202 PHYSICAL GEOLOGY 



(p. 221) and the channels (thoroughfares) back of them are kept open largely by tidal 

 scour, and the deep waterways in some bays are sometimes maintained in the same 

 way. The work accomplished by tidal currents consists more in the transportation 

 of material prepared by the waves than in the actual wear of the coast. 



The importance of tides to man is considerable. Many of the 

 important harbors of the world could not be entered without tides. 

 This is shown by the fact that ships must wait until the water is 

 deepened by high tide before entering. The washing out of harbors 

 by the tides twice a day is of great sanitary importance. The produc- 

 tion of power from tides has not as yet been financially successful, 

 but the possibility of the use of tidal power in the future in the gen- 

 eration of electrical energy is worthy of mention. 



Tidal Bores. — When a tide enters the mouth of a river which is obstructed by the 

 form of the entrance and by the shallows, its progress may be so retarded that its waters 

 will, for a while, be prevented from passing up the valley. When its height finally 

 becomes great enough, it rushes up in one or more great waves, which are called bores. 

 In the Tsientang River, China, and in the Amazon River, Brazil, waves 20 or more 

 feet in height are said to have been developed at times in this way. Smaller bores 

 occur in other rivers. These waves are characteristic of but few rivers and are not 

 of daily occurrence in any, but in such rivers as those cited they sometimes tear out 

 the banks, destroy forests along the shores, and wash away islands. 



Earthquake Waves. — Because of their great length, waves generated by earth- 

 quakes (p. 292) rise to great heights when they reach shelving shores. Such a wave 

 10 to 30, or perhaps more, feet in height struck the coast of Japan in 1896, killing 

 2 6,975 people, destroying $3,000,000 worth of property, and changing the shore line 

 in many places. Because of their infrequency, earthquake waves are of little impor- 

 tance in marine erosion as compared with storm waves. 



Ocean Currents. — The great currents of the ocean, such as the Gulf Stream, per- 

 form a very slight work of erosion or transportation, but are of vast importance in 

 modifying past and present climates of those regions near which they pass. This is 

 due to the influence of the winds, since they convey the warmth of the poleward cur- 

 rents and the cold of the equator-moving currents to the adjacent lands. 



Marine Erosion 



Factors in Marine Erosion. — The impression one receives on seeing 

 a wave strike a rocky shore is that the blow and the weight of the 

 water are the only forces which are important in marine erosion. 

 This, however, is an error. (1) When a wave is dashed against a 

 cliff, every crack and cranny is more or less filled with water, and the 

 hydrostatic pressure exerted tends to force the walls of the fissure 

 apart. Ibis force sometimes amounts to three tons on the square 

 foot ; a force which, often repeated, must accomplish an important 



