| OCR Text |
Show large- scale avalanches of heavy snow can veer nearly 90°. The Terminal Deposit: Component Snow Most snow avalanches stop when they reach a certain level of low grade on the mountain side, and only the massive ones pass beyond this, some being capable of piling snow up the opposite side of a narrow gully. The structure of the terminal deposit ( mound, " dump") depends essentially on the character of the avalanche snow. The loose and more friable this is, the thinner and more even will be the terminal deposit. But when the course of the avalanche has been long, or the relief is uneven, relatively solid, strong snow will be ground up and scattered down a slope as an even layer. The depth ( thickness) of a terminal deposit seldom exceeds 1 m and bears a direct relation to avalanche volume only in the case of certain relief features and with certain types of avalanche. However, if layers of extremely hard avalanche snow remain undamaged during slide movement, or this movement is accompanied by compression of the snow into solid masses, the terminal deposit may reach 10- 12 m in height. This type of terminal dump may be seen often with avalanches of wet granular snow and with those in which nearly the entire thickness of a snowcover is brought down. The avalanche front covers or fills in the sub- slide snow on the slope, trampling it under. The forward limit of the dead avalanche may be very distinct; one very often observes bands or strips thrown ahead, as though poured out from the terminal deposit. These " outpourings" register the dying out of the avalanche movement; they narrow toward the ends and may be tens of meters in length, but are seldom more than 0.5 m wide. Where these out- pour bands are of unequal length, the forefront of the terminal deposit appears broken and is not very distinct, especially where large snow masses are tossed ahead and fall on undisturbed snow. All this clearly indicates that an avalanche moves as a rolling pile or mound rather than as a mass preserving its internal relationships. An avalanche slows down at the right level of reduced grade on the slope, stops, and collapses forward at once. This is especially evident with avalanches of loose dry snow; here the terminal deposit may be 200 m and more in length and clearly show the loose or dispersed nature of its snow. In avalanches of wet, especially heavy snow, the forefront of the terminal deposit preserves even the form of a roller. Avalanche deposits are frequently crossed by zigzag contraction ( compression) fissures 1- 3 cm wide; these run at random and cross one another. One observes such crevices in terminal deposits of wet granular or pulverized ( abraded) heavy snow and also in those parts of a deposit that overlie breaks in grade. The basic portion of a terminal deposit consists of a homogeneous mass of uneven surface; within the thickness of the deposit, and especially at the surface, one observes formations of varying structure, the character of which is determined by the type of snow and, to some extent, by the avalanche volume and the terrain. These inclusions may be classified as follows: 1) heaped ball- like masses - fragile ovoid balls measuring 10- 20 cm, seldom up to 50 cm, formed in avalanches of freshly fallen or loose windblown snow, sorted and brought together by avalanche movement; 2) knurled ( milled) balls and blocks - firm formations with laminations counter- - 36- |
