The stones and grains of sand which get jammed between the ice and the rock over which it is moving are made to score and scratch this rock. They form a kind of rough polishing powder, whereby the glacier is continually grinding down the bottom and sides of its channel. If you creep in below the ice, or catch a sight of some part of the side from which the ice has retired a little, you will find the surface of the rock all rubbed away and covered with long scratches made by the sharp points of the stones and sand.
You will now see the reason why the river, which escapes from the end of a glacier, is always muddy. The bottom of the glacier is stuck all over with stones, which are scraping and wearing down the rock underneath. A great deal of fine mud is thus produced, which, carried along by streams of water flowing in channels under the glacier, emerges at the far end in the discolored torrents which there sweep from under the ice.
A glacier is not only busy grinding out a bed for itself through the mountains; it bears on its back down the valley enormous quantities of fallen rock, earth and stones, which have tumbled from the cliffs on either side. In this way blocks of rock as big as a house may be carried for many miles, and dropped where the ice melts. Thousands of tons of loose stones and mud are every year moved on the ice from the far snowy mountains away down into the valleys to which the glaciers reach.
The largest glaciers in the world are those of the polar regions. North Greenland, in truth, lies buried under one great glacier, which pushes long tongues of ice down the valleys and away out to sea. When a glacier advances into the sea, portions of it break off and float away as icebergs. So enormous are the glaciers in these cold tracts that the icebergs derived from them often rise several hundred feet above the waves which beat against their sides. And yet, in all such cases, about seven times more of the ice is immersed under water than the portion, large as it is, which appears above. You can realize how this happens if you take a piece of ice, put it in a tumbler of water, and watch how much of it rises out of the water. Sunk deep in the sea, therefore, the icebergs float to and fro until they melt, sometimes many hundreds of miles away from the glaciers which supplied them.