334 PHYSIOGRAPHY 



TYPES OF MOVEMENT 



The movements which result from the foregoing causes are (1) 

 waves, with the undertow and shore currents which they produce, 

 (2) ocean currents, (3) drift, or currents which are feeble and not 

 well marked, (4) tides, and (5) what we may call creep. 



Waves 



The work of waves and their effects on coast lines have been 

 outlined already (p. 152). It may be added here that the waves 

 of the sea make much land by deposition, and destroy more by 

 erosion. If nothing happened to prevent, the sea would finally 

 destroy all land by the continued cutting of its waves along the 

 shores. 



Since the water in waves does not commonly move forward, 

 waves do not produce a general circulation of ocean water. 



Currents and Drifts- 



There are more or less distinct streams of water, or currents, in 

 various parts of the ocean. This was first known by the effect of 

 the moving water on the course of sailing vessels. It was later 

 proved in various other ways, as by following the course of floating 

 objects set adrift for this purpose. 



The best known currents in the ocean are at its surface, and they 

 extend down to depths of several hundred feet. Ocean currents 

 are not so easily seen as the currents of running water on the land, 

 because their waters flow through a liquid, while, rivers flow in a 

 channel of solid material. A slow current, which is not very dis- 

 tinct, is often called drift. According to this use of the term, a 

 current may become a drift when it spreads out and becomes 

 slow. 



Fig. 265 shows the general course of movement of the surface 

 waters of the seas. The figure represents a large part- of the sur- 

 face water as moving. There are westward movements near the 

 equator in both the Atlantic and the Pacific Oceans. These are 

 the equatorial currents or drifts. In each ocean the drift is double, 

 and a narrow counter-current, or drift moves eastward between 



