On large mountains the important thing to know about a couloir beforehand, if we intend to use it, is whether it is filled with ice or with snow, and whether it is subject to stone fall.
To information on the first point, the study of the angle of the couloir is the first help. A steep angle generally implies that the lining will be ice and not snow.
But as snow, supported by retaining rock, may remain at a very high angle, we look to see if there are any furrows in the white surface. If there are furrows and they are in ice, they will show us ice reflections in sunlight. If there are no ice reflections and they are therefore in snow, or if they are only ice-backed and therefore in snow-covered ice, the depth of the furrows will tell us what depth of snow we may expect upon the ice. If there are no furrows, the edges of the supposed snow must be inspected for further information. Where the snow runs out on to the rock at the side in ice webs, the surface tone will appear of different qualities if it be snow on ice, or if it be ice throughout.
And after all our examination, we may be agreeably surprised to be able to ascend on hard snow in the early hours where we had calculated from the angle that we should be hacking in snow-ice; or as disagreeably disappointed to find snow of avalanche quality, on a late return or after warm wind, where we had located snow of the best bearing variety during a morning inspection.
The base of the couloir should be inspected for traces of previous snow avalanches, and for the character of its bergschrund. We see if we can whether the summit of the couloir is commanded by glacier or slope likely to use it for the discharge of snow or stones. If so, we note during what hours the presence of sun will increase the risk, and when shade to diminish it may be looked for.
In a big couloir we mark down islands of rock which, in case we are detained in the recesses until stone fall time, will provide us with screens below which to steer our line.
We study the lie of the strata and the containing walls for possible exits from the couloir. On big peaks the top of a couloir often opens upon an amphitheatre of slabs, too thinly ice coated for steps. It is then important to mark down a line for escape in time.
If the base of the couloir selected can be seen, the presence there of fallen stones is evidence that they have fallen; but their absence there is not conclusive that they have not. They may have been swallowed by the bergschrund, disappeared into some crevasse, or lodged in soft concealing snow. For this end we must examine the edges of the schrund and of any cleavages for traces, and the surface of the lower snow for pockling. If there are channels worn in the snow or ice at the base or back of the couloir, we shall get further evidence that something is accustomed to fall. These channels may have been made by stones, by ice trash or by water. In sunshine the difference is distinguishable. If the runnels are ‘silver-backed’ in sunlight, and there are no stones apparent, they have been made by water and are harmless.
If the couloir to be visited descends on to a visible glacier, but is itself invisible, the presence, absence or scarcity of fallen stones discoverable at the bottom end of the glacier, subject always to the possibility of the consumption by rift or crevasse, will demonstrate the couloir, or couloirs, commanding the visible glacier to be proportionately infected with, free from, or only in some cases liable to, the falling sickness.
Rock.