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._