AGENTS AT WORK IN THE OCEAN 225 
of the waves is done. If they are beating against 
a cliff, they gradually eat it away and cause it to 
retreat toward the land; if they break upon the beach, 
surging backward and forward, they grind the rock 
fragments to sand and clay. The coast line is a 
great mill, whose never-resting engine is the wind 
wave. 
Aid of Currents. Soon this work of the waves 
would be self-destructive if there were not another 
action in codperation. Just as those rivers furnished 
with more sediment than they can carry, are pre- 
vented from cutting their channels deeper, so waves 
that cannot dispose of the materials which they 
obtain, are hindered from wearing back the. land. 
On many parts of the coast south of New York 
this condition prevails, and the waves are building 
bars off the real coast, instead of eating back into 
the land (Plate 11). . 
Currents of various kinds, some of wind origin and 
others resulting from tidal action, are constantly remov- 
ing the finer fragments which the waves have prepared. 
The work of tidal currents is considered in its proper 
place (p. 231), and we have now merely to examine 
that of the undertow and the wind currents, which 
codperate with the tidal movement in the transporta- 
tion of sediment and its deposition over the ocean 
bottom. 
Q 
