Account for the fact that the morainic ridge rises considerably above the surface of the ice] The sand brought out by streams from beneath a glacier differs from river sand in that it consists of freshly broken angular grains. Why? The stream derives its water chiefly from the surface melting of the glacier. As the ice is touched by the rays of the morning sun in summer, water gathers in pools, and rills trickle and unite in brooklets which melt and cut shallow channels in the blue ice. The course of these streams is short. Soon they plunge into deep wells cut by their whirling waters where some crevasse has begun to open across their path. These wells lead into chambers and tunnels by which sooner or later their waters find way to the rock floor of the valley and there unite in a subglacial stream. [Illustration: Fig. 107. Subglacial Stream Issuing from Tunnel in the Ice, Norway] =The lower limit of glaciers.= The glaciers of a region do not by any means end at a uniform height above sea level. Each terminates where its supply is balanced by melting. Those therefore which are fed by the largest and deepest névés and those also which are best protected from the sun by a northward exposure or by the depth of their inclosing valleys flow to lower levels than those whose supply is less and whose exposure to the sun is greater. A series of cold, moist years, with an abundant snowfall, causes glaciers to thicken and advance; a series of warm, dry years causes them to wither and melt back. The variation in glaciers is now carefully observed in many parts of the world. The Muir glacier has retreated two miles in twenty years. The glaciers of the Swiss Alps are now for the most part melting back, although a well-known glacier of the eastern Alps, the Vernagt, advanced five hundred feet in the year 1900, and was then plowing up its terminal moraine. How soon would you expect a glacier to advance after its névé fields have been swollen with unusually heavy snows, as compared with the time needed for the flood of a large river to reach its mouth after heavy rains upon its headwaters? [Illustration: Fig. 108. A Glacier Table] On the surface of glaciers in summer time one may often see large stones supported by pillars of ice several feet in height (Fig. 108). These "glacier tables" commonly slope more or less strongly to the south, and thus may be used to indicate roughly the points of the compass. Can you explain their formation and the direction of their slope? On the other hand, a small and thin stone, or a patch of dust, lying on the ice, tends to sink a few inches into it. Why? In what respects is a valley glacier like a mountain stream which flows out upon desert plains? Two confluent glaciers do not mingle their currents as do two confluent rivers. What characteristics of surface moraines prove this fact? What effect would you expect the laws of glacier motion to have on the slant of the sides of transverse crevasses?