232 



EARTH FEATURES AND THEIR MEANING 



there is a continued loss of energy which is here not compensated 

 by added wind friction, and so the orbital motion grows smaller and 

 smaller, until at the depth of about a wave length it has completely 

 died out. This level of no motion is called the wave base. In 

 quiet weather the level of no motion is practically at the water's 

 surface, and inasmuch as the geological work of waves is in large 

 part accomplished during the great storms, the term "wave base" 

 refers to the lowest level of wave motion at the time of the heavi- 

 est storms. Upon the ocean the highest waves that have been 

 measured have an amplitude of about fifty feet and a wave 

 length of about six hundred feet. 



Free waves and breakers. So long as the depth of the water 

 is below wave base, there is obviously no possibility of interfer- 

 ence with the wave through friction upon the bottom. Under 

 these conditions waves are described as free waves, and their forms 

 are symmetrical except in so far as their crests are pulled over 

 and more or less dissipated in the spray of the " white caps " at 

 the time of high winds. 



As a wave approaches a shore, which generally has a gentle 

 outward sloping surface, there is interposed in the way of a free 

 forward movement the friction upon the bottom. This friction 

 begins when the depth of water is less than wave base, and its 

 effect is to hold back the wave at the bottom. Carried slowly 



FIG. 248. Diagram to illustrate the transformation of a free wave into a breaker 

 as it approaches the shore. 



upward in the water by the friction of particle upon particle, 

 the effect of this holding back is a piling up of the water, which in- 

 creases the wave height as it diminishes the wave length, and also 

 interferes with wave symmetry (Fig. 248). Moving forward 

 at the top under its inertia of motion and held back at the bottom 



