236 PHYSICAL GEOLOGY 



The great ocean currents are caused primarily by the 

 winds. Their courses are determined (1) by the direction 

 of the winds, (2) by the arrangement of the land masses, 

 (3) locally, by the configuration of the ocean bottom, and (4) 

 by the earth's rotation, which deflects them toward the right 

 hand in the northern hemisphere, and toward the left hand 

 in the southern hemisphere. The importance of ocean cur- 

 rents in connection with the distribution of temperature 

 has been referred to. Warm and cold currents influence 

 greatly the present distribu- 

 tion of marine life, and the 

 ocean currents of earlier geo- 

 logical periods, some of which 

 flowed across the centers of 

 the continents (then sub- 

 merged) , have to be taken into 

 account in explaining the dis- 

 FIG. 252. Breakers on the coast tribution of former life. The 

 ** **' Buch n ' mechanical work of ocean cur- 

 rents is in general unimpor- 

 tant. Deep currents in shallow places may scour the ocean 

 bottom, but the bottoms of currents are usually far above 

 that of the sea. Certain ocean currents carry away sediment 

 brought to them by the streams of the neighboring land, but 

 large quantities are never carried far. (Why are ocean cur- 

 rents not so efficient transporting agents as rivers on the 

 land?) The work of shore currents is discussed later (p. 246). 

 The regular rise and fall of the waters of the ocean, twice 

 in about twenty-four hours, constitute the tides. In the 

 open ocean the tides are imperceptible. Along the shores 

 the change of level ranges from 2 or 3 feet to 50 

 feet and more in narrow bays. For about six hours the 

 water rises and advances upon the shore (flood tide), and 

 then for an equal time falls and recedes (ebb tide). Wide 

 flats are in consequence often alternately exposed to the 

 atmosphere and covered by the sea. In V-shaped bays and 



