SUMMARY OF COASTAL IRREGULARITIES.
The horizontal irregularities of coasts are both large and small. Some of them, like Florida, Sandy Hook, etc., consist primarily of projections of land into the sea; others, like Chesapeake Bay, the Gulf of Mexico, and Puget Sound, are projections of the sea into the land; while still others, like the Gulf of California and its associated peninsula, cannot readily be put in either of the foregoing classes. Some of the irregularities of the land border, such as Yucatan, are more or less nearly normal to the general trend of the coast which they affect, while others, such as the “beaches” along the Atlantic and Gulf coasts of the United States (Figs. [319] and [320]), are more or less nearly parallel with it. Some of the irregularities, especially some of the small ones, are more or less angular in their outline ([Pl. XX] and parts of [Fig. 2, Pl. XXII]), while others are bounded by curves instead.
In many cases more than one factor has been involved in the development of irregularities. In the case of great irregularities, diastrophism has generally been the dominant factor. The Gulf of Mexico and the Mediterranean Sea perhaps represent differential subsidence, while Florida and the Iberian peninsula represent differential uplift (relative, though perhaps not absolute). The narrow bays which indent many coasts generally represent the subsidence of a region previously affected by valleys ([Fig. 297]). Many of them, such as Narragansett, Delaware, and Chesapeake Bays, are primarily the drowned ends of river valleys, while others, such as Puget Sound,[167] are primarily structural valleys (synclines). Many of the long and narrow bays or fiords common in the high latitudes of North America and Europe ([Fig. 266], [p. 293]) appear to be the drowned ends of valleys previously deepened by glaciers. The drowned ends of river canyons, and the submerged parts of valleys excavated (not sunk) beneath the sea by glaciers, would also be fiords.
PLATE XXIII.
U. S. Geol. Surv.
Scale, 1+ mile per inch.
Fig. 1. MASSACHUSETTS.
U. S. Geol. Surv.
Scale, 1+ mile per inch.
Fig. 2. MAINE.
PLATE XXIV.
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.