Over the equator the snow-line is about sixteen thousand feet high, or higher than Mont Blanc; so if Mont Blanc were situated on the equator it would not be a snow-clad mountain. At the north and south poles the snow-line is down at the sea-level. Summer warmth there does not suffice to get rid of winter ice and snow. Between the equator and the poles the snow-line varies in each country, according to that country’s position and climate.
Above the snow-line on lofty Swiss mountains heavy falls add perpetually to the mass of snow. During the summer a certain degree of thawing goes on, but never anything like enough to balance the wintry additions. Exactly at the snow-line the quantity of snow which falls and the quantity which thaws in the year are about equal. Lower down the yearly thawing exceeds the yearly snowfall.
But if, above that line, more snow is being added year by year than can thaw and flow away, must it not be that those mountains which wear perpetual snow are always growing higher?
It certainly would be so if the snow heaped upon those summits had no other outlet, no other means of escape to the Ocean—the goal of all Earth’s waters. But another mode of escape is found. Superabundant snow on mountain heights gets away by means of glaciers.
We have had to think about liquid rivers flowing in the ocean, and here are solid rivers flowing on the land. Liquid rivers with liquid banks; solid rivers with solid banks. A curious anomaly in either case.
The higher levels of land are drained by rivers pouring down to lower levels and thence into the sea; and it is the same with the mighty snowfields of Switzerland as with any gentle range of English hills, only here we have rivers of ice instead of rivers of water.
But why not rivers of snow, if a glacier means the draining away of snowfields?
Well, so they are—rivers of snow. But the heavy weight of overlying snow above, and the great pressure of descending masses later, welds the light and delicate snow into hard ice. Those tiny needles, of which the falling snow was made, are crushed closer and closer, till they form a solid block, which loses all resemblance to snow. In summer the melting of the surface by day, and its freezing again by night, help forward this transformation.
Thus a glacier is literally an Ice-River, a huge long tongue of ice, squeezed from beneath snowfields, and creeping down a valley.
Such rivers vary much in size. Some of the Swiss glaciers are between twenty and thirty miles long, in parts two or three miles wide, and often hundreds of feet deep. Starting above the snow-line they sometimes reach thousands of feet below it, the milder air failing to end their existence sooner. So enormous are the ice-masses, that not all the strength of the summer sun can make away with them. Of course, each square yard in turn does melt, and does help to feed the river of water which rushes away from the lower end of the glacier—sooner or later to reach the ocean.