t of the sun's rays softens the snow, when
by its great weight it packs more closely together until it is, in many
cases, formed into solid ice-cakes. If we take a quantity of snow or a
quantity of granulated ice and put it under a sufficient pressure we can
produce clear solid ice, and it is by this process that ice is formed
out of the snow and hail that falls continually upon the tops of these
glacial mountains. We have seen that ice possesses certain viscous or
semi-fluidic properties and that it will yield to pressure, but if we
put it under sufficient tensional strain it snaps like glass or any
other brittle substance. As the snows upon these mountains pile up
higher and higher the pressure becomes greater and greater until it
reaches a point where the mass begins to move gradually down the
mountain side, following the gulches and defiles that furnish a path of
least resistance to its flow.

At the sides and bottom, where there is contact with the earth, the
movement is slower than it is at the surface and in the middle of the
ice stream. If there were no curves in the ravine or gulch through which
it flows the point of greatest movement would be confined to the middle
of its width. But in flowing through a winding gulch the most rapid flow
follow the lines of greatest pressure, and this line is deflected from
side to side, so that the line of greatest flow is more winding than is
the bottom of the valley through which it flows. (The movement is called
a "flow," but it is very sluggish, only a few inches in a day, as will
appear later.)

If the bottom and sides of the valley were straight the surface of the
ice would be comparatively even; I say comparatively, for as compared
with a smooth surface it would be very rough; but there would be none of
the great crevasses or openings now to be found in the ice, which
sometimes are very large and extend to a great depth. If in its downward
course the bottom of the ravine suddenly becomes steeper, the top of the
ice is put under a tensional strain which causes it to break, thus
forming the crevasses.

If at the bottom of the descent the valley curves upward or preserves
the straight line for a considerable distance, these crevasses will
close at the top and perhaps open at the bottom, and the blocks of ice
will freeze together to such an extent that the water caused by the
melting ice will flow on top until it comes to another crevasse, where
it runs through to the bo