LAKES AND SHORES 153 



deep. The curved line represents the crest and trough of a wave, 

 while the little circles show the path of each particle of water in 

 the wave. The amount of motion in a wave is greatest at the 

 surface and diminishes rapidly downward. 



When a wave advances into shallow water near the shore, its 

 motion changes. This is because the motion of the water in the 

 lower part of the wave is hindered by dragging on the bottom, and 

 its top then pitches forward, as surf (Fig. 1, PI. XXXVI, p. 129). 

 In strong winds and in shallow water, therefore, there is a distinct 

 forward movement of some of the water of a wave. 



The water thrown against the shore in the wave runs back again, 

 and this from-shore motion is the undertow. Where the waves 

 come in against a shore obliquely, they produce a movement of 

 water along the shore. This is the shore or littoral current. 



Erosion by waves. The force of the wave as it is hurled against 

 the shore may be very great. In the open sea, storm waves have 

 a height of 20 to 30 feet in some cases. Surf has been thrown up 

 to heights of more than 100 feet with force enough to destroy light- 

 houses. It has been estimated that the strength of waves on the 

 coast of Great Britain is sometimes as much as three tons per square 

 foot, and that the average for winter waves is about one ton per 

 square foot. Such waves would move masses of rock tons in weight. 

 It is clear, therefore, that the force of waves is great enough to wear 

 shores, even of solid rock. The waves of lakes are never so strong 

 as the great waves of the sea, but the storm waves of large lakes 

 have great force, and sometimes wash away piers and breakwaters, 

 even in a single storm. 



Let us think of a regular shore composed of rock of unequal 

 resistance. Waves would wear the weaker rock of such a shore 

 more than the more resistant, and the coast would be made irregular. 

 Such irregularities are, in most cases, rather small. 



Again, let us think of a very irregular coast, the rock of which 

 is all of equal resistance. The waves would wear the projecting 

 points of such a shore more than they would wear the heads of the 

 bays. Wave erosion, therefore, tends to make a very irregular 

 coast less irregular, unless the projecting points are more resistant 

 than other parts of the coast. 



