On any surface over which water flows we shall find
that the softer materials have yielded first and most completely. Hard
dikes will be left standing out, while softer rocks around them are worn
away,--furrows will be eaten into more deeply,--fissures will be
widened,--clay-slates will be wasted,--while hard sandstone or limestone
and granite will show greater resistance. Not so with surfaces over
which the levelling plough of the glacier has passed. Wherever softer
and harder rocks alternate, they are brought to one outline; where dikes
intersect softer rock, they are cut to one level with it; where rents or
fissures traverse the rock, they do not seem to have been widened or
scooped out more deeply, but their edges are simply abraded on one line
with the adjoining surfaces. Whatever be the inequality in the hardness
of the materials of which the rock consists, even in the case of
pudding-stone, the surface is abraded so evenly as to leave the
impression that a rigid rasp has moved over all the undulations of the
land, advancing in one and the same direction and levelling all before
it.
Among the inequalities of the glacier-worn surfaces which deserve
especial notice, are the so-called "_roches moutonnees_." They are
knolls of a peculiar appearance, frequent in the Alps, and first noticed
by the illustrious De Saussure, who designated them by that name,
because, where they are numerous and seen from a distance, they resemble
the rounded backs of a flock of sheep resting on the ground.
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