to cut a more or less distinct
canon. As the basin becomes ancient, this element of the gorge tends
to disappear, the reason for this being that, while the river bed is
high above the sea, the current is swift and the down-cutting rapid,
while the slow subsidence of the country on either side--a process
which goes on at a uniform rate--causes the surface of that region to
be left behind in the race for the sea level. As the stream bed comes
nearer the sea level its rate of descent is diminished, and so the
outlying country gradually overtakes it.

In regions where the winters are very cold the effect of ice on the
development of the stream beds both in the torrent and river sections
of the valley is important. This work is accomplished in several
diverse ways. In the first place, where the stream is clear and the
current does not flow too swiftly, the stones on the bottom radiate
their heat through the water, and thus form ice on their surfaces,
which may attain considerable thickness. As ice is considerably
lighter than water, the effect is often to lift up the stones of the
bed if they be not too large; when thus detached from the bottom, they
are easily floated down stream until the ice melts away. The ice which
forms on the surface of the water likewise imprisons the pebbles along
the banks, and during the subsequent thaw may carry them hundreds of
miles toward the sea. It seems likely, from certain observations made
by the writer, that considerable stones may thus be carried from the
Alleghany River to the main Mississippi.

Perhaps the most important effect of ice on river channels is
accomplished when in a time of flood the ice field which covered the
stream, perhaps to the depth of some feet, is broken up into vast
floes, which drift downward with the current. When, as on the Ohio,
these fields sometimes have the area of several hundred acres, they
often collide with the shores, especially where the stream makes a
sharp bend. Urged by their momentum, these ice floes pack into the
semblance of a dam, which may have a thickness of twenty, thirty, or
even fifty feet. Beginning on the shore, where the collision takes
place, the dam may swiftly develop clear across the stream, so that in
a few minutes the way of the waters is completely blocked. The
on-coming ice shoots up upon the accumulation, increases its height,
and extends it up stream, so that in an hour the mass completely bars
the current. The waters then