Glacier

Glacier is a stream of ice that flows down the valleys in high alpine regions by its own weight and by pressure of snow from above. In warmer climates they must be sought for at high altitudes, about 16,000 feet near the Equator, from 1,000 to 2,000 feet in New Zealand, and from 4,000 to 6,000 feet in Switzerland; but in higher latitudes, such as Greenland, the ice comes down to sea-level. Icebergs are formed by fracture of the ice sheet as it passes into water. There remains much to be discovered concerning the nature of glaciers. It is generally in motion, the rate of progress varying much with the locality. Valuable experiments were made on the Mer de glace near Chamonix by Forbes, Tyndall, and others. It was found that this particular glacier moved at an average rate of 1 foot 10 inches per day. In Greenland 21 feet per day is a fair average. As with rivers of water, the rate of motion at the sides is less than that in the middle on account of retardation by friction. Similarly where the bed is narrowest the glacier is generedly deepest. A steep glacier flows more rapidly than one of gentle slope. If the bed is ridged along its length, more or less parallel cracks or crevasses appear on the surface. When the ridges are transverse, the crevasses are transverse. If the bed is much broken, both longitudinal and transverse crevasses may occur. It is probable that sharp depressions in the bed produce crevasses on the under surface of the glacier, which is thus rarely continuous throughout its mass. Sudden lowering of the bed produces an icefall, the glacier here being broken up into pinnacles and irregular blocks separated by crevasses of great width and often profound depth. Fresh snow may in many cases cover the crevasses, but unless the snow is very thick slight depressions of its surface will indicate the positions of the hidden crevasses. It is probable that the curiously continuous flow of glaciers is due to the process of regelation first pointed out by Forbes; ice under pressure is liable to become liquefied, and will freeze again when pressure is removed. If, therefore, the stream of ice has to pass over an obstruction, pressure from behind will cause partial liquefaction of those parts that are subjected to the greatest stress; flow is thereby rendered easier, andafter the obstruction is passed the ice becomes solid again. A glacier is fed by snow from high altitudes forcing its way down the hollows first eis hard snow or neve, but gradually hardening under pressure until it becomes ice, and tributary streams may unite and so form the main glacier. Where it leaves the steeper parts of the mountain its weight will generally cause fracture, and crevasses that are technically termed bergschrunds will mark off the glacier from the neve or steeper ice.

Depending upon various conditions of the glacier, crevasses in their passage downwards with the ice become more or less oblique; they may close up or may become enlarged.

Moraines are heaps or lines of stones and boulders that have fallen upon the glacier and have been carried down by the stream of ice. Medial moraines lie along the centre line of the glacier. Lateral moraines lie along its edges. Terminal moraines are carried forward to the lower end of the glacier. and sometimes pass far down into the valley. When two glaciers meet, it frequently happens that;

two of their lateral moraines meet, forming a medial moraine down the main glacier. Moraine heaps may be so great as to hide all traces of glacier, and it is sometimes difficult to tell whether ice exists under the mass of boulders.

Temperature conditions assign a lower limit to the glacier, which may thus change in position.

Though the glacier never flows backwards, its lower extremity or snout may seem to do so by being dissolved away more rapidly than the ice stream creeps downwards. Fluctuations in the length and depth of glaciers are observed to take place in periods extending over many years. The plate facing page 65 shows a glacier at a period of shrinkage.

Many watercourses may be carved out in the glacier, vertical plunges or moulins occurring where the water falls down perpendicular shafts in the ice, these probably starting initially as crevasses and wearing down under the action of the water. Lake basins may also be occasionally formed in the ice, and present a source of danger in critical situations, where the ice walls enclosing them are not sufficiently strong to withstand the fluid pressure. The catastrophe of 1892 at St. Gervais in the Haute-Savoie is partially explained by this fact.