Deposition by ocean-currents is limited by their transportation. Only where they erode their bottoms do they gather coarse materials, and only in the lee of such places are their deposits coarse. Since the material which they carry is generally fine, it is widely distributed before deposition.
Ocean-currents have little influence on the configuration of coast-lines.
DEPOSITS ON THE OCEAN-BED.
Something has already been said concerning the sediments which accumulate in the shallow waters along shores; but the area of marine sedimentation is as extensive as the ocean itself, and the deposits must now be reviewed from another point of view.
Oceanic deposits may be conveniently divided into two chief groups, dependent on the depth of the water in which they are made.[170] These groups are (1) shallow-water deposits, made in water less than some such depth as 100 fathoms, and (2) deep-sea deposits, laid down in water of greater depth. The selection of the 100-fathom line as the dividing depth is less arbitrary than it seems, for passing outward from the shore, it is at about this depth that the bottom ceases to be commonly disturbed by the action of currents and waves; that sunlight and vegetable life cease to be important at the bottom; and that the coarser sediments which predominate along shore give place, as a rule, to muds and oozes. Furthermore, the 100-fathom line (or some line very near it) is an important one in the physical relief of the globe, for it appears to mark, approximately, the junction of continental plateaus and ocean-basins. Only because the latter are a little over-full does the water run over their rims, covering about 10,000,000 square miles of the borders of the continents, converting them from land into epicontinental seas.
Aside from the deposits made by organisms, shallow-water deposits are divisible into two groups—(a) those immediately along the shore, the littoral deposits, and (b) those made between the littoral zone and the 100-fathom line. Both are terrigenous. The deep-sea deposits likewise are divisible into two groups, (a) terrigenous deposits formed close to land, and made up chiefly of materials derived immediately from the disintegration of land formations; and (b) the pelagic deposits, made up chiefly of the remains of pelagic organisms and the ultimate products arising from the decomposition of rocks and minerals. The former predominate in the less deep waters relatively near shore; the latter in the deeper water far from land. The shallow- and deep-water deposits grade into each other in a belt along the 100-fathom line.
Shallow-water Deposits.
Littoral deposits.—The littoral zone is the zone between high- and low-water marks. It is the zone in which bowlders, gravels, sands, and all coarser materials accumulate, though muds are occasionally met with in sheltered estuaries. Generally speaking, the nature of these deposits is determined by the character of the adjoining lands and the nature of the local organisms. “The heavier materials brought by rivers from high terrestrial regions, or thrown up by the tides and waves of the sea, are here arranged with great diversity of stratification through the alternate play of the winds and waves. Twice in the twenty-four hours the littoral zone is covered by water and exposed to the direct rays of the sun or the cooling effects of the night. There is a great range of temperature; mechanical agencies produce their maximum effects,”[171] and physical conditions in general are most varied. Still greater diversity is introduced by the fact that the zone is inhabited by both marine and terrestrial organisms, while the evaporation of the sea-water which flows over tidal marshes and lagoons leads to the formation of saline deposits. If the length of the coast-lines of the world be taken at 125,000 miles (about 200,000 kilometers), and the average width of this zone at half a mile, these deposits are now forming over an area of 62,500 square miles (about 160,000 square kilometers) of the earth’s surface.
Non-littoral, mechanical deposits in shallow water.—These deposits are laid down in the zone of the ocean between low-water mark and the 100-fathom line. They cover about 10,000,000 square miles.[172] Their composition is much the same as that of the littoral deposits, with which they are continuous, though on the whole they are finer. At their lower limit they pass insensibly into the fine deposits of the deep sea. Coarse material, such as gravel and sand, prevails, though in special situations, such as depressions and inclosed basins, muddy deposits are found. While some of the deposits are wholly composed of inorganic débris, organic remains are freely mingled with others. The mechanical effects of tides, currents, and waves are everywhere present, but become less and less well marked as the 100-fathom line is approached. The forms of vegetable and animal life are numerous, though the former decrease as depths which exclude the sunlight are approached.
Both littoral deposits and deposits in shallow water outside the littoral zone have already been referred to in connection with the work of waves and currents (pp. 355–66). A few additional points only need here be added.