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.

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?

Lateral and medial moraines.  The surface of a glacier is striped lengthwise by long dark bands of rock debris.  Those in the center are called the medial moraines.  The one on either margin is a lateral moraine, and is clearly formed of waste which has fallen on the edge of the ice from the valley slopes.  A medial moraine cannot be formed in this way, since no rock fragments can fall so far out from the sides.  But following it up the glacial stream, one finds that a medial moraine takes its beginning at the junction of the glacier and some tributary and is formed by the union of their two adjacent lateral moraines.  Each branch thus adds a medial moraine, and by counting the number of medial moraines of a trunk stream one may learn of how many branches it is composed.

Surface moraines appear in the lower course of the glacier as ridges, which may reach the exceptional height of one hundred feet.  The bulk of such a ridge is ice.  It has been protected from the sun by the veneer of moraine stuff; while the glacier surface on either side has melted down at least the distance of the height of the ridge.  In summer the lowering of the glacial surface by melting goes on rapidly.  In Swiss glaciers it has been estimated that the average lowering of the surface by melting and evaporation amounts to ten feet a year.  As a moraine ridge grows higher and more steep by the lowering of the surface of the surrounding ice, the stones of its cover tend to slip down its sides.  Thus moraines broaden, until near the terminus of a glacier they may coalesce in a wide field of stony waste.