OCR Text |
Show its path and spreads laterally. At first the deeper snow, even loose snow, underlying the slide remains intact. As the avalanche increases in volume, the loose snow does not simply fall apart; it is swept into a streamlike flow and may carry the underlying layer with it. Bound but not strong ( firm) snow, loose windblown snow, for instance, moves down slope with its entire thickness; but like a section of detached snow that falls apart at once, this type of snow also assumes a streamlike flow. Such snow, especially in large avalanches, exerts considerable force on the sub- slide layer, yet is also capable of slipping ( sliding) over relatively loose snow. On its descent wet snow rolls up into balls, which can form in such number that they constitute a stratum of sliding lumps and blocks. In slides of bound, firm ( strong) snow there is downslope movement of an entire detached section consisting of one or more layers. The detached snow bed at first moves as one, then a layer will give way. providing the slide plane for others. When a bottom layer is fragile, composed of granular or loose, coarse granular snow, the slide originates here, with its collapse. Upon sliding, a seam or bed of such firm, strong snow opens in crevices, due to the unevenness of the terrain, and breaks up into ragged plates which keep their relative position low in the slide. This type of slide has a wavelike movement, the waves piling up like surf across the avalanche with the snow plates at their base. As the avalanche gathers speed, these plates begin to outrun one another, are tossed backwards, and gradually fall to pieces. As the edges of the snow plates are broken off, the fragments acquire a rounded shape; and the origin of these plate fragments may be recognized only by their layered structure. The strongest layer of this type snow can be completely broken up when avalanche travel is long enough and the relief is not uniform. Only a few individual snow pieces arrive undamaged at the foot of the slide; these are of great value for the information they give regarding the origin of the snow and features of the avalanche as a whole. In the lower part of its course, the snow flows like a thick, boggy fluid; its composition varies vertically, and it sets up currents that move at different speeds. In the stream of a great avalanche, the lower levels of which move at less speed than the upper ones due to greater friction at the bottom, the snow, subject to grinding and compression, is progressively broken up into chunks which lose their edges and take on a rounded shape. Such chunks do not show regular layers; they consist of heterogeneous snow and sometimes show inclusions such as stones, tree branches and ice crust fragments. In the terminal deposit or " dump," the spaces between these chunks are often empty, and these voids are frequently observed to depths of 2- 3 m. Sometimes the forefront of a dense terminal deposit will break off and drop several meters ahead of the main mass. Where the avalanche flows around an obstacle, a deep trench will form below this, axial to slide movement. A trench 10- 15 m long often forms down slope from a tree 15- 20 cm thick. In most cases a snow surface is the base over which an avalanche slides; - 34- |