This envelope also is in movement, not indeed as a whole, but particle by particle.  The causes which set its particles in motion, and the different forms which the mantle comes to assume, we will now proceed to study.

MOVEMENTS OF THE MANTLE OF ROCK WASTE

At the sandstone ledges which we first visited we saw not only that the rocks were crumbling away, but also that grains and fragments of them were creeping down the slopes of the valley to the stream and were carried by it onward toward the sea.  This process is going on everywhere.  Slowly it may be, and with many interruptions, but surely, the waste of the land moves downward to the sea.  We may divide its course into two parts,—­the path to the stream, which we will now consider, and its carriage onward by the stream, which we will defer to a later chapter.

Gravity.  The chief agent concerned in the movement of waste is gravity.  Each particle of waste feels the unceasing downward pull of the earth’s mass and follows it when free to do so.  All 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 of an abbey built on a slope in Wales seven hundred years ago, the creeping waste has gathered on the uphill side to a depth of seven feet.  The slow-flowing sheet of waste is often dammed by fences and walls, whose uphill side gathers waste in a few years so as to show a distinctly higher surface than the downhill side, especially in plowed fields where the movement is least checked by vegetation.