=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). [Illustration: 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] [Illustration: 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. [Illustration: Fig. 98. Crevasses of a Glacier, Canada] =Crevasses.= Slight changes of slope in the glacier bed, and the different rates of motion in different parts, produce tensions under which the ice cracks and opens in great fissures called crevasses. At an abrupt descent in the bed the ice is shattered into great fragments, which unite again below the icefall. Crevasses are opened on lines at right angles to the direction of the tension. _Transverse crevasses_ are due to a convexity in the bed which stretches the ice lengthwise (Fig. 99). _Marginal crevasses_ are directed upstream and inwards; _radial crevasses_ are found where the ice stream deploys from some narrow valley and spreads upon some more open space. What is the direction of the tension which causes each and to what is it due? (Figs. 100 and 101).