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.
A. Ripple markings within an ancient sandstone (courtesy of U. S. Grant).
B. A wave breaking as it approaches the shore. (Photograph by Fairbanks.)
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.
Effect of the breaking wave upon a steep rocky shore—the notched cliff.—If the shore rises abruptly from deeper water, the top of the breaking wave is hurled against the cliff with the force of a battering ram. During storms the water of shore waves is charged with sand, and each sand particle is driven like a stone cutter’s tool under the stroke of his hammer. The effect is thus both to chip and to batter away the rock of the shore to the height reached by the wave, undermining it and notching the rock at its base ([Fig. 249]). When the notch has been cut in this manner to a sufficient depth, the overhanging rock falls by its own weight in blocks which are bounded by the ever present joints, leaving the upper cliff face essentially vertical.