THEORIES OF ORIGIN 621 



advantage, and wliicli increase tliat advantage as they grow; jnst as the 

 accidentally favored drainage basins increase in size and advantage at the 

 expense of those which began the contest with biit a slightly less favor- 

 able chance. The tendency of wave action will be to develop from initial 

 irregularities a smaller number of broad and shallow depressions on that 

 portion of the beach traversed by the swash. The depressions will be 

 broad, because they are thus better adapted to the movements of large 

 volumes of water, and shallow, because the elevations between the de- 

 pressions are also buried under the advancing and retreating waters and 

 are kept worn down to a moderate height. Only near the upper zone of 

 wave action, where the water invades the depressions but does not rise 

 high enough to override the intervening elevations, are the depressions 

 continually scoured deeper and the unworn elevations left as pronounced 

 ridges. Out toward the seaward margin of the submarine terrace, depo- 

 sition rather than erosion prevails, and the delta scallops may rise higher 

 than the seaward extension of the elevations which exist farther up the 

 beach. 



There is a limit to the width to which the depressions, or shallow 

 "channels," if we may so call them, can develop. Inasmuch as the en- 

 largement of some necessitates the obliteration of others, enlargement 

 will continue only so long as the impulse toward growth imposed on the 

 more favored channels is sufficient to overcome the tendency of their 

 neighbors to enlarge. Equilibrium • will be established when adjacent 

 channels are of approximately the same size, and at the same time of a 

 size appropriate to the volumes of water traversing them. If the waves 

 are low and the volumes of water consequently small, equilibrium will 

 be reached while the channels are yet small. But if the waves are high 

 and the volumes of water large, a perfect adjustment will not be reached 

 until the channels have attained considerable size. 



The remainder of the process is easily understood. With the water 

 advancing repeatedly up a beach which is faintly but systematically 

 channeled, as above indicated, there will be a constant tendency to push 

 gravel and other debris farther up the slope in the depressed areas than 

 in the intervening areas. Near the upper limit of wave action the de- 

 pressed areas alone are invaded by water and are scoured deeper as the 

 gravels are pushed back and the finer material dragged down to form the 

 delta scallops. The intervening areas are fashioned into beach cusps, 

 whose sharpened points divide the waters of the advancing waves and 

 concentrate the attack toward the heads of the depressions. The coarse 

 material is constantly pushed into the cusp areas, the channels swept 

 relatively clean. With a rising tide, both channels and cusps are pushed 



