U. S. Geol. Surv.
Scale, 1+ mile per inch.
PORTION OF THE COAST OF MAINE.
The processes which develop coastal indentations, together with the antecedent subaërial and the subsequent wave gradation, account for most of the islands which affect indented coasts. Some of them are high and some low for reasons which will be readily understood. The long narrow belts of land constituting irregularities parallel to the general trend of the coast (Figs. [319] and [320]) are usually the result of deposition in shallow water. They are usually sand or coral reefs, built up above water-level by waves. The deposits at the debouchures of streams give rise to projecting deltas. Most small irregularities of angular form, especially if high ([Pl. XX]), indicate wave-erosion, and their details of form are determined by the structure of the rock along shore, while most irregularities of curved outline involve something of shore-deposition, if not due wholly to it. Glaciation, or glaciation and subsidence, may also give rise to peninsulas, capes, and islands of curved outlines ([Pl. XXIV], coast of Maine). Curving outlines may, however, be developed by erosion alone in weak rock structures. This is illustrated by the weak rock structures (clay, sand, etc.) of most of the Atlantic coastal plain. Thus inspection of the horizontal configuration of coasts will often indicate the processes which have been dominant there in recent times. On the other hand, the interpretations of many coastal irregularities, such as Hudson Bay, Puget Sound, the Gulf of California, the Baltic Sea, etc., are not to be read from the map. In such cases, diastrophism and gradation have usually coöperated, but the relative importance of the two processes can only be determined by detailed study in the field. When it is remembered that the tendency of shore-erosion is to reduce great irregularities of horizontal configuration, though not to obliterate small ones if the coast be heterogeneous in composition ([p. 353]), and that the tendency of shore-deposition is also to regularity, it is clear that the great irregularities of coast-lines are due neither to shore-erosion nor to shore-deposition, though minor ones may be due to either.
THE WORK OF OCEAN-CURRENTS.
As agents of erosion, ocean-currents are not, in general, of great importance. Currents which reach the bottom are comparable, in their effects, to rivers of the same velocity and volume; but most ocean-currents do not touch bottom, and, therefore, do not erode it. Where the current agitates the bottom sensibly, as it often does in shallow water, the bottom is abraded, and in the lee of such places it is doubtless aggraded. Since ocean-currents do not, for the most part, flow in shallow water, their erosive work is, on the whole, relatively slight; but where they are forced through narrow and shallow passageways, their abrasive work may be considerable. Thus the Gulf Stream, where it issues from the Gulf, has a velocity of four or five miles per hour, and its shallow and narrow channel is current-swept.
A rough test of the abrasive work of an ocean-current is found in the nature of the bottom beneath it. If this be hard, it indicates that the loose sediment on the floor of the ocean has been swept away, while the presence of fine detritus indicates that the current is not wearing. Thus the abrasive power of the Gulf Stream is known to continue somewhat beyond its narrow channel, for on the Blake plateau (between the Bahamas and Cape Hatteras), where the water is 600 fathoms and less in depth, “the bottom of the Gulf Stream ... is swept clean of lime and ooze and is nearly barren of animal life.”[168] Other illustrations of the erosive power of currents have been noted near Gibraltar in water 500 fathoms deep, and between the Canary Islands at depths of 1000 fathoms.[169] In spite of these examples, and of many others which probably exist in similar situations, it yet remains true that ocean-currents are on the whole but feeble agents of erosion.
As agents of transportation, ocean-currents are scarcely more important than as agents of corrasion, for they transport only what they erode, if the life which inhabits them be left out of consideration. This phase of their work has probably been exaggerated through a confusion of transporting energy and actual transportation. Ocean-currents which do not touch bottom roll no sediment and carry only what may be held in suspension. A river’s power of transporting sediment in suspension is due largely to the cross-currents occasioned by the unevenness of its resistant bottom ([p. 117]). If a particle of mud in suspension in a river drops to the bottom, as it frequently does, it may be picked up again and carried forward. If, on the other hand, a particle in suspension in an ocean-current once escapes the moving water by settling through it, the current which does not drag bottom has no chance to pick it up again. Very fine sediment may be carried by an ocean-current far beyond the point where it was acquired, but currents which do not touch bottom are rarely strong enough to hold any but the finest material for any considerable length of time. As transporters of sediment, therefore, ocean-currents are at a great disadvantage as compared with rivers.
How readily particles of extreme fineness may be kept in suspension, and how little agitation is necessary to keep them from sinking, is shown by the experiments of Sorby, who showed that while a sand grain ¹⁄₁₀₀ of an inch in diameter will settle one foot per second in still water, fine particles of clay require days to sink through the same distance. The Challenger found fine sediment derived from the land 400 miles from the coast of Africa, and that not opposite the debouchure of any large river. Sediment settles more readily in salt water than in fresh, despite the fact that the former is heavier. This is presumably because the salt diminishes the cohesion of the water.