FORMATION OF BARRIER SAND REEFS 191 



occur whcMi the sea is affected only by in-shore winds of sli_L^ht 

 energy, break very near the waterdine. Heavier surges — 

 say, those having a height of three or four feet — comb and 

 fall over at a distance of some scores of feet from the actual 

 margin of the sea; while waves of the greatest volume, such 

 as are formed at rare times of heavy tempests, may break a 

 mile or more away from the strand. Wherever the over- 

 turning occurs, the power of the wave is broken and whatever 

 debris it may have been urging forward is left upon the 

 bottom. If we clearly perceive these features in the action 

 of waves, it is easy to understand how the bar islands which 

 enclose lagoons are formed. Thus in case our southern shore 

 should be depressed below the level of the sea, so that the 

 barrier sand reefs were covered, a result which would be pro- 

 duced if the region were lowered to the depth of thirty or 

 forty feet below its present level, the immediate effect would 

 be to brine the ocean waters into free contact with the shore 

 of the mainland in a manner found on coast-lines where there 

 are no such outlying islands. At once the submerged barrier 

 would be taken to pieces by the waves, and the accumula- 

 tions of sand would be spread over the bottom of the sea, 

 still further shallowing the water near the shore. W^ith the 

 advent of the next ensuing great storm the waves would 

 break at a distance from the shore ; it might be even some 

 miles away from it, and on this new line of breakers the con- 

 struction of a new series of barrier sand reefs would begin. 

 If the storm were great, so that the waves, were of the first 

 magnitude, the breaking might take place in water having a 

 depth of as much as fifty feet. Waves of a volume to break 

 in water of this depth would carry a good deal of sand to the 

 point where they topple over. Here this transported detritus 



