374 



THE WORK OF RUNNING WATER 



shallow bottoms and build reefs and sand bars ; and they form 

 dangerous cliffs along the shore. Waves beat unceasingly against 

 the shore (Fig. 241) and, armed with sand, pebbles, and even 

 bowlders, erode it and cause the shore line to retreat. Large 

 waves are more powerful than small ones and coasts whose surf 

 is heavy are worn more than coasts whose surf is light. A coast 

 of sand, pebbles, and loose material like that at Atlantic City is 



FIG. 243. Thick masses of seaweed help to protect a coast. 



cut into more than a rocky coast. An island off the north- 

 ern coast of Germany has been so worn away that it is now 

 only one twentieth of its former size. Irregular, dangerous 

 coasts are made where rocks of hard and soft materials are 

 beaten by the sea. The softer portions are worn away, 

 but the more resistant parts remain and endanger ships. 

 Sometimes cliffs of enormous size tower high above the waves 



