THE GLACIER’S SURFACE FEATURES AND THE DEPOSITS UPON ITS BED

The glacier flow.—The downward flow of the ice within a mountain glacier has been the subject of many investigations and the topic of many heated discussions since the time when Louis Agassiz and his companions set a line of stakes across the Aar glacier and numbered the surface bowlders in preparation for repeated observations. Their first observation was that the line of stakes, which had run straight across the glacier, was distorted into a curve which was convex downstream (Fig. 416, A´), thus showing that the surface layers have more rapid motion in proportion as they are distant from the side margins. Summarizing these and later studies, it may be stated that the glacier increases its rate of motion from its side margin towards its center line, from its bed upwards towards its surface, and below the nÉvÉ the velocity is greatest where the fall is greatest and also wherever the cross section diminishes. In all these particulars, then, the ice of the glacier behaves like a stream of water. The average rate of flow of Alpine glaciers varies from a few inches to a few feet per day, and is greater during the warm summer season. The Muir glacier of Alaska has been shown to move at the rate of about seven feet per day.

In traveling from the nÉvÉ downward to the glacier foot, the snow not only changes into ice, but it undergoes a granulating process with continued increase in the size of the nodules until at the foot of the glacier these may be picked out of the partially melted ice as articulating balls the size of the fist or larger. Glacier ice has therefore a structure quite different from that of lake ice, since the latter is developed in parallel needles perpendicular to the freezing surface.

Crevasses and sÉracs.—Prominent surface indications of glacier movement are found in the open cracks or crevasses, which are the marks of its yielding to tensional stresses. Crevasses are apt to run either directly across the glacier, wherever there is a steep descent upon its bed, or diagonally, running in from the margin and directed up-glacier (r, r, r, of Fig. 416), though they occasionally run longitudinally with the glacier when there is a rock terrace at the side of the valley beneath the ice. The diagonal crevasses at the glacier margin are due to the more sluggish movement where the ice is held back by friction upon the walls of the valley, as will be clear from Fig. 416. The square a has by this movement been distorted into the lozenge a´, so that the line xy has been extended into x´y´, with the obvious tendency to open cracks in the direction ss.

Every glacier surface below its nÉvÉ is marked by steps or terraces, which are well understood to overlie corresponding steps of the cascade stairway to be seen in all vacated glacier valleys (plate 19). The steep risers of these steps are usually marked by parallel crevasses which cross the glacier. Under the rays of the sun, which strike them more from one side than from the other, the slices into which the ice is divided are transformed into sharpened blades and needles which are known as sÉracs (Fig. 401, p. 376, and Fig. 417).

The numerous crevasses tell us that the ice is many times wrenched apart during its journey down the glacier. This has been illustrated by somewhat grewsome incidents connected with accidents to Alpinists, but as they illustrate in some measure both the mode and the rate of motion of Swiss glaciers, they are worthy of our consideration.

Bodies given up by the Glacier des Bossons.—In the year 1820, during one of the earlier ascents of Mont Blanc, three guides were buried beneath an avalanche near the Rochers Rouges in the nÉvÉ of the Glacier des Bossons (Fig. 418). In 1858 Dr. Forbes, who had measured the rate of flow of a number of Alpine glaciers, predicted that the bodies of the victims of this accident would be given up by the glacier after being entombed from thirty-five to forty years. In the year 1861, or forty-one years after the disaster, the heads of the three guides, separated from their bodies, with some hands and fragments of clothing, appeared at the foot of the Glacier des Bossons, and in such a state of preservation that they were easily recognized by a guide who had known them in life. Inasmuch as these fragments of the bodies had required forty-one years to travel in the ice the three thousand meters which separate the place of the accident from the foot of the glacier, the rate of movement was twenty centimeters, or eight inches, per day.

Various separated parts of the body of Captain Arkwright, who had been lost in 1866 upon the nÉvÉ of the same glacier, reappeared at its foot after entombment in the ice for a period of thirty-one years. To-day the time of reappearance of portions of the bodies of persons lost upon Mont Blanc is rather accurately predicted, so that friends repair to Chamonix to await the giving up of its victims by the Glacier des Bossons.

The moraines.—The horns and comb ridges which rise above the glacier surface are continually subject to frost weathering, and from time to time drop their separated fragments upon the glacier. Falling as these do from considerable heights, they reach the ice under a high velocity, and rebounding, sometimes travel well out upon its surface before coming to a temporary rest. Upon a fresh snow surface of the nÉvÉ their tracks may sometimes be followed with the eye for considerable distances, and their fall is a constant menace to Alpine climbers. Below the nÉvÉ the larger number of such fragments remain near the cliff, and the lines of flow of the ice within the glacier surface are such that blocks which reach points farther out upon the glacier are later gathered in beneath the cliff at the side (Fig. 419). The ridge of angular rock dÉbris which thus forms at the side of the glacier is called a lateral moraine (see Fig. 411, p. 385, and Fig. 420).

At the junction of two glacier streams, the lateral moraines are joined, and there move out upon the ice surface of the resultant glacier as a medial moraine. Thus from the number of medial moraines upon a glacier surface it is possible to say that the important tributary glaciers number one more (Fig. 420).

The plucking and abrading processes in operation beneath the glacier, quarry the rock upon its bed, and after shaping and smoothing the separated rock fragments, these are incorporated within the lower layers of the ice as englacial rock dÉbris. In spaces favorable for its accumulation, a portion of this material, together with much finer dÉbris and rock flour, is left behind as a ground moraine upon the bed of the glacier (see Fig. 421).

At the foot of the glacier the relatively angular rock dÉbris, which has been carried upon the surface, and the soled and polished englacial material from near the bottom, are alike deposited in a common marginal ridge known as the terminal or end moraine (plate 21 B).

Selective melting upon the glacier surface.—The white surface of the glacier generally reflects a large proportion of the sun’s rays which reach it, and its more rapid melting is largely accomplished through the agency of rock fragments spread upon its surface. Such fragments, however, promote or retard the melting process in inverse proportion to their size up to a certain limit, and above that size their action is always to protect the glacier from the sun. This nice adjustment to the size of the rock fragments will be clear from examination of Fig. 422, for rock is a poor conductor of heat, and in even the longest summer day a thin outer layer only is appreciably warmed. Large rock blocks, grouped in the medial and lateral moraines, hold back the process of lowering the glacier surface during the summer, so that late in the season these moraines stand fifty feet or more above the glacier as armored ice ridges.

Isolated and large rock slabs, as the season advances, may come to form the capping of an ice pedestal which they overhang and are known as glacier tables (Fig. 423). Such tables the sun attacks more upon one side than upon the other, so that the slab inclines more and more to the south and may eventually slip down until its edges rest against the glacier surface. Rounded bowlders, which less frequently become perched upon ice pedestals, may, from a similar process, slide down upon the southern side and leave a pyramid of ice furrowed upon this side and known as an ice pyramid.

Fine dirt when scattered over the glacier surface is, on the other hand, most effective in lowering its level by melting. Use was made of this knowledge to lower the great drifts of snow which had to be removed each season during the construction of the new Bergen railway of southern Norway. Each dirt particle, being warmed throughout by the sun’s rays, melts its way rapidly into the glacier surface until the dust well which it has formed is so deep that the slanting rays of the sun no longer reach it. When the dirt particles are near together, the thin walls which separate the dust wells are attacked from the sides in the warm air of summer days, thus producing from a patch of dirt upon the glacier surface a bath tub (Fig. 424 d). At night the water which fills these basins is frozen to form a lining of ice needles projecting inward from the wall, and this, repeated in succeeding nights, may entirely close the basin with water ice and produce the familiar glacier star (Fig. 424 c).

If the dirt upon the glacier surface, instead of being scattered, is so disposed as to make a patch completely covering the ice to the thickness of an inch or more, the effect is altogether different. Protecting as it now does the ice below, a local ice hillock rises upon its site as the surrounding surface is lowered, and as this grows in height its declivities increase and a portion of the dirt slides down the side. The final product of this shaping is an almost perfectly conical ice hill encased in dirt and known as a dÉbris, sand, or dirt cone (Fig. 425). The novice in glacier study is apt to assume that these black cones contain only dirt, but is rudely awakened to the reality when he attempts to kick them to pieces. Both glacier tubs and dÉbris cones may assume large dimensions; as, for example, in Alaska, where they may be properly described as lakes and hills.

A patch of hard and dense snow which is less easily melted than that upon which it rests may lead to the formation of snow cones upon the glacier surface similar in size and shape to the better known dÉbris cones. Such cones of snow have, with doubtful propriety, been designated “penitents”, for it is pretty clear that the interesting bowed snow figures, which really resemble penitents and which were first described from the southern Andes under the name of nieves penitentes, are of somewhat different character.

One further ice feature shaped by differential melting around rock particles remains to be mentioned. Wherever the seasonal snowfalls of the nÉvÉ are exposed in crevasses, they are generally found to be separated by layers of dirt, and lines of pebbles similarly separate those ice layers which are revealed at the foot of the glacier. In either case, if the sun’s rays can reach these layers in an opened crevasse, the half-buried rock fragments are warmed by the sun upon their exposed surfaces and slowly melt their way down the ice surface, thus removing from it a thin layer of snow or ice and causing that part above the pebble layer to project like a cornice. This process will go on until the overhanging cornice protects the pebbles from any further warming by the sun, but each lower pebble layer that is reached by the sun will produce an additional cornice, so that the original surface may at the bottom have been retired by the process a number of inches. These features are described as glacier cornices (Fig. 426).

Glacier drainage.—Already in the early morning of every warm summer day, active melting has begun upon the surface of the Swiss glaciers. Rills of icy water soon make their way along depressions upon the surface, and are joined to one another so that they sometimes form brooks of considerable size (Fig. 427). Such streams continue their serpentine courses until these are intersected by a crevasse down which the waters plunge in a whirling vortex which soon develops a vertical shaft of circular section within the ice. Such shafts with their descending columns of whirling water are the well-known moulins, or “mills”, which may be detected from a distance by their gurgling sounds. The first plunge of the water may not reach to the bottom of the glacier, in which case the stream finds a passageway below the surface but above the floor until another crevasse is encountered and a new plunge made, here perhaps to the bottom. Once upon the valley floor the stream is joined by others, and pursues its course within an ice tunnel of its own making (Fig. 421, p. 394) until it issues at the glacier front.

The coarser of the rock dÉbris which was gathered up by the stream upon the glacier surface is deposited within the tunnel in imperfect assortment (gravel and sand), while all finer material and that lifted from the floor (rock flour) is retained in suspension and gives to the escaping stream its opaque white appearance. This glacier milk may generally be traced far down the valleys or out upon the foreland, and is often the traveler’s first indication that a range which he is approaching supports glaciers.

Deposits within the vacated valley.—For every excavation of the higher portions of the upland through glacial sculpture, there is a corresponding deposit of the excavated materials in lower levels. So far as these materials are deposited directly by the ice, they form the lateral, medial, ground, and terminal moraines already described. A considerable proportion of them are, however, deposited by the water outside the terminal moraine; but as with the shrinking glacier the ice front retires in halting movements over the area earlier ice-covered, the terminal moraines are ranged along the vacated valley as recessional moraines, each with a valley train of outwash below. About the apron of the piedmont glacier, such deposits are particularly heavy (Fig. 428). During the “ice age” the Swiss glaciers extended down the valleys below the existing ice remnants and spread upon the Swiss foreland as great piedmont glaciers such as may now be seen in Alaska. To-day we find there moraines and glacial outwash, a lake in the middle of the apron site, and sometimes a group of radiating drumlins like those found within the ice lobes of the continental glacier in southern Wisconsin (Fig. 429, and Fig. 344, p. 317).

Behind the recessional moraines within the glaciated valley are found the valley moraine lakes (Fig. 448, p. 413), in association with the rock basin lakes due to glacial sculpture (Fig. 447, p. 412). After the glacier has vacated its valley, the precipitous side walls become the prey of frostwork and are the scenes of disastrous avalanches or landslides. Within the cirques, drifts of snow are nourished long after the ice has disappeared, and as a consequence the amphitheater walls succumb to the process of solifluxion (p. 153).

Diversions and reversals of drainage, which are so characteristic of the work of continental glaciers, are hardly less common to glaciated mountain districts. Many of our most beautiful waterfalls have resulted from either the temporary or permanent obstruction of earlier valleys above the falls. The famous Yosemite Falls offers an interesting illustration of the shifting of an earlier waterfall, itself no doubt due to ice blocking in a still earlier glaciation (plate 22 B).

Marks of the earlier occupation of mountains by glaciers.—It is well that we should now bring together within a small compass those evidences by which the existence of earlier mountain glaciers may be proven in any district. These marks are so deeply stamped upon the landscape that no one need err in their interpretation.

MARKS OF MOUNTAIN GLACIERS

High-level sculpture. The grooved upland with its cirques, or the fretted upland with its cirques, cols, horns, and comb ridges.

Low-level sculpture. The U-shaped main valley, the hanging side valleys with their ribbon falls, the glacier staircase with its rock bars and gorges, the rounded, polished, and striated rock floor.

Deposits. The recessional moraines of till and the valley trains of sand and gravel, the soled erratic blocks derived always from higher levels of the valley.

Lakes. The valley moraine lakes and the chains of rock basin lakes.

Reading References for Chapter XXVIII

Glacier movement:—

L. Agassiz. Nouvelles Études et ExpÉriences sur les Glaciers Actuels, etc., Paris, 1847, pp. 435-539.

H. Hess. Die Gletscher, Braunschweig, 1904, pp. 115-150.

H. F. Reid. The Mechanics of Glaciers, Jour. Geol., vol. 4, 1896, pp. 912-928; Glacier Bay and Its Glaciers, 16th Ann. Rept. U. S Geol. Surv., Pt. i, 1898, pp. 445-448.

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