THE WORK OF THE OCEAN. 



357 



occurs when the waves are strong. To this Hne of breakers, material 



is shifted from both directions: 

 from shore by undertow, and from 

 seaward by the weaves. Accumulat- 

 ing here, it builds up a low ridge. 

 This is a harrier (Fig. 314). If it is built up above the surface of the 

 water by storm-waves, it may shut in a lagoon behind it, and this may 

 ultimately be filled by sediment washed down from the land. At one 

 stage in the fiUing, the lagoon becomes a marsh. ^ In the part which 

 the barrier plays in the history of a coast, it is identical with the beach. 

 The spit, the bar, and the loop. — The disposition of shore-deposits 

 depends largely on the currents at and near shore. If the coast-hne is 

 deeply indented, the littoral current usually fails to follow the reentrants. 

 In holding its course across the mouth of a small bay, a shore-current 

 usually passes into deeper water. Here its velocity is checked because 

 its motion is communicated to the water beneath it, and a larger amount 

 of water being involved in the motion, the motion of each part is dimin- 

 ished. If sediment was being moved along its bottom before the cur- 

 rent was checked, some part of it is dropped when and where the cur- 

 rent is slackened. It follows that deposition commonly takes place 

 beneath a littoral current as it crosses the mouth of a bay. The belt 



Fig. 315. — A recurved spit. Dutch Point, Grand Traverse Bay, Lake Michigan. 



of deposition is often narrow, and the result is the construction of a 

 ridge beneath the water in the direction of the current. The current 

 would never build the embankment up to the water-level, but when 



^ Shaler, Sea Coast Swamps of the U. S., 6th Ann. Rept. U. S. Geol. Surv. ; and 

 MerriU, Pop. Sci. Mo., Oct., 1890. 



