COASTAL WORKS 241
violence. This effect was discovered by the first Eddystone
Lighthouse (1700). The door had been strongly supported to
resist the waves, but it was blown outward by the expansion
of air compressed within the lighthouse.
The wearing back of the coast is aided by subaerial denuda-
tion, which generally produces an upward slope inland above
the sea-cut part of the cliff.
Wave action is aided by animals and plants. Seaweeds
growing on a rock enable a wave to move it. Animals
bore into rocks and their acid secretions dissolve the cement ;
Pholas bores into limestone; the shipworm, the Teredo,
burrows through timber; and sea-urchins (Echinoidea),
browsing on films of seaweed, wear pits even in granite.
TransPorT OF Brace Mareriar—The material obtained
by the wearing back of a cliff is rolled to and fro by the waves
and reduced to shingle, which drifts along the shore. Ac-
cording to one view this movement is due entirely to waves
made by the wind; but according to another it is due to
tidal currents. Both agencies act in varying degrees; the
movement of pebbles is usually by the waves, but guided
by the persistent currents. Beach shingle and river shingle
may be distinguished by the shape of the cobbles (stones of
about 4 to § inches in diameter), which in a river are rolled
along with the long axis at right angles to the current, and
are typically ovoid. Cobbles on a sea-beach are spun around
by the tide, the base is worn flat, and the upper side is rounded
by the scour of sand, until they become bun-shaped.
Marine abrasion forms a shelf or plain of marine denuda-
tion between high and low tides. ‘Stacks’ or pillars of
hard rock may rise above it. The * Old Man of Hoy” in
the Orkneys is a pillar of Old Red Sandstone about 450 feet
high. Some bands of rock with the layers on end and lying
in contorted shale were attributed to glacial action, until
Murray Macgregor recognized them as stacks that had fallen
on a muddy shore.
Sea-caves are formed where rocks are eaten into hollows.
Raised lines of caves often give evidence of the uplift of a
coast.
CONTINENTAL SHELF—A broad shelf of mud, sand, and
shell beds with occasional exposures of rock, borders the
continents and extends from the shore till, at the depth of
