OCR Text |
Show sites are easily recognized by the residues of melting dumps containing stones, tree branches, layers of dirt, etc. Attention should be drawn to the body of a terminal dump consisting ( entirely) pf clean snow, because the snow melts much later here than at the periphery of the dump. Even where the dump has completely melted, the growth of vegetation is retarded in this spot; and for several weeks, sometimes until fall, the dump site can be recognized by the clear green color against the general background. Damaged vegetation is also a clear- cut sign of avalanching. Avalanches can easily snap off a young pine tree, but more often just bend the more flexible birches, especially the young trees. Because of this, gaps showing a growth of young birches bent over valleyward appear in pine wood zones on avalanche slopes. The bark of such birches is damaged or stripped. Where avalanches are infrequent in a given location, occurring only once in several decades, pine woods can grow in the intervals. When rotting tree trunks with their tops directed downward to the valley litter the ground in such renewed pine woods, this is usually a sign of an old slide. On the south side of Mount Ukspor, between the village of Uksporyok and Big Vudyavr Lake, is such a pine forest with numbers of trees felled long ago by avalanche action. This forest plot is opposite a cirque from which no avalanche descended during the eight years of our observation. It is to be noted that the north edge of this pine wood does not clearly show the limits of any avalanche dump, for loose snow can overrun several tens of meters of pine wood without causing any damage, as was the case with a sanatorium and an experimental station workshop. " Outlyer" or " pour- over" cones, rising like mounds at the foot of slopes, composed of rock debris, are also indications of avalanching. The amount of rock brought down mountain sides by avalanches is considerable. For example, data of P. F. Somerov show that an avalanche of December 22, 1936, brought no less than 800 tons of rock debris down to the valley from the sides of Mount Aykuayventchorr. Rock material and rubble are deposited in this way on slopes by the melting of avalanche dumps, especially by those of wet snow and " lump snow" avalanches. These terminal dumps are sometimes covered with turf or are even wooded, showing that avalanching is rare here. Some " outlyer" or " pour-over" cones, especially those dumped opposite shallow loges, are not easy to make out, since they lack the characteristic form possessed by terminal deposits lying below deep loges and cirques and consist of rather small aggregations of stones. Although, to be sure, deposits of rock at the foot of slopes may be the result of rock slides or transport by spring freshets, it is still often possible to distinguish these from rock carried down slope by avalanche action. As a ( snow) avalanche dump melts, the sand, gravel and rock fragments in the upper layers of the dump settle or precipitate upon the underlying rocks, which often come to rest in unstable positions. But in rock slides a great percentage of the component rocks have rolled and can be covered only with dust, never with gravel. Mud slides pass over dense avalanche snow, but soak into and permeate natural precipitated snowcover. Even when thawing is exceptionally heavy, as in the spring of 1934, mountain freshets lose themselves ( i. e., unload) in the stone aggregations at the foot of slopes. The spring freshets in the Chibin - 59- |