THE WORK OF THE OCEAN 



297 



Waves 



Wave-motion. 1 The most common waves are those generated 

 by winds. During the passage of a wave, each particle affected by 

 it rises and falls and moves forward and backward, describing an 

 orbit in a vertical plane. If the passing wave is a swell, the orbit 

 of the particle is closed and is either a circle or an ellipse; but in 

 the case of a wind-wave the orbit is not closed, for in such a wave 

 the water, as well as the undulation, moves forward. On the crest 

 of the wind-wave each particle of water moves forward, and in the 

 trough it moves less rapidly backward, and the excess of the forward 

 movement over the backward gives the water a slight residual 

 advance. This advance is the initiatory element of current. As a 

 result of this advance, the upper layer of water is carried forward 

 with reference to the layer below, in the direction toward which 

 the wind blows. The waves of any considerable or long-continued 

 wind, therefore, generate a 

 current tending in the same 

 direction as the wind. 



The wave motion is prop- 

 agated indefinitely down- 

 ward, but the amo.unt of 

 motion diminishes rapidly 

 with increasing depth (Fig. 

 242.) Engineering opera- 

 tions have shown that sub- 

 marine structures are little 



Fig. 242. Diagram illustrating the de- 

 creasing size of the orbits of water 

 particles in a wave, with increasing 

 depth. (Fenneman.) 



disturbed at depths of five 



meters in the Mediterranean, 



and eight meters in the Atlantic. On the other hand, debris 



as coarse as gravel, which is transported by rolling on the bottom, 



is not infrequently carried out to depths of 50 feet, and sometimes 



even to 150 feet. Fine sediment, like silt, is disturbed at still 



1 In the following pages concerning the waves and their work, Gilbert's 

 discussion of shore features, in the Fifth Annual Report of the U. S. Geol. 

 Survey, pp. 80-100, is freely drawn on. Another incisive discussion of certain 

 shore phenomena is that of Fenneman, Jour, of Geol., Vol. X, pp. 1-32. 



