2. Longitudinal Schema.—A. On the flat, or approximately, those glaciers show convex surfaces.
B. When resting on a slope they are concave in the upper basin, which feeds them and become convex as they reach lower and wider channels.
This second type is the normal glacier type.
A diagram or section of the convex portion of the glacier—an ideal diagram of course—would show the mechanical and static forces at work in a fan-shaped formation radiating from a point on the not geometrical, but mechanical, centre line of the glacier, this point being situated on its bed, where the side-pressures converge and annihilate each other’s progress.
From this point the bottom ice works its way up to the melting-surface—but obliquely, being the whole time carried down by the slope—and throws up side moraines and one or several spinal moraines in the process. The spinal moraines always rest on pure ice. The ice seams have been thrown up from the inside.
Crevasses may occur in an outward, open, surface-formation, as in séracs when they are grouped together, or else they are the result of accidental deflections or temporary oppositions in mechanical and static forces at work in the ice.
We said a while ago that there was no reason why, at the height of 9,000 feet and upwards, snow accumulations should be more stable and constant on ice surfaces than on rock. The cause for this is simply that rock and ice are too near to each other and at altitudes too closely alike for serious differences in temperature.
Let us now pass to the matter of avalanches. If snow is utterly unstable on rock, so it is on ice. Rock and ice constitute an avalanche area, which in winter extends down so as to include all steepnesses on which snow may lodge and whence it may be dislodged by the forces of Nature.
Avalanches may be periodic or accidental.
A periodic avalanche is the kind that comes down regularly at a known spot, each time sufficient cause is brought into play. Maps of the Alps exist on which those periodic avalanches are noted. Almost every Alpine village has a periodic avalanche on its territory. The peasants know when and where to expect it. It is called the avalanche of so and so, and your business is to find out, each time you propose going out on an expedition, whether it has come down or not, and all about it.