CHAPTER XV

COAST-LINES

ULAR

COASTS-INFLUENCE OF

FORM AND GENERAL TREND OF COAST-LINES-SMOOTH OR REGGEOLOGICAL STRUCTURE ON VARIOUS FORMS ASSUMED BY CLIFFS-CLIFFS CUT IN Bedded AND IN AMORPHOUS ROCKS-SEA-CAVES-FLAT COAST-LINES AND COASTAL PLAINS--INDENTED OR IRREGULAR COASTSGENERAL TRENDS OF COAST-LINES DETERMINED BY FORM SUBORDINATE INFLUENCE OF MARINE

OF LAND-SURFACE

EROSION.

THE

HE coast-lines of the globe-the varied forms they assume and the directions they followare an interesting study. Wandering alongshore and observing the effects of wave-action, we are soon convinced that here, as in landward areas, hard rocks and strong structures tend to resist erosion, while soft rocks and weak structures more readily succumb. When we so frequently find the former projecting seawards in capes and headlands, while the latter are often cut back in bays and inlets, it might almost seem as if both the shape and the direction of coastlines had been determined solely by marine action. But this cannot be altogether true. If bays and all other inlets and arms of the sea were the result of

marine erosion alone, the most highly indented coasts should also be the oldest. If not, then they should occupy positions peculiarly exposed to the battering and undermining of waves and breakers, or they should be excavated in the softest and most yielding rocks. The very opposite of all this, however, is the case. Not only are highly indented coast-lines of relatively recent age, but they frequently consist of the hardest kinds of rock, and they are, moreover, not subject to wave-action in any greater degree than coasts which are smooth and regular. If indentations were always due to marine erosion, the sea should be still eating its way into the land at the head of most fiords, estuaries, and other inlets. Instead of advancing in such places, however, it is more frequently receding. Rivers entering the heads of estuaries and sea-lochs gradually push their deltas outwards. Not only so, but in long, narrow inlets and fiords waves and breakers do very little work— they are practically powerless. Since such inlets, therefore, are neither extended nor widened by the sea, they cannot owe their origin to its action. However potent an agent of erosion it may be, we cannot credit it with the formation of the numerous deep indentations of such a coast as that of Norway. In point of fact, the general tendency of marine erosion is to reduce irregularities-to cut back headlands, to silt up intervening bays, and to stretch banks and ridges across the mouths of estuaries and other notable indentations of the land, so as eventually to shut

these off more or less completely. Hence all coasts which can be shown to be of relatively great age have a gently sinuous or profusely curved outline. Conversely, as we have indicated, highly indented coasts are of recent origin-the sea has not yet had time to reduce their irregularities.

We must distinguish between the form and the general trend of a coast-line. The varying shape of cliff and low shore is no doubt largely determined by the manner in which the rocks yield to the sea, but the general direction followed by a coast obviously depends on the form of the land. If the latter be mountainous, with great valleys opening on the sea, the coast-line will usually be more or less deeply indented. If, on the other hand, it be a low-lying, gently undulating land, there will be a general absence of deep and long inlets, although broad and shallow bays may be numerous. Such a land may be margined by steep cliffs or it may be bordered by low plains, or by both. In short, however much the sea may modify the form of its coasts, it is evident that it has had but a small share in determining their direction. The latter obviously depends on the position of the sea-level and the shape of the land. Hence a very slight elevation or depression of the land would in many cases completely change the direction of the coast-lines. An elevation of 300 feet, for example, would lay dry the bed of the North Sea and the English Channel, while an elevation of 600 feet would not only join the British Islands to the

Continent, but cause the shores of Europe to advance some 50 or 60 miles beyond the Outer Hebrides and Ireland.

We shall first, therefore, treat of the various forms assumed by coast-lines, and thereafter the causes which have determined their general trends will be more particularly considered. When we run our eye over a map of the world we are struck by the fact that in some places the coasts are relatively smooth and unbroken, while in other regions they are more or less deeply indented. We have thus at least two principal types, which we may classify as (a) smooth or regular coasts, and (b) indented or irregular coasts.

Smooth or Regular Coasts. These may be high and steep, or low and gently shelving, the one kind often alternating with the other. Their chief characteristic is the absence of prominent inlets. A steep, regular coast, as shown upon a small-scale map, has a softly undulating or sinuous course, or presents a succession of smaller and larger curves. It need hardly be said that when such a long line of cliffs is examined in detail, many minor irregularities make their appearance. In some places the cliffs project boldly beyond the average coast-line to form headlands, elsewhere they curve backwards, or their continuity may be interrupted by more or less numerous creeks, gullies, and small inlets, which could only be represented upon a map of a very large scale. The cliffs, moreover, may vary in form at relatively short intervals, or they may preserve great uniformity of char

acter for long stretches. All such inequalities and differences are due to the nature of the rocks and the mode of their arrangement. Bedded rocks, for example, owing to the regularity of their joints, tend to form cliffs with even faces. If the strata be horizontal, it is obvious that the cliffs must be vertical, or nearly so, since the rocks naturally yield along their approximately vertical division-planes. When a slice has been detached from the cliff, the new surface exposed is an even wall of rock. But as the beds entering into the formation of such a cliff are likely to yield unequally to weathering, the smooth wall of rock sooner or later becomes etched and furrowed. (Fig. 86.) Now and again, however, owing to the

FIG. 86. SEA-CLIFF CUT IN HORIZONTAL STRATA.

jj, joints.

nature of the rocks, or to the rapid retreat of the cliffs, weathering has not sufficient time to effect any marked modification of the surface. When the strata, instead of being horizontal, are inclined, and the dip is inland, or away from the coast, the joint-planes necessarily have an inclination towards the sea, and the cliffs naturally slope in the same direction. (Fig. 87, p. 320.) On the other hand, should the strata