228 ELEMENTARY GEOLOGY 
Results of Wave Action. By chemical action, direct 
wave attack, and the hurling of rock fragments 
against the coast, the shore is driven back. The line 
of chief activity is at the water level, and here the 
waves and water saw their way into the rock. By this 
PIG: 12h: 
Limestone cliffs in the Bermuda Islands, undercut by waves. 
action hard rocks are sometimes cut into the form of 
cliffs, undercut at the water line (Fig. 127). In other 
cases the waves work locally, with sufficient energy to 
carve out a sea cave (Fig. 128). Soon the cliff above 
becomes unstable, and a part must fall. Hence the cliff 
