INFLUENCE OF MIGRATING SANDS 219 



the direct action of the waves we have to take account of tlie 

 movement effected by the currents which the winds procUice 

 alonof a shore. Such wind-induced streams, more or less reen- 

 forced by tidal movements, often flow with a speed of ordinary 

 rivers, and are equally effective in conveying detritus. The 

 action of these varying currents can often be noticed in cases of 

 shipwreck, when the debris from the castaway vessel is in the 

 course of a few hours scattered along many miles of shore 

 which lies leeward of the point where the disaster occurred. 

 The effect of these currents is to transport the detritus which 

 the waves grind up along the shore until the drifting material 

 attains a point where it may enter some recess which is so 

 deeply embayed that no change in the direction of the wind can 

 cause it to be removed by wave action. As this indentation of 

 the coast-line is apt to be a natural harbor, the effect of the 

 coastwise movement of detritus is often in hiirh measure destruc- 

 tive to the usefulness of ports. 



The damage done by wandering sands is greatest where 

 there is no tidal action, as along the shores of the larger lakes 

 and lesser seas. Along such coasts as are not tide-swept, the 

 wave- and current-borne detritus rapidly finds its way into all 

 the recesses. In course of time the waters of the embayments 

 become so far shallowed as to be unserviceable to ships. The 

 effect of this action is conspicuous on the shores of our great 

 lakes of North America, particularly those which lie in the 

 valley of the St. Lawrence River. Although the present shore- 

 line of these lakes has been where it now is for but a short 

 geologic time, so that the harborages are relatively very new, 

 the influence of migrating sands has been already effective in 

 closing many originally good ports. The damage done in this 

 way is most conspicuous in the lower lakes of this series. In 



