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Avalanches of snow, their mode and frequency of occurrence in mountains of high relief, the menace they constitute to life and property, and their eroding action on cliffs and mountain slopes, have long been subjects of scientific study in certain parts of Europe; but in the United States, whose lofty western ranges are but sparsely inhabited, and where the economic need for their study has thus far been relatively slight, avalanches have received until recently hardly any attention from either geographers or geologists. During the summers of 1935 and 1936, however, the author, while engaged in geomorphologic and glaciologic studies in the Sierra Nevada of California, became aware of the important part which avalanches there play, and have played throughout probably all of Pleistocene time, as eroding agents, more especially in the region above the timber line—the alpine zone—where they produce a distinctive type of cliff sculpture. On and in the vicinity of Mount Whitney, the culminating peak of the range, and the highest in the continental United States, that type of cliff sculpture attains an exceptional degree of perfection and regularity, and accordingly he feels impelled to offer this brief note as a firstling contribution from the United States to the study of avalanche action.
Perhaps it will duplicate in some measure the excellent work of European observers, notably that of Allix,1 but he feels justified nevertheless in presenting the results of his own studies, because they were made in a mountain district where topography, rock structure and climatic conditions together have permitted the three erosional processes characteristic of the alpine zone—glacial action, avalanche action and nivation—to produce their respective type forms with great distinctness, and where because of slow rock weathering those forms remain exceptionally well preserved.
In order that the reasons for this singularly fine development of type forms may be understood it should be explained, first, that the mountains in question are, in spite of their great altitude—which ranges from 12,000 to over 14,000 feet—prevailingly full-bodied, having escaped complete dissection during the cycle of stream erosion which preceded glaciation. Many of them possess, in consequence, gently sloping summit platforms of considerable extent that are undivided by streamcut gulches but sharply bounded by the headwalls of glacial cirques hewn in the sides of the mountains. These summit platforms, although surrounded by the very sources of the Pleistocene glaciers, have themselves remained unglaciated, being windswept throughout. The snow that fell upon them in glacial times was, as it is now, in large part blown away by the fierce gales of winter, and was never permitted to accumulate to sufficient depth to form gravitating glaciers. It lay only in stationary drifts and fields of moderate thickness, promoting with its melt-water the process of nivation.
The summit platforms of Mount Whitney and of the other tabular peaks in its vicinity are therefore typical nivated areas, where the rock has been riven into fragments, and is still being riven, by recurrent freezings and thawings, but where no effective transporting agent in the form of stream or glacier was available during glacial times, nor is at present, to remove the frost-split fragments.
The cirques and canyons below, again, appear never to have been filled with glacial ice to their full capacity, as were the cirques and canyons farther north in the Sierra Nevada. The great majority of them were filled only to approximately one-half their depth, some of them even to less than one-half. This fact is shown clearly on their walls by the height of the upper limit of glaciation, which, as may be seen in the photographic illustrations, is almost everywhere conspicuous and unmistakable, because of the contrast between the angular sculpture of the cliffs above it and the smooth forms of the glaciated rock surfaces below; also because the canyon walls have suffered on the whole but little from postglacial weathering. The maximum depth which the glaciers of the Wisconsin (Würm) stage2 attained in the cirques and canyons of the Mount Whitney district can therefore be definitely determined as having ranged between 500 and 1200 feet. As a rule it was less than 1000 feet,3 and as a consequence the rock walls rose from 500 to 1500 feet above the glaciers.
It is on these unglaciated rock walls, at the heads of the cirques as well as on
the sides of the canyons, that the evidences of avalanche erosion are best displayed. These rock walls owe their steep profiles primarily to the undercutting
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The gently sloping tabular summits of the Mount Whitney district are nivated but not
glaciated. Their walls above the limit of glaciation are deeply fluted by alternating snow chutes
and rock ribs. By François Matthes
The last named process deserves, in the author’s opinion, more recognition
than it has been generally accorded thus far; for his observations have shown
him that the distinctive type of cliff sculpture which it produces occurs in a
large percentage of the glacial cirques and canyons, not only in the Mount
Whitney district but elsewhere in the Sierra Nevada, and likewise in certain
parts of the Rocky Mountains and the Cascade Range. It occurs, in fact, wherever
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Typical snow chutes on the north face of Mount Hitchcock. The snow chutes are carved to
depths of 50 to 100 feet in disregard of the joint structure of the granite. All terminate at the
upper limit of glaciation which is clearly marked. Below is the glaciated floor of Whitney
Canyon. By François Matthes
Avalanche sculpture, wherever it is well developed, gives the cliffs a distinctly fluted appearance, there being an alternation of smoothly concave gullies and sharp rock ribs, all trending downward roughly parallel to one another in the direction of greatest declivity. That the smooth gullies are actually the pathways of recurring avalanches is known from direct observation and, besides, it is evident from the fact that a large pile of snow usually lies beneath each of them in the spring or early summer. In addition there is on the slope below each gully a cone of rock fragments carried down by the rushing snow masses.
Allix has referred to the alternating gullies and rock ribs, appropriately, as
“cannelures.”4 That word, however, cannot well be taken over into the English
vocabulary, and accordingly it seems desirable to find a suitable English word
to denote the features in question. The word gully, used above in a preliminary
way, is decidedly inappropriate, being associated in the minds of geographers and
geologists with the action of running water. It is therefore proposed to employ
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Postglacial weathering and avalanche sculpture along zones of shearing.
East face of Mount Hitchcock. By François Matthes
Snow chutes, as they will here be called, are not necessarily the equivalent of “chimneys” and “couloirs” of which alpinists make use in scaling peaks. Those narrow recesses in the flanks of mountains are produced primarily by differential weathering along vertical or nearly vertical zones of faulting or sheeted structures in the rock. The positions of many snow chutes, it is true, are determined by such structures, but it is equally true that many snow chutes occur in places where no lines or zones of weakness exist in the rock. On the cliffs of granitic rocks in the Sierra Nevada they were observed to cross the jointing at various angles. The most perfect forms were found in massive granite, wholly devoid of joint fractures. Such forms, produced solely by abrasion, are extremely smooth, with semi-circular or parabolic cross sections. To the mountaineer they offer no convenient routes for an ascent; on the contrary they are to be avoided by him, being too smooth and too slippery. Even in horizontally stratified rocks of sedimetary origin very smooth and regular chutes are often worn by avalanches. Excellent examples of this kind were observed by the author in Glacier National Park, Montana.
How entirely regardless of structure snow chutes may develop in some cliffs is well exemplified by the photograph on page 158. It shows a remarkably regular series of chutes worn in the north side of Mount Hitchcock (Sequoia National Park, California), which is composed of vertically sheeted granite, the strike of the fractures being from east to west—that is, at right angles to the direction of the chutes. The latter are, by estimation, fully 50 feet deep. A trickle of water is seen descending through one of the chutes. It appears as a dark line that might readily be mistaken for a little stream-cut trench. There is, however, no such trench. The water merely finds its way down over the smooth, concave bottom of the chute; it is in no wise instrumental in deepening the chute.
Inspection of the photograph shows that many of the chutes are wider at the top than at the bottom, and that not a few branch upward into two or more minor chutes, separated by small, attentuated rock ribs. The reason appears to be that the bulk of the windblown snow accumulates at the top of the cliff in the form of a massive cornice, and it is the sudden breaking and slipping down of sections of this cornice that initiates the avalanches. As the latter progress downward, more and more of their volume is dissipated into whirling spray, and as a consequence the lower parts of the chutes receive less wear than the upper parts and remain relatively narrow. Not infrequently, moreover, an avalanche, after rushing down through the upper half of a chute, comes to rest in the lower half, owing to the retarding action of snow that clogs its path.
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Gigantic snow chute carved in massive granite, near Bearpaw Camp.
Sequoia National Park. By François Matthes
It deserves to be pointed out, finally, that the snow chutes on a given canyon wall all terminate at approximately the same height above the canyon floor— namely at the upper limit of glaciation. This is illustrated on page 158, but it can be observed in scores of glacial cirques and canyons throughout the Sierra Nevada. It demonstrates clearly that the chutes were formed largely, almost wholly, during glacial times, when the canyons were about one-half filled with ice. Avalanche action has of course continued since the disappearance of the glaciers and is frequent every winter at the present time; but the amount of erosional work that avalanches have accomplished in the resistant granitic rocks of the Sierra Nevada during postglacial time is decidedly small and in places almost negligible. As may be seen on page 158 cones of rock fragments beneath the chutes on Mount Hitchcock, which give a measure of postglacial avalanche erosion, are of small or at best, moderate volume; they contain but a small fraction of the total amount of material that has been carved out of the chutes. It is evident, moreover, that the chutes themselves have been prolonged downward but little below the upper limit of glaciation.
In mountain regions where the rocks weather more rapidly than in the Sierra Nevada, and where the canyon walls are dismantled at a correspondingly rapid rate, avalanche sculpture soon loses its characteristic forms and is likely to escape detection. Moreover, wherever the rock structure is highly irregular or the cliff sculpture is controlled by rock masses that vary greatly in resistance, the chances for the development of typical snow chutes are relatively small, and such forms may never become distinct. Such is the case in many parts of the Rocky Mountains, of the Cascade Range and of the great ranges of Alaska. Such is the case also in many parts of the European Alps; and so it is, doubtless, that avalanche sculpture has not been clearly recognized thus far, save in a few favorable localities, and has not yet taken its place along with the type forms of glaciation and nivation as a characteristic part of alpine morphology.
Reprinted from “Transactions of the Meetings of the International Commissions of Snow and of Glaciers,” Edinburgh, September 1936. International Association of Hydrology, Bulletin 23, pages 631-637. Riga, 1938.
1The author freely confesses that he was not acquainted with the results of Allix’s studies when he made his own observations in the Sierra Nevada. His attention was invited to Allix’s work only recently, but, being out in the field and unable to visit a library, he has had to proceed with the writing of this note—in camp—without having had opportunity to familiarize himself with Allix’s observations and conclusions.
2Referred to in other essays as the later glacial stage, in distinction from the earlier of the two well-recorded stages (the El Portal) in this part of the Sierra Nevada. “Würm” is the European equivalent.—Ed.
3These relatively shallow glaciers, it should be explained, were nevertheless many miles in length and were tributary to the great Kern Glacier, which filled the Kern Canyon to depths of 2500 feet and more, and attained a length of 26 miles.
4Allix, André, “La morphologic glaciaire en Vercors,” Recueil des Travaux de L’Institut de Géographic Alpine de l’Université de Grenoble, Vol. II, 1914, pp. 109-110.
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