Glacial ice

“ And when the temperature under which these vast frozen masses had been formed rose again, the wasting ice must have yielded first on its southern boundary, gradually and uniformly retreating to the Arctic regions, without breaking up into distinct glacial regions, separated from one another, each with its local distribution of erratic boulders and glacier-marks radiating from circumscribed areas on higher levels, as they occur everywhere in Europe. ”

“ Where the glacial markings have been most studiously investigated, it appears that an ice-sheet invariably occupies a definite area; that a mer de glace accumulates in the higher central portions, from which glaciers move outwardly in all directions, extending as far as the descending ground will permit, or the oceanic currents can convey the icebergs broken off from the frozen mass without melting ”

“ All the Greenland glaciers become confluent in the interior, so that it is not a typical mer de glace, but a sheet of ice, which overspreads the highest mountains. It is only near the edge that the several streams manifest their independent existence. Thence the usual phenomena of Alpine glaciers manifest themselves. The sub-glacial streams pouring into the sea produce certain peculiarities. Immense supplies of heat penetrate the ice from the sun's rays, which melt very much ice and discharge muddy torrents of great size. ”

“ To cause extensive glaciation we must have, in addition to favorable eccentricity, a large precipitation. With these two factors combined we get an ice age, but not otherwise. The result has an important bearing on geologic glacial periods and their explanation.
Once formed, an ice-sheet cools everything about it and chills the climate of its hemisphere. ”

“ Presumably the creature who chipped the flints found in those rocks near Foxhal, England, could do so.
Then came the first of the great glacial advances of the ice age about 1,000,000 years ago. Five times the northern lands were buried beneath a mantel of moving ice. Five times the ice melted until the glaciers were at least as small as those now remaining on Greenland and in the valleys of Alaska. ”

“ This intense heat during the day melts the surface of the ice, which forms streams that run along on the top of a glacier until they come to a crevasse or riffle in the ice river, where they plunge down and become a part of the glacial stream that is flowing underneath the ice. ”

“ That it was cradled in some deep recess of the land, and there remained until it had grown to such a thickness that no summer's sun or water's washing could wholly obliterate it before the winter cold came again, is most probable. After this it grows as the glacier grows, from above, and is, like the glacier, wholly composed of fresh ice,—that is, of frozen snow. It will be thus seen that the accumulation of ice upon the mountain tops is not different from the accumulation which takes place upon these floating fields, and each recurring year marks an addition to their depth. ”

“ Hayes a lively description of the Sermiatsialik glacier, which he thinks must at some places be more than seven hundred and fifty feet in depth, overflowing the borders of the valley like a swollen torrent. For upwards of four leagues, the icebergs which throng the fiord, or gulf, are those of the glacier itself, and terminating in a wedge-like outline, disappear in the vast sea of ice expanding to right and left above the loftiest summits, and drawing irresistibly the eye to its rippled surface,—boundless, apparently, like that of ocean. ”

“ A low temperature alone can do nothing without moisture, and this fact quickly disposes of the popular notion that glaciers invariably exist in cold countries. Thibet, for instance, and also some parts of Arctic North America are destitute of ice streams, though eternal cold may be said to reign supreme in these parts.
Imagine for a moment the higher mountains clear of snow and ice, and then watch for the formation of a glacier. Snow falls and fills up all the valleys and gullies, avalanches descend from the higher parts, and a great accumulation gathers in all hollows. ”

“ The primary material of glacier-ice is the snow which falls in the high regions of the mountain. The yearly addition of snow in the higher cold regions gradually forces the snow down the valley; here, subject to alternate thawing and freezing, it undergoes a second crystallization into what is called névé snow, and still farther down, under increased pressure, comes transformed into a granular, opaque, bulbous ice, called névé ice, which at last changes into the compact blue ice of the glacier proper. ”

“ Compressed by the immense weight of the superior strata, that lower portion of the névé which feeds the glacier is, at the very start, transformed into a solid ice, whose particles are cemented by the alternate melting and freezing which go on through all but the coldest weather of the year. Whether it is the pressure of the névé, or the irresistible expansion caused by the action of cold upon the water pervading its great mass, that drives off the glacier, is a question respecting which the most intelligent observers are by no means unanimous. ”

“ As the decades pass, its level is greatly lowered by the melting of the ice. More and more, earth and rocks accumulate upon the surface, as it travels onward, and are scattered over it by the rains and melting snow. At last, in its old age, when far down its canyon, the glacier is completely hidden, save where crevasses reveal the ice. Only at its snout, where it breaks off, as a rule, in a high wall of ice, do we realize how huge a volume and weight it must have, far above toward its sources, or why so many of the crevasses on the upper ice fields seem almost bottomless. ”

“ How shall we account for the masses of polar ice, for the immense icebergs, and the glaciation of Greenland? The snowfall of the polar regions is light. The air is nowhere drier than over the cold glacier ice, as is proved every day in Switzterland by the quickness with which clothes dry when hung over it. At the same time the ice is covered with extremely fine, hardly visible snow crystals. If we boil water in a retort which is connected with another vessel containing a piece of ice, all the steam will pass over on to the ice and deposit itself as ice upon it. ”

“ The pressure of the great body of snow above, the change of the atmospheric conditions, assisted by gravity, are the causes which enter into the formation of the solid glacier ice. These conditions may be increased or diminished by earthquakes and mild winters. Like a great river it advances toward the mouth of the valley, and as the immense body of ice moves downward, it brings with it by erosion huge pieces of rock, earth, and trees. ”

“ Besides this action in winter, which goes on more or less in every country wherever the temperature sinks sufficiently low to permit of the freezing of water, ice effects many changes on the surfaces of rocks when it takes the form of glaciers and icebergs. We have already noted the operation of a glacier during its slow progress in crushing down large fragments of stone, scratching and abrading the rocks over which it passes, and eventually producing a vast quantity of mud, which is carried - 160 - down by streams to form new accumulations either in lakes or seas. ”

“ Well, many glaciers, after quitting the snow-beds by which they are fed, suddenly find themselves at the top of a precipice. Under these circumstances, as they are unable to stand still, there is but one thing for them to do, viz., go over; and as ice, though plastic, is by no means so to the extent that treacle—to which glacier ice has so often been likened—is, it is obvious that a slice will break off the advancing [Pg 88] tongue of the glacier, and come thundering down the rocks, to form material for another glacier below, or, if the quantity is insufficient, to melt gradually away. ”

“ The best known existing great ice sheets are those of Greenland and Antarctica, especially the former, which covers about 500,000 square miles. This glacier is so large and deep that only an occasional high rocky mountain projects above its surface, and the ice is known to be slowly moving outward in all directions from the interior to the margins of Greenland. Along the margins, where melting is more rapid, some land is exposed, and often the ice flows out into the ocean where it breaks off to form large icebergs. ”

“ We may see it in the early dawn, before the new ice of the preceding night begins to yield to the action of the sun, and the surface of the glacier is veined and inlaid with the water poured into its holes and fissures during the day and transformed into pure, fresh ice during the night,—or when the noonday heat has wakened all its streams, and rivulets sometimes as large as rivers rush along its surface, find their way to the lower extremity of the glacier, or, dashing down some gaping crevasse or open well, are lost beneath the ice. ”

“ Under these conditions, all the ice in the zone of constant temperature, except that at its bottom, must be permanently below the melting-point, but it is perhaps worthy of especial note that much of it is but little below. In alpine glaciers the part of the ice affected by this constant low temperature (below freezing) is presumed to be chiefly that which lies beneath the snow-fields. In polar glaciers the low temperature probably prevails beneath the surface, not only throughout the great ice-caps, but also in the marginal glaciers which descend from them. ”

“ Under the ice river is always flowing a stream of water, and many air holes and openings are found upon the surface at different points, no doubt produced by the influence of spring water coming in of temperature above freezing or at the usual fifty-two degrees, about the average of fresh water springs in all latitudes. This conglomeration of influences to make a glacier shows the absurdity of having such solid masses break off, as claimed to be seen by Arctic explorers, large enough to remain intact well down into the Atlantic Ocean. ”

“ But lower down in the glacier proper, where it is completely transformed into ice, its surface becomes convex, for the following reason: The rocky walls of the valley, as they approach the plain, partake of its higher temperature. They become heated by the sun during the day in summer, so that the margins of the glacier melt rapidly in contact with them. In consequence of this, there is always in the lower part of the glacier a broad depression between the ice and the rocky walls, while, as this effect is not felt in the centre of the glacier, it there retains a higher level. ”

“ But glacier-ice presents some other curious peculiarities. Every abundant snow-fall on the summit of the mountains forms a layer easily distinguishable from preceding layers—which, in most cases, have already passed into the névé condition. This stratification becomes more apparent when the whiteness of the surface has been sullied by dirt or dust wafted on “the wings of the wind.” It is perceptible also in ice; but here we must not confound it with another phenomenon of which the cause is different, the veined structure. ”

“ The flakes of snow are gradually transformed into granules because the points and angles of the original flakes melt and evaporate more readily than the more solid central portions, which become aggregated round some master flake that continues to grow in the névé at the expense of its smaller neighbours, and increases in size until finally the glacier ice is composed of a mass of interlocked crystalline granules, some as large as a walnut, closely compacted under pressure with the principal crystalline axes in various directions. ”

“ Indeed, if all the spaces in the mass of the glacier, not occupied by continuous ice, could be graphically represented, I believe it would be seen that cold air surrounds the glacier-ice itself in every direction, so that probably no masses of a greater thickness than that already known to be permeable to cold at the surface would escape this contact with the external temperature. ”

“ These layers of ice become numerous and are parallel to each other, like the layers of ice formed from slosh. Such crusts of ice I have myself observed again and again upon the glacier. This stratified snowy ice is now the bottom on which the first autumnal snow-falls accumulate. These sheets of ice may be formed not only annually before the winter snows set in, but may recur at intervals whenever water accumulating upon an extensive snow-surface, either in consequence of melting or of rain, is frozen under a sharp frost before another deposit of snow takes place. ”

“ We may see it when the heat and other disintegrating influences have acted upon the ice to a certain superficial depth, so that its surface is covered with a decomposed crust of broken, snowy ice, so permeated with air that it has a dead-white color, like pounded ice or glass. Those who see the glacier in this state miss the blue tint so often described as characteristic of its appearance in its lower portion, and as giving such a peculiar beauty to its caverns and vaults. ”

“ Polar ice sheets are permanent only so long as the lands remain on which they rest. Mountain glaciers can stay only the brief time during which the ranges continue young and high. As lofty mountains, such as the Selkirks and the Alps, are lowered by frost and glacier ice, the snowfall will decrease, the line of permanent snow will rise, and as the mountain hollows in which snow may gather are worn beneath the snow line, the glaciers must disappear. ”