osition of its waters is
expended in the endless twistings and turnings of its stream, which
result only in the development of heat which flies away into the
atmosphere. In the ice stream, owing to its slow movement and to the
detritus which it forces along the bottom, a vastly greater part of
the energy which impels it down the slope is applied to rock cutting.
None of the boulders, even if they are yards in diameter, obstruct its
motion; small and great alike are to it good instruments wherewith to
attack the bed rocks. The fragments are never left to waste by
atmospheric decay, but are to a very great extent used up in
mechanical work, while the most of the detritus which comes to a
torrent is left in a coarse state when it is delivered to the stream;
the larger part of that which the glacier transports is worn out in
its journey. To a great extent it is used up in attacking the bed
rock. In most cases the _debris_ in the terminal moraine is evidently
but a small part of what entered the ice during its journey from the
uplands; the greater part has been worn out in the rude experiences to
which it has been subjected.

It is evident that even in the regions now most extensively occupied
by glaciers the drainage systems have been shaped by the movement of
ordinary streams--in other words, ice action is almost everywhere,
even in the regions about the poles, an incidental feature in the work
of water, coming in only to modify the topography, which is mainly
moulded by the action of fluid water. When, owing to climatal changes,
a valley such as those of the Alps is occupied by a glacial stream,
the new current proceeds at once, according to its evident needs, to
modify the shape of its channel. An ordinary torrent, because of the
swiftness of its motion, which may, in general, be estimated at from
three to five miles an hour, can convey away the precipitation over a
very narrow bed. Therefore its channel is usually not a hundredth part
as wide as the gorge or valley in which it lies. But when the
discharge takes place by a glacier, the speed of which rarely exceeds
four or five feet a day, the ice stream because of its slow motion has
to fill the trough from side to side, it has to be some thousand times
as deep and wide as the torrent. The result is that as soon as the
glacial condition arises in a country the ice streams proceed to
change the old V-shaped torrent beds into those which have a broad
U-like form. The pra