In this way the bottom of the cliff is
undermined, and so great pieces tumble down from time to time,
and keep the fall upright instead of its being sloped away at the
top, and becoming a mere steam. Secondly, you may often see
curious cup-shaped holes, called "pot-holes," in the rocks on the
sides of a waterfall, and these also are concerned in its
formation. In these holes you will generally find two or three
small pebbles, and you have here a beautiful example of how water
uses stones to grind away the face of the earth. These holes are
made entirely by the falling water eddying round and round in a
small hollow of the rock, and grinding the pebbles which it has
brought down, against the bottom and sides of this hollow, just as
you grind round a pestle in a mortar. By degrees the hole grows
deeper and deeper and though the first pebbles are probably ground
down to powder, others fall in, and so in time there is a great
hole perforated right through, helping to make the rock break and
fall away.
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