ice. In some cases nature does this on a large scale.
Where mountains are sufficiently elevated to raise their heads
above the snow line we know they are white all the year around
with snow. What is not blown away, evaporated, or, as an
avalanche, precipitated to lower heights, must accumulate from
year to year. But the weight pressing on the lower portions of
this snow-field must soon be considerable, and at length become
so great, that the snow changes to the form of ice. But as ice
it is no longer fixed and immovable. We need not stop to explain
just how this ice-field moves, but the fact is that, though
moving very slowly, it acts like a liquid body. It will steal
away over any incline however small, down which water would
flow. Like a river it fills the valleys leading down from the
mountains. But, of course, the lower down it flows the higher
the temperature it meets, and it will sooner or later reach a
point where it will melt as fast as it advances. This stream of
ice flowing down from snow-clad mountains is called a glacier.
Those we are best acquainted with are but puny things compared
with those of the polar regions, where in one case a great river
of ice sixty miles wide, flowing from an unknown distance, some
thousands of feet in depth (or height), pours out into
the sea.<20>

We at once perceive that such a mass of ice could not pour down
a valley without leaving unmistakable signs of its passage.
The sides of the mountains would be deeply scarred and smoothed.
Projecting knobs would be worn away. The surface of the
valley, exposed to the enormous grinding power of the moving
ice, would be crushed, pulverized, and dragged along with it.
Pieces of stone, like that here represented, would form part of