THE OCEANS, THEIR STRUCTURES AND FEATURES 



35 



extremely fast moving, averaging about 450 miles per 

 hour, and not likely to be noticed at all. As they ap- 

 proach land the friction of the shallovi' sea bottom 

 causes the waves to pile up on a titanic scale, with 

 marked depressions of the waters to their front and 

 back. The two depressions account for the peculiar 

 recession of shore waters during the cycle when a 

 tidal wave strikes the land. The first indication of this 

 cycle is the marked withdrawal of shore waters, 

 because the forward depression causes recession, 

 generally so great that the ocean floor is uncovered 

 way beyond the normal lowest tide level. Next, the 

 receding waters appear to hesitate. Then; there is a 

 landward surge and the tidal wave carries water for 

 some distance beyond the normal highest high tide 

 level. In some cases, elevations of more than 50 feet 

 may be flooded. Naturally, this is the phase that 

 causes so much death and destruction. The third 

 phase of the cycle is another marked recession of 

 waters and is associated with the depression behind 

 the wave. However, the retreat is never as great as 

 was the first withdrawal. The final phase consists of 

 many, ever-decreasing oscillations and eventual 

 return to normal conditions. 



Frequently, many tidal waves follow one another. 

 In any single series, individual waves generally are 

 spaced more than 1 5 minutes apart. In such cases the 

 first tidal wave typically is neither the largest nor the 

 most destructive. 



Tidal wave destruction is not due to the force of the 

 wave alone. Of considerable importance is the nature 

 of the shoreline. If the shoreline is flat and straight, 

 the most destructive waves rise about 60 feet; but if 

 the shoreline is a V-shaped inlet, the waves may rise 

 over 100 feet. 



WAVES AND SHORE 



Waves near the shore produce characteristic fea- 

 tures at the point of junction between sea and land. 

 The general features are a straight shoreline and an 

 underwater profile that curves downward, first rather 

 steeply from the shoreline, but progressively flattens 

 out and finally merges into the gradual slope charac- 

 teristic of the continental shelf. However, this ma- 

 ture stage of shoreline is not the only type of seascape, 

 because crustal movements are constantly causing 

 shores to rise and fall. This vertical mobility of most 

 coastal areas provides landscapes that are being 

 eroded by wave action. 



EROSION MECHANISMS 



Waves erode the shoreline mainly by forcing, 

 quarrying, abrading, undermining, and dissolving. 

 Wave impact is suflicient to break parts of the shore 

 and quarrying can remove masses of rock to deeper 

 water. Because both quarrying and impact are the 

 result of wave force, the term "hydraulicking" is 

 sometimes used for a single erosion mechanism. 

 Abrading is the wearing effect of sand and other par- 

 ticles carried by the waves. The process of under- 

 mining causes areas of the shore to collapse into the 

 ocean waves, thereby supplying more materials for 

 hydraulicking. 



SHORE CURRENTS 



Water movements along the shore carry materials 

 eroded by waves from the erosion site to a new area. 

 Along the Pacific Coast of North America these.move- 

 ments cause a constant southward progression of 

 sand and other sediments. These so-called longshore 

 currents operate as follows: 



After waves become surf, water recedes into the sea 

 under the incoming waves. This undertow, the gravi- 

 tational attraction upon surf, carries particles of solid 

 materials a short distance out to sea. The important 

 thing about undertow is that it is directly downslope. 

 Because waves usually approach the shore from an 

 angle, water brought in by waves has traveled down 

 the shore when it completes an undertow movement. 

 The downshore travel contributes to the offshore, or 

 longshore, current (Figure 3.7). 



It should be emphasized that waves do not follow 

 a straight line from sea to shore. Near the shore the 

 ocean bottom deflects them from a straight path and 

 produces a final arcing one that more closely approxi- 

 mates a perpendicular to the shore. This reduces the 

 speed of the longshore current. This reduced speed 

 means that the arcing of an incoming wave and its 

 undertow create a less rapid downshore movement of 

 sediments. However, sand and other materials are 

 carried by these currents. 



SHORE DEPOSITS 



Shore deposits come primarily from two sources. 

 First, when waves destroy shore features in one area, 

 the eroded materials are transported by the undertow 

 into the offshore current. Then the erosion products 



