Ice sheet

“ In places where the ice must have moved more or less rapidly, as on considerable slopes, no accumulation took place, while in the rear of projecting crags and knobs of rock, sub-glacial materials often gathered deeply. Again, over low-lying tracts, where the motion of the ice would necessarily be retarded, clay, sand, and stones tended to collect. And this particularly appears to have been the case in those regions where the slow-creeping and gradually thinning ice-sheet approached its terminal line. ”

“ 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 ”

“ As the edge of an ice sheet, or as the end of a glacier, retreats, the land which it has previously covered is laid bare, and the effects which the passage of the ice produced may be seen. In some cases one may actually go back a short distance beneath the ice now in motion, and see its mode of work and the results it is effecting. ”

“ A sheet of ice, of this kind, floating upon water, dissolves in the same manner, melting wholly from the surface, if the process be sufficiently rapid, or falling to pieces, if the air-bubbles are gradually raised in their temperature sufficiently to render the whole mass cavernous and incoherent. If we now compare these facts with what is known of the structure of land-ice, we shall see that the mode of formation in the two cases differs essentially.
Land-ice, of which both the ice-fields of the Arctics and glaciers consist, is produced by the slow and gradual transformation of snow into ice ”

“ Thus a given thickness of ice may cause a suspension of the freezing process, and the first ice-stratum may even be partially thawed before the cold is renewed with such intensity as to continue the thickening of the ice-sheet by the addition of fresh layers. The strata or beds of ice increase gradually in this manner, their separation being rendered still more distinct by the accumulation of air-bubbles, which, during a hot and clear day, may rise from a muddy bottom in great numbers. ”

“ The ice-mantle becomes continental in its extent. Its thickness reaches hundreds of feet. Toward the pole this may be increased to thousands or tens of thousands. It has a great weight. It presses upon itself. Its lower portions yield to the inequalities of the rocky surface. The mass seeks the valleys. It slides down the mountain-sides, carrying the débris which it detaches in its descent. It covers the broad plains. The accumulations toward the north, ever increasing, press out toward the south, the foot of the ice-sheet. ”

“ The ice-sheet, which at this day wraps the poles, began to extend southward, and an icy sea, such as now washes the shores of Greenland and the North Cape, rolled over the northern half of the globe.
The Ice period has occasioned much dispute and wonderment among men of science. Its remaining traces are chiefly drift, ice-markings such as ice-worn rocks and boulders, with the bones of animals and the traces of Arctic plants found far south of the regions which such organisms occupy at the present time, or did occupy in an age earlier than the Glacial. ”

“ Sometimes when those floating pieces or plates meet with any obstruction in the channel of the river, they accumulate in such quantities as to cover the surface of the water, and become frozen together in one large sheet, but this kind of ice may be always readily distinguished from that produced in the usual way by the action of the cold air on the surface, which is smooth, transparent, and of an uniform texture ”

“ In wind sheltered areas thickening ice usually forms a continuous sheet before it can develop into the characteristic ice cake form. When sea ice reaches a thickness of between 10 to 30 centimeters it is referred to as gray and gray-white ice, or collectively as young ice, and is the transition stage between nilas and first-year ice. First-year ice usually attains a thickness of between 30 centimeters and 2 meters in its first winter’s growth. ”

“ When a floating sheet of ice grows to this or a greater thickness and extends over a great horizontal distance, it is called an ice shelf. Massive ice shelves, where the ice thickness reaches several hundred meters, are found in both the Arctic and Antarctic. ”

“ They may be divided into two groups: 1, Those where the temperatures are lower than the normal, without becoming low enough for ice to form; and 2, Those where the temperature sinks so low, that ice forms.
It is difficult to make definite divisions among the various forms of natural refrigerators, but it is correct, probably, to classify them under five heads, in accordance with the different kinds of formations of the hollows in the rocks:
1. Gullies, gorges, and troughs where ice and snow remain.
2. Soil or rocks overlaying ice sheets.
3. Taluses and boulder heaps retaining ice. ”

“ Bulletin de la classe physico-mathématique de l’Académie Impériale des Sciences de St. Pétersbourg. 1853. Vol. XI, pages 305-316.
Subterranean Ice Sheets.—In several places in different parts of the world there are underground ice sheets which extend over large spaces; they are common under the tundras of Alaska; and there are fine examples on Kotzebue Sound, [16] on the Kowak River, [17] and along the Yukon River. [18] The “Ice Spring” in Oregon « 116 » seems to be a formation of the same kind. ”

“ But the angle was too wide to give security to this position, the more especially that with shortened axes they were compelled to hack out enough of the ice to reveal the rock below. In such places the ice is but loosely adherent, being raised up from the face much as pie-crust dissociates itself from the fruit beneath under the influence of the oven. Strike lightly with the axe, and a hollow sound is yielded without much impression on the ice; strike hard, and the whole mass breaks away. ”

“ In the course of a tramp to-day about noon, the thermometer standing at -18°, I came across a wonderful instance of the yielding elasticity of ice under intense pressure. About two hundred yards from the brig, on her starboard quarter, was an unbroken plain of level ice, which before our recent break-up used to form one of my daily walks. It measured one hundred and thirty paces in its longer diameter and eighty-five in its shorter, and its thickness I ascertained this morning was over five feet. ”

“ The origin of the shelf ice is believed to be a sea-ice which because not easily detached at the time of the spring “break-up” is thickened in succeeding seasons chiefly by the deposition of precipitated and drifted snow upon its surface, so that it is bowed down under the weight and sunk to greater and greater depths in the water. To some extent, also, it is fed upon its inner margin by overflow of glacier ice from the inland ice masses. ”

“ This broken-up ice is known as "Pack ice." During the breaking up of the ice, the floes crush together and their edges are broken and curled and piled with the pressure. Farther away from the open sea, well in among the solid floes, this pressure is very heavy and one floe may run over and another under the other. The edges of an extended crack, that has formed in a weak place, curl over and over and a long ridge of broken-up ice is the result. ”

“ When it is to be removed by the field planer, the ice field is first laid out and plowed to the depth of the snow or sap ice to be removed. The snow ice planer follows the plowed grooves, cutting off the refuse ice.
Thickness of Ice.—Generally speaking, the ice is desired at an average thickness of fourteen inches; this being convenient for subsequent handling. In practice, the thickness of the ice, as it is stored or shipped from the water, varies with the exigencies of the season and the average thickness formed at the locality. ”

“ If it is deep it can be readily handled, by cutting holes through the ice, or tapping the ice field in a number of places; the water, being heavier than the ice, will sink and raise the ice, if it has opportunity of so doing. Tapping should have prompt attention, and the water removed without delay. Even if the water freezes on top, the ice is not of good quality and is not part of the ice below. It forms a layer which is attached to the old ice, but yet is not solidly a part of it, and gives trouble by splitting out in barring off. ”

“ Standing on our little deck, and looking out on the floe, we had the strange spectacle of an undulating solidity, a propagated wave borne in swell-like ridges, as if our ice was a carpet shaken by Titans. I can not convey the effect of this sublime spectacle. The ice, broken into polyhedric masses, gave at a few hundred yards no indications to the eye of the lines of separation; besides which, the infiltration of salt water had no doubt increased the plasticity of the material. ”

“ But, under all these various circumstances, frost will transform the crystalline snow into more or less compact ice, the mass of which will be composed of an infinite number of aggregated snow-particles, very unequal in regularity of outline, and cemented by ice of another kind, derived from the freezing of the infiltrated moisture, the whole being interspersed with air. Let the temperature rise, and such a mass, rigid before, will resolve itself again into disconnected ice-particles, like grains more or less rounded. ”

“ Ice, capable of being thus moulded, is practically incapable of being stretched. The condition essential to success is that the particles of the ice operated on shall be kept in close contact, so that when old attachments have been severed new ones may be established.
11. The nearer the ice is to its melting point in temperature, the more easily are the above results obtained; when ice is many degrees below its freezing point it is crushed by pressure to a white powder, and is not capable of being moulded as above. ”

“ 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. ”

“ It can be seen that this ice is much more plastic than fresh-water ice. Fresh-water ice is relatively brittle, even in thin layers. Fresh-water ice, of the thickness of about 21⁄2 or 3 inches, will bear the weight of a man, but a child's foot would sink through sea ice of similar thickness, as if going through a layer of tough glue. ”

“ We have butted several times against projecting floes, but it is as unmoved as solid rock. Very foggy: Rescue not visible. Thermometer at 29 degrees.
“We recognize, among the floe fragments around us, old play-fellows. Here we played foot-ball; there we skated; by this hummock crag stood my thermometers; and here I shot a bear. We are passing slowly from them, or they from us. Now and then a rubbish pile will show itself, cresting the pure ice. Even an old champagne basket, full of nothing but sadly-pleasant associations, is recognized upon a distant floe. ”

“ The weather now determines the lot of the ice dealer. As the cold breezes whistle over the water, stirring it into ripples, and breaking its surface into waves, a wonderful change is rapidly transforming its liquid pearls into flinty diamonds. Gradually the heat in the water is radiated into the air. As fast as the surface water is cooled, it is condensed, and sinks to the bottom, its place being taken by the warmer and lighter water from beneath. ”

“ The irregularity of the forms of ice is so great, that no deduction can safely be drawn from them; cases may occur where a recently-formed ice-floe, which has been attached to old ice, is forced by its neighbour to sink under the normal level ”

“ Summer and winter the snow is precipitated upon these mountains, and from time to time the heat of the sun's rays softens the snow, when by its great weight it packs more closely together until it is, in many cases, formed into solid ice-cakes. If we take a quantity of snow or a quantity of granulated ice and put it under a sufficient pressure we can produce clear solid ice, and it is by this process that ice is formed out of the snow and hail that falls continually upon the tops of these glacial mountains. ”

“ As the ice is taken from the house, the covering of sawdust should be kept in place over the ice as far as possible. If58 the space over the ice is sealed up, the air, being above the freezing point, becomes saturated with moisture, which settles upon the ice, softening and melting it. When the outside air is cooler than that over the ice or in the loft, the moist air is driven out, if the ventilators are open. ”

“ This snow ice is white, being very porous and filled with air, and detracts from the quality of the crop, its thickness depending on the depth of snow on the field, amount of water, and the temperature. At the top of this snow ice, where it merges into the17 snow, will be found a stiff, crusty layer, more or less firmly united to the ice below, which adds to the difficulty of removing the snow on top. An inch or two of snow ice will lessen the loss by breakage of cakes, in stowing, and the ice also comes out of the house in better shape, and will stand shipping better. ”

“ I take it for granted, also, that no one, familiar with the presence of the numerous ice-seams parallel to the layers of snow in these upper regions of the glacier, can doubt that they, as well as the névé, are the result of frost. But be this as it may, the difference between the porous ice of the upper region of the glacier and the compact blue ice of its lower track seems to me evidence direct that at times the whole mass must assume the rigidity imparted to it by a temperature inferior to the freezing-point. ”