Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/295

Rh ICE.] Snow exhibits two different kinds of geological behaviour, one conservative, tl1e other destructive. Lying stationary and unmelted it exercises a protective influence on the face of the land, shielding rocks, soils, and vegetation from the effects of frost. On low grounds this is doubtless its chief function. Whe11 snow falls in a partially melted state it is apt to accumulate on branches and leaves, until by its weight it breaks them off, or even bears down entire trees. Snow which falls thickly on steep mountain slopes is fre- quently during spring and summer detached i11 large sheets. These rusl1 down the declivities as avalanches, and often create much destruction of trees, soil, crops, and houses in their course. Another indirect effect of s11ow is seen in the sudden rise of the rivers when warm weather rapidly melts the mountain snows. Many summer ﬂoods are caused in this way in Switzerland. 5. G'laciers.—(l.) Nature and Origin.—A glacier is a river of ice formed by the slow movement and compression of the snow which by gravitation creeps downward into a valley descending fron1 a snow-ﬁeld. The structure and physics of glaciers are described elsewhere (see GLACIERS). From a geological point of view these ice-rivers may be re- garded as the drainage of the snow-fall above the snow—line, as rivers are the drainage of the rainfall. In a mountainous region, such as the Alps, or a table-land like Scandinavia, where a considerable mass of ground lies above the snow- line, three varieties of glaciers have been observed. Glaciers of the first order, where the ice—river comes down well below the snow and extends into the valley, even it may be far below the upper limits of cultivation, or in northern regions approaches or even reaches the sea. In the Alps such glaciers may be 20 or 30 miles long, by a mile or more wide, and 600 feet or 111ore deep. Glaciers of the second order, which hardly creep beyond the high recesses wherein they are formed, and do not therefore reach as far as the nearest valley. Many beautiful examples of this type may be seen along the steep declivities which GEOLOGY 1 l escapes from under the melting extremity of the ice. 281 through accurate measurement by J. D. Forbes, who found that in the Mer de Glace of Chamouni, the mean daily rate of motion in the summer and autumn was from ‘20 to 27 inches in the centre, and from 13 to 19.1; near the side. The consequence of this differential motion is seen in the arrangement of the lines of rubbish thrown down at the end of a glacier, which often present a horse-shoe shape, corresponding to that of the end of the ice by which they were discharged. There are some features of geological importance also in the behaviour of the ice as it descends its valley. When it has to travel over a very uneven ﬂoor, some portions may get embayed, while overlying parts slide over them. A massive ice-sheet may thus have many local eddies in i-ts lower portions, the ice there even travelling for various distances, according to the nature of the ground, obliquely to the general flow of the mainmass. In descending by a steep slope from an upper to a lower and more level part of its course, a glacier becomes a mass of ﬁssured ice in great confusion. It descends by a slowly creeping ice-fall, where a river would shoot over in a rushing waterfall. A little below the fall the fractured ice is pressed together again into a solid mass as before. The body of the glacier throughout its le11gtl1 is traversed by a set of ﬁssures called crevasses, which, though at first as close-fitting as cracks in a sheet of glass, widen by degrees as the glacier moves on, till they form wide yawning chasms, reaching, it may be, to the bottom of the ice, and travelling down with the _ glacier, but apt to be effaced by the pressing of their walls together again as the glacier winds down its valley. The glacier continues to descend until it reaches that point where the supply of ice is just equalled by the liquefaction. There it ends, and its place down the rest of the valley is taken by the tumultuous river of muddy water which A prolonged augmentation of the snow-fall will send the foot of the glacier further down the valley; a diminution of the intervene between the snow—covered plateau of Arctic Nor— ] snowfall with a general rise of temperature will cause it to way and the sea. of fragments which fall from an ice—clitf crowning preci- pices of rock, and are re-frozen at the bottom into a solid mass, creeping downward as a glacier usually of the second order. Probably the best illustrations in Europe are fur- nishexl by the Nus Fjord, and other parts of the north of Norway. In some cases a cliff of blue ice appears at the top of the precipice,-—the edge of the great “ snce-fond,” or snow-ﬁeld,—while several hundred feet below, in the corrie or cwm at the bottom, lies the re—cemented glacier (glacier rem.(uu'é of the Swiss), white at its upper edge, but acquir- ing somewhat of the characteristic blue gleam of compact ice as it moves towards its lower margin. But it is in high Arctic, and still n1ore in Antarctic, latitudes that land-ice, formed from the drainage of agreat snow-ﬁeld, attains its greatest dimensions. The land in these regions is completely buried under an ice-cap, which ranges in thickness up to a depth (in the South Polar ' circle) of 10,000 feet (2 miles) or even more. Greenland lies under such a pall of snow that all its inequalities, save the 1nere steep mountain peaks, are concealed. The snow creeping down the slopes, and mounting over the minor hills, passes beneath by pressure into compact ice. From the main valleys great glaciers like vast tongues of ice, 2000 or 3000 feet thick, and sometimes 50 miles or more in breadth, push out to sea, where they break off in huge fragments, which ﬂoat away as icebergs. A glacier, like a river, is always in motion, though so slowly that it seems to be solid and stationary. The motion also, like that of a river, and for the san1e reason, is unequal in the different parts, the centre moving faster than the sides and bottom. This important fact was ﬁrst ascertained (c.) Re-cemented glaciers, consisting | retreat farther up. (2.) Work done by Glaciers.——Glaciers have two import- ant geological tasks to perfor1n,—(1) to carry the debris of the mountains down to lower levels; and to erode their beds. a.. T-rcmsport.—Tl1is takes place chieﬂy on the surface of the glacier. Descending its valley, the glacier receives and bears along 011 its margin the earth, stones, and rubbish which, loosened by frost, or washed down by rain a11d rills, slip from the cliffs a11d slopes to the level of the ice. In this part of its work the glacier resembles a river which carries down branches and leaves from the woods on its banks. Most of the detritus rests on the surface of the ice. It includes huge masses of rock, sometimes as big as a large cottage, all which, though seemingly at rest, are slowly travelling down the valley with the ice, and liable at any moment to slip i11to the crevasses which may open below them. When they thus disappear they may descend to the bottom of the ice, and move with it along the rocky floor, which is no doubt the fate of the smaller sto11es and sand. But the large stones seem sometimes at least to be cast up again by the ice to the surface of the glacier at a lower part of its course. Whether, therefore, on the ice, iii the ice, or under the ice, a vast quantity of detritus is con- tinually travelling with the glacier down towards the plains. The rubbish lying on the surface is called moraine stuﬂ’. Naturally it accumulates on either side of the glacier, where it forms the so-called lateral morai-nes. When two glaciers unite, their two adjacent lateral moraines are bronght together, and travel thereafter down the centre of the glacier as a medial moraine. A glacier, formed by the union of many tributaries in its upper _parts, ngay have A. — 3