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The Elements of Geology by William Harmon Norton
page 65 of 414 (15%)
helped by the work of the weather. Especially at low water more or
less of the bed is exposed to the action of frost and heat and
cold, joints are opened, rocks are pried loose and broken up and
made ready to be swept away by the stream at time of flood.

POTHOLES. In rapids streams also drill out their rocky beds. Where
some slight depression gives rise to an eddy, the pebbles which
gather in it are whirled round and round, and, acting like the bit
of an auger, bore out a cylindrical pit called a pothole. Potholes
sometimes reach a depth of a score of feet. Where they are
numerous they aid materially in deepening the channel, as the
walls between them are worn away and they coalesce.

WATERFALLS. One of the most effective means of erosion which the
river possesses is the waterfall. The plunging water dislodges
stones from the face of the ledge over which it pours, and often
undermines it by excavating a deep pit at its base. Slice after
slice is thus thrown down from the front of the cliff, and the
cataract cuts its way upstream leaving a gorge behind it.

NIAGARA FALLS. The Niagara River flows from Lake Erie at Buffalo in
a broad channel which it has cut but a few feet below the level of
the region. Some thirteen miles from the outlet it plunges over a
ledge one hundred and seventy feet high into the head of a narrow
gorge which extends for seven miles to the escarpment of the
upland in which the gorge is cut. The strata which compose the
upland dip gently upstream and consist at top of a massive
limestone, at the Falls about eighty feet thick, and below of soft
and easily weathered shale. Beneath the Falls the underlying shale
is cut and washed away by the descending water and retreats also
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