Not only are these lines
frequently uninterrupted for a distance of many yards, but they are also
parallel, except when some change takes place in the thickness of the
ice, which may slightly modify the trend of the mass, or where lines in
a variety of directions are produced by the intermittent action of
separate glaciers running successively at different angles over the same
surfaces. The deeper grooves sometimes present a succession of short
staccato touches, just as when one presses the finger vertically along
some surface where the resistance is sufficient to interrupt the action
without actually stopping it,--a kind of grating motion, showing how
firmly the instrument which produced it must have been held in the
moving mass. No currents or sudden freshets carrying hard materials with
them, even moving along straight paths down hill-sides or
mountain-slopes, have ever been known to draw any such lines. They could
be made only by some instrument held fast as in a vice by the moving
power. Something of the kind is occasionally produced by the drag of a
wheel grating over rocks covered with loose materials.
It has been said that grounded ice or icebergs floating along a rocky
shore might produce similar marks; but they will chiefly be at the level
of high-water mark, and, if grounded, they will trend in various
directions, owing to the rocking or rotating movement of the iceberg.
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