of mechanical and solvent denudation. We shall, later on, make anestimate of the magnitude of the quantities actually involved.
In the Swiss valleys we see torrents of muddy water hurrying
along, and if we follow them up, we trace them to glaciers high
among the mountains. From beneath the foot of the glacier, we
find, the torrent has birth. The first debris given to the river
is derived from the wearing of the rocky bed along which the
glacier moves. The river of ice bequeaths to the river of
water—of which it is the parent—the spoils which it has won from
the rocks
31
The work of mechanical disintegration is, however, not restricted
to the glacier's bed. It proceeds everywhere over the surface of
the rocks. It is aided by the most diverse actions. For instance,
the freezing and expansion of water in the chinks and cracks in
those alpine heights where between sunrise and sunset the heat of
summer reigns, and between sunset and sunrise the cold of winter.
Again, under these conditions the mere change of surface
temperature from night to day severely stresses the surface
layers of the rocks, and, on the same principles as we explain
the fracture of an unequally heated glass vessel, the rocks
cleave off in slabs which slip down the steeps of the mountain
and collect as screes in the valley. At lower levels the
expansive force of vegetable growth is not unimportant, as all
will admit who have seen the strong roots of the pines
penetrating the crannies of the rocks. Nor does the river which
flows in the bed of the valley act as a carrier only. Listening
carefully we may detect beneath the roar of the alpine torrent
the crunching and knocking of descending boulders. And in the
potholes scooped by its whirling waters we recognise the abrasive
action of the suspended sand upon the river bed.
A view from an Alpine summit reveals a scene of remarkable
desolation (Pl. V, p. 40). Screes lie piled against the steep
slopes. Cliffs stand shattered and ready to fall in ruins. And
here the forces at work readily reveal themselves. An occasional
wreath of white smoke among
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the far-off peaks, followed by a rumbling reverberation, marks
the fall of an avalanche. Water everywhere trickles through the
shaly _débris_ scattered around. In the full sunshine the rocks are
almost too hot to bear touching. A few hours later the cold is
deadly, and all becomes a frozen silence. In such scenes of
desolation and destruction, detrital sediments are actively being
generated. As we descend into the valley we hear the deep voice
of the torrents which are continually hurrying the disintegrated
rocks to the ocean.
A remarkable demonstration of the activity of mechanical
denudation is shown by the phenomenon of "earth pillars." The
photograph (Pl. IV.) of the earth pillars of the Val d'Hérens
(Switzerland) shows the peculiar appearance these objects
present. They arise under conditions where large stones or
boulders are scattered in a deep deposit of clay, and where much
of the denudation is due to water scour. The large boulders not
only act as shelter against rain, but they bind and consolidate
by their mere weight the clay upon which they rest. Hence the
materials underlying the boulders become more resistant, and as
the surrounding clays are gradually washed away and carried to
the streams, these compacted parts persist, and, finally, stand
like walls or pillars above the general level. After a time the
great boulders fall off and the underlying clay becomes worn by
the rainwash to fantastic spikes and ridges. In the Val d'Hérens
the earth pillars are formed
33
of the deep moraine stuff which thickly overlies the slopes of
the valley. The wall of pillars runs across the axis of the
valley, down the slope of the hill, and crosses the road, so that
it has to be tunnelled to permit the passage of traffic. It is
not improbable that some additional influence—possibly the
presence of lime—has hardened the material forming the pillars,
and tended to their preservation.
Denudation has, however, other methods of work than purely
mechanical; methods more noiseless and gentle, but not less
effective, as the victories of peace ate no less than those of
war.
Over the immense tracts of the continents chemical work proceeds
relentlessly. The rock in general, more especially the primary
igneous rock, is not stable in presence of the atmosphere and of
water. Some of the minerals, such as certain silicates and
carbonates, dissolve relatively fast, others with extreme
slowness. In the process of solution chemical actions are
