Excerpt from Avalanches and Forest Cover in the Northern Cascades

In any mountainous region where the slopes are steep and the snowfall is heavy there is likelihood of avalanches. Avalanches, or snowslides, are masses of snow which, becoming dislodged from the upper slopes of a mountain side, slide or fall into the lower country. Because of the enormous amount of damage they do to everything in their path, whether forest, railroad structure, or habitation, ava lanches must be looked upon as calamities in a class with floods, hur ricanes, and great conflagrations.

Avalanches are of several distinct types - dust avalanches, glacier avalanches, ground avalanches, and top avalanches. Dust avalanches, so called because they fill the air along their coursepwith a mass of fine snow, are composed of very dry snow, and start usually at high alti tudes during very cold weather. Glacier avalanches, as the name suggests, are the masses of ice or compact. Snow that break loose from glacier and névé fields in very precipitous country, usually during the warm weather of summer. Ground avalanches are formations, generallyof wet snow, which slide over the ground. They occur when the snow has become excessively heavy from a thaw or rain, or the surface of the ground slippery, and are therefore most fre quent in late winter and early spring. Ground avalanches often accumulate in their course great quantities of earth, rock, and forest débris. Top avalanches are in most respects similar to ground ava lanches, except that they slide over a crust of older snow, instead of 'over the ground. Where a top avalanche breaks through the crust and slides on the surface of the ground it becomes a ground ava lanche. Every gradation between these four types may be found, and a single avalanche may have the characteristics of two or more of the types.

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