Page:The Journal of geology (IA journalofgeology11893univers).pdf/56
to the shore, whatever force the waves may have is expended at the water's edge. On a bold coast they carve sea-cliffs and grind shingle with sand. Such are the coasts of New England, Oregon, California, and of all the Pacific side of South America. The resulting sediments are composed of worn but fresh rock fragments and thus bear witness to rapid mechanical erosion, like the products of rock breaking on steep declivities. On a shore of incoherent materials waves stir, wash and separate fine and coarse, light and heavy particles. Under favorable conditions of depth of water and long fetch, waves thus sort a heterogeneous mass of gravel or of residual sand and clay more efficiently than any other agent, and leave clean cross-stratified beach sand and gravel with boulders, while the finer materials are swept away. The southeastern shore of Long Island presents a conspicuous example of this, and the westward drift of the beach-sands is illustrated in the fact that shingle beaches prevail toward the eastern end of Montauk point, and the sands there washed from the bluffs of glacial gravel form long barriers along the coast to the westward.
If, on the other hand, waves break in shallow waters at a distance from shore they there build a barrier, and the height to which they build it above high tide is the measure of their maximum power during great storms. Within the barrier then extends a lagoon. The whole Atlantic coast from Long Island to Florida is thus fringed by the features of prevalent wave action, due to the great fetch from off the ocean and the gradual slope of the continental platform.
(b) Prolonged transportation.—Sorting is also accomplished to some extent, though less perfectly, by deep water and continuous currents. Sediments settle unequally according to size and specific gravity of particles; therefore the largest and heaviest reach bottom first, the finer and lighter later, and the finest and lightest last. If the conditions of supply or current be intermittent over any area then each incident of deposition will be marked by a layer composed of coarsest grains below and finest grains on top. This is the nature of deposition in tidal estua-