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the fragments, great and small, that alternate
freezing and thawing can separate from the rock
walls of the valley. Some of these find their way
to the bottom of the glacier, are bedded in the ice,
where it rests upon the rock, and there act like
file-teeth or plane-irons upon the bottom of the
valley, which thus becomes filed or planed down.
Whenever a glacier recedes (as many glaciers
do at intervals), and exposes its bed, this bed is
seen to be polished by the ice and the minute
sandy particles, and grooved and scratched in
parallel lines by the larger teeth. This smoothing
and parallel striation are so characteristic and
decided, that by such vestiges we are able to
recognize the paths of ancient glaciers that have long
ago ceased to exist.
Where a file or plane has done its work there
must be filings or shavings; the matter thus
ground from the rock must be very fine, and must
go somewhere. All ordinary glaciers are sources
of rivers of greater or smaller magnitude. A
rivulet flows through a tunnel at the lowest part
of the bottom of every glacier, emerging from an
icy arch at its foot. Such rivulets are always
more or less turbid or milky, owing to the
abundance of the rock filings they contain. These
suspended particles are carried on and deposited
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