i86 



WORK OF THE OCEAN 



Fig. 102. A recurved spit, Dutch Point, Grand Traverse Bay, Lake Michigan. 

 (U. S. Geol. Surv.) 



water-level; but when its surface approaches the level of effective 

 agitation, the waves may build it up to the surface of the water, or 

 even above it. So long as the end of such an embankment 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 de- 

 pression (deep water) begins. The embankment thus begun is 

 extended by carrying out new material, which is left at the end of 

 the dump already made, as at the end of a railway grade. 



The spit is normally 

 either straight or parallel 

 with the general course of 

 the shore-current, but since 

 the littoral current is sub- 

 ject to change with shifting 

 winds, the spit may be- 

 come curved or hooked 

 (Fig. 192). 



If the spit is lengthened 

 until it crosses, or nearly 

 crosses, the bay, shutting it 

 off from the open water it 

 becomes a bar. Bars have 

 shut in lakes, ponds, and 

 lagoons at numerous 



Fig. 193. 'Map of'the head of Lake Superior. P oints both on the Atlan ' 



(U. S. Geol. Surv.) 



tic and the Pacific coasts 



