Post 24748
Created:
Modified:
The other kinds of slides may be called the Annual and the Century. In places of rough surface or moderate slope there must be a large accumulation of snow before a slide will start. Weeks or even months may pass before storm and wind assemble sufficient snow for a slide. Places of this kind commonly furnish but one slide a year, and this one in the springtime. At last the snow-drifts reach their maximum; warmth assists starting by melting snow-cornices that have held on through the winter; these drop, and by dropping often start things going. Crags wedged off by winter ice are also released in spring; and these, in going recklessly down, often knock hesitating snow-drifts into action. A fitting name for those slides that regularly run at the close of winter would be Spring, or Annual. These are composed of the winter’s local accumulation of snow and slide rock, and carry a much heavier percentage of rock-débris than the Storm slide carries. They transport from the starting-place much of the annual crumbling and the weatherings of air and water, along with the tribute pried off by winter’s ice levers; with this material from the heights also goes the year’s channel accumulation of débris. The Annual slide does man but little damage and, like the Flood slide, it follows the gulches and the water-courses.
In snowy zones the avalanche is commonly called a snow-slide, or simply a slide. A slide, with its comet tail of powdered snow, makes an intense impression on all who see one. It appears out of order with the scheme of things; but, as a matter of fact, it is one of gravity’s working ways, a demonstration of the laws of sliding bodies. A smooth, steep slope which receives a heavy fall of snow will promptly produce or throw off a sliding mass of snow. Raise, lower, or roughen this slope, increase or decrease the annual snow-fall, or change the direction of the wind,–and thus the position of snow-drifts,–and there will follow corresponding slide-action. Wind and calm, gravity, friction, adhesion, cohesion, geology, temperature and precipitation, all have a part and place in snow-piling and in slide-starting.
The Century slides are the damaging ones. These occur not only at unexpected times but in unexpected places. The Century slide is the deadly one. It usually comes down a course not before traversed by a slide, and sometimes crashes through a forest or a village. It may be produced by a record-breaking snow or by snow-drifts formed in new places by winds from an unusual quarter; but commonly the mass is of material slowly accumulated. This may contain the remnant snows and the wreckage spoils of a hundred years or more. Ten thousand snows have added to its slowly growing pile; tons of rock-dust have been swept into it by the winds; gravel has been deposited in it by water; and gravity has conducted to it the crumbling rocks from above. At last–largely ice–it breaks away. In rushing down, it gathers material from its predestined way.