rigated, and the water power is also
used for mills, the streams seeming to rush on, as Ruskin says, "eager
for their work at the mill, or their ministry to the meadows."

Apart from the action of running water, snow and frost are continually
disintegrating the rocks, and at the base of almost any steep cliff may
be seen a slope of debris (as in Figs. 25, 26). This stands at a regular
angle--the angle of repose--and unless it is continually removed by a
stream at the base, gradually creeps up higher and higher, until at last
the cliff entirely disappears.

[Illustration: Fig. 25.--Section of a river valley. The dotted line
shows a slope or talus of debris.]

Sometimes the two sides of the valley approach so near that there is not
even room for the river and the road: in that case Nature claims the
supremacy, and the road has to be carried in a cutting, or perhaps in a
tunnel through the rock. In other cases Nature is not at one with
herself. In many places the debris from the rocks above would reach
right across the valley and dam up the stream. Then arises a struggle
between rock and river, but the river is always victorious in the end;
even if dammed back for a while, it concentrates its forces, rises up
the rampart of rock, rushes over triumphantly, resumes its original
course, and gradually carries the enemy away.

[Illustration: Fig. 26.--Valley of the Rhone, with the waterfall of
Sallenches, showing talus of debris.]

Another prominent feature in many valleys is afforded by the old river,
or lake, terraces, which were formed at a time when the river ran at a
level far above its present bed.

Thus many a mountain valley gives some such section as the following.

[Illustration: Fig. 27.--_A_, present river valley; _B_, old river
terrace.]

First, a face of rock, very steep, and in some places almost
perpendicular; secondly, a regular talus of fallen rocks, stones, etc.,
as shown in the view of the Rhone Valley (Fig. 26), which takes what is
known as the slope of repose, at an angle which depends on the character
of the material. As a rule for loose rock fragments it may be taken
roughly to be an angle of about 45 deg.. Then an irregular slope followed in
many places by one or more terraces, and lastly the present bed of the
river.

[Illustration: Fig. 28.--Diagram of an Alpine valley showing a river
cone. Front view.]

The width or narrowness of the valley in relation to its depth depends
greatly on the