Fig. 253.—The Marble Islands, stacks in Lake Buenos Aires, southern Andes (after F. P. Moreno).
The cut rock terrace.—When waves first begin their attack upon a steep, rocky shore, the lower limit of the action is near the wave base. The action at this depth is, however, less efficient, and as the recession of the cliff is one of the most rapid of erosional processes, the rock floor outside the receding cliff comes to slope gradually downward from the cliff to a maximum depth at the edge of the terrace, approximately equal to wave base ([Fig. 255]). This cut terrace is extended seaward or lakeward, as the case may be, in a built terrace constructed from a portion of the rock débris acquired from the cliff.
Fig. 254.—Squared stacks which reveal the position of the joint planes which have controlled in the process of carving by the waves. Pt. Buchon, California (after a photograph by Fairbanks).
Fig. 255.—Ideal section of a steep rocky shore carved by waves into a notched cliff and cut terrace, and extended by a built terrace.
The broken wave, after rising upon the terrace under the inertia of its motion until all its energy has been dissipated, slides outward by gravity, and though checked and overridden by succeeding breakers, it continues its outward slide as the “undertow” until it reaches the end of the terrace. Here it suddenly enters deep water, and losing its velocity, drops its burden of rock, and builds the terrace seaward after the manner of construction of an embankment. As we are to see, the larger portion of the wave-quarried material is diverted to a different quarter.
Fig. 256.—Map showing the outlines of the Island of Heligoland at different stages in its recent history. The peripheries given are in miles.