THE WORK OF THE OCEAN 313 



flows past the end of a spit, it may cut away its extremity and 

 rebuild the materials into a smaller spit, joining the main one at 

 an angle. This gives rise to a hook (Fig. 259). Successive storms 

 may develop successive hooks along the side of a growing spit. 

 The end of a hook may be so extended as to join the mainland, 

 when it becomes a loop. If the spit is lengthened until it crosses, 

 or nearly crosses, the bay, shutting it off from the open water, it 



Fig. 260. Map of the head of Lake Superior. (U. S. Geol. Surv.) 



becomes a bar. Bars have shut in lakes (ponds) on the coast of 

 Martha's Vineyard, Mass. (PI. XX), and lakes, ponds, and lagoons 

 at numerous points both on the Atlantic and the Pacific coasts. 

 The same phenomena are to be seen along many lake shores 

 (Fig. 260). Bars sometimes tie islands to the mainland (PI. 

 XIX, Fig. 2). 



If the bay across which the bar is built receives abundant 

 drainage from the land, the outflow from the bay may be sufficient 

 to prevent the completion of the bar, for when the growth of the 

 bar has sufficiently narrowed the outlet of the bay, the sediment 

 brought to the end of the spit by the littoral current will be swept 

 out beyond the spit by the current setting out from the bay. The 

 completion of a bar may be interfered with also by tidal currents, 



