffmann Glacier attained in the earlier ice stages the proportions of a true "mer de glace." It had a length of 13 miles and a breadth ranging from 2 to nearly 4 miles. Its surface area comprised about 45 square miles. So completely did the glacier fill the valley of Yosemite Creek that the ice overtopped the hills as well as the gaps in the northwesterly divide over a stretch of several miles and poured in considerable volume into the valleys tributary to the Middle Fork of the Tuolumne River. One of the largest diversions of ice took place through the pass now occupied by Lukens Lake, a body of water which owes its existence to the concentrated erosive action of the ice current in the defile. The ice thus diverted met and united with the overflow from the Tuolumne Glacier, which overspread the upland south of the Grand Canyon of the Tuolumne for many miles.
When the Hoffmann Glacier reached its culminating stage it joined the Yosemite Glacier without appreciable break of surface. It coalesced also with the partly stagnant ice mass in the basin of Indian Creek, and thus was formed a vast expanse of ice that stretched continuously from the south rim of the Yosemite Valley northward to Mount Hoffmann and northeastward to the main crest of the range. The Hoffmann Glacier at this stage also was thick enough and powerful enough to push a lobe of ice upgrade into the Eagle Peak Meadows and over the divides to the south, thus connecting with the Yosemite Glacier and surrounding the craggy summit of Eagle Peak.
As the glacier subsided it left on the unnamed summit west of the upper Yosemite Fall the moraine that includes the boulder perched on the 5-foot pedestal already referred to. (See pl. 38, A.) Subsiding still further, it became detached from the ice in the Indian Creek Basin and deposited successive lateral moraines on the long, straight divide that terminates in Yosemite Point.
The crown of El Capitan and the ridge north of it known as Boundary Hill, were high enough to stand above the highest ice flood. They constitute the first unglaciated area of any considerable extent on the north side of the Yosemite Valley and west of Clouds Rest. They were, however, completely surrounded by the ice, for at the culminating stage the Hoffmann Glacier sent a lobe of ice through the saddle north of Boundary Ridge, and this lobe, reinforced by small glaciers of local origin, formed the Ribbon Creek Glacier.
Cascade Glacier.The most westerly ice stream tributary to the Yosemite Glacier on the north side was the Cascade Glacier. It was nourished wholly by cirques on the divide west of Yosemite Creek and existed independently of the Hoffman Glacier, yet it was by no means insignificant. It had a length of 5 miles and, in its lower course, a thickness of 1,500 feet. It was thus comparable in magnitude to some of the finest glaciers now to be seen on Mount Rainier. The fact that an ice body of this size could be generated by only half a dozen small cirques in the side of a ridge not over 9,000 feet high speaks eloquently of the abundance of snow and the severity of climate that prevailed in the Yosemite region during the earlier stages of the glacial epoch.
Yosemite Glacier.During the phases of maximum glaciation the Yosemite Glacier filled the Yosemite Valley literally to the brim and wound its way, like a broad, majestic river, down through the Merced Canyon below, as far as a point somewhat beyond the site of El Portal. At the head of the Yosemite Valley its surface probably was continuous with that of the vast ice sea that overspread the Little Yosemite, Tenaya Canyon, and the Illilouette Basin. So great was the congestion of ice caused by the bottle neck between North Dome and Glacier Point that the abrupt increase in depth at the head of the Yosemite Valley did not give rise to any significant break in the surface of the ice. Before and after the climax of each of the earlier glacial stages, however, when the ice had less thickness, yet sufficient to overtop the broad platform at the west base of Half Dome, there was a marked break in its surface at the head wall of the valley, for there the ice plunged down in the form of a grand cataract. This slow-moving Niagara of ice, which must have dwarfed the cascades by which the Merced Glacier descended from the giant stairway, persisted probably for considerable periods of time and, as explained more fully on page 97, was a powerful factor in the excavation of the Yosemite chasm.
From the vicinity of Glacier Point westward the surface of the Yosemite Glacier probably declined rather evenly. The ice nowhere spilled over the upland on the south side of the valley, even during the culminating phases of glaciation, but it did not fall far short of reaching the brink, for it passed completely over the Cathedral Rocks and even over the high brushy summit south of them, as is plainly attested by the boulders which it left on those heights. Having overwhelmed the Cathedral Rocks, the ice, filled the V-shaped gulch of Bridalveil Creek and passed westward over the summit of the Leaning Tower. This entire group of features, to-day so conspicuous in the landscape, was submerged beneath the dazzling sea of ice.
Except for the Bridalveil Glacier, which was but a shallow ice stream, the Yosemite Glacier received no tributaries from the south, and its southern margin was therefore sharply defined throughout, by cliffs as far as Old Inspiration Point and by lateral moraines beyond. The northern margin of the Yosemite Glacier was, by contrast, ill-defined, at least as far west as El Capitan, owing to the fact that the ice which mantled the northern upland sloped down gradually to the level of its surface. Eagle Peak was the first sharp nunatak to split the ice sea, but a short distance farther west rose the bare rounded crowns of El Capitan and Boundary Ridge. West of the small Ribbon Glacier the Yosemite Glacier was flanked by another area of unglaciated upland, of which Fireplace Bluff and the hanging vale of Fireplace Creek were the principal features. At the junction of the Cascade Glacier the northern margin of the trunk glacier was indefinite for a stretch of three-quarters of a mile, but beyond this stretch there were no more tributary glaciers and the margin of the trunk glacier continued well defined down to the end.
Below the elbow bend of the Merced Gorge, where the northern upland breaks down to a mere row of hills, the Yosemite Glacier ceased to be confined to the gorge. Sweeping over the low hills, as a river in flood might sweep over a broken levee, a large share of its ice invaded the Big Meadow flat. There the ice spread out in the form of a broad lobe that thinned rapidly toward its northern and western margins. The main ice stream, depleted by this diversion of its substance, declined rapidly in thickness. The great breadth of surface exposed to the sun, moreover, increased its losses by melting, and so, although a small volume of ice from the Big Meadow flat rejoined it through the gulch of Crane Creek, the Yosemite Glacier came to an end a short distance below El Portal.
From the junction of the Merced and Tenaya Glaciers to its terminus the ice stream measured 16 miles in length. It had a breadth averaging 2 miles, save in its lower course, where it expanded to almost double that breadth. In the Yosemite Valley the glacier attained the greatest thicknessfrom 2,500 feet over the Bridalveil Meadow to more than 3,000 feet opposite Glacier Point. Measured from its ultimate sources on Mount Lyell the Yosemite Glacier attained a maximum length of 37 miles. It was for the Sierra Nevada but a moderately large ice stream, however. Its neighbors on the north and the south, the Tuolumne Glacier and the San Joaquin Glacier, each attained a length of about 60 miles. The Yosemite Glacier, nevertheless, reached a lower level on the flank of the range than most of the other great trunk glaciers. It terminated at an altitude of about 2,000 feet.
Last Updated: 28-Nov-2006