AGENTS AT WORK IN THE OCEAN 233 
which at high tide are covered, at low tide are ex- 
_ posed to the air; and so the zone of wave attack is 
swung up and down twice each day. Ordinarily this 
increase in vertical range is not great; but where 
the tide rises a score or two of feet, it causes a 
marked influence on the nature of the wave work. 
By this means, too, in the winter, where the climate 
is cold, ice is formed in the crevices of a rock from 
whose face the tide is falling. This, freezing at low 
tide, and melting when covered by the water, prys 
open the crevices and breaks off particles in a manner 
analogous to frost work in weathering (p. 112). Ice is 
formed every winter on some coasts; and as the tides 
rise and fall, or as the waves dash against the shore, 
these icy masses, in which pebbles and bowlders are 
often frozen, are ground against the coast, aidmg in 
the work of destruction. 
Effect of Organisms. — In the sea the most impor- 
tant work of organisms is the construction of beds 
of sediment; but along the sea-coast they sometimes 
aid or interfere with the erosive work. Sea-weed, 
barnacles and other kinds of plants and animals, 
cling to the rocky coasts in many places (Fig. 159). 
These aid erosion by prying off fragments as they 
grow, just as is done by plants on the land. Some 
sea-weeds also buoy up pebbles, and allow the waves 
to more easily drive them on the beach. How- 
