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

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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 debris 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’Herens (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.