A glacier is formed in this way: There is a heavy fall of snow which lies in basins and little valleys high up on the mountain side. The air is too cold for it to melt, and as more falls on the top of it the mass gets pressed down. Now, if you take a lump of snow in your hand and press it, you get an icy snow-ball. If you squeeze anything you make it warmer. The pressing down of the great mass of snow is like the squeezing of the ball in your hand. It makes it warmer, so that the snow first half melts and then gradually becomes ice. You bring about this change in your snow-ball in a moment. Nature, in making a glacier, takes much longer, so that what was snow one year is only partly ice the next—it is known as nevé—and it is not until after several seasons that it becomes the pure ice we see in the lower part of a glacier.
One would fancy that if a quantity of snow falls every winter and does not all melt, the mountains must grow higher. But though only a little of the snow melts, it disappears in other ways. Some is evaporated into the atmosphere; some falls off in avalanches. Most of it slowly flows down after forming itself into glaciers. For glaciers are always moving. The force of gravity makes them slide down over their rocky beds. They flow so slowly that we cannot see them move, in fact most of them advance only a few inches a day. But if a line of stakes is driven into the ice straight across a glacier, we shall notice in a few weeks that they have moved down. And the most interesting part of it is that they will not have moved evenly, but those nearest the centre will have advanced further than those at the side. In short, a glacier flows like a river, the banks keeping back the ice at the side, as the banks of a river prevent it from running so fast at the edge as in the middle.
General View on the Lower Part of a Large Glacier.
The surface is ice, not snow. The snow-line may be seen further up.
A large glacier is fed by such a gigantic mass of snow that it is in its upper part hundreds of feet thick. Of course when it reaches warmer places it begins to melt. But the quantity of ice composing it is so great that it takes a long time before it disappears, and a big glacier sometimes flows down far below the wild and rocky parts of mountains and reaches the neighbourhood of forests and corn-fields. It is very beautiful at Chamonix to see the white, glittering ice of the Glacier des Bossons flowing in a silent stream through green meadows.
The reason that mountaineers have to be careful in crossing glaciers is on account of the holes, cracks, or, to call them by their proper name, crevasses, which are met with on them. Ice, unlike water, is brittle, so it splits up into crevasses whenever the glacier flows over a steep or uneven rocky bed. High up, where snow still lies, these chasms in the ice are often bridged over, and if a person ventures on one of these snow bridges it may break, and he may fall down the crevasse, which may be so deep that no bottom can be found to it. He is then either killed by the fall or frozen to death. If, as I have explained before, several climbers are roped together, they form a long string, like the tail of a kite, and not more than one is likely to break through at a time. As the rope is—or ought to be—kept tightly stretched, he cannot fall far, and is easily pulled out again.
The snow melts away off the surface of the glacier further down in summer. It is on this bare, icy stream, scarred all over with little channels full of water running merrily down the melting rough surface, that the ordinary tourist is taken when he visits a glacier during his summer trip to Switzerland.