Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

THE WORK OF GLACIERS.

Erosion and transportation.

The work accomplished by glaciers is distinctive, for while like rivers, they abrade the valleys through which they pass, carry forward the material which they remove from the surface, and wear, grind, and ultimately deposit it, and while their work therefore includes erosion, transportation, and deposition, their method is peculiar.

Getting load.—If the surface on which the snow-field which is to become a glacier accumulates be rough and covered with abundant rock débris, as such surfaces usually are, the glacier already has a basal load when its movement begins, for the snow covers, surrounds, and includes such loose blocks of rock as project above the general surface and envelops all projecting points of rock within its field. When the snow becomes ice and the ice begins to move, it carries forward the loose rock already imbedded in it, and tears off the weak points of the enveloped rock-projections. It may perhaps also move some of the soil and mantle rock of the original surface to which it is frozen. In addition to the subglacial load which the glacier thus has at the outset, there may be a surface load which has fallen on the snow or ice from cliffs above. This is especially true of mountain-valley glaciers. If this has been buried by snow and ice it is englacial; if it lies on the surface it is superglacial.

Once in movement, the ice carries away the débris to which it was originally attached, and at the same time gathers new load from the same area. The new load is acquired partly by the rasping effect of the rock-shod ice on its bed, and partly by its rending power which, under favorable circumstances, may quarry out considerable blocks of rock. This “plucking” process is at its best where the ice passes over cliffs of jointed rock or steep-sloped hills.

As the ice advances into new territory it acquires additional basal load, partly by rasping, partly by plucking, partly by freezing to it, and partly in other ways. One of these ways may be illustrated by the sequence of events when the end of a glacier advances on a very large bowlder. As the ice approaches it, the reflection of heat from it melts the adjacent edge of the ice, making a slight reëntrant. With farther advance, the ice closes in against and around the bowlder, and finally carries it along in the bottom of the moving mass. In some cases, especially when its advance is rapid, the ice may push débris in front of itself. Even where this is the case, the amount of material pushed forward is generally slight, partly because the extreme edge of the ice often fails to rest on the land in summer, when the movement is greatest, being melted from below by the heat of the surface over which it is spreading (see [Fig. 252]), and partly because the earth in front of the glacier is frozen during a large part of the year. In this condition, the earthy matter has greater resistance than the ice, and the latter rides over it. Superglacial material may be acquired during movement by the fall of débris from cliffs, or by the descent of avalanches.

Fig. 252.—Diagram showing lack of contact of the edge of the ice with its bed.

Conditions influencing rate of erosion.—An obstructive attitude of the surface toward the movement of the ice is as necessary for effective erosion as the movement of the ice itself. Advancing over a flat surface, ice ordinarily inflicts but little wear, since there is little for it to get hold of. So slight is the abrasive power of ice under these conditions that it frequently overrides and buries the soil with more or less of its herbaceous vegetation. But while a certain measure of roughness of surface is favorable for glacial erosion, the topography may be so uneven as to seriously impede the ice. Erosion is probably at its maximum, so far as influenced by topography, when the roughness of the surface is such as to offer notable catchment for the basal ice, but not such as to impede its motion very seriously. The amount of relief favorable for the greatest erosion increases with increasing thickness of the ice.

Other conditions which influence erosion by ice are (1) the amount of loose or slightly attached débris on the surface, (2) the resistance of the rock, (3) the slope of the surface, (4) the thickness of the ice, (5) the rate of movement, and (6) the abundance and character of the débris which the ice has to work with. The effect of the first five of these conditions is evident. The effect of the last is less simple. Clean ice passing over a smooth surface of solid rock has little effect upon it; but a rock-shod glacier will abrade the same surface notably. The effect of this abrasion is shown in the grooves and scratches (striæ) which the stones in the bottom of the ice inflict on the surface of the rock over which they pass (Figs. [253], [255], and [256]). At the same time the stones in the ice are themselves worn both by abrasion with the bottom, and with one another ([Fig. 254]). It does not follow, however, that the more material in the bottom of the ice the greater the erosion it effects; for with increase of débris there may be decrease of motion[131] and, beyond a certain point, the decrease of motion seriously interferes with the efficiency of erosion. When any considerable thickness of ice at the bottom of the glacier is full of débris, this loaded basal portion may approach stagnancy, and the lower limit of considerable movement may lie between the loaded ice below and the relatively clean ice above. A moderate, but not an excessive load of débris is, therefore, favorable for great erosion. Something depends, too, on the character of the load. Coarse, hard, and angular débris is a more effective instrument of erosion than fine, soft, or rounded material. The adverse influence of the overloading of the ice on its motion has been likened to the stiffening of a viscous liquid by the addition of foreign matter, but it may better perhaps be referred to the destruction of the granular-crystalline continuity on which glacier motion probably depends.

Fig. 253.—Glacial striæ and bruises. The block to the right shows two sets of striæ: that to the left shows the peculiar curved fractures known as Chatter Marks.

Fig. 254.—Bowlders showing glacial striation. (Drawn by Miss Matz.)

Fig. 255.—Striæ on bed rock, Kingston, Des Moines Co., Ia. (Iowa Geol. Surv.)

From the preceding statement, it is evident that erosion is not equally effective at all points beneath a glacier. So far as concerns the ice itself, erosion is not most effective at the end of a valley glacier, or at the edge of an ice sheet, for here the strength of movement is too slight and the load too great; nor is the most effective erosion at the source or near it, for though the ice may here be thick, the movement is slow and the load likely to be slight. Ice conditions only being considered, erosion is most effective somewhere between the source and the terminus, and probably much nearer the latter than the former. The conditions of the surface over which the ice passes may be such as to vary the place of greatest erosion widely. Thus in an Alpine glacier, erosion may be most effective at the Bergschrund because the slope here favors “plucking.” Here, notable amphitheatres (cirques) are sometimes excavated. After the glacier disappears, the bottom of the cirque is often seen to contain rock basins ([Fig. 257]). Glacial cirques abound in mountains where glaciers once existed, but from which they have now disappeared. The cirques of the Bighorn mountains of Wyoming ([Pl. XIX]) are examples.

Fig. 256.—Striæ, grooves, etc., in a canyon tributary to Big Cottonwood Canyon, Wasatch Mountains. (Church.)

Summary.—In summary it may be said that rapidly moving ice of sufficient thickness to be working under goodly pressure, shod with a sufficient but not excessive quantity of hard-rock material, passing over incoherent or soft formations possessing a topography of sufficient relief to offer some resistance, and yet too little to retard seriously the progress of the ice, will erode most effectively.

Varied nature of glacial débris.—From its mode of erosion it will readily be seen that the bottom of a glacier may be charged with various sorts of material. There may be (1) bowlders which the ice has picked up from the surface, or which it has broken off from projecting points of rock over which it has passed; (2) smaller pieces of rock of the size of cobbles, pebbles, etc., either picked up by the ice from its bed or broken off from larger masses; (3) the fine products (rock-flour) produced by the grinding of the débris in the ice on the rock-bed over which it passes, and similar products resulting from the rubbing of stones in the ice against one another; and (4) sand, clay, soil, vegetation, etc., derived from the surface overridden. Thus the materials which the ice carries (drift) are of all grades of coarseness and fineness, from large bowlders to fine clay. The coarser material may be angular or round at the outset, and its form may be changed and its surface striated as it is moved forward. Whether one sort of material or another predominates, depends primarily on the nature of the surface overridden.

PLATE XIX.

U. S. Geol. Surv.

Scale, 2+ miles per inch.

PART OF THE BIGHORN MOUNTAIN RANGE, WYOMING.

PLATE XX.

U. S. Geol. Surv.

Scale, 1+ miles per inch.

A SECTION OF THE CALIFORNIA COAST NEAR SAN MATEO, CALIFORNIA.

Fig. 257.—A glacial cirque. The lake occupies a rock basin, produced by glacier erosion. Head of Little Timber Creek, Montana.

The topographic effects of glacial erosion.—In passing through its valley, an alpine glacier deepens and widens its bottom and smooths its slopes up to the upper limit of the ice. It tends to change a V-shaped valley ([Fig. 258]) into a U-shaped one ([Fig. 259]). The change in topography at the upper limit of glaciation is often marked (Figs. [260] and [261]).

Fig. 258.—The Valley of the American Fork. A V-shaped non-glaciated valley in the Wasatch Mountains of Utah. Compare [Fig. 259]. (Church.)

Fig. 259.—U-shaped valley resulting from glaciation. Little Cottonwood Canyon, Wasatch Mountains. (Church.)

Fig. 260.—Contrast between glaciated topography below and non-glaciated topography above. The minarets in the Sierras, Cal.

Fig. 261.—Contrast between glaciated topography below, and non-glaciated topography above. Needles Mountains, from slope west of Hidden Lake. (Cross, U. S. Geol. Surv.)

The deepening of a valley by glacial erosion may throw its tributaries out of topographic adjustment. Thus if a main valley is lowered 100 feet by glacial erosion while its tributary is not deepened, the lower end of the latter will be 100 feet above the former when the ice disappears. Such a valley is called a hanging valley (Figs. [262] and [263]). Such valleys are of common occurrence in regions recently glaciated, but now ice-free. Examples are common in the western mountains of North America and elsewhere.

Ice-caps which overspread the surface irrespective of valleys and hills, tend to reduce the angularities of the surface. Hills and ridges are cut down and smoothed (Figs. [264] and [265]); but since valleys parallel to the direction of movement are deepened at the same time, it is doubtful if the relief of the surface is commonly reduced by the erosion of an ice-cap.

Fig. 262.—A hanging valley. East side of Lake Kootenai, B. C. All except the highest summits glaciated. (Atwood.)

Fiords.—A glacier descending into the head of a narrow bay may gouge out the bay to a very considerable depth, causing its head to recede. When the ice finally melts, the bay may be a fiord. Thus have arisen the glacial features of many of the fiords of high-latitude coasts, and many of the glaciers of those coasts are now making fiords ([Fig. 266]). Fiords also arise in other ways. Coasts indented by fiords are likely to be bordered by islands.

The positions in which débris is carried.—As a result of the methods by which a glacier becomes loaded, there are three positions in which the débris is carried: (1) the basal or subglacial, (2) the englacial, and (3) the superglacial. The material picked up or rubbed off from the surface over which the ice moves is normally carried forward in the base of the ice; while that which falls on the surface is usually carried in the form of surface moraines. In the former position the drift is basal; in the latter, superglacial. It is doubtful if much débris is moved along beneath (that is, strictly below the bottom of) the ice, though the movement of the latter would have a tendency to drag or urge along with it the loose material of its bed. If drift were carried forward in such positions, it would be strictly subglacial.

Fig. 263.—A hanging valley. The water falls (Bridal Veil) from a hanging valley. (Wineman.)

The basal load of a glacier is constantly being mixed with new accessions derived from ground over which the ice is passing, and this admixture tells the story of the work done by the bottom of the ice. The englacial and superglacial material, on the other hand, is normally borne from the place of origin to the place of deposition without such intermixture. It is a case of “local” versus “through” transportation.

Transfers of load.—While the origin of the load usually determines its position, exceptions and complications arise from the transfer of load from one position to another, and from the gradation of one horizon into another.

Fig. 264.—A non-glaciated hill. Dalrymple Island. North Greenland.

Fig. 265.—A glaciated hill. Southeastern Carey Island. About 30 miles west-northwest of Dalrymple Island.

Most of the débris gathered by ice is acquired at its bottom. While such material is basal at the outset, some of it may find itself above the bottom a little later. Thus when ice passes over a hill ([Fig. 267]) the bottom of the ice rends débris from the top of the hill. When it descends from one level to another there is a similar result ([Fig. 268]). To the lee of the hill the ice from either side may close in under that which came over the top, in which case the débris derived from the top of the hill by the bottom of the overriding ice will be well up in the ice. It has passed from an initial basal to a subsequent englacial position. The change does not usually involve an actual rise of the material, but rather a decline. If carried upward at all, the upward movement is temporary only, and incident to the passage of the ice over the hill, or to other local causes. The englacial débris may be little or much above the basal zone according to the height of the elevation overridden.

Fig. 266.—Alaskan fiords. The shaded areas represent land. (From charts of the C. & G. Surv.)

Fig. 267.—Diagram to illustrate the taking of débris from a hill-top. It also illustrates how englacial débris may become superglacial as the result of surface ablation.

Fig. 268.—Taking débris from a protuberance of the bed.

Superglacial débris may obviously become englacial by falling into crevasses or by being carried down by descending waters. Either superglacial or englacial débris may become basal by the same means.

From their form and position, there is less ice-free land in immediate association with ice-caps than with valley glaciers. Furthermore, the ice-free land about the borders of an ice-cap is less likely to be in the form of cliffs above it. As a result, the surfaces of ice-caps are comparatively clean, except at their edges where the ice is thin.

Fig. 269.—Side view of end of a glacier on the south side of Olriks Bay, North Greenland.

Fig. 270.—Closer view of a part of the ice shown in [Fig. 269].

Englacial material may become superglacial by surface ablation. In this case the drift does not rise, but melting brings the surface of the ice down to its level. This occurs chiefly at the end or edge of the ice, where the surface melting is greatest. Englacial débris, especially that near the bottom, may also become basal by the melting of the bottom of the ice.

Englacial material plucked or rasped from an elevation over which the ice has passed is liable to be disposed in a longitudinal belt in the ice in the lee of the elevation itself. By surface ablation this material may reach the surface at some point below its source, and be disposed as a medial moraine. Such a moraine has an origin very different from that of a medial moraine formed by the junction of two lateral moraines of superglacial origin.

Much less in the natural order of things is the transfer of material from a basal to an englacial and from an englacial to a superglacial position by upward movement of the débris itself. Such transfer is remarkable because the specific gravity of rock is from two and a half to three times as great as that of ice, so that its normal tendency is to sink.

Fig. 271.—Surface terminal moraines due to upturning. Edge of the ice-sheet, North Greenland.

In arctic glaciers, and probably in others, some material which has been basal becomes englacial by being sheared forward over ice in front of it. So far as observed this takes place chiefly where the ice in front of the plane of shearing lies at a lower level than that behind, as where the surface of an upland falls off into a valley, or where a boss of rock shelters the ice in its lee from the thrust of the overriding ice ([Fig. 268]).

Fig. 272.—Diagram illustrating the upturning of the layers of ice at the end of an arctic glacier as seen in end-section. The bottom line represents sea level.

At the borders of arctic glaciers the lower layers are not infrequently upturned, as shown in Figs. [269] to [272]. Where the layers turn up at the end of a glacier (Figs. [269] and [270]), basal and englacial débris is carried to the surface by actual upward movement, and a terminal moraine or a series of terminal moraines sometimes aggregated where the upturned layers of ice outcrop at the surface ([Fig. 271]). That the material of these moraines was originally basal is abundantly demonstrated by the bruised and scratched condition of the bowlders and pebbles, and sometimes by the nature of the material itself. For example, in two cases in North Greenland where glaciers descend into the heads of shallow bays and move forward on their bottoms, moraines formed by the upturning of the layers were seen to contain abundant molluscan shells derived from the bottom of the bay. The upturning sometimes affects the side-edges of ice-tongues ([Fig. 272]) as well as their ends, and the material thus brought to the surface gives origin to lateral moraines altogether different in origin from the lateral moraines formed by the falling of débris upon the glaciers. Sometimes also there is an upturning of the ice along a longitudinal zone well back from the lateral margins ([Fig. 273]), and the material so borne to the surface in such a zone gives rise to a moraine resembling the medial moraine formed by the union of lateral moraines, but of wholly different origin.

Fig. 273.—Diagram illustrating the same point as 272, where the structure is more complex. The bottom line of the figure represents sea level.

The phenomenon of upturning here referred to has been observed only at or near the terminus of the ice, and is perhaps due in most part to the resistance of frozen morainic or other material beneath and in front of the edge. To this should probably be added the effect of the increased rigidity of the ice at its borders, due to the low external temperature during the larger part of the year, while the interior, with its higher temperature, remains more fluent. But even this probably leaves the explanation inadequate. In not a few instances the upturning is associated with a notable thickening of the layers toward their edges ([Fig. 274]). This suggests that perhaps there is an exceptional growth of the granular crystals of the ice near the edge of the layers, owing to the penetration of the surface-waters which are much more abundant at the borders than elsewhere, and which in the arctic glaciers probably do not penetrate deeply before they reach a freezing temperature.

Wear of drift in transit.—Drift carried at the bottom of the ice is subject to notable wear. The materials in transportation abrade one another and are abraded by the bed over which they pass. Englacial drift is subject to less wear because it is commonly more scattered. Superglacial drift is worn little or none while it lies on the surface of the ice; but in so far as superglacial or englacial drift is derived from the basal load, it may show the same evidences of wear as the basal drift itself. Superglacial drift often reveals its history in this way.

Fig. 274.—Thickening of the upturned layers of ice.

Deposition of the Drift.

1. Beneath the body of the ice.—During the advance of a glacier, deposition may take place both beneath the body of the ice and beneath its end and edges. Deposition beneath the body of the ice is liable to take place wherever the topography favors lodgment, or wherever the ice is overloaded. The topography favoring deposition is much the same as that favoring erosion, but the two processes are not favored at the same point. Erosion is greatest on the “stoss” side of an obstruction (the side against which the ice advances), and deposition on the lee side. The ice is likely to be overloaded (1) just beyond a place where conditions have favored the gathering of a heavy load, and (2) where the ice is rapidly thinning. On the whole, however, the deposition of material beneath the main body of a glacier is much more than balanced by erosion in the same position.

Fig. 275.—Glacier building an embankment. Southeast side of McCormick Bay, North Greenland.

2. At ends and edges of glaciers.—At and near the end of a glacier the conditions of deposition are somewhat different. Here deposition beneath the ice goes on faster than elsewhere, chiefly because of the more rapid melting and the more rapid thinning and weakening of the ice. If the end of the glacier be stationary in position, drift is being continually brought to it and left there, for though the end is stationary, the ice continues to move. If the glacier moves forward 500 feet per year, and if its end is melted at the same rate, all the débris in the 500 feet of ice which has been melted has been deposited, and all except that which has been washed away has been deposited at and beneath the end of the glacier. If the end of the glacier is retreating, the retreat means that the waste at the end exceeds the forward movement. If the ice advances 300 feet per year, and is melted back 500 feet in the same time, all the débris carried by the 500 feet which has been melted has been deposited, and largely in the narrow zone (200 feet) from which the ice has receded. Even in this case, therefore, there is a notable tendency to marginal accumulation. If the end of the glacier is advancing 500 feet per year while it is being melted but 300 feet, all the drift in the 300 feet melted has been deposited, and chiefly at or beneath the immediate margin of the ice. To the marginal and sub-marginal accumulations made in this way, the material carried on the ice is added whenever the ice is melted from beneath it. This addition is sometimes considerable and sometimes meagre. If the edge of the ice is without much fluctuation in position, the material dumped over its end may take the form of a narrow ridge or bowlder-wall (Geschiebe-wall). If a glacier pushes material in front of it, this, too, becomes a part of the general terminal aggregation of drift.

Fig. 276.—Embankment completed. Near the last.