heap up until they break their way over
the obstacle, washing its top away, until the whole is light enough
to be forced down the stream, where, by the friction it encounters on
the bottom and sides of the channel, it is broken to pieces. It is
easy to see that such moving dams of ice may sweep the bed of a river
as with a great broom.

Sometimes where the gorges do not form a stationary dam large cakes of
ice become turned on edge and pack together so that they roll down the
stream like great wheels, grinding the bed rock as they go.

In high northern countries, as in Siberia, the rivers, even the
deepest, often become so far frozen that their channels are entirely
obstructed. Where, as in the case of these Siberian rivers, the flow
is from south to north, it often happens that the spring thaw sets in
before the more northern beds of the main stream are released from
their bondage of frost. In this case the inundations have to find new
paths on either side of the obstructed way. The result is a type of
valleys characterized by very irregular and changeable stream beds,
the rivers having no chance to organize themselves into the shapely
curves which they ordinarily follow.

The supply which finds its way to a river is composed, as has been
already incidentally noted, in part of the water which courses
underground for a greater or less distance before it emerges to the
surface, and in part of that which moves directly over the ground.
These two shares of water have somewhat different histories. On the
share of these two depends the stability of the flow. Where, as in New
England and other glaciated countries, the surface of the earth is
covered with a thick layer of sand and gravel, which, except when
frozen, readily admits the water; the rainfall is to a very great
extent absorbed by the earth, and only yielded slowly to the streams.
In these cases floods are rare and of no great destructive power.
Again, where also the river basin is covered by a dense mantle of
forests, the ground beneath which is coated, as is the case in
primeval woods, with a layer of decomposing vegetation a foot or more
in depth, this spongy mass retains the water even more effectively
than the open-textured glacial deposits above referred to. When the
woods, however, are removed from such an area, the rain may descend to
the streams almost as speedily as it finds its way to the gutters from
the house roofs. It thus comes about that all region