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?
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?