732 ELEMENTARY GEOLOGY 
ocean surface will cause currents to flow backward and 
forward through this. Thus on many coasts, par- 
ticularly those that are irregular, the tide, which 
naturally is merely a rising and falling of the water, 
becomes a true moving current. These currents may 
srow powerful enough to destroy land, and they are 
Hig. 13k 
Cliffs and beach enclosing a bay at Grand Menan. 
always at work transporting sand and clay along the 
coast, or out to sea. 
Therefore one of the most important effects of the 
tides, is to move sediment about from place to place. 
They also increase the range of the wave work, be- 
cause they raise and lower the ocean level. Rocks 
