THE OCEAN AND ITS WORK 



221 



(Fig. 213). Sometimes the end of a hook is curved so far around 

 as to form a loop. Provincetown harbor, Massachusetts, is an 

 example. 



Bars are often of great disadvantage to navigation, since they so 

 shallow the water that vessels are compelled to wait until high tide 

 before they can pass 



^ J 



Fig. 213. — Hook Bay near the north entrance to 

 Chignik Bay, Alaska. The hook was formed by shore 

 currents. (Atwood, U. S. Geol. Surv.) 



over them. In other 

 cases constant dredg- 

 ing, maintained at 

 great expense, is nec- 

 essary to keep a 

 channel open. Spits 

 and hooks sometimes 

 serve as breakwaters 

 and are of consider- 

 able value to ship- 

 ping in time of 

 storm. 



The effect of the 

 formation of bay- 

 head beaches and of 

 bars by the shore 

 currents is to shorten the coast line and give it a smoother outline. 



Sand Reefs or Barrier Beaches. — When the water offshore is 

 shallow, the waves drag bottom and build up a ridge of sand or gravel 

 some distance from the shore, which is as high as the storm waves can 

 lift the material. After the surface is reached the height is further 

 increased through the piling up of sand dunes by the wind. Sand 

 reefs or barrier beaches (Fig. 214 A, B) are therefore formed on shelv- 

 ing shores, along a line to which material is brought seaward by the 

 undertow and landward by the drag of the waves. Such sand reefs 

 are separated from the mainland by narrow lagoons, or if they have 

 been in existence for a long time by marshes (p. 223). Sand reefs 

 are approximately parallel to the low shores which they border. They 

 are seldom continuous for many miles (Fig. 215), but are broken by 

 " inlets " which are kept open by tidal scour or by water which is 

 brought into the lagoons by the streams, or by a combination of the 

 two. Inlets occur at intervals of from two to twenty miles on the 

 Atlantic coast of the United States. After a sand reef is formed, it 

 sometimes happens that a second reef is built up in front of it, leaving 



