They are composed of heavy, compacted
snow, which has been subjected to frequent alternations of freezing and
thawing. They are developed on canyon and mountain-sides at an elevation
of from nine to ten thousand feet, where the slopes are inclined at an
angle too low to shed off the dry winter snow, and which accumulates
until the spring thaws sap their foundations and make them slippery;
then away in grand style go the ponderous icy masses without any fine
snow-dust. Those of Clouds' Rest descend like thunderbolts for more than
a mile.
The great century avalanches and the kind that mow wide swaths through
the upper forests occur on mountain-sides about ten or twelve thousand
feet high, where under ordinary weather conditions the snow accumulated
from winter to winter lies at rest for many years, allowing trees, fifty
to a hundred feet high, to grow undisturbed on the slopes beneath them.
On their way down through the woods they seldom fail to make a perfectly
clean sweep, stripping off the soil as well as the trees, clearing paths
two or three hundred yards wide from the timber line to the glacier
meadows or lakes, and piling their uprooted trees, head downward, in
rows along the sides of the gaps like lateral moraines. Scars and broken
branches of the trees standing on the sides of the gaps record the depth
of the overwhelming flood; and when we come to count the annual
wood-rings on the uprooted trees we learn that some of these immense
avalanches occur only once in a century or even at still wider
intervals.
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