These pieces are in their turn ground down to
pebbles which serve to batter against the remaining rock.
Professor Geikie tells us that the waves beat in a storm against
the Bell Rock Lighthouse with as much force as if you dashed a
weight of 3 tons against every square inch of the rock, and
Stevenson found stones of 2 tons' weight which had been thrown
during storms right over the ledge of the lighthouse. Think what
force there must be in waves which can lift up such a rock and
throw it, and such force as this beats upon our sea-coasts and
eats away the land.
Fig. 28 is a sketch on the shores of Arbroath which I made some
years ago. You will not find it difficult to picture to
yourselves how the sea has eaten away these cliffs till some of
the strongest pieces which have resisted the waves stand out by
themselves in the sea. That cave in the left-hand corner ends in a
narrow dark passage from which you come out on the other side of
the rocks into another bay. Such caves as these are made chiefly
by the force of the waves and the air, bringing down pieces of
rock from under the cliff and so making a cavity, and then as the
waves roll these pieces over and over and grind them against the
sides, the hole is made larger.
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