THE SEA AS AN AGENT OF DESTRUCTION Just as the wavelets in our miniature sea "wasted" the beach here, and added to it there ; undermined a cliff at one point, and formed an island at another ; so also the waves of the Ocean incessantly dash on the exposed margin of the land, rapidly disintegrating the softer materials, and surely, although much more slowly, wasting away the hardest rocks. The peculiar grating sound which always accompanies the advance and retreat of the waves on a shingly shore, plainly indicates the incessant "planing," as it were, of the land, more especially that portion between high and low-water mark. The rounded pebbles are mute, but at the same time most expressive, evidences of continual attrition—of being dragged up and down the beach, rubbed against one another—until their angularities have entirely disappeared. Being almost constantly in motion, the pebbles are reduced to coarse gravel, then into sand, and lastly into fine mud, which, easily held in suspension for a considerable time, is removed by tidal waves and currents, but finally also settles down on the floor of the ocean.
We have already (Art. 112) noticed the great force of waves. Such indeed is their power, that immense blocks have been displaced, lighthouses swept away, and breakwaters broken down. The pressure of the Atlantic breakers on the coasts of the Western Hebrides has been experimentally proved to be, on an average, 611 lbs. per square foot in sum mer, and 2,086 lbs. in winter.i The spray has been occasion ally driven right over the lantern of the Bell Rock Lighthouse, 112 feet high, and even over that of the Eddystone Light house, 140 feet high. that the lantern at Dunnet Head, 270 feet above the level of the sea, has been cracked by pebbles hurled from the beach by the waves ; and that during very violent storms, the Atlantic waves rush up the sides of the cliff at Hoy Head, in Orkney, to a height of nearly 600 feet. The pressure on the more exposed parts
during violent storms must therefore be from 6,000 to 8,000 lbs. per square foot ! This fact alone would suffice to prove the tremendous power of waves, and the destructiveness of their action, not only on the softer, but also on the most rocky, coast exposed to their fury.
But it must be remembered that the denuding power of waves does not lie simply in their weight or force of impact, but also in the use they make of the loose pebbles and frag ments of stones strewn on the beach. These are swept upwards by the advancing wave, and dashed against the cliffs, broken in pieces themselves, but also breaking down or further weak ening some part of the rock. Wave after wave, similarly charged, dash on and hurl their missiles against the cliff, which is thus constantly battered, and ultimately undermined; huge fragments are loosened, and fall into the sea. And since the hardest and most rocky coasts thus suffer, it will be evident that the softer cliffs and beaches will be loosened and demolished much more rapidly ; and, where the coast is formed of materials of unequal hardness, the softer portions are swept away, leaving the harder rocks partially detached or quite isolated. The characteristic irregularity of contour of the land-masses is thus a result of the unequal hardness of the strata exposed to the action of the sea. The harder coasts are bold and irregular, while the softer portions are regularly carved out into bays and coves. The limits of this little work, however, prevents us from following out this part of our subject fully : we must therefore restrict ourselves to a brief statement of the principal instances of the changes, due to the action of the sea, on our own and other coasts.