Chapter VI: The Work of the Ocean (1)

The general facts concerning the depth of the ocean and the distribution of its water have been given on a preceding page (p. 8), and the origin of the ocean and the ocean basins is discussed in the second volume. This chapter has to do primarily with the processes now going on in the sea and its borders, in so far as they are of importance in the interpretation of geologic history. The study of these processes is prefaced by a few words concerning the amount and composition of the sea-water, the life of the ocean, and the topography of its bed.[138]

=Volume and composition.=—Every 1000 parts of sea-water contain about 34.40 parts by weight of mineral matter in solution. The principal solids, acids, and bases, combined according to the principles laid down by Dittmar, are shown in the following table:[2]

Chloride of sodium 77.758
Chloride of magnesium 10.878
Sulphate of magnesium 4.737
Sulphate of calcium 3.600
Sulphate of potassium 2.465
Bromide of magnesium 0.217
Carbonate of calcium 0.345
-------
Total salts 100.000

Expressed in terms of tons per cubic mile of sea-water, the composition is as follows:[139]

Tons per Cubic Mile.
Chloride of sodium (NaCl) 117,434,000
Chloride of magnesium (MgCl₂) 16,428,000
Sulphate of magnesium (MgSO₄) 7,154,000
Sulphate of calcium (CaSO₄) 5,437,000
Sulphate of potassium (K₂SO₄) 3,723,000
Bromide of magnesium (MgBr₂) 328,000
Carbonate of calcium (CaCO₃) 521,000
-----------
For sea-water, total dissolved matter 151,025,000

Aside from the ingredients shown in the above tables, the presence of the following has been proved: iodine, fluorine, phosphorus, silicon, boron, silver, lead, copper, zinc, cobalt, nickel, iron, manganese, aluminum, barium, strontium, arsenic, lithium, cæsium, rubidium, and gold. Oxygen, nitrogen, and carbonic acid gas are also present in quantity. The amount of carbonic acid is estimated to be 18 times as great as in the atmosphere.[140]

The amount of sea-water is estimated by Murray at 323,722,150 cubic miles,[141] or about 15 times the volume of the land above sea-level. The volume and composition of the sea-water being known, the amount of mineral matter which it contains may be readily calculated. Assuming the average specific gravity of the mineral matter in solution to be 2.5, the 3.5% by weight becomes 1.4% by volume, and 1.4% of 323,722,150 cubic miles is 4,532,110 cubic miles. This then represents the aggregate volume of mineral matter in the sea if it were precipitated and compacted so as to have an average specific gravity of 2.5. Assuming the average depth of the sea to be 2076 fathoms (12,456 feet), as given by Murray, the mineral matter in solution, if precipitated, would cover the ocean bottom to a depth of about 175 feet. Assuming the area of the land to be to that of the sea as 28 to 72, this amount of mineral matter would make a layer about 450 feet deep over the land. Its amount is equal to about 20% of that of all lands above sea-level, and it falls but little short of that in all lands below 600 feet in altitude. If it were precipitated and concentrated in the shallow waters about the borders of the lands, it would fill the sea out to the depth of about 4000 feet, and would diminish its area by some 19,000,000 to 20,000,000 square miles, an area which is more than ⅓ of the present land surface. In other words, if the mineral matter in the sea-water were precipitated and concentrated in the shallow waters about the lands, it would restore the continental shelves to the land areas, and add an almost equal area beyond.

These comparisons may perhaps help to give some idea of the amount of mineral matter in solution in the sea, but they give no more than a hint of the importance of the solvent power of water in the general processes of rock decay, for most of the substances carried to the sea in solution by rivers are extracted from the water about as rapidly as they are supplied. Thus calcium carbonate is about twenty times as abundant as sodium chloride in river-water,[142] but is only ¹⁄₁₂₅ as abundant in sea-water.

The total river discharge into the sea is estimated at 6524 cubic miles of water per year.[143] This water is estimated to carry to the sea annually about half a cubic mile of mineral matter in solution. At this rate it would take about 9,000,000 years for the streams to bring to the sea an amount of mineral matter equal to that it now contains, but the proportions of the ingredients would be very different.

The sodium chloride makes up about 2.4% of the mineral matter in river-water and nearly 78% of the mineral matter of the sea. At this rate it would take nearly 300,000,000 years for the salt of the sea to have been contributed by the rivers. It is not to be understood, however, that this figure indicates the age of the ocean. The salt is not all brought in by the rivers; the rivers have probably not always contributed at the present rate; and much salt once in the sea has been precipitated. Nevertheless the above figure gives some suggestion as to the order of magnitude of the figures which represent the age of the ocean.

In contrast with the salt, the amount of calcium carbonate in the sea is so small that at their present rate of contribution, it would be brought to the sea by rivers in about 62,000 years.

=Topography of bed.=—The general relations of ocean basins to continents are suggested by Fig. 296. The borders of the continental platforms are covered by the _epicontinental sea_, while the _abysmal sea_ occupies the ocean basins proper. From the figure it is seen that an ocean basin is pronouncedly convex upward, and so departs as widely as may be from the current notion of the homely utensil from which it is named. Only when it is remembered that a level surface (on the earth) is one which has the mean curvature of the earth, and that the deeper parts of the ocean basin are well below the mean sphere level, does the current name seem justified.[144] The figure also shows that the depth of an ocean basin is slight compared with the radius of the earth.

The bed of the ocean, like the face of the land, is affected by elevations and depressions, and its deepest points are about as far below its surface as the highest mountains are above it. There are areas of the sea bottom which, as a whole, may be compared to the plains of the land, and others which may be likened to plateaus, and the lines of gradation between them are as indistinct as they often are on the land. There are mountain peaks, chiefly of volcanic origin, and depressions comparable to the great basins on the land. But apart from these general features, there is little in common between the topography of the sea bottom and that of the land. Mountain systems are, for the most part, absent, though certain islands, like Cuba and some of its associates, may be regarded as the crests of systems which are chiefly submerged. If the water were drawn off from the ocean’s bed so that it could be seen as the land is, its most impressive feature would be its monotony. The familiar hills and valleys which, in all their multitudinous forms, give the land surface its most characteristic features are essentially absent. A large part of its surface would be found to be so nearly flat that the eye would not detect its departure from planeness.

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.

Bays may be developed by local subsidence as well as by the submerging of valleys, though decisive examples are not readily cited. Bays may also be produced by uplift of the surface on either side of an area which does not change its level. For example, uplift on either side of the Gulf of California has probably been one element, though probably not the only one, in the development of this indentation. The general outline of a great bay produced by coastal warping might be regular, though it would be likely to be marked by small irregularities where the streams enter. It is not to be understood that all, or even most, bays are due to local diastrophism.

Diastrophism, then, as it affects the ocean-bottoms and the ocean-borders, may make the water of any ocean shallower or deeper; it may cause the emergence or submergence of land; it may make coast-lines regular or irregular; it may shift the habitat of life, and through these changes may greatly influence the processes of gradation, which are especially active along the contact of sea and land.

=Vulcanism= affects the sea-bottom much as it affects the land. At the volcanic centers, where the great body of extruded matter accumulates, mounds and mountains are built up. Most of the mountain peaks of the sea-bottom, whether their crests are islands, or whether they are wholly submerged, have had a volcanic origin. The rock material ejected from submarine vents is probably less widely distributed than that from vents on land, and so far forth, the volcanic cones in the oceans are steeper than those on land. Where volcanic cones are built up near the surface of the sea, they often furnish a home for shallow-water life, such as polyps. Wherever built up so as to be within the reach of waves, gradational processes are stimulated.

The processes of vulcanism do not commonly influence coasts of continents directly, for few volcanoes lie immediately on coasts. In places, however, as at various points in and about Italy, the configuration of the coast is influenced by the building of volcanoes. Indirectly, vulcanism influences the shape of coast-lines, for the resistance of igneous rock is often different from that of the rock with which it is associated, and under the influences of the forces of gradation it may come to form projecting points or reëntrants, as the case may be.

The number of active volcanoes on islands is about 200, or about two-thirds of all now known. Since the area of the sea is about three times that of the land, the known active volcanoes in the sea are rather less numerous per unit area than those on the land. The number of active vents beneath the sea is altogether unknown. A few submarine eruptions have been observed, and those observed are probably but a small percentage of those which have taken place in historic time. Slight eruptions in deep water might not manifest themselves at the surface in an unequivocal way, even were observers stationed near them. Volcanic cones which fail to reach the surface are known, and the forms of many sea-bottom mountain peaks are such as to make it probable that they are volcanic. These phenomena, as well as the numerous volcanic islands, give some indication of the importance of submarine eruptions in past time.

Ocean volcanoes, and especially submarine volcanoes, affect both the temperature and the composition of the sea-water. Both the increase of temperature and the solution of volcanic gases increase the capacity of the water for mineral matter, and both the change in temperature and composition affect the life of the adjacent waters. The destruction of life during eruptions occasions the generation of the products of organic decomposition, and these stimulate further chemical changes. The diffusion of affected waters occasions chemical changes wherever they go. The effects of oceanic volcanoes on the sea-water are, therefore, appreciable, when long periods of time are considered. The deposition of the finer parts of volcanic discharges will be considered in connection with the deposits of the deep sea.

=Gradation.=—The gradational processes of the land and the sea are in striking contrast. On the land, degradation predominates, and aggradation is subordinate. In the sea, aggradation predominates, and degradation is subordinate. On the land, degradation is, on the whole, greatest where the land is highest, while aggradation is of consequence only where the land is low, or where steep slopes give place to gentle ones. In the sea, degradation is virtually confined to shallow water, or to what might be called the highlands of the sea, while aggradation is nearly universal, but most considerable in shallow water, or where shallow water gives place to deep. Both the degradational and aggradational work of the sea are greatest near its shores. Opposed as the gradational work of the land and sea are, they yet tend to a common end—the leveling of the surface of the lithosphere.

The gradational processes which affect the sea-bottom may be divided into three categories: (1) Those effected by mechanical means, (2) those effected by chemical means, and (3) those effected by organic agencies.

The mechanical work of gradation in the sea is effected chiefly by the movements of the water, and, very subordinately, by the movements of the ice which the water carries. The results of these movements may be degradational wherever the water is sufficiently shallow for the motion to affect the bottom. Elsewhere it is aggradational.

The direct gradational work effected by chemical means is likewise partly degradational and partly aggradational. If at any time or place the water becomes supersaturated with any mineral substance, precipitation takes place, and the precipitate accumulates as sediment on the bottom. This sometimes happens in lagoons and other small inclosures, and perhaps in open water. On the other hand, wherever solution is effected, degradation is the result. Solution is most important where the bottom consists of relatively soluble rock, such as lime carbonate.

Organic agencies are, on the whole, aggradational. Accumulations of coral, coral débris, shells, etc., help to build up the sea-bottom, and most rapidly in shallow water where the proper forms of life are most abundant. Here also should be mentioned the accumulations of carbonaceous matter, especially in the form of plant bodies. In the aggradation effected directly by organic agencies, the sea is passive. Its only part is to support the life which gives rise to the solid matter, and incidentally to float a part of it in its currents.

MOVEMENTS OF THE SEA-WATER.

The movements of the sea-water fall into several categories. There is (1) a general circulation of sea-water, determined chiefly by three factors: differences in density in the sea-water, differences of level, and the general movements of the atmosphere; (2) periodic movements which are not primarily circulatory, brought about by the attraction of the sun and moon; and (3) aperiodic movements, due to occasional causes, such as earthquakes, volcanic explosions, landslides, etc., which determine local and temporary movements, often of exceptional strength.

=Differences in density and their results.=—Differences in density result from differences in temperature and salinity. Temperature alone considered, water would be densest where it is coldest, namely in the polar regions. Differences in salinity result from differences in evaporation and from inequalities in the supply of fresh water. Evaporation alone considered, the sea-water should be densest where evaporation is greatest; but the equatorial region, where evaporation is greatest, is also a region where precipitation is heavy, and precipitation, by freshening the water, opposes the effect of great evaporation. The greatest differences in density due to the unequal supply of fresh water are to be found near the borders of continents, where the precipitation on the land is discharged into the sea. In the polar regions, the great supply of fresh water, especially during the season when the ice is melting, opposes the effect of the low temperature, so far as the density of the water is concerned. The result of the operation of these factors affecting the density of the sea-water is to insure a general circulation, directed to the end of equalizing the densities; and since the disturbing factors are constantly in operation, equilibrium is never established, and the movements of the water are perpetual.

The pressure gradients resulting from differences of density are so slight that the resulting movements are scarcely more than a creep of the waters. In general they are far too slow to be of importance in gradational work; but the earth’s rotation deflects the creeping waters and tends to concentrate the equator-ward movement into currents on the east sides of the continents, and the pole-ward movement on the west sides. In favorable situations these currents may be competent to produce sensible mechanical results. Even where this is not the case the circulation helps to equalize the temperatures of the sea, and so of the air above and of the land about. Indirectly, therefore, the circulation of the ocean-waters affects every geological process which is sensitive to climate.

=Differences in level and their results.=—While the surface of the ocean is the common datum plane to which elevations and depressions are referred, it is to be remembered that the sea has “a very complicated undulating surface in consequence of the attraction which the heterogeneous and elevated portions of the lithosphere exercise on the liquid hydrosphere. In the opinion of geodesists, the geoid may in some places depart from the figure of the spheroid by 1000 feet.”[146] These variations in level would, however, not occasion circulation. The differences in level which determine circulation are much more trivial. Every stream which pours fresh water into the sea tends to raise the level of the water where it enters. The waters brought to the ocean by the Amazon, the Mississippi, and other great rivers would appreciably change the level of the sea at their debouchures, if the excess did not promptly flow away. The ready mobility of the water, however, prevents its accumulation, and the discharge of every stream generates widespread movement. This movement is strongest at the debouchure, and weakens with increasing distance from it, though in the case of great streams, such as the Amazon, the movement is traceable, by means of the sediment which the water carries, hundreds of miles out to sea.

Changes of level are also brought about by the winds, which pile up water along the shore against which they blow. The level of the water is said to have risen 24 feet at Calcutta on October 5, 1864, as the result of a severe storm. While this is exceptional, a rise of 2 feet is not rare. This piling up of the waters along shore insures a compensating movement (undertow, littoral currents, etc.) in some other direction. Unequal evaporation and precipitation likewise disturb the level of the sea and occasion movement. In the open sea the movements generated by differences of level, like those generated by differences of density, are chiefly slow, creeping movements, but movements which never cease. In bays and gulfs, on the other hand, the surface of the water may be so raised, either as the result of wind, river discharge, or heavy precipitation, as to give rise to strong outward currents. There is little doubt at the present time that the Gulf Stream owes its origin primarily to the difference of level between the Gulf of Mexico and the Atlantic.[147]

=Movements generated by winds.=—The circulation resulting from the tendency of the winds to change the level of the sea-water has already been mentioned, but the wind also works in other ways. Where the winds have a somewhat constant direction and are at the same time strong, they determine a general movement of the surface-waters in their own direction, the surface-water being dragged along at a rate somewhat less than that of the wind itself. The constant trades appear to be the chief generators of the equatorial ocean-currents. Once generated, these currents may be concentrated and their courses modified. The currents generated by trades are turned north and south when directed against a continent; they are modified by the configuration of the bottom if the water be shallow, and always and everywhere, except, at the equator, they are deflected by the rotation of the earth, in the northern hemisphere to the right, and in the southern to the left. The pole-ward currents generated in the equatorial region by the trades, and directed by the winds, the lands, the configuration of the bottom, and the rotation of the earth, determine compensating currents from high latitudes to low, and the same influences which control the course of the former direct the latter as well.

Since the atmospheric movements are so far constant that there is a prevailing direction of winds in all latitudes, the winds, as well as differences of density and differences of level, insure a general and continual circulation of sea-water. The geological effects of this circulation are direct and indirect; direct, by gradation of the bottom over which they flow, and indirect, by the modifications of climate they produce. Since rotation deflects the pole-ward currents to the east sides of the oceans (west sides of the continents) and the equator-ward movements to the west sides of the oceans (east sides of the continents), the east shores of the oceans are warmer than the west in corresponding latitudes, and the west sides of the continents are both warmer and moister[148] than the east sides.

The most obvious disturbance of sea-water resulting from the winds is the generation of _waves_. Waves are not primarily parts of the general oceanic circulation. Since they are generated in other ways than by winds, and since the gradational effects of waves are independent of their origin, the effects of wind-waves will not be considered separately.

=Movements generated by attraction.=—One of the movements of the sea-water which is not primarily circulatory results from the attraction of the moon and sun. The tide is really the result of the _inequalities_ of the attraction of these bodies on different parts of the earth. The lunar tide is more important than the solar, not because the attraction of the moon is greater, for it is not, but because its differential attraction, the result of its lesser distance, is greater.

The distance of the moon from the earth is about 240,000 miles. If this be taken as the distance from the center of the moon to the center of the earth, 236,000 and 244,000 miles respectively are the distances from the center of the moon to the nearest and most distant points on the earth. The distance of the sun from the earth is about 93,000,000 miles. If this be taken as the distance between the centers of these bodies, then the distances from the center of the sun to the nearest and most distant points on the earth’s surface are 92,996,000 and 93,004,000 miles respectively. The ratio of 4000 to 236,000 or to 244,000 is much greater than the ratio of 4000 to 92,996,000 or to 93,004,000. Hence the tide-producing force of the moon is greater than that of the sun.

The tides show themselves along shores in the form of waves which, in shallow water, become translatory. They differ from the wind-waves in their periodicity, and locally in their greater height. The effects of the tidal waves on the shores of the sea, and on the bottom in shallow water, are the same as the effect of wind-waves of equal strength, and need not be separately considered in connection with the gradation of the sea-bottom. In passing through narrow straits or narrow passes of any sort, the tidal movement becomes a current which, under favorable conditions, abrades or “scours” the bottom effectively. The tidal currents in the narrow passes about New York harbor may serve as an illustration.

=Aperiodic movements.=—In addition to the wind-waves which are essentially constant and universal, and to the tidal waves, which are periodic, there are accidental waves which are locally and temporarily of importance. Such are earthquake-waves, which are sometimes extremely destructive. Thus an earthquake-wave on the coast of Peru in 1746 swept a frigate several miles inland and deluged Lima, seven miles from the shore. The havoc of most earthquakes affecting coasts, such as that of Lisbon in 1755, is greatly aggravated by accompanying sea-waves. Earthquake-waves differ from ordinary waves in being translatory, and so in being more effective on the bottom in deep water. Their greatest force, however, is felt in shallow water and on shores. Volcanic eruptions likewise give rise to exceptional aperiodic waves. The same is true of landslides where they affect the coast or any part of the sea-bottom. The fall of glacier ends and the capsizing of icebergs likewise generate strong waves. To the category of exceptional waves also belong those generated by the winds of exceptional storms, such as that which devastated Galveston in 1900.[149]

=Summary.=—From the point of view of their direct geological results in shallow water, all movements of the sea-water may be grouped into two main classes—(1) waves, with the undertow and the littoral currents they generate, and (2) ocean-currents.[150]

WAVES.

=Wave-motion.=[151]—The most common waves, and from the present point of view the most important, are those generated by winds. During the passage of a wave, each particle affected by it rises and falls, and moves forward and backward describing an orbit in a vertical plane. If the passing wave is a swell, the orbit of the particle is closed and is either a circle or an ellipse; but in the case of a wind-wave the orbit is not closed. In such a wave two things move forward, the undulation and the water. The velocity of the undulation is relatively rapid; that of the water, slow and rhythmic. On the crest of the wind-wave each particle of water moves forward, and in the trough it moves less rapidly backward, and the excess of the forward movement over the backward gives it a slight residual advance. This residual advance is the initiatory element of current. By virtue of it, the upper layer of water is carried forward with reference to the layer below, in the direction toward which the wind blows. The waves of any considerable or long-continued wind, therefore, generate a current tending in the same direction as the wind.

The agitation of which waves are the superficial manifestation is not restricted to the surface, but is propagated indefinitely downward. Near the surface the amount of motion diminishes rapidly with increasing depth (Fig. 299), but the rate of diminution itself diminishes, and there seems no theoretic reason for assigning any definite limit to the downward propagation of the oscillation.

At the surface, the radius of the circular orbit which a particle of water in a wave tends to describe is half the height of the wave. At a depth equal to one wave-length, the radius of the circle described by a particle is ¹⁄₅₃₅ as great as at the surface, and at a depth equal to two wave-lengths, ¹⁄₃₀₀₀₀₀. If the height of a wave be 43 feet, the radius of the circle described by a surface particle is 21½ feet. If the length of the wave be 300 feet, the radius of a particle at a depth of 300 feet is only about ⁴⁄₁₀ of an inch, and at 600 feet ¹⁄₁₂₀₀ of an inch.[152] These figures make it clear that effective agitation of the water does not extend to great depths.

So long as the velocity of the wind remains constant, the velocity of the current which the wind-waves generate is less than that of the wind, and there is always a differential movement of the water, each layer moving faster than the one beneath. The friction is thus distributed through the whole vertical column of the water in movement, and is even borne in part by the sea-bottom if the movement extends so far down. The greater the depth, the smaller the share of the friction each layer of water is called upon to bear, and the greater the velocity of the current generated by a given wind. But while the wave-motion extends indefinitely downward, the lower limit of agitation effective in erosion is soon reached. Engineering operations have shown that submarine structures are little disturbed at depths of five meters in the Mediterranean and eight meters in the Atlantic.[153] On the other hand, débris as coarse as gravel, which is transported by rolling on the bottom, is not infrequently carried out to depths of 50 feet, and sometimes even to 150 feet. Fine sediment, like silt, is disturbed at still greater depths, for ripple-marks, which indicate agitation of the water, are said to have been found at depths of 100 fathoms.[154]

When a wave approaches a shelving shore, its habit is changed. The velocity of the undulation is diminished, while the velocity of the advancing particle of water in the crest is increased; the wave-length, measured from trough to trough, is diminished, and the wave-height is increased; the crest becomes acute, with the front steeper than the back, and these changes culminate in the breaking of the crest, when the undulation proper ceases. Waves of a given height break in about the same depth of water, and the line along which incoming waves break is the line of _breakers_. The line of breakers is in deeper water and farther from shore when the waves are strong than when they are weak. Waves are reported to have broken in 100 fathoms of water,[155] but this must be regarded as very exceptional. The return of the water thrown forward in the crests of waves is accomplished by a current along the bottom called the _undertow_. The undertow is sensibly normal to the coast when uninfluenced by oblique waves, and is efficient in removing the products of erosion.

Since the incoming wave affects water which is at the same time under the influence of the undertow, it gives to that current a pulsating character, for the wave-motion sometimes supports and sometimes opposes the undertow, and thus endows it with a higher transporting power than belongs to its mean velocity. Near the breaker-line, the oscillations communicated by the wave may momentarily overcome and even reverse the movement of the undertow. Inside the breaker-line, irregular oscillation only is communicated. The broken wave-crest, dashing forward, overcomes the undertow and throws it back, and the water returns as a simple current descending a slope. The power of the undertow diminishes rapidly from the breaker-line outward as the depth of the water increases.

When waves advance on the shore obliquely, a shore-current is developed as illustrated by Fig. 300, where _ab_ represents the direction of the incoming wave, _bc_ the direction of the littoral current, and _bd_ the direction of the undertow. Where they strike the borders of land, the wind-waves, therefore, generate two other movements, the undertow and the littoral current. Any particle of water near shore may be affected by any two or by all three of these movements at the same moment. The effect of littoral current and undertow is to give a particle of water on which both are working a direction between the two, as _be_. The effect of other combinations can be readily inferred. These various combinations are of consequence in the transportation of débris.

WORK OF THE WAVES.

_Erosion._

The general effects of the waves and the other movements to which they give rise along shores are (1) the wear of the shores; (2) the transportation for greater or less distances of the products of wear; and (3) the deposition of the transported materials.

=By waves and undertow.=—In the dash of the waves against the shore, the chief wear is effected by the impact of the water and of the débris which the water carries. Lesser results are accomplished in other ways.

When the land at the margin of the water consists of unconsolidated material, or of fragmental material but slightly cemented, the impact of the water is sufficient to displace or erode it. If weak rock be associated with resistant rock within the zone of wave-work, the removal of the former may lead to the disruption and fall of the latter, especially when weak rock is washed out from beneath the strong. The impact of the water is competent also to break up and remove rock which was once resistant, but which has been superficially weakened by changes of temperature. Rock affected by numerous open joints is likewise attacked with success, for by the dash of the waves the blocks between the joints may be loosened and literally quarried out. It may, however, be doubted whether the dash of waves of clear water, even when their force is many tons to the square foot, has any appreciable power to wear rock which is thoroughly solid.

The impact of the waves is generally reinforced and made effective by the impact of the detritus they carry. The sand, the pebbles, and such stones as the waves can move are used as weapons of attack, being turned against one another and against the shore. Masses of rock too large for the waves to move (Fig. 301) are worn by the detritus

driven back and forth over them, and in time reduced to movable dimensions (Fig. 302). They then become the tools of the waves, and in use, are reduced to smaller and smaller size. Thus bowlders are reduced to cobbles, cobbles to pebbles, pebbles to sand, and sand to silt. The silt is readily held in suspension in agitated water, and thus is carried out beyond the range of breakers, and settles in water so deep as not to be effectively agitated to its bottom. Thus one generation of bowlders after another is worn out, and the comminuted products are carried out from the immediate shore and deposited in deeper water.

The effectiveness of waves, whether they work by impact of water alone, or by impact of water and detritus, is dependent on their strength and on the concentration of their blows.[156] The strength of waves is dependent on the strength of the winds (or other generating cause) and the depth and expanse of the water, and the concentration of their blows is conditioned by the slope against which they break. On exposed ocean-coasts the fetch of the waves is always great. The winds are variable. For a given coast they have an average strength, but the effectiveness of wave-erosion is determined less by the average strength of waves than by the strength of the storm-waves. This is often very great. On the Atlantic and North Sea coasts of Britain, winter breakers which exert a pressure of three tons per square foot are not infrequent.[157] So great is the force of exceptional storm-waves that blocks of rock exceeding 100 tons in weight are known to have been moved by them. Ground-swells, “even when no wind is blowing, often cover the cliffs of north Scotland with sheets of water and foam up to heights of 100 or even nearly 200 feet. During northeasterly gales the windows of the Dunnet Head lighthouse, at a height of upwards of 300 feet above high-water mark, are said to be sometimes broken by stones swept up the cliffs by sheets of sea-water.”[158] The average force of waves on the Atlantic coast of Britain has been found to be 611 lbs. per square foot in summer, and 2086 lbs. in winter.[159]

Where deep water extends up to the shore, the force of the wave is almost wholly expended near the water line; where shallow water borders the land, the force of the waves is expended over a greater area. Waves are, therefore, most efficient on bold coasts bordered by broad expanses of deep water.

The less familiar phases of wave-work are accomplished by hydraulic pressure, compressed air, the use of ice, etc. When the water of a wave is driven into an open joint or a cave, the hydraulic pressure is great, and if the structure be weak, the rock may be broken. When water is driven with force into a cave, the compression of the air may be great if the wave be high enough to close the entrance. When the water runs out of a cave, the air within may be greatly rarefied, while that above exerts its normal pressure. In either case the roof of the cave, if it be weak, may be broken. At certain seasons of the year, especially during the spring, waves make destructive use of the ice which is then breaking up, but it is only in high latitudes that sea-ice is of consequence in this way. In general, the effect of its presence in keeping down waves overbalances its effect as an agent of erosion.

The direct effect of wave-erosion is restricted to a zone which is narrow both horizontally and vertically. There is no impact of breakers at levels lower than the troughs of the waves, though erosion may extend down to the limit of effective agitation (p. 341). The efficient impact of waves is limited upward by the level of the wave-crests, although the dash of the water produces feebler blows at higher levels. The rise and fall of the water during the flow and ebb of the tides gives the waves a greater vertical range than wind-waves alone would have. The vertical zone of direct wave-work is therefore limited above by the level of wave-crests, and below by the depth of wave-troughs (nearly). The indirect work of waves is limited only by the height of the shore, for as the zone of excavation is carried landward, masses higher up the slope are undermined and fall. The fallen rock temporarily protects the shore against the waves, but are themselves eventually broken up.

The pulsating current of the undertow (p. 341) has both an erosive and a transporting function. It carries the detritus of the shore to and fro, and dragging it over the bottom, continues downward the erosion initiated by the breakers. This downward erosion is the necessary concomitant of the shoreward progress of wave-erosion; for, if the land were merely planed away to the level of the wave-troughs, the incoming waves would break where shoal water was first reached, and become ineffective at the water margin. The rate of erosion by the undertow becomes less and less as the surface it affects is lowered. Littoral currents do little erosive work beyond that inflicted on the material which they transport.

The general result of wave-erosion is the advance of the sea on the land, the rate of advance being determined chiefly by the nature of the material attacked and the strength of the waves. Numerous as examples are of the retreat of coast-lines before the advance of the sea, it is not to be understood that the advance of the sea on the land is universal or uninterrupted. Numerous instances may be cited of the encroachment of the land on the sea. At Long Branch the advance of the sea, in spite of elaborate breakwaters, has been so rapid in recent years as to menace important buildings, while a few miles to the north and south, the land is advancing in the face of the waves. The low coast of the Middle Netherlands has retreated two miles or more in historic times,[160] but the opposite tendency is shown at other points in the same region. On the coast of England the sites of villages have disappeared by the advance of the sea within historic times,[161] but the coast of the same island affords illustrations of land advance. On the south side of Nantucket island, the sea-cliff has been known to retreat before the waves as much as six feet in a single year.[162] Almost every considerable stretch of coast affords illustrations both of the advance of the sea on the land and of land on the sea; but in the long run, the former must exceed the latter, diastrophic movements aside.

_Topographic Features Developed by Wave-erosion._

=The sea-cliff.=—The action of the waves, cutting as they do along a definite horizontal zone, has been compared to the action of a horizontal saw. As the waves cut into the shore at and near the water-level, the material above, being unsupported, falls, leaving a steep face above the line of cutting. This steep face is known as _the sea-cliff_ (Figs. 301 to 306). The same term is sometimes applied to the cliffs of lakes. The principles involved in the development of the sea-cliff are applicable to any broad stretch of water.

The height of the cliff depends on the height of the land on which the sea is advancing. Its slope may be steep or gentle (compare Figs. 303 to 306), according to the nature of the material of which it is composed and the rapidity of the cutting. Rapid cutting tends to produce steep cliffs and slow cutting gentle ones, for in the latter case weathering is more important relative to the cutting, and at sea-level (low altitudes) weathering generally tends to reduce the angle of slope. In general, the more resistant the material the steeper the slope of the cliff. Incoherent materials, such as sand and clay, are not likely to form steep cliffs; but if the cutting be very rapid, bold faces may be developed even in such materials (Fig. 307). If beds of slight resistance at sea-level underlie beds of greater resistance, the development of steep cliffs is favored. The structure of the cliff-rock also has an influence on the slope. The rock may be massive or bedded. If bedded, the beds may be horizontal, or they may dip at any angle, in any direction. The rock, whether stratified or not, may be abundantly or sparsely jointed. All these structures influence the slope and configuration of the sea-cliff (see Figs. 305 to 308).

=Chimney-rocks, etc.=—By working in along the joints of the rock, widening them and quarrying out the intervening blocks, pillars of rock (“chimney-rocks,” “pulpit-rocks”) or even considerable islets are sometimes isolated by the waves. This is most readily accomplished where the joints converge back from the shore. A well-known example of this sort is the “Old Man of Hoy” (Fig. 309) on the coast of the Orkneys. A pulpit-rock or other island, or any jutting point of rock may be pierced, giving an arch or bridge. La Roche Percée, a steep-faced isle near Gaspé Harbor, is an example.

=Sea-caves.=—Waves sometimes excavate caves at the bases of cliffs. This is especially likely to occur where the rock is much jointed and where the joints are not continued to the surface in a single plane. The bottom and roof of a sea-cave usually have a pronounced inclination landward. If the cliff be low, the cave may be extended landward until its roof is pierced. Through such an opening in the top of the cliff the water of the incoming waves may be forced in the form of spray. On the New England coast such holes are sometimes known as “spouting horns.” Similar openings may be made, as already pointed out, by the compression or rarefaction of the air in the cave as the wave enters or retreats. If the roof of the cave be partially destroyed, the portion which remains may form an arch or _bridge_. Such a bridge occurs on Santa Cruz Island, California (Fig. 310).

The cave, the “spouting horn,” the “bridge,” the “pulpit-rock,” and other isolated islets, are all closely associated with the sea-cliff in origin.

=The wave-cut terrace.=—The bottom of the sea-cliff is bordered by a submerged platform over which the water is shallow. This platform, or at any rate its landward portion, represents the area over which the water has advanced as the result of wave-cutting, and is, therefore, known as the _wave-cut terrace_. From the method of cliff development it will be seen that the wave-cut terrace is its necessary accompaniment. Such a terrace has a gentle slope to seaward, for its outer and older edge has been degraded longer and more. Its slope is influenced by the strength of the waves, being greater where they are stronger. The outer edge of the wave-cut terrace is often marked by an abrupt descent. Fig. 303 represents the wave-cut terrace in its relation to the sea-cliff above.

So long as wave-cut terraces are submerged, they do not appear on topographic maps of the land, though they appear on the charts of the coasts; but if a coastal tract with wave-cut terraces be elevated, or if the sea-level be drawn down, the terraces become land. Elevated sea-cliffs and wave-cut terraces are among the best evidences of change of relative level between water and land (Fig. 311).

=Wave-erosion and horizontal configuration.=—The structure of the rock along shore has as much to do with the horizontal configuration of the wave-shaped coast, as with its relief. In general, waves develop reëntrants in the less resistant portions of the shore, leaving the more resistant parts as headlands (San Pedro Point and Devil’s Slide, Pl. XX, Coast of California). It is to be noted that the resistance of rock to wave-erosion is not determined by its hardness alone. Every division plane, whether due to bedding, to jointing, or to irregular fracture, is a source of weakness to the rock, and rock of great hardness may be so broken as to offer relatively little resistance. Inequalities of resistance, whatever their cause, give origin to inequalities of coastal configuration where wave-erosion is in progress. Given a coast of marked regularity and equal exposure, but composed of unequally resistant material, the waves will make it irregular by cutting most where the material is least resistant. A regular coast of uniform material, but unequal exposure, will be made irregular by the greater cutting at the points of greater exposure. A coast of marked irregularity and homogeneous material will be made more regular by the cutting off of the projecting points, because they are most exposed. With a given set of conditions, waves tend to develop a certain sort of shore-line which, so far as its horizontal form is concerned, is relatively stable. Such a shore-line may be said to be _mature_[163] so far as wave-erosion is concerned. Since coastal lands are, in general, both heterogeneous and unequally exposed, a mature coast-line is somewhat irregular. Its maturity is attained when the lesser exposure in the reëntrants developed in the less resistant parts, balances the superior exposure of the projections of the more resistant portions.

Since the conditions of erosion along coasts are constantly, even if slowly, changing, maturity is constantly being approached, but rarely reached. Other forces and processes, such as those of aggradation, vulcanism, and diastrophism, are in operation along coasts, and their results are sometimes antagonistic to those of the waves. The horizontal configuration of coasts is, therefore, the result of many coöperating forces, of which waves are but one. It is, nevertheless, important to note the goal to which the waves are working, even though they are continually defeated in their attempt to reach it. Their immediate goal is an equilibrium of erosion-rate and maturity of configuration; their final goal is the destruction of the land and the deposition of its substance in the sea, that is, in a position nearer the center of gravity of the earth.

_Transportation by Waves._

The material eroded from the shore by the waves in the shaping of the cliff and terrace is carried away by the joint action of the waves, undertow, and shore-currents.