WAVE CUTTING 297 



The currents moving the loose material with them some- 

 times form it into bars which inclose or tie islands to the 

 mainland or extend into the sea free ends, forming what 

 are called spits. A famous example of a land-tied island 



NAHANT, MASSACHUSETTS. 

 . A land-tied island. 



is that of the great English fortress at Gibraltar; although 

 now a promontory, it was once an island detached from 

 the coast of Spain. Shifting sand bars, especially if cov- 

 ered with water, are exceedingly dangerous to vessels, and 

 coasts where these are abundant need especial protection 



