OCEANS AND LAKES 235 



Throughout the geological ages a chief service of the ocean 

 has been to receive, arrange, and preserve the materials 

 from which new land areas were later formed. While aggra- 

 dation has always been the dominant gradational process in 

 the ocean, degradation has always been of chief importance 

 upon the land. 



(4) Finally, the sea has always been engaged in eroding 

 portions of its shores. Thus it tends persistently to reduce 

 the area of the land, and to increase its own extent. 



The movements of sea waters. The geologically im- 

 portant movements of the sea are wind waves, currents, and 

 tidal waves. Earthquake waves and certain other occa- 

 sional and unusual movements are at times important. 



Wind waves are caused by the pressure of the wind upon 

 the surface of the water. In the open sea, the water is 

 pushed forward very little and slowly, even though the 

 wave form advances with rapidity. Each particle moves 

 through an elliptical path every time that a wave passes, but 

 returns essentially to the point of starting. The movement 

 of the water particles in a wave has been likened frequently 

 to that in a field of tall grass across which the wind is blow- 

 ing. Each blade is bent up and down, back and forth, yet 

 retains its place. Waves are propagated with gradually 

 lessening height far beyond the area of the storm which 

 generated them; here the diminishing waves are called 

 swells. 



On approaching land, waves drag bottom and the oscilla- 

 tory movement passes into a true onward movement. The 

 unimpeded top of the wave moves faster than the lower 

 part, which is retarded by friction with the bottom, and the 

 front of the wave accordingly becomes increasingly steep, 

 until the crest topples over and the wave breaks with all 

 its weight upon the shallow bottom or upon the shore line 

 (Fig. 252). The water of the broken wave rushes up the 

 beach, and then returns seaward under gravity, forming the 

 undertow. 



