=Free waves and breakers.=—So long as the depth of the water is below wave base, there is obviously no possibility of interference 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. [Illustration: FIG. 248.—Diagram to illustrate the transformation of a free wave into a breaker as it approaches the shore.] 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 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 increases 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 by constantly increasing friction, a strong turning motion or couple is started about a horizontal axis, the immediate effect of which is to steepen the forward slope of the wave, and this continues until it overhangs, and, falling, “breaks” into surf. Such a breaking wave is called a “comber” or “breaker” (plate 11 B). ┌─────────────────────────────────────────────────────────────────┐ │ PLATE 11. │ │ │ │ [Illustration: _A._ Ripple markings within an ancient sandstone │ │ (courtesy of U. S. Grant).] │ │ │ │ [Illustration: _B._ A wave breaking as it approaches the shore. │ │ (_Photograph by Fairbanks._)] │ └─────────────────────────────────────────────────────────────────┘ [Illustration: FIG. 249.—Notched rock cliff cut by waves and the fallen blocks derived from the cliff through undermining. Profile Rock at Farwell’s Point near Madison, Wisconsin.]