thickness of the glacier to the ground below.

The surface of the glacier above is not smooth and glassy like the ice
of a freshly-frozen river or pond; but is white, like a field of snow.
This appearance is produced in part by the snow which falls upon the
glacier, and in part by the melting of the surface of the ice by the
sun. From this latter cause, too, the surface of the glacier is covered,
in a summer's day, with streams of water, which flow, like little
brooks, in long and winding channels which they themselves have worn,
until at length they reach some fissure, or crevasse, into which they
fall and disappear. The waters of these brooks--many thousands in
all--form a large stream, which flows along on the surface of the ground
under the glacier, and comes out at last, in a wild, and roaring, and
turbid torrent, from an immense archway in the ice at the lower end,
where the glacier terminates among the green fields and blooming flowers
of the lower valley.

The glaciers are formed from the avalanches which fall into the upper
valleys in cases where the valleys are so deep and narrow and so
secluded from the sun that the snows which slide into them cannot melt.
In such case, the immense accumulations which gather there harden and
solidify, and become ice; and, what is very astonishing, the whole mass,
solid as it is, moves slowly onward down the valley, following all the
turns and indentations of its bed, until finally it comes down into the
warm regions of the lower valleys, where the end of it is melted away by
the sun as fast as the mass behind crowds it forward. It is certainly
very astonishing that a substance so solid as ice can flow in this way,
along a rocky and tortuous bed, as if it were semi-fluid; and it was a
long time before men would believe that such a thing could be possible.
It was, however, at length proved beyond all question that this motion
exists; and the rate of it in different glaciers at different periods of
the day or of the year has been accurately measured.

If you go to the end of the glacier, where it comes out into the lower
valley, and look up to the icy cliffs which form the termination of it,
and watch there for a few minutes, you soon see masses of ice breaking
off from the brink and falling down with a thundering sound to the rocks
below. This is because the ice at the extremity is all the time pressed
forward by the mass behind it; and, as it comes to the brink, it breaks