agencies which produce waste tend to set its particles free and in
motion, and therefore cooperate with gravity. On cliffs, rocks
fall when wedged off by frost or by roots of trees, and when
detached by any other agency. On slopes of waste, water freezes in
chinks between stones, and in pores between particles of soil, and
wedges them apart. Animals and plants stir the waste, heat expands
it, cold contracts it, the strokes of the raindrops drive loose
particles down the slope and the wind lifts and lets them fall. Of
all these movements, gravity assists those which are downhill and
retards those which are uphill. On the whole, therefore, the
downhill movements prevail, and the mantle of waste, block by
block and grain by grain, creeps along the downhill path.

A slab of sandstone laid on another of the same kind at an angle
of 17 degrees and left in the open air was found to creep down the
slope at the rate of a little more than a millimeter a month.
Explain why it did so.

RAIN. The most efficient agent in the carriage of waste to the
streams is the rain. It moves particles of soil by the force of
the blows of the falling drops, and washes them down all slopes to
within reach of permanent streams. On surfaces unprotected by
vegetation, as on plowed fields and in arid regions, the rain
wears furrows and gullies both in the mantle of waste and in
exposures of unaltered rock (Fig. 17).

At the foot of a hill we may find that the soil has accumulated by
creep and wash to the depth of several feet; while where the
hillside is steepest the soil may be exceedingly thin, or quite
absent, because removed about as fast as formed. Against the walls