Page:The Journal of geology (IA journalofgeology11893univers).pdf/60
carries up material stirred from the bottom. The finer particles are swept back by the undertow, the coarse are placed by the greater waves beyond the reach of the lesser. Thus waves, constantly in advancing, take material from the lower part of the slope to carry it up, and in retreating sweep back more or less of their load with them. If the slope be gentle they thus take from the lower to add to the upper part, and therefore they increase the declivity until the seaward profile becomes so steep that the load carried in retreat balances that advanced. This is the profile of equilibrium, which waves perpendicular to the trend of the beach do not change, unless they are of unusual force. Waves oblique to the beach-slope, scour, transport and deposit the same sands repeatedly, and if the oblique advance be prevailingly from one direction the effect is to move the beach longitudinally. Then the beach, in any one section, continues, while the supply of sand is continuous; but when the supply ceases the beach is gradually moved onward in the direction of the prevailing wave action, and the material beneath the beach sands is exposed to wave erosion.
A beach itself is but a narrow zone; it cannot constitute a wide-spread formation any more than a line can constitute a plane. But if a line be moved in one direction parallel to itself it will develop the plane, and in the same manner if a beach advances landward it may spread a formation. This advance may be a result of wave erosion, which carving a sea cliff on a bold shore planes a surface of marine denudation. The beach deposit is then a basal conglomerate. Or, the land reduced to a low surface by subaërial erosion may subside slightly in reference to sea level, and the sea, transgressing, will rearrange the superficial formations. If the waves have power to handle the material the sea is margined by beach sands. If they cannot efficiently sort it the land will merge in tide-flats with the water.
A beach is not only narrow, it is also shallow; waves build on the surface over which they break, and the height to which they may build does not exceed a few feet. Therefore, beach deposits cannot form thick strata.