224. ELEMENTARY GEOLOGY 
time there is an outward movement of the water 
at the bottom, which is known as the wndertow. 
Wave Attack. One of the ways in which the wave 
does work, is to batter at the dense rocks of the coast. 
In these there are usually crevices filled with air or 
water; and as the wave beats against the rocks, it 
Fie. 125. 
Surf at Long Branch. 
produces a pressure upon these which exerts suffi- 
cient force to rend off fragments of great and 
small size. 
With these rock fragments, added to others fur- 
nished by weathering, by streams, or any other cause, 
the waves batter at the coast and wear it away. 
It is by means of these tools that the greatest work 
