he tiny rills join the greater, all
their channels sway to and fro as directed this way and that by chance
irregularities, until something like river basins are carved out,
those gentle slopes which form broad valleys where the carving has
been due to the wanderings of many streams. If the field be large,
considerable though temporary brooks may be created, which cut
channels perhaps a foot in depth. At the end of this miniature stream
system we always find some part of the waste which has been carved
out. If the streamlet discharges into a pool, we find the tiny
representative of deltas, which form such an important feature on the
coast line where large rivers enter seas or lakes. Along the lines of
the stream we may observe here and there little benches, which are the
equivalent in all save size of the terraces that are generally to be
observed along the greater streams. In fact, these accidents of an
acre help in a most effective way the student to understand the
greater and more complicated processes of continental erosion.

A normal river--in fact, all the greater streams of the
earth--originates in high country, generally in a region of mountains.
Here, because of the elevation of the region, the streams have cut
deep gorges or extensive valleys, all of which have slopes leading
steeply downward to torrent beds. Down these inclined surfaces the
particles worn off from the hard rock by frost and by chemical decay
gradually work their way until they attain the bed of the stream. The
agents which assist gravitation in bearing this detritus downward are
many, but they all work together for the same end. The stroke of the
raindrop accomplishes something, though but little; the direct washing
action of the brooklets which form during times of heavy rain, but dry
out at the close of the storm, do a good deal of the work; thawing and
freezing of the water contained in the mass of detritus help the
movement, for, although the thrust is in both directions, it is most
effective downhill; the wedges of tree roots, which often penetrate
between and under the stones, and there expand in their process of
growth, likewise assist the downward motion. The result is that on
ordinary mountain slopes the layer of fragments constituting the rude
soil is often creeping at the rate of from some inches to some feet a
year toward the torrent bed. If there be cliffs at the top of the
slope, as is often the case, very extensive falls of rock