FORMATION OF POCKET-BEACHES 22 r 



immediately in front of the port they are apt to yield detritus 

 which may be, indeed generally is, carried directly into the 

 port by the ocean waves. In this respect they are much 

 more daneerous than headlands, for the latter often have 

 conditions which favor the development of pocket-beaches 

 on one side of their faces, but all the waste worn from an 

 island is apt to tail around either end of its mass and march 

 straight away toward the channel of the port. Along our 

 northern shores it not infrequently happens that insular masses 

 of glacial drift, frequently in the form of drumlins, lie at the 

 mouth of the haven or within it. As the drift materials of 

 which they are composed are easily broken up, even by slight 

 waves, and much of the matter is in the form of clay, which 

 can be borne to great distances by currents, such shores as 

 these isles afford are a constant menace to the neighboring 

 harborages. 



Almost all the important ports of the world have to con- 

 tend against the inconveniences which arise from the immi- 

 gration of detritus which wanders toward them from along the 

 neiehborine shores. One of the first tasks of the engineer 

 who has charge of the defences of such harbors against their 

 natural enemies, the waves, is to secure protection as far as 

 possible by constructing walls or other revetments which may 

 serve to keep the sea from assailing cliffs of a nature to be 

 easily washed away. In this manner much can be effected to 

 aid the work of the natural pocket-beaches. In other cases 

 artificial pockets may be constructed by building moles from the 

 shore ; but while with such contrivances it is generally possible 

 to prevent the incursion of the coarser sands and pebbles, the 

 finer mud which sweeps along the bottom in even tolerably 

 deep water cannot thus be debarred entrance to the harbor. 



