Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

GLACIATION OF THE SIERRA NEVADA

NATURE OF ICE MANTLE

During the great ice age, which began presumably early in the Quaternary period (see table of geologic time divisions, p. 50), the climate of the Sierra Nevada turned wintry, and the higher parts of the range became heavily mantled with snow and ice, much like the higher ranges in Canada and Alaska at the present time. As a consequence the streams in the High Sierra and in most of the Yosemite region were superseded as valley-cutting agents by glaciers, and thus these regions entered upon a new epoch in the history of their development.

The ice mantle of the Sierra Nevada was not, as some of the pioneer observers supposed, part of the vast continental ice sheet which overspread most of Canada and the northern United States east of the Rocky Mountains. It was a distinct and wholly separate mass that originated locally, on the range itself. Indeed, the Sierra Nevada in glacial time was, like most of the higher mountain ranges of the Far West, a separate center of snow accumulation. That it receives heavy snowfalls to-day, owing to its great height and its position across the path of the moisture-laden winds that blow in from the Pacific Ocean, has already been explained (p. 10). In glacial time, under essentially analogous conditions, the range was the recipient probably of still greater quantities of snow. In any event the heat of summer then did not suffice to melt away all the snow that fell in winter, and as a consequence an annual residue was left, so that in the course of time great fields of compacted snow and, ultimately, glaciers were formed. A true ice cap, such as completely buries the mountains of Greenland, however, there was not. At all times, even during the culminating stages of the glacial epoch, the highest peaks and crests of the Sierra Nevada stood out partly bare. That fact is attested by the best of evidence, as will be shown further on.

Comparative study of the glaciated areas in the western mountain ranges of the United States discloses the fact that the Sierra Nevada had a more extensive ice mantle than any other range in the same latitude. Its ice mantle was larger, even, than that of the Cascade Range, which adjoins the Sierra Nevada on the north. This fact seems incredible at first, for one would naturally suppose that the ice of glacial time was most extensive in the northern latitudes and dwindled gradually southward. However, two other factors besides latitude determine the extent which snow fields and glaciers may attain on mountains—namely, altitude and precipitation—and their influence was preponderant throughout most of the Sierra-Cascade chain. A brief survey of that chain will readily show how greatly its glacial covering varied locally in extent as a result of the combined influences of latitude, altitude, and precipitation and will help to make clear why the Yosemite Valley lay in the midst of the largest separate area of glaciation west of the Rocky Mountains.

DISTRIBUTION OF ICE ON SIERRA-CASCADE CHAIN

The Sierra-Cascade chain, 1,000 miles in length, extends over 14 degrees of latitude, from the 49th parallel down to the 35th, and traverses regions of the utmost geographic and climatic diversity. Beginning near the Canadian boundary, in a region of extremely wet, snowy climate, it terminates at the edge of the Mohave Desert, which is one of the driest and most torrid areas on the continent.

Naturally its glacial covering was very extensive at the north end, in the vicinity of Mount Baker, Mount Rainier, and Glacier Peak, where the great height and breadth of the Cascade Range, the enormous quantities of snow supplied by the westerly winds, and the prolonged winters together produced conditions exceptionally favorable for glaciation. This part of the chain lay fairly smothered under snow and ice and sent forth glaciers 80 to 100 miles in length.

South of Mount Rainier, however, the ice mantle contracted rapidly in breadth, mainly as a result of declining altitude. Throughout southern Washington and northern Oregon, where the crest line, not counting the isolated volcanic peaks, rises scarcely above 5,000 feet, the glaciers attained lengths of only a dozen to a score of miles. Still farther south, throughout that 200-mile stretch of which Crater Lake, Mount Shasta, and Lassen Peak are the dominant landmarks, there was no ice mantle properly so called but only detached glaciers and snow fields that lay in sheltered canyons high up under the main peaks. This dearth of ice, this state of semiaridity, was due not to a further decline in altitude, for the range here again rises to 6,000 and in places even to 8,000 feet, but to deficient snowfall caused by the presence between the Cascade Range and the Pacific Ocean of a large complex of mountains—the Siskiyou, Salmon, Trinity, and Contra Costa Ranges—which intercepted a considerable share of the moisture from the westerly winds.

Southward from the canyon of Feather River, however, in the northern part of the Sierra Nevada, glaciation developed on an increasingly large scale, owing both to greater altitude and greater snowfall, the intercepting power of the Coast Ranges here diminishing with decline in height. In the vicinity of Lake Tahoe, where the Sierra Nevada attains altitudes of more than 9,000 feet, the ice fields and ice streams were large enough to coalesce and produce trunk glaciers from 15 to 20 miles in length. And in the stretch from Lake Tahoe to Mount Lyell, in which the crest rises progressively to altitudes of 11,000, 12,000, and 13,000 feet, the snows were so abundant as to mantle the range continuously over a breadth of 20 to 30 miles and to create trunk glaciers 40 to 60 miles in length. In short, there was reached in this central part of the Sierra Nevada, where the Yosemite is situated, another climax of glaciation—a climax second only to that which was attained near the Canadian boundary, 800 miles to the north.

Over a stretch of fully 100 miles—from Mount Lyell to Mount Whitney—the glacial mantle extended almost undiminished in breadth. It covered all those parts of the High Sierra which are drained by the San Joaquin, Kings, Kaweah, and Kern Rivers and which are crowned by the culminating peaks of the range. A short distance south of Mount Whitney, however, the glacial mantle came abruptly to an end. Beyond the Kern Glacier there were only small detached ice bodies, the southernmost of which lay on Olancha Peak, in latitude 36° 15', at an altitude of about 10,000 feet. South of this point no glaciers could exist, owing to the rapidly declining height of the range, the lack of sufficient snow precipitation, and the great losses of snow by evaporation in the heated and parched air that was wafted up from the torrid southern part of the Great Valley of California and from the Mohave Desert.

ORIGIN AND GROWTH OF ANCIENT SIERRA GLACIERS

How, it may be asked, did the glacial mantle of the Sierra Nevada originate, and how did it grow to such vast extent? These are matters that deserve a word of elucidation in passing, the more so as the term "glacial mantle," which has been used repeatedly here, is somewhat misleading. That term might seem to imply a thick layer of snow and ice spread rather evenly over the ups and downs of the landscape, but in reality the bulk of the snow and ice in a region of rugged mountains in process of being glaciated is concentrated in the valleys, where it may attain depths of hundreds or even thousands of feet, whereas on the steep-sided peaks and crests there may be but a thin veneer of snow, or in places none whatever. Such, doubtless, was the state of things also in the Sierra Nevada during the ice age, and its glacial mantle, therefore, is to be conceived as a composite mass that was made up of a multitude of independently formed ice bodies lying mostly in the valleys and basins between the peaks.

The snow in a region of high mountains tends from the start to accumulate unevenly—in the first place, because on southerly slopes the snow wastes away rapidly, whereas on northerly slopes, especially in places shaded by peaks or cliffs, it is conserved for long periods, owing to the fact, primarily, that at high altitudes the radiant heat of the sun is almost the sole melting agent, the thin and dry air remaining cold, even in midsummer; in the second place, because the gales of winter sweep the snow, while still in a powdery state, from the slopes exposed to their blasts and drop it on the lee sides of peaks and crests, and, as the gales come prevailingly from the same quarter, they tend to heap the snow ever in the same sheltered spots, the "wind shadows," as they have been termed; and in the third place, because on very steep slopes snow can not accumulate indefinitely but is removed by avalanches.

Now in the Sierra Nevada, as in many other mountain ranges, these circumstances governing the distribution of the snow, especially the first two mentioned, though entirely independent from one another, in large measure enhance one another's effect. As the prevailing winds are westerly or southwesterly, the wind shadows lie mostly on the easterly or northeasterly sides of the mountains and thus coincide in large measure with the sun shadows. Avalanches are likely to be most frequent on northerly and northeasterly slopes because these are the most heavily laden with snow. The tendency for both wind and avalanches is thus in general to concentrate the snow in the very spots where it will be exposed to the least amount of solar heat, and where the chances for its conservation are the best.

The results may be readily observed in the High Sierra at the present time, although the snow there no longer accumulates from year to year, the gains made in winter being offset by the losses sustained in the ensuing summer. The two sides of a northwestward-trending crest, such as the Clark Range, the Cathedral Range, or the main divide, stand in striking contrast to each other: the sunny and wind-swept southwest side as a rule is wholly bared of snow by the middle of July, whereas the shady and sheltered northeast side remains checkered with drifts and fields of snow until the end of the summer. (See pl. 13, B.) The deeply inset valley heads that open to the north, the northeast, or the northwest naturally hold the largest accumulations; a few still contain small glaciers. It was in these favorably situated valley heads that the first glaciers of the ice age originated; and even to-day the conditions in them closely approach an equilibrium between accumulation and wastage. The losses by melting in summer only slightly outbalance the gains by deposition in winter, and in occasional years the gains slightly exceed the losses. No one conversant with these facts through repeated visits to the High Sierra can doubt that it would require but a trifling change in the present climate to bring on again a period of continued glacier growth.

One contributory reason for this state of things, however, remains to be pointed out: the valley heads mentioned, owing to prolonged and vigorous glaciation, now have amphitheater-like forms, semicircular or horseshoe-shaped in plan, and encircled by precipitous cliffs. (See pl. 28.) They have been transformed into what the Swiss mountaineers term "cirques," and no sculptural forms are better designed than these for the effective entrapment of snow, by drifting and by avalanches, nor for its conservation in well-shaded compact bodies. Throughout the High Sierra these glacier cradles abound. There are literally thousands of them, and they constitute a characteristic and highly picturesque element of the landscape. It will readily be seen, therefore, that the configuration of the High Sierra is now far more propitious for the development of glaciers than it was at the beginning of the ice age.

PLATE 28.—A (top), EMPTY CIRQUE ON SHADY NORTHEAST SIDE OF KUNA CREST. Deep amphitheaterlike hollows of this kind were the sources of the ancient Sierra glaciers. Originally sharp-cut valley heads, they have been enlarged to their present capacious forms by the excavating action peculiar to the glaciers. Most of them have rock-rimmed lake basins in their floors. Photograph by G. K. Gilbert. B (bottom), DANA GLACIER. The Dana Glacier occupies the shaded side of a cirque cut by a much larger glacier of the ice age. At its front are several moraine ridges composed of rock débris. Photograph by G. K. Gilbert.

With this insight into the conditions that govern the accumulation of snow on high mountains, one may readily picture in imagination how the glaciers in the Sierra Nevada originated and how they expanded by degrees into a continuous glacial mantle. At first there were only drifts and fields of compacted snow that survived from year to year in valley heads with northerly or northeasterly exposures; by degrees these perpetual bodies of snow ice, or "névé," attained depths of a hundred feet or more, until under the influence of the force of gravity they acquired an imperceptibly slow, flowlike motion and, being constantly replenished at their heads, lengthened forward in tonguelike ice streams or glaciers. Each of these small ice streams followed a preglacial stream-worn valley, and where such valleys united the glaciers became confluent, forming broader and deeper ice streams. These in turn coalesced farther down into powerful ice rivers, or trunk glaciers, that advanced down the main Sierra canyons. As the glacial climate increased in severity, moreover, glaciers were formed also in the valley heads on the less favored southwestern flanks of the mountain crests. Thus at length every valley in the High Sierra came to hold an ice stream, and there resulted a vast system of glaciers whose ramifications corresponded closely to those of the system of preglacial valleys and canyons.

During the periods of maximum glaciation the snows were so abundant that the trunk glaciers, though thousands of feet in thickness, could not carry the snow-ice away from the gathering grounds in the High Sierra as fast as it accumulated. The branch glaciers in consequence overflowed, submerged the low divides between them, and coalesced to form unbroken expanses scores of miles in extent. The glacier currents at this stage no longer followed everywhere the axes of the preglacial valleys but were diverted through gaps and over low ridges and even advanced broadly over the billowy uplands, such as those of the Yosemite region, regardless of the trend of the lesser features. Only the major peaks and crests stood out above the ice flood, like dark rocky islands above a dazzling sea.

EXTENT REACHED BY ANCIENT SIERRA GLACIERS

The extent of the glacial covering of the Sierra Nevada has long been a subject of conjecture and dispute. Some writers have maintained that the glaciers enveloped the entire range down to its very base and even spread far beyond, over the lowlands of California. Others have insisted that the glaciers were restricted to the higher parts of the range. This wide divergence of opinion was due to the fact that the evidences of glaciation, though conspicuous and unmistakable in the High Sierra, are obscure in the heavily forested middle zone of the western flank and had not been systematically traced to their outer limits.

To-day the situation is quite different, for, as a result of a systematic survey of glacial deposits, the margin of the glacial mantle is definitely mapped, and the dimensions of the individual glaciers are known over a considerable part of the Sierra Nevada. Even where the ice margin remains unmapped, its position can now be determined tentatively within narrow limits. There is thus no further doubt that the glacial mantle was confined wholly to the upper parts of the Sierra, and that it reached at no point down to, or even near to, the western base of the range. In the south-central portion of the range, where the glacial mantle was broadest, its western margin descended to altitudes of somewhat under 5,000 feet. The trunk glaciers, of course, descended to still lower levels, yet even they fell far short of reaching the foot of the range. The Tuolumne Glacier, which was the longest ice stream north of the Yosemite region, attained a maximum length of 60 miles and projected about a dozen miles beyond the margin of the ice mantle that lay on the adjoining uplands. It terminated, however, fully 30 miles from the foot of the range and at an altitude of about 2,000 feet. The Yosemite Glacier at the time of maximum glaciation was 37 miles long and projected 7 miles beyond the margin of the ice mantle. Its terminus lay in the Merced Canyon just below the site of El Portal, about 50 miles from the foot of the range and about 2,000 feet above it. The San Joaquin Glacier was nearly as long as the Tuolumne Glacier, but it advanced only a few miles beyond the margin of the ice mantle on the flanking uplands. The glacier halted 45 miles from the mouth of its canyon, at an altitude of about 3,000 feet. The Kings Glacier, despite the great altitude of the crest region which it drained, attained a length of only 44 miles (measured along its middle branch) and came to an end about 37 miles miles from the base of the range at an altitude of 2,500 feet.

The low levels reached by these trunk glaciers seem truly remarkable when it is considered that their lower portions lay wholly in the zone of wastage, where even in the shaded spots the heat of summer was abundantly able to remove the snows of winter. The Yosemite Glacier, for instance, reached more than a mile below the level (somewhat above 8,000 feet) in which glaciers were formed in the Yosemite region. The Tuolumne Glacier reached 6,000 feet below this level; the San Joaquin Glacier, about 5,300 feet; the Kings Glacier, about 6,000 feet. (The level of glacier generation rose gradually southward.)

The ability of these glaciers to reach such low levels in spite of the warmth that prevailed in the zone of wastage affords impressive testimony of the immense surplus of snow ice that descended from the higher parts of the range. However, it is accounted for also in part by the protection from the sun's rays, that was afforded to the glaciers by the high walls of the canyons; by the relatively small surface areas proportionate to their bulk which the glaciers, 3,000 feet to over 4,000 feet in thickness, presented to the melting agencies; and the relatively rapid movement of the ice, which in the thicker glaciers must have averaged several feet a day.

There can be little doubt, further, that the trunk glaciers penetrated also far below the upper limits of vegetation and even well into the zone of forests, like so many of the glaciers now existing in the Alps of Switzerland and in Canada. A comparison with the present glaciers on Mount Rainier, which are readily accessible to the public, is particularly instructive in this connection. These glaciers originate at altitudes considerably above 7,000 feet, yet the larger ones descend to altitudes as low as 3,900 and even 3,400 feet—that is, 2,000 to 2,500 feet below the timber line.⁴⁴

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⁴⁴That the Sierra Nevada in glacial time bore forests on its lower slopes seems scarcely open to doubt. There is reason to believe, moreover, that its forests then contained sequoias or "big trees." None of the present sequoias, of course, witnessed the reign of ice in the Sierra, for the oldest of them probably do not exceed 4,000 years in age, whereas the glacial epoch came to an end between 15,000 and 20,000 years ago; but the present trees are doubtless descendants of those which lived in the glacial epoch, and their ancestry goes back much farther still, to the Tertiary period, as is evident from the occurrence of fragments of fossil sequoia wood in the sediments of lava-entombed stream beds of that period.

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On its eastern flank also the Sierra Nevada bore a great array of glaciers, there being a glacier in almost every canyon; but these glaciers were in general much shorter than those on the western flank, owing to the abruptness of the escarpment, the shortness of the canyons, and the small extent of glacier-generating territory at their heads. Most of these glaciers, nevertheless, reached down to the eastern foot of the range, and not a few projected well out into the adjoining lowlands. This was true especially in the north-central and south-central parts of the range, where the adjoining lowlands have altitudes of 5,000 to nearly 7,000 feet. Doubtless the abundant waters that issued from the glaciers were instrumental in transforming these now arid wastes into fertile, verdant land.

The basin of Mono Lake was invaded by no less than six ice tongues, each of which extended several miles out from the range—indeed, to the shores of Mono Lake itself, which at that time was considerably larger than it now is. In the regions south of Mono Lake the glaciers projected as a rule but little beyond the mouths of their canyons, and along the border of Owens Valley the glaciers were confined mostly to the upper parts of the canyons; still farther south there were only scattered snow fields, and the array of ice bodies came to an end.