These
knolls are the result of the prolonged abrasion of masses of rocks
separated by deep indentations wide enough to be filled up by large
glaciers, overtopping the summits of the intervening prominences, and
passing over them like a river, or like tide-currents flowing over a
submerged ledge of rock. It is evident that water rushing over such
sunken hills or ledges, adapting itself readily to all the inequalities
over which it flows, and forming eddies against the obstacles in its
course, will scoop out tortuous furrows upon the bottom, and hollow out
rounded cavities against the walls, acting especially along preexisting
fissures and upon the softer parts of the rock,--while the glacier,
moving as a solid mass, and carrying on its under side its gigantic file
set in a fine paste, will in course of time abrade uniformly the angles
against which it strikes, equalize the depressions between the prominent
masses, and round them off until they present those smooth bulging
knolls known as the "_roches moutonnees_" in the Alps, and so
characteristic everywhere of glacier-action. A comparison of any
tide-worn hummock with such a glacier-worn mound will convince the
observer that its smooth and evenly rounded surface was never produced
by water.
Besides their peculiar form, the _roches moutonnees_ present all the
characteristic features of glacier-action in their polished surfaces
accompanied with the straight lines, grooves, and furrows above
described.
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