Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The reason for this profound difference is readily found. On the land, the dominant processes which shape the details of the surface are degradational, and though the final result of degradation is flatness (base-level), the immediate result is relief, and, most commonly, relief of the hill-and-valley type. In the sea, the dominant processes are aggradational, and tend to monotonous planeness.

Distribution of marine life.—Marine life has been of such importance in the history of the earth that the elementary facts concerning its distribution and the principles which control it are here recalled. The distribution of marine life is influenced by many factors, chief among which are temperature and depth of water. Not only is life more abundant in the warmer parts of the ocean than in the colder, but the species inhabiting cold waters are different from those in warm, and few species range through great variations. Many forms of life are restricted to shallow water. Many more, especially those which do not live on the bottom, swim about freely without reference to the depth of the water beneath them, while relatively few are restricted to great depths. Many species are also influenced by the salinity of the water, which varies notably along coasts where the fresh waters from the land are discharged; by the character of the sediment at the bottom, some species preferring mud, others sand, and others gravel; by the movement of the waters, some species preferring still waters and others rough; and some species by the abundance and nature of the food-supply, and by rival and hostile species.

Subject to the exceptions determined by temperature, etc., plant life abounds in shallow water out to depths of 100 fathoms or so, and is found in abundance at the surface where the depth is much greater. Animal life abounds in shallow water, both at the bottom and above it, out to depths of 200 or 300 fathoms, and occurs in great profusion in the surface-waters of temperate and tropical regions without regard to the depth. The great body of the ocean water lying below a depth of some few hundred fathoms is nearly tenantless, though life reappears sparingly at the bottom, even where the depth is great. For further discussion of this topic, see [Chapter XI].

PROCESSES IN OPERATION IN THE SEA.

Within the area of the sea, as on the area of the land, three sets of processes are at work—diastrophism, vulcanism, and gradation.

Diastrophism ([p. 2]) affects the sea-bottom as the land, but the results are notably different in certain respects. So far as the lithosphere is concerned, the sea-level may be said to be the critical level. At and above it, many processes are in operation which do not appear below, and below it, many which do not take place above. Changes of level which do not involve the submergence of areas which were land, or the emergence of areas which were under water, are relatively unimportant, compared with those which effect such changes. The rise of the bottom of the sea from a depth of 500 fathoms to a depth of 200 fathoms would not lead to important consequences, so far as the area itself is concerned, while an equal rise of the bottom beneath 200 fathoms of water, or an equal subsidence of land 500 feet high, would be attended by more striking consequences. It follows that the changes effected by diastrophism are much more obvious along coasts than in the deep seas. Emergence or submergence shifts the zones of aggradation and degradation, shifts the zone of contact of ocean and land, and changes the region concerned from one appropriate for sea life to one appropriate for terrestrial forms, or vice versa.

Over the continental shelves the water is shallow and the bottom relatively smooth. If a coastal region be elevated evenly, or if the sea-level be drawn down, the new shore-line on the smooth surface of the former submerged shelf will be relatively regular, even though the coast was notably irregular before the change. Thus in [Fig. 297] the coast-line is notably irregular. A sea-withdrawal or a land-uplift of 120 feet would change the coast-line to the position of the 20-fathom line, when it would be notably less irregular than now. If it were shifted to the 100-fathom line, few irregularities would remain. In so far as new coast-lines formed by the lowering of the sea (or rise of the crust) depart from straightness, it is usually by broad, smooth curves. Local uplifts of coastal lands, and especially uplifts along axes normal to the trend of the coast, would give rise to projections of land, and so to coastal irregularities; but such uplifts are rarely so localized as to give origin to minor projections. It follows that rising coasts, and those which have recently risen, or more likely, coasts along which the sea-level is sinking or has recently sunk, are likely to be regular so far as details of outline are concerned. Subsidence of a coast-line (or rise of the sea-level) tends to the opposite results, for in this case the sea advances on a surface which has more or less relief, and the water takes possession of every depression brought to its level. The lower parts of the valleys are converted into bays, the length and width of which depend on the slope and width of the valleys drowned. The numerous bays at the debouchures of the streams along the Atlantic coast of the United States, from Long Island Sound to Carolina, such as the Delaware, Chesapeake, ([Fig. 297]) and numerous smaller bays, are the results of recent sinking, which has allowed the sea to invade the lower ends of river valleys. The ragged coast of Maine is another example, though glaciation as well as subsidence has been operative here. From the present configuration of coast-lines, it has been inferred that the present is, on the whole, an era of continental depression.[145] River valleys, the lower ends of which are embayed, are sometimes found to be continuous with submerged valleys beyond the coast-line ([Fig. 298]). Submerged river valleys show that the surface in which they lie was once land.

Fig. 297.—Sketch of the eastern coast of the United States from Cape May to Cape Henry, showing coastal irregularities. The figures represent the depths of water in fathoms. (From charts of C. and G. Surv.)