Fig. 95. Sea Wall of the Muir Glacier, Alaska

The bergschrund. The névé is in slow motion. It breaks itself loose from the thinner snows about it, too shallow to share its motion, and from the rock rim which surrounds it, forming a deep fissure called the bergschrund, sometimes a score and more feet wide ([Fig. 94]).

Size of glaciers. The ice streams of the Alps vary in size according to the amount of precipitation and the area of the névé fields which they drain. The largest of Alpine glaciers, the Aletsch, is nearly ten miles long and has an average width of about a mile. The thickness of some of the glaciers of the Alps is as much as a thousand feet. Giant glaciers more than twice the length of the longest in the Alps occur on the south slope of the Himalaya Mountains, which receive frequent precipitations of snow from moist winds from the Indian Ocean. The best known of the many immense glaciers of Alaska, the Muir, has an area of about eight hundred square miles ([Fig. 95]).
Fig. 96. Diagram showing Movement of Row of Stakes a, set in a direct line across the surface of a glacier; b, c, and d, successive later positions of the stakes Fig. 97. Diagram showing Movement of Vertical Row of Stakes a, set on side of glacier

Glacier motion. The motion of the glaciers of the Alps seldom exceeds one or two feet a day. Large glaciers, because of the enormous pressure of their weight and because of less marginal resistance, move faster than small ones. The Muir advances at the rate of seven feet a day, and some of the larger tide glaciers of Greenland are reported to move at the exceptional rate of fifty feet and more in the same time. Glaciers move faster by day than by night, and in summer than in winter. Other laws of glacier motion may be discovered by a study of Figures [96] and [97]. It is important to remember that glaciers do not slide bodily over their beds, but urged by gravity move slowly down valley in somewhat the same way as would a stream of thick mud. Although small pieces of ice are brittle, the large mass of granular ice which composes a glacier acts as a viscous substance.

Fig. 98. Crevasses of a Glacier, Canada