then a large stone, which falls splashing into the stream below.  If you look at the glacier in the Frontispiece, you will see that these stones come from those long lines of stones and boulders stretching along the sides and centre of the glacier.  It is easy to understand where the stones at the side come from; for we have seen that damp and frost cause pieces to break off the surface of the rocks, and it is natural that these pieces should roll down the steep sides of the mountains on to the glacier.  But the middle row requires some explanation.  Look to the back of the picture, and you will see that this line of stones is made of two side rows, which come from the valleys above.  Two glaciers, you see, have there joined into one, and so made a heap of stones all along their line of junction.

These stones are being continually, though slowly, conveyed by the glacier, from all the mountains along its sides, down to the place where it melts.  Here it lets them fall, and they are gradually piled up till they form great walls of stone, which are called moraines.  Some of the moraines left by the larger glaciers of olden time, in the country near Turin, form high hills, rising up even to 1500 feet.

Therefore, if ice did no more than carry these stone blocks, it would alter the face of the country; but it does much more than this.  As the glacier moves along, it often cracks for a considerable way across its surface, and this crack widens and widens, until at last it becomes a great gaping chasm, or crevasse as it is called, so that you can look down it right to the bottom of the glacier.  Into these crevasses large blocks of rock fall, and when the chasm is closed again as the ice presses on, these masses are frozen firmly into the bottom of the glacier, much in the same way as a steel cutter is fixed in the bottom of a plane.  And they do just the same kind of work; for as the glacier slides down the valley, they scratch and grind the rocks underneath them, rubbing themselves away, it is true, but also scraping away the ground over which they move.  In this way the glacier becomes a cutting instrument, and carves out the valleys deeper and deeper as it passes through them.

You may always know where a glacier has been, even if no trace of ice remains; for you will see rocks with scratches along them which have been cut by these stones; and even where the rocks have not been ground away, you will find them rounded like those in the left-hand of the Frontispiece, showing that the glacier-plane has been over them.  These rounded rocks are called “roches moutonnees,” because at the distance they look like sheep lying down.

You have only to look at the stream flowing from the mouth of a glacier to see what a quantity of soil it has ground off from the bottom of the valley; for the water is thick, and coloured a deep yellow by the mud it carries.  This mud soon reaches the rivers into which the streams run; and such rivers as the Rhone and the Rhine are thick with matter brought down from the Alps.  The Rhone leaves this mud in the Lake of Geneva, flowing out at the other end quite clear and pure.  A mile and a half of land has been formed at the head of the lake since the time of the Romans by the mud thus brought down from the mountains.