The structure of glacial ice is characteristically different from that produced by the freezing of a body of water. The latter is made up of parallel crystals with optical axes perpendicular to the surface of the water. Glacial ice, on the other hand, consists of crystalline grains, which increase in size toward the lower end of the glacier, with optical axes disposed irregularly. The banded' structure of the glacier, often so conspicuous, is a kind of stratification and is derived from the successive snow layers of the neve.

The temperature of the interior of a glacier corresponds at every depth to the melting-point of the ice for the pressure at that depth. The melting-point of ice is lowered by pressure, and therefore pressure changes within and at the bottom of the glacier cause melting and refreezing without corresponding temperature changes. This fact that glacial ice is so nearly at the melting-point indicates that the maximum thickness of the glacier cannot exceed 1600 feet, which in truth appears to be the thickness of the Antarctic ice-cap. A greater thickness would cause melting by pressure of the bottom parts.

It follows from their mode of formation that glaciers can be formed only where the snow accumulates to great thicknesses, and cannot be disposed of by either melting or evaporation Hence, glaciers are rare or absent in dry regions, as in most of the Rocky Mountains within the limits of the United States. It also follows that the ground upon which the snow lies must be so shaped as to allow great masses of it to gather.

A glacier moves in much the same way as a river, but at a very much slower rate. The middle portion moves faster than the sides, because the latter are retarded by the friction of the banks, and, for the same reason, the top moves faster than the bottom. While behaving like a plastic substance under pressure, ice yields readily to tension, and even a slight change in the slope of the bed will cause a great transverse crack, or crevasse, to form, which, like an eddy in a stream, seems to be stationary, because always formed again at the same spot. Other systems of cracks, the marginal crevasses, are formed along the sides of the glacier, and are due to the more swiftly moving middle pulling away from the retarded sides.

The rate of glacier movement depends upon the snow supply, upon the slope of the ground, and the temperature of the season. The comparatively small glaciers of the Alps move at rates varying from two to fifty inches per day in summer and at about half that rate in winter, while the vastly larger glaciers of the polar lands have a correspondingly swifter flow. The great stream of ice which enters Glacier Bay in Alaska has a summer velocity of seventy feet per day in the middle.

Fig. 63. - Moraine-covered surface of the Malaspina Glacier, Alaska. (U. S. G. S).

Southeastern Alaska is a region where glaciers are developed on a very extensive scale. The Malaspina is an immense ice-sheet, having an area of 1500 square miles, which is formed at the foot of the St. Elias Alps by the confluence of several great glaciers from the neighbouring mountains. Parts of this vast accumulation of ice are stagnant and deeply covered with rock debris, upon which there is a luxuriant growth of vegetation, with not less than 1000 feet of ice beneath it.

In Greenland and the Antarctic continent the accumulations of ice are on a scale not elsewhere found, and these regions present conditions of great geological interest. Greenland, except for a narrow strip along the coasts, is buried beneath a vast ice-sheet, from which great glaciers descend to the sea. In the interior only a few isolated mountain peaks, or nunataks, rise through the ice mantle; except for these, nothing is visible but illimitable fields of snow. The snowfall is not very great; but so little of it is disposed of by evaporation or melting, that there is a large excess which goes to the growth of the ice-sheet, and keeps up the supply for the innumerable glaciers which flow to the sea.

Fig. 64. - Nunatak rising through the ice-cap, Greenland. (Photograph by Libbey).

The Antarctic ice-cap is estimated to be nearly seven times as large as that of Greenland.

The source of a glacier is always above the snow-line, but the ice-stream itself may descend far below that line, slowly melting and diminishing in thickness as it flows. The lower end is at the point where the rate of melting and the rate of flow balance, so that changes in the temperature of the seasons or in the amount of the snow supply will cause the glacier to advance or retreat, as one or other of these factors prevails. Thus the Alaskan glaciers have retreated notably within the last century, while some of the Norwegian ones are advancing. From the lower end of a glacier there always issues a stream of water, which flows under the ice, often in great volume, and even in winter, for the thick ice is a non-conductor and protects the stream from the intense cold of the air.

There are various forms of moving bodies of land ice corresponding to bodies of water. We have (1) Alpine glaciers, of which those in the Alps are types, and are relatively small streams occupying narrow mountain valleys, each connected with a particular basin or gathering ground of snow. (2) Hanging glaciers, which descend but little below the snow-line and are small glaciers occupying steep clefts near the mountain tops. In some cases they are not connected with snow-fields, but are fed by avalanches. The glaciers of the northern Rocky Mountains and the Sierra Nevada are mostly of this class. (3) Ice fields, exemplified in Scandinavia; these are extensive and continuous areas of thick ice, with gently curved surface, from the margins of which numerous, but mostly small, glaciers descend through rocky gorges. (4) Piedmont glaciers, like the Malaspina of Alaska. These are great accumulations or lakes of ice which form at the foot of mountains, by the coalescence of numerous glaciers of the Alpine, or valley, type. (5) Continental glaciers are those which cover enormous areas of land, such as the ice-sheet under which nearly all of Greenland is buried and that which covers the Antarctic land.