northern United States, if we except
some of the mountains of the West, the winter snows entirely disappear
long before the coming of summer. But the climate of this region
has not always been so pleasant and mild. Lands now densely peopled
were once buried under a thick mantle of ice which lasted through
many thousands of years.

Scattered over the surface of the northern United States are vast
numbers of boulders and rock fragments which are not at all like
the solid rocks beneath the soil. The history of these materials
takes us back to the Glacial period, which can be best understood
from a study of some one of the glaciers now existing upon the
mountains of the northwestern part of our country.

Among the lofty mountain ranges of the Cordilleran region there
are many peaks upon which perpetual snow-banks nestle, defying
the long summer days. Where the winters are long and cold and the
storms are severe, immense drifts of snow collect in the hollows
and canons of the mountain slopes. Each summer all or a part of
this snow melts. Upon the northern slopes the melting process is
slower, and if there happens to be a large basin upon that side,
an extensive field of snow remains until the winter storms come
again. Each winter new snow is added to the surface, while the
older snow, becoming hard and firm through repeated freezing and
thawing, at last turns to ice.

This mass of snow and ice does not remain stationary, as might
be expected from its apparent solidity. Under the influence of
its own weight and of alternations of heat and cold, it flows down
the incline like a very thick liquid. During the winter the ice
melts but little, and the movement is slow, but in the summer,
under the influence of the warm days and cool nights, both the
melting and the rate of flow of the ice are increased. A moving
body of snow and ice of this sort is called a "glacier." It creeps
down the mountain slope and into some canon, until, in the warmer
air of the lower mountains, the rate of advance is exactly balanced
by the rate of melting at the lower end of the mass. The glaciers
in the United States are at present comparatively small, but once
these icy masses stretched over the mountains and lowlands of a
large portion of the continent.

In the southern Sierra Nevada mountains no permanent snow exists
below an elevation of about eleven thousand feet, but as we go
north snow-fields are found lower and lower, until in the fiords