GLACIER, a current of ice formed from compressed snow. Water, changed into vapor by sun-heat and carried by the winds over frosty highlands, is crystallized into snow. Glaciers take their rise in regions which he above the snow-line. Upon these regions from their geographical position and elevation, the quantity of snow that falls exceeds the quantity melted and immediately evaporated. The sur plus, instead of accumulating indefinitely, is changed by the pressure of its weight into ice, which, though hard and apparently as brittle and inflexible as glass, flows down toward the sea in beautiful swaying undulating lines, as if soft like honey or tar. Thus the overbur dened regions above the snow-line are relieved and a continuous circulation is maintained,— ocean water flying away through the air in the form of vapor, but in returning creeping along the ground in the form of ice, grinding and crushing the rocks that lie in its way, and leav ing, a heavier track than anything else that moves on the face of the earth.
In general a glacier flows like a river, and drains off snow as a river drains off rain. At different places it moves at different rates, not only along its cross-sections, but along its length and also from surface to bottom, as friction and the declivity of its bed varies. The velocity of the swiftest parts of the largest glaciers of the Alps is about from one foot to three feet per day; of the smallest, about as many inches. The lower central part of the Muir Glacier of Alaska flows about 10 feet a day. Some of the Greenland glaciers are said to flow much faster, from 59 feet to 100 feet having been recorded for 24 hours. Glacier motion, however slow, is continuous. It is less in winter than in summer, and slightly less in frosty nights than in warm and rainy days. Differences are noted also where the neighbor ing peaks constrict the ice flow to a narrower path. At such points the speed is increased.
Though obedient to the laws of liquid motion in general, a glacier refuses to stretch, as is shown by its breaking sharply asunder at right angles to tension strains, thus forming the so-called transversal, longitudinal, marginal and bergschrund crevasses. The first two are caused by unevenness of the channel, the marginal by differential motion, the berg schrund by the glacier flowing away from the motionless snow attached to the head of its basin. The last is of course a feature of all
glaciers; so are the marginal crevasses, since the middle of all glaciers flows faster than the sides; but large central areas, where the bed is regular in slope or slightly concave, are free from crevasses. The largest crevasses are sev eral miles long, 1,000 feet deep or more, and 30 or 40 feet wide, though at first they are usually too narrow to admit a knife-blade. In some places all sorts of crevasses are interlaced, forming labyrinths of yawning gulfs defying the skill and will of the bravest mountaineer who tries to hew a way through them. The ridges between closely spaced crevasses are known as seracs.
The brittleness of ice, with its flowing motion, is partly explained by regelation (refreezing). In 1850 Faraday discovered that when two pieces of thawing ice are placed together they freeze at the points of contact. Snow at a temperature of 32° F., stuffed into a mold and squeezed, becomes trans parent ice. So also fragments of ice pressed in a mold break, are crushed and recongealed into a solid mass of the form of the mold, illus trating the breaking of glaciers and their regela tion when from change of position the sides of the chasms, great or small, are pressed together.
The life of a glacier is one eternal grind. Its draining streams are always milky with rock mud rubbed off its bed, and separated from the large detached masses by the waters. Moraines, lateral, medial and terminal, are the general detritus of a glacier and the weathered heights about it, drawn out and arranged by the ice currents, and located as their names indicate. The medial moraines, of which each glacier usually has one fewer than the number of its tributary glaciers, are formed by the union of two laterals at the confluence of the tributaries, and extend down the trunk in beautiful order. The terminal moraine is made up of parts of all the others. The moraine material, clay, sand and boulders, of the great continental glaciers of the Ice Age, is often called drift. The detached rock masses, borne along by the ice currents and left in the terminal moraines, or if the glacier reaches the sea, dropped perhaps hundreds of miles away by iceberg, are called erratics.