[Illustration: 18. THE FALLS OF TOSA, VAL FORMAZZA. Said to be the grandest in the Alps, 470 feet high. The Tosa falls in three cascades. The first only is shown in the picture.]
The great importance of glaciers in mountain formation is the part they play as carrying agents. There is practically no limit to the weight of rock they will bear down with them in their steady uninterrupted flow. Whatever falls upon the glacier at any part of its course is carried down by it and ultimately dumped off its sides or end. A stone that falls on the highest rim of the snowfield will presently be covered up by newly-fallen snow and will be carried down at, or close to, the floor of the glacier, where it will either be ground to powder or will not emerge till it is melted out at the end of the glacier's snout. A stone that plunges in a crevasse to the bottom of the glacier will have similar experiences. Stones that tumble on to the glacier surface further down will not be so deeply covered by annual accumulations of snow, and will therefore sooner emerge again on to the surface by the melting away of the accumulation above them. Stones that fall on to the glacier below the snow-line will not be covered up at all, but will simply be carried down on the surface.
The visible collections of stone rubbish carried by a glacier are called its moraines. As the surface of a glacier tends to become convex the moraine-stuff tends to be rolled off towards the sides, where it forms the right and left lateral moraines. Where two glaciers flow together and unite, the right lateral moraine of the one and the left of the other will join and be carried down as a medial moraine on the surface of the united glacier. Such medial moraines may be observed in considerable numbers flowing down, side by side, on glaciers formed by the union of a number of higher tributaries. First comers to the Alps, beholding them from a distance, or seeing them in photographs, sometimes have thought they were cart-ruts, thus showing how false a scale of size their unaccustomed vision applies to mountain views.
A given kind of rock subjected to the action of frost and the other disintegrating forces operative at high levels, usually breaks up into debris of a roughly uniform average size. There will, of course, be some large masses and a lot of dust and gravel, but the average lump will be fairly uniform. A climber in a given district comes to know what to expect on a moraine, and he will immediately notice if the average size of the debris is much larger or smaller than usual. Thus, when he sees a debris-slope or a moraine from a distance, he is instinctively conscious that its granulated aspect represents great blocks of rock. That gives him a roughly correct scale for the view. The lowlander, who has never been in contact with a moraine, has no such sense, and can imagine that the brown streak he sees a few miles away is, as it looks to be, a mere line of dust. It was through the aspect of the moraines and debris-slopes that I first obtained an approach to a direct visual understanding of the vaster scale of the Himalayas than that of the Alps.