312 GEOLOGY 



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

 diminished. If sediment was being moved along its bottom be- 

 fore the current was checked, some part of it is dropped when and 

 where the current is slackened. It follows that deposition com- 

 monly takes place beneath a littoral current as it crosses the mouth 

 of a bay. The belt 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 its surface approaches the level 

 of effective agitation, the waves may build it up to, and even above 



Fig. 259. A recurved spit, Dutch Point, Grand Traverse Bay, Lake Michi- 

 gan. (U. S. Geoi. Surv.) 



the surface of the water. So long as the end of such an embank- 

 ment is free, it is a spit. The construction of a spit has been aptly 

 compared to the construction of a railway embankment across a 

 depression. The material is first carried out from the bordering 

 upland (in this case the shallow water) and dumped where the 

 slope to the depression (deep water) begins. The embankment 

 thus begun is extended by the carrying out of new material, which 

 is left at the end of the dump already made, as at the end of a rail- 

 way grade. 



The spit is normally either straight or parallel with the general 

 course of the shore-current, but since the littoral current is subject 

 to change with the shifting of the winds, the spit may depart from 

 straightness. Winds which simply reverse the direction of the 

 littoral current retard its construction; but if a strong current 



