National Geographic : 1896 Oct
THE ECONOMIC ASPECTS OF SOIL EROSION rarely, if ever, do the great waves of the wide ocean attain the continental shores. The decay of the wave is due to the application of its energy to the erosion work which it has done on the sea floor. The loss is shown not so much by the decrease in the height of the surge as in the shortening of its width and the slowing of its motion. A good share of its height is preserved in a peculiar manner: as the undulation comes over the shallowing floor of the sea the hindrance to its ongoing is proportionate to the dimi nution of the depth. The result is that the front of the wave, being in the least depth, is held back to a greater degree than the rear which is in deeper water. The two sides of the wave are thus crowded together, so that the crest of the arch is rela tively uplifted. For all this, however, the wave when it over turns-that is, when the top, or part least held back by the friction on the bottom, shoots over the base and falls in the re current cataract of the surf-probably never exceeds twenty feet in height and the energy left in the surging water may be reck oned at less than one-tenth of that which is held by the greater waves of the open sea. When the wave delivers its finishing stroke in the surf line and its splash front, the modes in which its energy is applied suddenly become changed. The falling mass of water strikes a powerful blow, which, coming upon firm-set rock or sand, has but little effect; but when, as is often the case, the beach is covered with loose stones, these fragments are driven about in a violent manner and strike heavy blows. When the wave over turns, the mass of water sweeps up the slope of the strand, urging before it all the rock fragments which it can drive onward. If the upper edge of the beach is bordered by cliffs, as is generally the case along rock-bound shores, the swash and secondary waves which gather inside the tumble of the surf send the boulders with each stroke to batter the base of the bluff Although the waves have in all cases lost a large part of their energy before they are able to do this work of battering the shore cliffs they are still, when armed with rock fragments, competent to accomplish a great deal of erosion. Whenever the cliff is composed of ordinary hard rock, the battering at its base cuts a recess, causing the cliff to overhang. In time the weight of the mass which is thus unsupported brings it in ruins to the beach, where the fragments are ground into sand or mud by the action of the waves and removed to the deep sea or the distant reaches of the shore.