The Elements of Geology

by William Harmon Norton

The débris torn by waves from rocky shores is far less in amount than the waste of the land brought down to the sea by rivers, being only one thirty-third as great, according to a conservative estimate. Both mingle alongshore in all the forms of beach and bar that have been described, and both are together slowly carried out to sea. On the shelving ocean floor waste is agitated by various movements of the unquiet water,—by the undertow (an outward‑running bottom current near the shore), by the ebb and flow of tides, by ocean currents where they approach the land, and by waves and ground swells, whose effects are sometimes felt to a depth of six hundred feet. By all these means the waste is slowly washed to and fro, and as it is thus ground finer and finer and its soluble parts are more and more dissolved, it drifts farther and farther out from land. It is by no steady and rapid movement that waste is swept from the shore to its final resting place. Day after day and century after century the grains of sand and particles of mud are shifted to and fro, winnowed and spread in layers, which are destroyed and rebuilt again and again before they are buried safe from further disturbance.

These processes which are hidden from the eye are among the most important of those with which our science has to do; for it is they which have given shape to by far the largest part of the stratified rocks of which the land is made.

The continental delta. This fitting term has been recently suggested for the sheet of waste slowly accumulating along the borders of the continents. Within a narrow belt, which rarely exceeds two or three hundred miles, except near the mouths of muddy rivers such as the Amazon and Congo, nearly all the waste of the continent, whether worn from its surface by the weather, by streams, by glaciers, or by the wind, or from its edge by the chafing of the waves, comes at last to its final resting place. The agencies which spread the material of the continental delta grow more and more feeble as they pass into deeper and more quiet water away from shore. Coarse materials are therefore soon dropped along narrow belts near land. Gravels and coarse sands lie in thick, wedge-shaped masses which thin out seaward rapidly and give place to sheets of finer sand.

Sea muds. Outermost of the sediments derived from the waste of the continents is a wide belt of mud; for fine clays settle so slowly, even in sea water,—whose saltness causes them to sink much faster than they would in fresh water,—that they are wafted far before they reach a bottom where they may remain undisturbed. Muds are also found near shore, carpeting the floors of estuaries, and among stretches of sandy deposits in hollows where the more quiet water has permitted the finer silt to rest.

Sea muds are commonly bluish and consolidate to bluish shales; the red coloring matter brought from land waste—iron oxide—is altered to other iron compounds by decomposing organic matter in the presence of sea water. Yellow and red muds occur where the amount of iron oxide in the silt brought down to the sea by rivers is too great to be reduced, or decomposed, by the organic matter present.

Green muds and green sand owe their color to certain chemical changes which take place where waste from the land accumulates on the sea floor with extreme slowness. A greenish mineral called glauconite—a silicate of iron and alumina—is then formed. Such deposits, known as green sand, are now in process of making in several patches off the Atlantic coast, and are found on the coastal plain of New Jersey among the offshore deposits of earlier geological ages.

Organic deposits. Living creatures swarm along the shore and on the shallows out from land as nowhere else in the ocean. Seaweed often mantles the rock of the sea cliff between the levels of high and low tide, protecting it to some degree from the blows of waves. On the rock bench each little pool left by the ebbing tide is an aquarium abounding in the lowly forms of marine life. Below low-tide level occur beds of molluscous shells, such as the oyster, with countless numbers of other humble organisms. Their harder parts—the shells of mollusks, the white framework of corals, the carapaces of crabs and other crustaceans, the shells of sea urchins, the bones and teeth of fishes—are gradually buried within the accumulating sheets of sediment, either whole or, far more often, broken into fragments by the waves.

By means of these organic remains each layer of beach deposits and those of the continental delta may contain a record of the life of the time when it was laid. Such a record has been made ever since living creatures with hard parts appeared upon the globe. We shall find it sealed away in the stratified rocks of the continents,— parts of ancient sea deposits now raised to form the dry land. Thus we have in the traces of living creatures found in the rocks, i.e. in fossils, a history of the progress of life upon the planet.