Fig. 309.—“Old Man of Hoy.” (Geikie.)
Chimney-rocks, etc.—By working in along the joints of the rock, widening them and quarrying out the intervening blocks, pillars of rock (“chimney-rocks,” “pulpit-rocks”) or even considerable islets are sometimes isolated by the waves. This is most readily accomplished where the joints converge back from the shore. A well-known example of this sort is the “Old Man of Hoy” ([Fig. 309]) on the coast of the Orkneys. A pulpit-rock or other island, or any jutting point of rock may be pierced, giving an arch or bridge. La Roche Percée, a steep-faced isle near Gaspé Harbor, is an example.
Sea-caves.—Waves sometimes excavate caves at the bases of cliffs. This is especially likely to occur where the rock is much jointed and where the joints are not continued to the surface in a single plane. The bottom and roof of a sea-cave usually have a pronounced inclination landward. If the cliff be low, the cave may be extended landward until its roof is pierced. Through such an opening in the top of the cliff the water of the incoming waves may be forced in the form of spray. On the New England coast such holes are sometimes known as “spouting horns.” Similar openings may be made, as already pointed out, by the compression or rarefaction of the air in the cave as the wave enters or retreats. If the roof of the cave be partially destroyed, the portion which remains may form an arch or bridge. Such a bridge occurs on Santa Cruz Island, California ([Fig. 310]).
Fig. 310.—An arch developed by waves. Santa Cruz Island, Cal. (Law.)
The cave, the “spouting horn,” the “bridge,” the “pulpit-rock,” and other isolated islets, are all closely associated with the sea-cliff in origin.
The wave-cut terrace.—The bottom of the sea-cliff is bordered by a submerged platform over which the water is shallow. This platform, or at any rate its landward portion, represents the area over which the water has advanced as the result of wave-cutting, and is, therefore, known as the wave-cut terrace. From the method of cliff development it will be seen that the wave-cut terrace is its necessary accompaniment. Such a terrace has a gentle slope to seaward, for its outer and older edge has been degraded longer and more. Its slope is influenced by the strength of the waves, being greater where they are stronger. The outer edge of the wave-cut terrace is often marked by an abrupt descent. [Fig. 303] represents the wave-cut terrace in its relation to the sea-cliff above.
Fig. 311.—An elevated cliff above Great Salt Lake. In this case the water-level has been lowered. (Gilbert, U. S. Geol. Surv.)