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Show of the trough wall is composed of dense chunks and balls, forming a declivity about 1: 1 in depth. As it pours from the gully, the avalanche torrent grows and the distance between its sides gradually increases. The sides of the stream, moving more slowly than the main stream because of side friction, planes out new walls parallel to the previous ones. Eighteen parallel side walls, leaning against one another, were found by digging a cross trench through the deposit of the avalanche of December 22, 1936, on Mount Aykuayventchorr. These walls varied between 40 and 120 cm in thickness and between 6 and 8 m in height; their length was about 200 m. The inner surfaces of many were sheeted with transparent ice 5 cm thick; the remainder were covered with icy snow. The outer surface of the moving snow stream shows the same type of icing. At least at first, the trough walls are not formed in the moving snow stream itself, but in the snowcover through which it passes, otherwise the trough would show kneading, and wrinkle and crevice formation because of the passage of the avalanche over uneven relief. Not to be excluded is the possibility of disruption of a wide snow stream, as when speed in midstream differs from that on the sides, and as when a fast stream invades a slower one, creating ( internal) troughs. On the lower portion of a slope, where avalanche movement slows down and the height of the snow stream greatly decreases, even an insignificant obstacle can set up wall formation. Small walls begin to form downstream from bushes in some cases. In some areas the iced trough may be quite free of avalanche snow ( following a slide). Here the effects of an avalanche on the sub- slide snow can be clearly observed. In the trough floor rocks are carried away, protrusions are flattened, pine trees and large birches are shorn off as with a knife, and small, pliable birches are bent over and frozen into the iced surface. The trough floor is so slick that it is impossible to walk on a 9° grade with soft shoes. Avalanche troughs are not always formed, being observed only with fairly large- scale slides in which the entire thickness of a snowcover is carried down. The highest trough wall noted was in an avalanche of December 17, 1936, at Hack-mann Pass, where a precipitous wall was raised 8 m above the avalanche snow remaining in the trough to a depth of several meters. Hummocks of snow are raised to heights of 6- 12 m, 50- 150 m from the mouth of a loge, as an avalanche emerges, in some cases. The avalanche divides in two and flows around these formations, flattening their sides and shaping them into the shape of a wedge or flatiron with the tip pointing at the gully. This type of accumulation was observed only twice, in avalanc'hes that had formed iced troughs and passed over protrusions. Air Waves A fast- moving avalanche sets up an air wave which pushes tens and even hundreds of meters ahead of a slide. Toppling trees and destroying buildings, the air wave may cause more damage than the avalanche itself. Air waves were observed in the Chibin area only with avalanches of dry snow, and these were mostly - 40- |
