ALGONEIAN ; CAMI3RIAN ; PERMIAN ; PLEISTO CENE EPOCH.
Work of the Ocean.—Practically three fourths of the earth's surface is covered by oceans, which have a maximum depth of about 30,000 feet, and an average depth of about two miles. Sea water contains 3.4 per cent of dis solved matter, of which about three-fourths is common salt, (NaC1) and about 0.3 per cent calcium carbonate.
Of all the movements of ocean water waves are geologically the most striking and their chief work lies in modification of the shore lint.
Wave impact striking blows of 600 to 2,000 pounds per square foot and armed with sand and pebbles gathered on the shallow bottom may become powerful agents of erosion, wear ing back steep shores with ,a constant tendency to undercut and form cliffs, grinding finer and finer the mantle of sand and pebbles on gentler beaches. The eroded material is gradually rolled and dragged out into deeper water by undertow and currents. Incidental to the erosion produced by waves are many peculiar shore line features such as sea caves, isolated stacks or pillars, natural archways and wave cut terraces. Along the chalk cliffs of Eng land and France the waves wear back the shore line in places at the rate of several feet a year. The island of Heligoland once very much larger than it is now has been nearly cut away hy wave work. (See HELIGOLAND). Many geologists believe that such marine planation is even more important than river peneplana tion in the wearing away of continental masses. The detritus formed by wave wear is drifted by the undertow or by currents parallel to the shore and built up into sand beaches. On gently inclined shores where the waves break some distance from land barrier beaches or islands often form. Aided by the work of the wind in drifting the sand into dunes, such barriers may effectually isolate bodies of ocean water which then become lagoons or salt marshes. These fill rapidly with vegetation which may form peat.
Shore lines are frequently modified by changes of level. Since the ocean bottom is not subject to erosion it is much less rough than land. An uplift would cause smooth ocean bottom to become land resulting in long straight coasts accompanied by beaches elevated far above the reach of waves. A sinking of the coast would allow the sea to enter many valleys and result in an extremely irregular shore line. See section on Diastrophism.
The material contributed to the ocean by rivers plus that worn from shore lines by waves accumulates steadily on the ocean floor, there to rest till uplift exposes it again to weathering and erosion. Along the littoral belt, that is, between the reach of highest and lowest tide, coarse gravel or sand is the rule, with here and there areas of finer muds near the mouth of some large river. Such sediments are likely to change rapidly from point to point and to show crossbedding and ripple marks. (See section on STRUCTURAL GEOLOGY). Further out in the shallow water belt the sediments become finer, the coarser material derived from the land settling out nearer shore. Beyond the gravels are sands and beyond these clays and muds. These deposits are much more constant in character than those near shore, with no such rapid changes in the coarseness or the nature of the material. Very minute lime secreting animals live in this zone, where the waters are clear. Their skeletons settling to the bottom form accumulations of lime oozes that later become limestone, which may be in terstratified with beds of clay or sand. Other oozes are also known. (See OozE). Where the water is especially clear, coral polyps live both in the littoral and shoal water zones and build coral reefs. and islands of large dimen sions. (See CORAL AND CORAL ISLANDS).
Little material derived from the land ever gets into the abysmal ocean. Insoluble windblown material largely of volcanic origin, meteoritic dust, and insoluble remains of marine organ isms accumulate very slowly forming the red clays of, the deep sea. Since it is believed that the ocean deeps have remained great deeps throughout much of geologic time, it is thought that these deposits are rarely exposed on land; but the shales (clays), sandstones (sends), and limestones (lime oozes) formed in the shallower ocean frequently are exposed by up lift, and a large proportion of our present sedimentary formations are of marine origin, in contrast to the eolian, alluvial, and glacial deposits already described, which are classed as terrestrial or continental. See BEACHES; LIMESTONE; OCEAN; SEDIMENTARY • RocKs; SHORE LINES, etc.