LAND SCULPTURE BY MOUNTAIN GLACIERS

Contrasted sculpturing of continental and mountain glaciers.—In discussing in a previous chapter the rock pavement lately uncovered by the Greenland glacier, we learned that this surface had been lowered by the processes of plucking and abrasion, the combined effect of which is always to reduce the irregularities of the surface, soften its outlines, and from sharply projecting masses to develop rounded shoulders of rock—roches moutonnÉes.

Though the same processes act in much the same manner beneath mountain glaciers, though here upon all parts of the bed, they are, in the earlier stages at least, subordinated to a third process more important than the two acting together. Sculpture by mountain glaciers, instead of reducing surface irregularities and softening outlines, increases the accent of the relief and produces the most sharply rugged topography that is known. In nearly all places where Alpinists resort for difficult rock climbing, mountain glaciers are to be seen, or the evidence for their former presence may be read in unmistakable characters.

Wind distribution of the snow which falls in mountains.—Until quite recently students of glaciation have concerned themselves but little with the work of the wind in lifting and redistributing the snow after it has fallen. We have already seen that, for the continental glaciers, wind appears to be the chief transporting agent, if we except the marginal lobes where glacier flow assumes large importance. In the case of mountain glaciers, also, we are to find that for the earlier stages particularly wind is of the first importance as a redistributing agent. In the higher levels snow is swept up from the ground by all high winds, and does not find a resting place until it is dropped beneath an eddy in some irregularity of the surface; and if the inherited surface be relatively smooth, this will be found in most cases upon the lee of the mountain crest.

In normal cases at least the inherited irregularities of the higher zones of mountain upland are the gentle depressions which develop at the heads of streams. These become, then, the sites of snowdrifts that are augmented in size from year to year, though at first they melt away in the late summer.

The niches which form on snowdrift sites.—Wherever a drift is formed, a process is set in operation, the effect of which is to hollow out and lower the ground beneath it, a process which has been called nivation. The drift shown in Fig. 390 was photographed in late summer at an elevation of some 9000 feet in the Yellowstone National Park. The very gently sloping surface surrounding the drift is covered with grass, but within a zone a few feet in width on the borders of the drift no grass is growing, and in its place is found a fine brown soil which is fast becoming the prey of the moving water derived by melting of the drift. This is explained by the water permeating the crevices of the rock and being rent by the nightly freezing. Farther from the drift the ground is dry, and no such action is possible. With each succeeding spring the augmented drift as it melts carries all finely comminuted rock material down slopes beneath the snow to emerge at the lowest margin and be there deposited in the form of a delta. By the operation of this process of nivation the higher parts of the drift site are lowered as deposition goes on upon the lower. The combined effect is thus to produce a niche or faintly etched amphitheater upon the slope of the mountain (Fig. 391).

The augmented snowdrift moves down the valley—birth of the glacier.—In still lower air temperatures the drifts enlarge with each succeeding year until they endure throughout the summer season. From this stage on, an increment of snow is left from each succeeding season. No longer entirely wasted by melting, the time soon comes when the upper snow layers will by their weight compress the lower into ice, and the mass will begin to creep down the slope along the course of the inherited valley. The enlarged snowdrift which feeds this ice stream is called the nÉvÉ or firn.

Against the sloping cliff which had been shaped by nivation at the upper margin of the snowdrift, that snow which is not of sufficient depth to begin a movement towards the valley separates from the moving portion, opening as it does so a cleft or crevasse parallel to the wall. This crack in the snow is called by its German name Bergschrund or Randspalte, and may perhaps be referred to as the marginal crevasse (Fig. 392).

The excavation of the glacial amphitheater or cirque.—It has been found that the marginal crevasse plays a most important rÔle in the sculpture of mountains by glaciers, for the great amphitheater which is everywhere the collecting basin for the nourishment of mountain glaciers is not an inherited feature, but the handiwork of the ice itself. This was the discovery of Mr. W. D. Johnson, an American topographer and geologist, who, in order to solve the problem of the amphitheater allowed himself to be lowered into such a crevasse upon the Mount Lyell glacier of the Sierra Nevadas in California.

Let down a distance of a hundred and fifty feet, he reached the bottom of the crack, and in a drizzling rain of thaw water stood upon a floor composed of rock masses in part dislodged from a wall which extended some twenty feet upwards upon the cliff side of the crevasse. It was evident that the warm air of the day produced the thaw water which was constantly dripping and which filled every crack and cranny of the rock surface. With the sinking of the sun below the peaks the sudden chill, so characteristic of the end of the day in high mountains, causes this water to freeze and thus rend the rock along its planes of jointing. Broad and thin plates of ice, loosened by melting at the walls, could be extracted from the crevices of the rock as mute witnesses to the powerful stresses developed by this most vigorous of weathering processes.

In short, the rock wall above the glacier, which in its initial stage was the upper wall of the niche hollowed beneath the snowdrift, is first steepened and later continually both recessed and deepened by an intensive frost rending which is in operation at the base of the marginal crevasse. The same process does not go on as rapidly above the surface of the nÉvÉ for the reason that the necessary wetting of the rock surface does not there so generally result from the daily summer thaw. At the bottom of the marginal crevasse alone is this condition fully realized. Intensive frost action where the rock is wet with thaw water daily is thus a fundamental cause, both of the hollowing of the early drift site to form the niche, and of the later enlargement of this niche into an amphitheater or cirque when the drift has been transformed into the nÉvÉ of a glacier. Inasmuch as the crevasse forms where the snow and ice pull away from the rock toward the middle of the depression, the cirque wall in its early stage has the outline of a semicircle. In the Bighorn Mountains of Wyoming, all stages, from the unmodified valley heads to the full-formed cirque, may be seen near one another (Fig. 393). It will be noted that wherever a glacier has formed, as indicated by the cirque, there is a series of lakes which have developed in the valley below (see p. 412).

Life history of the cirque.—In its earliest stage the cirque is more or less uniformly supplied with snow from all sides, and so it enlarges by recession in a manner to retain its early semicircular outline. In a later stage a larger proportion of the snow reaches the cirque at its sides so that its further enlargement causes it to broaden and to flatten somewhat that part of its outline which represents the head of the valley (Fig. 389, p. 364). As the territory of the upland is still further invested by the cirques, their nourishment becomes still more irregular, and the circular outline gives place to a scalloped border, as the amphitheater becomes differentiated into subordinate smaller cirques, each of which corresponds to a scallop of the outline (Fig. 398 and Fig. 394).

Grooved and fretted uplands.—The partial investment by cirques of a mountain upland yields a type of topography quite unlike that produced by any other geological process. The irregularly connected remnants of the inherited upland resemble nothing so much as a layer of dough from which biscuits have been cut (Fig. 395). The surface as a whole, furrowed as it is below the cirques, may be described as a grooved upland (plate 19 A). A further continuation of the process removes all traces of the earlier upland, for the cirques intersect from opposite sides and thus yield palisades of sharp rock pinnacles which rise on precipitous walls from a terraced floor. This ultimate product of cirque sculpture by glaciers is called a fretted upland (plate 18 A and 19 B).

The features carved above the glacier.—The ranges of pinnacles carved out by mountain glaciers have become known by various names of foreign derivation, such as arÊte, grat, aiguille mountains, “files of gendarmes”, etc. They may, perhaps, be best referred to as comb ridges, and according to their position they are differentiated into main and lateral comb ridges, as will be clear from the second map of plate 19.

With the gradual invasion of the upland upon which the cirques have made their attack, the area from which winds may gather up the snow is steadily diminished, and hence cirque recession is correspondingly retarded. Cirques which have approached each other from opposite sides of the ridge until they have become tangent at one point may, however, still receive nourishment at the sides and so continue to cut down the intervening rock wall to form a pass or col. The theoretical curve which results from this intersection is that known as the hyperbola, of which an illustration is afforded by Fig. 396. An approximation to this form is clearly furnished by most of the mountain passes in glaciated mountain districts, and a particularly good illustration is furnished from the vicinity of Glacier on the line of the Canadian Pacific Railway (Fig. 397).

Upon either side of the col the land mass is left in high relief, rising from a more or less triangular base (Fig. 398, III) into a sharp horn or tooth. An illustration of such a horn is furnished by the Matterhorn in the Swiss Alps, or by Mount Sir Donald in the Selkirks, though less noteworthy examples may be found in every maturely glaciated mountain district.

The features shaped beneath the glacier.—Those features which are carved above the glacier—the comb ridge, the col, and the horn—are all shaped as a result of intensive weathering upon the cirque wall. The shaping at lower levels is accomplished by processes in operation below the glacier surface, where weathering is excluded and where plucking and abrasion work together to tear away and grind off the rock surface. By their joint action the valley is both deepened and widened, directly to the height of the glacier surface, and indirectly through undermining as far up as rock extends. Thus the valley is transformed into one of broad and flat bed and precipitous side walls—the U-shaped section illustrated by valleys of the Swiss Alps and in fact in all districts which have been strongly glaciated by mountain glaciers (Fig. 399).

As high up in the valley as it has been occupied by the glacier, the bed is rounded, smoothed, and polished, and marked by the characteristic glacial scorings or striÆ which point down the valley. Above the level of the glacier’s upper surface, on the other hand, erosion is accomplished through undermining or sapping, a process which always leaves precipitous slopes of ragged surface made up of the joint planes on which the fallen blocks have separated from the cliff. Thus there is found a sharp line which separates the smoothly rounded rock surface below from the jagged and precipitous one above (Fig. 400). Inasmuch as this boundary usually separates the scalable from the inaccessible slopes above, snow is apt to lodge at this level and make it strikingly apparent.

If uplift of the land occurs while glaciers occupy the valleys of mountains, an increased capacity for deepening the valley is imparted to these ice streams, and we find, as a result, a deep central valley of U cross section excavated within a relatively broad trough visible above the shoulder on either side of the later furrow. Save only for its characteristic curves, such a valley bears close resemblance to a mature stream valley which has been rejuvenated (see p. 173). The remnants of the earlier glacier-carved valley are, as already stated, gently curving high terraces so common in Switzerland, where they are known as albs or high mountain meadows. These albs may be seen to special advantage on the sides of the Chamonix valley (Fig. 401), the Lauterbrunnen valley, or in fact almost any of the larger Alpine valleys.

The cascade stairway in glacier-carved valleys.—If now, instead of giving our attention to the cross section, we follow the course of the valley that has been occupied by a glacier, we find that it descends by a series of steps or terraces having many backwardly directed treads (plate 19), whereas a normal and well-established river valley has only forward grades. Because of these backward grades the stream waters are impounded, and so lakes are found strung along the valley in chains as the larger beads are found in a rosary, and these are the characteristic rock basin lakes sometimes referred to as “Paternoster Lakes” (see p. 412 and Fig. 402).

When the backward grades upon the valley floor are especially steep, the rock step becomes a rock bar, or Riegel, of which nearly every Alpine valley has its examples. In a walk from the Grimsel to Meiringen many such bars are passed. Carrying in suspension the sharp rock sand from the glacier deposits along its bed, the stream which succeeds to the glacier as it vacates its valley saws its way through these obstructions with a rapidity that is amazing, thus producing narrow defiles, of which the Gorge of the Aar near Meiringen and that of the Gorner near Zermatt are such well-known examples (Fig. 403).

It is characteristic of rivers that the tributaries cut their valleys more rapidly than does the main stream within the neighboring section, though they cannot cut lower than their outlets—the side streams enter accordantly. This is easily explained because the grades of the tributary streams are the steeper, and, as we well know, the corrosion of a valley is augmented at a most amazing rate for each increase of its grade. No such law controls the processes of plucking and abrasion by which the glacier lowers its floor, for these processes appear to depend for their efficiency upon the depth of the ice, and the supply of cutting tools, quite as much as upon the grade of the bed. To apply a homely illustration, the hollowing of flagstones upon our walks is dependent more upon the number of persons that pass over them, and upon their size and the number of protruding nails in their boot heels, than upon the grades upon which they are placed. At all events we find that the main glacier valleys are cut deeper than the side valleys, so that the latter become hanging valleys—they enter the main valley, not upon its bed, but some distance above it (Fig. 404).

The U-shaped hanging valleys, like the main valley, are much too large for the streams which now fill them, and these diminutive side streams plunge over the steep wall of the main valley in ribbon-like falls so thin that the wind turns them aside and disperses the water in the spray of a “bridal veil.” Such falls are found by the hundred in every glaciated mountain district, imparting to it one of the greatest of its scenic charms.

The character profiles which result from sculpture by mountain glaciers.—The lines which are repeated in landscapes carved by mountain glaciers are easy to recognize (Fig. 405). The highest horizon lines are the outlines of horns which are separated by cols. Minaret-like palisades, or “files of gendarmes”, often run for long distances as the characteristic comb ridges. Lower down and lacking the lighter background of the sky, we make out with less distinctness the U-valley, either with or without the albs to show that the sculpturing process has been interrupted by uplift.

The sculpture accomplished by ice caps.—In the case of ice caps, the only rock exposed is found in the neighborhood of the margin—the projecting islands known as nunataks. It is essential for the existence of the ice cap that the rock base should have relatively slight irregularities compared to the dimensions of the cap itself. Except in very high latitudes this base must be somewhat elevated, for like mountain glaciers ice caps are nourished by the surface air currents, and their snows are deposited above the snow line.

The Norwegian tind or beehive mountain.—Within temperate or tropical climes the snow line lies so high that only the loftier mountains are able to support glaciers. It follows that those which are formed flow upon relatively high grades with correspondingly high rate of movement and increased cutting power. Within high latitudes the snow is found nearer the sea level, and glaciers are for the most part correspondingly sluggish in their movements as well as less active denuding agents.

To this condition characteristic of high latitude glaciers, there is added in Norway another in the peculiar shape of the basement beneath the recent and the still existing glaciers. The plateau of Norway is intersected by a network of deep and steep walled fjords, and the glaciers have developed as small ice caps perched upon veritable pedestals of rock, over the margins of which their outlet tongues of ice descend on steep slopes into the fjord. The tops of the pedestals thus come to be shaped by the plucking and abrading processes into flat domes (Fig. 406), while the knobs of rock, which as nunataks reach above the surface of the ice, divide the outflowing ice tongues at the margin of the pedestal. These tongues being much more active denuding agents, because of their steep gradients, continually lower their beds, thus transforming the earlier knobs of rock into high and steep mountains of more or less circular base. Such “beehive” mountains upon the margins of the fjords are the characteristic Norwegian tinds (Fig. 407).

Reading References for Chapter XXVI

I. C. Russell. Quaternary History of Mono Valley, California, 8th Ann. Rept. U. S. Geol. Surv., 1889, pp. 329-371, pls. 27-37.

F. E. Matthes. Glacial Sculpture of the Bighorn Mountains, Wyoming, 21st Ann. Rept. U. S. Geol. Surv., 1900, Pt. ii, pp. 179-185, pl. 23.

W. D. Johnson. Maturity in Alpine Glacial Erosion, Jour. Geol., vol. 12, 1904, pp. 569-578.

G. K. Gilbert. Systematic Asymmetry of Crest Lines in the High Sierras of California, ibid., pp. 579-588.

Emm. de Martonne. Sur la Formation des Cirques, Ann. de GÉogr., vol. 10, 1901, pp. 10-16.

W. M. Davis. Glacial Erosion in North Wales, Quart. Jour. Geol. Soc. Lond., vol. 65, 1909, pp. 281-350, pl. 14.

Ed. BrÜckner. Die Glazialen ZÜge im Antlitz der Alpen, Naturw. Wochenschr., N. F., vol. 8, 1909.

William H. Hobbs. Characteristics of Existing Glaciers, pp. 1-96.

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