Изображения страниц
PDF
EPUB

sea and there terminated their existence, just as the Antarctic glaciers terminate at the present Antarctic ice-wall.

What must happen when a glacier is thus thrust out to sea? This question is usually answered by assuming that it slides along the bottom until it reaches such a depth that flotation commences, and then it breaks off or "calves" as icebergs. This view is strongly expressed by Mr. Geikie (p. 47) when he says that "The seaward portion of an Arctic glacier cannot by any possibility be floated up without sundering its connection with the frozen mass behind. So long as the bulk of the glacier much exceeds the depth of the sea, the ice will of course rest upon the bed of the fjord or bay without being subjected to any strain or tension. But when the glacier creeps outwards to greater depths, then the superior specific gravity of the sea-water will tend to press the ice upward. That ice, however, is a hard continuous mass, with sufficient cohesion to oppose for a time this pressure, and hence the glacier crawls on to a depth far beyond the point at which, had it been free, it would have risen to the surface and floated. If at this great depth the whole mass of the glacier could be buoyed up without breaking off, it would certainly go to prove that the ice of Arctic regions, unlike ice anywhere else, had the property of yielding to mechanical strain without rupturing. But the great tension to which it is subjected takes effect in the usual way, and the ice yields, not by bending and stretching, but by breaking." Mr. Geikie illustrates this by a diagram showing the "calving" of an iceberg.

In spite of my respect for Mr. Geikie as a geological authority, I have no hesitation in contradicting some of the physical assumptions included in the above.

Ice has no such rigidity as here stated. It does possess in a high degree "the property of yielding to mechanical strain without rupturing." We need not go far for evidence of this. Everybody who has skated or seen others skating on ice that is but just thick enough to "bear" must have felt or seen it yield to the mechanical strain of the skater's weight. Under these conditions it not only bends under him, but it afterwards yields to the reaction of the water below, rising and falling in visible undulations, demonstrating most unequivocally a considerable degree of flexibility. It may be said that in this case the flexibility is due to the thinness of the ice; but this argument is unsound, inasmuch as the manifestation of such flexibility does not depend upon absolute thickness or thinness, but upon the

relation of thickness to superficial extension. If a thin sheet of ice can be bent to a given arc, a thick sheet may be bent in the same degree, but the thicker ice demands a greater radius and proportionate extension of circumference. But we have direct evidence that ice of great thicknessactual glaciers-may bend to a considerable curvature before breaking. This is seen very strikingly when the uncrevassed ice-sheet of a slightly inclined nevé suddenly reaches a precipice and is thrust over it. If Mr. Geikie were right, the projecting cornice thus formed should stand straight out, and then, when the transverse strain due to the weight of this rigid overhang exceeded the resistance of tenacity, it should break off short, exposing a face at right angles to the general surface of the supported body of ice. Had Mr. Geikie ever seen and carefully observed such an overhang or cornice of ice, I suspect that the above quoted passage would not have been written.

Some very fine examples of such ice-cornices are visible from the ridge separating the Handspikjen Fjelde from the head of the Jostedal, where a fine view of the great nevé or sneefornd is obtained. This side of the nevé terminates in precipitous rock-walls; at the foot of one of these is a dreary lake, the Styggevand. The overflow of the nevé here forms great bending sheets that reach a short way down, and then break off and drop as small icebergs into the lake.*

The ordinary course of glaciers afford abundant illustrations of the plasticity of such masses of ice. It spreads out where the valley widens, contracts where the valley narrows, and follows all the convexities or concavities of the axial line of its bed. If the bending thus enforced exceeds a certain degree of abruptness crevasses are formed, but a considerable bending occurs before the rupture is effected, and crevasses of considerable magnitude are commonly formed without severing one part of a glacier from another. They are usually V-shaped, in vertical section, and in many the rupture does not reach the bottom of the glacier. Very rarely indeed does a crevass cross the whole breadth of a glacier in such a manner as to completely separate, even temporarily, the lower from the upper part of the glacier.

If a glacier can thus bend downwards without "sundering its connection with the frozen mass behind," surely it may bend upwards in a corresponding degree, either with or

* See "Through Norway with a Knapsack," chapters 11 and 12, for further descriptions of these.

without the formation of crevasses, according to the thickness of the ice and the degree of curvature.

A glacier reaching the sea by a very steep incline would probably break off, in accordance with Mr. Geikie's description, just as an Alpine glacier is ruptured fairly across when it makes a cascade over a suddenly precipitous bend of its path. One entering the sea at an inclination somewhat less precipitous than the minor limit of the effective rupture gradient it would be crevassed in a contrary manner to the crevassing of Alpine glaciers. Its crevasses would gape downwards instead of upwards-have an A-shaped instead of a V-shaped section.

With a still more moderate slope, the up-floating of the termination of the glacier, and a concurrent general uplifting or upbending of the submerged portion of the glacier might occur without even a partial rupture or crevasse formation occurring.

Let us now follow out some of the necessary results of these conditions of glacier existence or glacial prolongation. The first and most notable, by its contrast with ordinary glaciers, is the absence of lateral, medial, or terminal moraines. The larger masses of débris, the chippings that may have fallen from the exposed escarpments of the mountains upon the surface of the upper regions of the glacier, instead of remaining on the surface of the ice and standing above its general level by protecting to some extent the ice on which they rest from the general snowthaw, would become buried by the upward accretion of the ice due to the unthawed stratum of each year's snow-fall.

The only thinning agency at work upon such glaciers during their journey over the terra firma being the outflow of terrestrial heat, and that due to their friction upon their beds, the thinning must all take place from below, and thus, as the glacier proceeds downwards, these rock fragments. must be continually approaching the bottom instead of continually approaching the top, as in the case of modern Alpine glaciers flowing below the snow-line.

An important consequence of this must be that the erosive power of these ancient glaciers was, cæteris paribus, greater than that of modern Alpine glaciers, especially if we accept those theories which ascribe an actual internal growth or regeneration of glaciers by the relegation below of some of the water resulting from the surface-thaw.

It follows, therefore, that such glaciers could not deposit any moraines such as are in course of deposition by existing Alpine and Scandinavian glaciers.

What, then, must become of the chips and filings of these outfloating glaciers? They must be carried along with the ice so long as that ice rests upon the land; for this débris must consist partly of fragments imbedded in the ice, and partly of ground and re-ground excessively subdivided particles, forming a slimy mud that must either cake into what I may call ice-mud, and become a part of the glacier, or flow as liquid mud or turbid water beneath it, as with ordinary glaciers. The quantity of water being relatively small under the supposed conditions, the greater part would be carried forward to the sea by the ice rather than by the water.

As the glacier with its lower accumulation advanced into deeper and deeper water, its pressure upon its bed must progressively diminish until it reaches a line where it would just graze the bottom with a touch of feathery lightness. Somewhere before reaching this it would begin to deposit its burden on the sea-bottom, the commencement of this deposition being determined by the depth whereat the tenacity of the deposit, or its friction against the sea-bottom, or both combined, becomes sufficient to overpower the nowdiminished pressure and forward thrusting, or erosive power of the glacier.

Farther forward, in deeper water, where the ice becomes. fairly raised above the original sea-bottom, a rapid thawing must occur by the action of the sea-water, and if any communication existed between this ice-covered sea and the waters of warmer latitudes a further thawing must result from the currents that would necessarily be formed by the interchange of water of varying specific gravities. Deposition would thus take place in this deeper water, continually shallowing it or bringing up the sea-bottom nearer to the ice-bottom.

This raising of the sea-bottom must occur not only here, but farther back, i.e., from the line at which any deposition commenced. This line or region, whereat the depth is just sufficient to allow the ice to rest lightly on its own deposit and slide over it without either sweeping it forward or depositing any more upon it, becomes an interesting critical region, subject to continuous forward extension during the lifetime of the glacier, as the deposition beyond it must continually raise the sea-bottom until it reaches this critical depth at which this deposition must cease. This would constitute what I may designate the normal depth of the glaciated sea, or the depth to which it would be continually tending, during a great glacial epoch, by the formation of a submarine bank or plain of glacier deposit, over which the

[graphic]

FIG. 1.-Low GLACIATED ROCKS OF THE THRENEN ISLANDS, NEAR THE COAST.

FIG. 2.-OUTLYING LOFODENS, NOT GLACIATED.

W. WELCH

[graphic][subsumed]
« ПредыдущаяПродолжить »