The nature of the underlying surface, apart from its contour and gradient, is a factor of vital importance, especially in the early winter and in the spring. At intervening periods most avalanches are superficial, and slide from an underlying surface of hard snow; but in the early winter and in the late spring the whole snow slope slides away, so that the nature of the underlying surface and the probable support that it affords is of great importance.
Steep grass-slopes form a dangerous under-surface, especially where it is never or seldom mown; for long unmown grass generally lies facing downwards, and offers a most slippery surface.
Grass which is regularly mown is usually short and stubby in winter, and gives better purchase to the snow. A slope covered by stony boulders, bushes or trees is usually fairly safe, though a big avalanche, once it is fairly under weigh, will sweep over shrubs and even over small trees. Fairly dense wood may usually be considered as safe, provided one avoids the long open clearings made by old avalanches, which so often run down the middle of a forest.
An elementary knowledge of geology is useful; the excellent geological maps published by the Swiss Survey can often be consulted with benefit. Rocks which suffer much surface disintegration provide a better purchase for snow than very hard and consequently very smooth rocks. The hard ‘Hochgebirgskalk,’ an alpine variety of limestone, which is very common, especially in high regions, is slippery, and instead of disintegrating gradually, as gneiss or granite disintegrates, has a habit of breaking away along vertical and horizontal joints.
The common rock known as ‘Flysch,’ common in the lower Alps, provides a much safer surface.
Glacier-polished rocks are, of course, especially dangerous, and the whole Grimsel region is consequently swept by avalanches throughout the winter.
The lie of the strata is an important factor. Where, as is usually the case, the strata are inclined, one slope of a mountain will usually be safer than the other. Diagrams I and II represent the north and south slopes of a ridge running more or less east and west. The ridge is formed by parallel but inclined bands of strata. Rock climbers know that the slope in Diagram I, though of the same gradient as the slope in Diagram II, is very much more difficult to climb. It is also much more liable to avalanche, as the outcrop of the strata provide a natural check to avalanches in Diagram II; whereas in Diagram I each outcrop forms a small steep snow slope quite unsupported. If the outcrops are of reasonable breadth, there will be belts running across the face of the slope at A’, B’, C’, D’ which will be inclined into the slope, and provide a safe line of traverse; whereas there is no safe line for a ski-runner desiring to traverse or ascend the slope A, B, C, D.
Diagram I.
Diagram II.
Geological maps indicate the ‘strike’ of the strata, and therefore provide useful clues as to the varying liability of slopes to avalanche.