Chapter VI: The Work of the Ocean (2)

The in-coming wave begins to shift material where it begins to drag bottom, that is, a little outside the line of breakers. From the line where transportation begins, to the line of breakers, bottom detritus is shifted shoreward by the waves, while the undertow tends to carry it back again. Between the breakers and the shore there is also a tendency for the on-shore movement to carry débris to the water’s edge, and for the ebbing wave to carry it back again. The result of these opposed tendencies is to keep sediment in transit between the shore and the line of breakers. If the in-coming waves have a direction normal to the shore, the advance and recoil of the water move particles toward and from the shore, but effect no transfer along the shore; but the results which waves normal to the shore would achieve are always modified by other waves and by littoral currents.

If the in-coming wave is oblique to the shore, it shifts material in its own direction. The transfer by undertow, taken alone, would be sensibly normal to the shore, but the effect of the oblique waves is to slightly modify this direction. There is thus a slow transportation along shore, even in the absence of steady currents. A great amount of transportation would be effected in this way, though it would be carried on at a slow rate. Oblique waves also tend to develop a definite shore-current (p. 342) which affects both the amount and direction of the transportation. Any particle in suspension, or in motion on the bottom as the result of the wave or undertow, is shifted along shore by the littoral current, which affects the same water (Fig. 300). By the coöperation of wave- and shore-current, more and heavier material can be moved than by either alone, and the direction of movement is more nearly parallel to the shore than that of the wave. Similarly, by the coöperation of undertow and shore-current, more and heavier material can be moved than by either alone. The direction of movement is readily inferred from Fig. 300. The direction in which débris is shifted by wave- and shore-current is modified by the undertow, and the direction which would result from undertow and current is modified by the wave. It is often the waves of storms, rather than those of the prevailing winds, which determine the direction of greatest shore transportation.

The waves, the undertow, and the littoral currents work together in assorting the detritus of the shore. The coarsest parts may be beyond the power of all but the strongest waves. They accumulate where agitation is great. Less coarse parts are shifted farther from the site of greatest agitation, but no materials which are classed as coarse are carried beyond the depth of sensible movement. The coarse material which covers the bottom where the agitation of the water at the bottom is effective, constitutes _shore drift_.

Shore drift is not all derived from the shore by the cutting of the waves. A part of it is brought to the sea by streams and mingled with that eroded from the cliffs. The material which is fine enough to be held in suspension is measurably independent of depth. This is shown during storms when the water becomes turbid far beyond the line of breakers, and clears only after the waves have died away.

This sorting of shore drift, effected while it is in transportation, is often very perfect. The conditions favoring assortment are (1) vigorous wave-action, (2) prolonged transportation, and (3) a moderate volume of sediment.[164] The effect of these several conditions will be readily understood.

Extensive transportation of shore drift of a given degree of coarseness is favored by (1) strong waves and undertow, (2) continuous currents, and (3) shallow water, deepening but gradually off shore.

_Deposition by Waves, Undertow, and Shore-currents._[165]

=The beach.=—The zone occupied by the shore drift in transit is the _beach_. The lower margin is beneath the water, a little beyond the line where the great storm-waves break. Its upper margin is at the level reached by storm-waves, and is usually a few feet above the level of still water. To the beach, material is brought from seaward by the in-coming waves, and from it detritus is carried out by the undertow. The cross-section of a beach is shown in Fig. 312. In horizontal position the beach follows the general boundary between water and land, though it does not conform to its minor irregularities (Fig. 313). The beach or barrier ridge often causes the deflection of the lower courses of streams descending to it (Pl. XXI).

NEW JERSEY
U. S. Geol. Surv.]

Fig. 1. PORTION OF SOUTH COAST OF MARTHAS VINEYARD, MASSACHUSETTS.
U. S. Geol. Surv.

Fig. 2. PORTION OF THE CALIFORNIA COAST NEAR TAMALPAIS.
U S. Geol. Surv.]

=The barrier.=—When the agitation of the water along shore becomes insufficient to carry the material, it is dropped. In its deposition it assumes various forms. Where the bottom of the lake or sea near shore has a very gentle inclination, the in-coming waves break some distance from the shore-line, and it is here that the most violent agitation occurs when the waves are strong. To this line of breakers, material is shifted from both directions: from shore by undertow, and from seaward by the waves. Accumulating here, it builds up a low ridge. This is a _barrier_ (Fig. 314). If it is built up above the surface of the water by storm-waves, it may shut in a lagoon behind it, and this may ultimately be filled by sediment washed down from the land. At one stage in the filling, the lagoon becomes a marsh.[166] In the part which the barrier plays in the history of a coast, it is identical with the beach.

=The spit, the bar, and the loop.=—The disposition of shore-deposits depends largely on the currents at and near shore. If the coast-line is deeply indented, the littoral current usually fails to follow the reëntrants. In holding its course across the mouth of a small bay, a shore-current usually passes into deeper water. Here its velocity is checked because its motion is communicated to the water beneath it, and a larger amount of water being involved in the motion, the motion of each part is diminished. If sediment was being moved along its bottom before the current was checked, some part of it is dropped when and where the current is slackened. It follows that deposition commonly takes place beneath a littoral current as it crosses the mouth of a bay. The belt of deposition is often narrow, and the result is the construction of a ridge beneath the water in the direction of the current. The current would never build the embankment up to the water-level, but when its surface approaches the level of effective agitation, the waves may begin to work on it, as on a barrier, and may build it up to, and even above, the surface of the water. So long as the end of such an embankment is free, it is a _spit_ (Fig. 315 and Pl. XXI). If the spit be lengthened until it crosses, or nearly crosses, the bay, shutting it off from the open water, it becomes a bar. Bars have shut in lakes (ponds) on the coast of Martha’s Vineyard, Mass. (Fig. 1, Pl. XXII), and lakes and lagoons at numerous points both on the Atlantic and the Pacific coasts (Fig. 2, Pl. XXII, Rodeo lagoon). The same phenomena are to be seen along many lake shores. Bars sometimes tie islands to the mainland (Pl. XXIII, Fig. 1, Nahant, Mass.; Fig. 2, near Biddeford, Me.). The structure of a bar as seen in cross-section is shown in Fig. 316.

The construction of a spit has been aptly compared to the construction of a railway embankment across a depression. The material is first carried out from the bordering upland (shallow water) and dumped where the slope to the depression (deep water) begins. The embankment thus begun is extended by the carrying out of new material, which is left at the end of the dump already made.

If the bay across which the bar is built receives abundant drainage from the land, the outflow from the bay may be sufficient to prevent the completion of the bar (Fig. 2, Pl. XXII), for when the growth of the spit has sufficiently narrowed the outlet of the bay, the sediment brought to the end of the spit by the littoral current will be swept out beyond the spit by the current setting out from the bay.

The completion of a bar may be interfered with by tidal currents, even without land-drainage. Currents generated by the tides may sweep in or out of the bay with increased force as the entrance is narrowed, carrying in or out the sediment which the littoral current would have left at the end of the spit. The scour of the tides often insures deep entrances (inlets) to bays, and maintains definite channels or “thorofares” in the lagoon marshes behind barriers and spits. The sediment brought down from the land, as well as that washed in by tidal currents and waves, tends to fill up the lagoon behind a barrier, a spit, or a bar, converting it into land (Fig. 317).

Since spits and bars are built only where there is shore-drift in transit, they are always built out from a beach or barrier. The distal end of the bar may also join a beach or barrier. Traced back to its source, the beach from which a spit leads out is often found to terminate in the cliff from which the material of the beach and the spit were derived (Pl. XX and Fig. 2, Pl. XXII). In such cases the sediment of the beach has been shifted but a short distance; but in other cases it has traveled far.

The spit is usually either straight or in conformity with the general course of the shore-current, but since the littoral current itself is subject to alteration as the result of shifting winds, the spit may depart from straightness. Winds which simply reverse the direction of the littoral current retard its construction, but may not otherwise affect it; but if a strong current be made to flow past the end of a spit, it may cut away its extremity and rebuild the materials into a smaller spit, joining the main one at an angle. This gives rise to a _hook_ (Fig. 315). Successive storms may develop successive hooks along the side of a growing spit. The end of a hook may be so extended as to join the mainland, when it becomes a _loop_.

=Wave-built terraces.=—Under the influence of off-shore currents, littoral currents may be drawn from the coast-line. If such a current continues as a well-defined surface-current, it builds a spit, but if it spreads, it tends to build a terrace. The accumulation then is not at the end of a beach, as in the case of a spit, but on its side, and the result of the deposition is to carry the beach seaward. The undertow abets the process. The widened beach is a _wave-built terrace_. The wave-built terrace often borders the wave-cut terrace along its seaward margin (Figs. 303 and 318). With the help of waves, the surface of the terrace may be built up into land by the expansion of the crest of the beach. Terrace-cutting and terrace-building are both involved in the development of the continental shelves.

Beach ridges, spits, bars, etc., like sea-cliffs and wave-cut terraces, are often preserved after the relative level of sea and land has changed. If the shore has risen, relatively or absolutely, these features are relied on as evidences of the change. If shore features be submerged instead of elevated, they furnish less accessible, though not less real, evidence of the change of level. Similar features about lakes have a like significance, but in this case it is often demonstrable that it is the water rather than the land which has changed its level.

_Effect of Shore-deposition on Coastal Configuration._

The tendency of shore-deposition is to cut off bays and to straighten and simplify the shore-lines. This is abundantly illustrated along the Atlantic and Gulf coasts of the United States (see Fig. 319 and Pl. XXII). It is to be noted, however, that in the simplification of the shore-line through deposition, the initial stages often result in great irregularity (Fig. 320 and Pl. XXIII). In some cases, the irregularities are not temporary. Thus deltas (p. 198), though not wholly the work of sea- (or lake-) water, often constitute irregularities of a more or less permanent nature. This is the case where they project beyond the general trend of the coast-line. Where, on the other hand, they are built at the heads of bays, they tend to simplify the coast-line by obliterating the indentation. The delta at the head of the Gulf of California is an example. So too is the delta of the Mississippi, the real head of which is far above the present debouchure of the stream. The form of the delta in ground-plan depends on the horizontal configuration of the coast where it is developed, on the strength of the waves and shore-currents, and on their relation to the amount of detritus contributed by the stream concerned. Good illustrations are furnished by the Gulf of Mexico where the deltas of the Mississippi and Rio Grande are in contrast.

So far as concerns the vertical configuration of coasts, erosion and deposition are in contrast, for while the former tends to develop steep, irregular, and often high slopes (p. 349) from the land to the sea, the latter tends to develop gentle, regular, and low ones. A partial exception to the latter part of this general statement comes about through the building of dunes, the material for which is furnished by the waves.

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.

Fig. 1. MASSACHUSETTS.
U. S. Geol. Surv.

Fig. 2. MAINE.
U. S. Geol. Surv.]

PORTION OF THE COAST OF MAINE.
U. S. Geol. Surv.]

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.

Deposition by ocean-currents is limited by their transportation. Only where they erode their bottoms do they gather coarse materials, and only in the lee of such places are their deposits coarse. Since the material which they carry is generally fine, it is widely distributed before deposition.

Ocean-currents have little influence on the configuration of coast-lines.

DEPOSITS ON THE OCEAN-BED.

Something has already been said concerning the sediments which accumulate in the shallow waters along shores; but the area of marine sedimentation is as extensive as the ocean itself, and the deposits must now be reviewed from another point of view.

Oceanic deposits may be conveniently divided into two chief groups, dependent on the depth of the water in which they are made.[170] These groups are (1) _shallow-water deposits_, made in water less than some such depth as 100 fathoms, and (2) _deep-sea deposits_, laid down in water of greater depth. The selection of the 100-fathom line as the dividing depth is less arbitrary than it seems, for passing outward from the shore, it is at about this depth that the bottom ceases to be commonly disturbed by the action of currents and waves; that sunlight and vegetable life cease to be important at the bottom; and that the coarser sediments which predominate along shore give place, as a rule, to muds and oozes. Furthermore, the 100-fathom line (or some line very near it) is an important one in the physical relief of the globe, for it appears to mark, approximately, the junction of continental plateaus and ocean-basins. Only because the latter are a little over-full does the water run over their rims, covering about 10,000,000 square miles of the borders of the continents, converting them from land into epicontinental seas.

Aside from the deposits made by organisms, shallow-water deposits are divisible into two groups—(_a_) those immediately along the shore, the _littoral deposits_, and (_b_) those made between the littoral zone and the 100-fathom line. Both are terrigenous. The deep-sea deposits likewise are divisible into two groups, (_a_) _terrigenous deposits_ formed close to land, and made up chiefly of materials derived immediately from the disintegration of land formations; and (_b_) the _pelagic deposits_, made up chiefly of the remains of pelagic organisms and the ultimate products arising from the decomposition of rocks and minerals. The former predominate in the less deep waters relatively near shore; the latter in the deeper water far from land. The shallow- and deep-water deposits grade into each other in a belt along the 100-fathom line.

_Shallow-water Deposits._

=Littoral deposits.=—The littoral zone is the zone between high- and low-water marks. It is the zone in which bowlders, gravels, sands, and all coarser materials accumulate, though muds are occasionally met with in sheltered estuaries. Generally speaking, the nature of these deposits is determined by the character of the adjoining lands and the nature of the local organisms. “The heavier materials brought by rivers from high terrestrial regions, or thrown up by the tides and waves of the sea, are here arranged with great diversity of stratification through the alternate play of the winds and waves. Twice in the twenty-four hours the littoral zone is covered by water and exposed to the direct rays of the sun or the cooling effects of the night. There is a great range of temperature; mechanical agencies produce their maximum effects,”[171] and physical conditions in general are most varied. Still greater diversity is introduced by the fact that the zone is inhabited by both marine and terrestrial organisms, while the evaporation of the sea-water which flows over tidal marshes and lagoons leads to the formation of saline deposits. If the length of the coast-lines of the world be taken at 125,000 miles (about 200,000 kilometers), and the average width of this zone at half a mile, these deposits are now forming over an area of 62,500 square miles (about 160,000 square kilometers) of the earth’s surface.

=Non-littoral, mechanical deposits in shallow water.=—These deposits are laid down in the zone of the ocean between low-water mark and the 100-fathom line. They cover about 10,000,000 square miles.[172] Their composition is much the same as that of the littoral deposits, with which they are continuous, though on the whole they are finer. At their lower limit they pass insensibly into the fine deposits of the deep sea. Coarse material, such as gravel and sand, prevails, though in special situations, such as depressions and inclosed basins, muddy deposits are found. While some of the deposits are wholly composed of inorganic débris, organic remains are freely mingled with others. The mechanical effects of tides, currents, and waves are everywhere present, but become less and less well marked as the 100-fathom line is approached. The forms of vegetable and animal life are numerous, though the former decrease as depths which exclude the sunlight are approached.

Both littoral deposits and deposits in shallow water outside the littoral zone have already been referred to in connection with the work of waves and currents (pp. 355–66). A few additional points only need here be added.

In general the coarser sediments are lodged near shore and those farther from the land become progressively finer. Even the coarser part of the material carried in suspension by the undertow is partly left in the shallow water. On the other hand, waves of exceptional strength may carry coarse material into water of some depth. Thus coarse shingle (gravel) and even bowlders have been found at depths of 10 fathoms.[173] Coarse deposits may extend far out from land if the waves are strong, and especially if the water is shallow, and since the zone of shallow water may be extended seaward by the aggradation of the bottom, shallow-water deposits may cover extensive areas. They may become deep at the same time, for as the outer border of the shallow-water zone is shifted seaward by aggradation, the vertical space to be filled becomes greater (compare Figs. 321 and 322). Again, if the coast be sinking, new deposits of coarse material may be made on older ones. In this way also great thicknesses of sediment may be accumulated, all parts of which were deposited in shallow water. The great thickness of some of the conglomerate beds of the past shows how far this process may go.

As a rule, no definite line marks the seaward terminus of the coarse detritus, since coarse material is carried farther out when the waves run high (and the undertow is strong) than when they are feeble. In calm weather, therefore, fine sediment may be deposited where coarse had been laid down in the preceding storm, only to be covered in turn by other deposits of a different character. Thus gravel grades off into sand, with more or less overlapping or interwedging, and sand grades off into silt in the same way. This is diagrammatically illustrated by Fig. 323.

=Characteristics of shallow-water deposits.=—Clastic sediments laid down in shallow water have several distinctive characteristics. While they are, in the aggregate, coarse, they are characterized by frequent variations in coarseness. The surfaces of successive beds are likely to be ripple- and rill-marked (Figs. 324, 325, 326), and cross-bedding (Fig. 327) is of common occurrence. Clayey sediments accumulated between high and low water are often sun-cracked (Fig. 328), and the tracks of land animals are sometimes preserved on their surfaces. Shallow-water deposits often contain fossils of organisms which live in waters of slight depth. These characteristics are sufficient to differentiate sedimentary formations made in shallow water from those made in deep water, even after they have been converted into solid rock and after the rock has emerged from the sea. Many of these characteristics are, however, shared by deposits made by streams on the land. Subaërial and lacustrine sediments are usually distinguishable from those made in the sea by their fossils, and sometimes by their distribution.

=Topography of shallow-water deposits.=—The shallow-water deposits have, on the whole, a rather plane surface, though there are some notable departures from flatness. The steep slopes of the delta fronts and of wave-built terraces have already been spoken of. Barriers often shut in depressions, and the disposition of the material deposited is sometimes uneven, owing to shore and tidal currents. The result is that the surface of the shallow-water deposits is often affected by low elevations and by shallow depressions. The elevations and depressions may be elongate, circular, or irregular in form. These general facts are shown in Figs. 319, 320, and 329. This topography is sometimes preserved on newly emerged lands, as at various points on the Coastal Plain of the United States.

=Chemical and organic deposits.=—There is no sharp line of distinction between the deposits usually classed as chemical and those regarded as organic. The latter are chemical in the broader sense of the term, but as they are immediately associated with life and are dependent upon it, it is a matter of practical convenience to separate them. Aside from the organic deposits, the chemical deposits made in shallow sea-water embrace (1) those due to reactions between constituents so brought together that new and insoluble compounds are formed and precipitated, and (2) those due to evaporation. The points of saturation for the various substances dissolved in sea-water are reached at different stages, and hence they are deposited more or less in succession.

The chemical deposits made in the shallow water of the sea, or in shallow bodies of water isolated from the sea, are chiefly simple precipitates resulting from evaporation; but new combinations are sometimes made in the process of concentration and precipitation. All substances in solution are necessarily precipitated on complete evaporation, but since the sea-water is in general far from saturation, so far as all its leading salts are concerned, only a few are thrown down in quantity sufficient to have geological importance where evaporation is incomplete. The leading deposits are lime carbonate (CaCO₃), lime sulphate (gypsum, CaSO₄,2H₂O), common salt (rock-salt, NaCl), and the magnesium salts, usually the chlorides and sulphates, which are later changed to carbonates. In investigations on Mediterranean water[174] which had an initial density of 1.02, no deposit took place until concentration by evaporation had brought the water to a specific gravity of 1.05. Between this density and that of 1.13, lime carbonate and some iron oxide were deposited. Between 1.13 and 1.22, lime sulphate was the most abundant precipitate, while between 1.22 and 1.31, 95% of the deposit was common salt. With still further concentration, the remaining substances in solution, especially the magnesium salts, were thrown down.

While there is somewhat more than ten times as much lime sulphate as lime carbonate in the ocean (p. 324), the deposits of the carbonate (including the organic) have been very much greater than those of the sulphate. This is due partly to the fact that the sulphate is much more soluble in natural waters than the carbonate. Rivers bring much more carbonate than sulphate to the sea, so that the point of saturation for the sulphate would normally be reached much later than that of the carbonate. The more important fact, however, is that marine plants and animals use lime carbonate freely for skeletal and housing purposes. It is held by some that they get their lime from the sulphate, but if so they convert it into carbonate before it takes the form of shells, coral, etc., the sulphuric acid set free in the process reproducing, directly or indirectly, more sulphate. The secretion of lime carbonate by organisms is not dependent on the saturation of the water, but may be carried on when the amount in solution is very small.

There can be little doubt that the chief deposits of lime carbonate have been and are being made through the agency of plants and animals in the form of shells, coral, bones, teeth, and other devices for supporting, stiffening, housing, protecting, and arming themselves; but while it is agreed that the larger part of the lime carbonate deposited in the open sea is of organic origin, it is equally clear that in closed seas subject to concentration from evaporation, simple precipitation takes place freely. There is some difference of opinion as to the importance of these two classes of deposits, past and present. The debated point is whether simple precipitation takes place in any appreciable degree under the usual oceanic conditions. There is much more evidence of solution by sea-water than of precipitation from it. The ocean appears to be under-saturated with lime carbonate on the whole, though it is still possible that deposition may take place in favorable situations, as, for example, where the very calcareous waters of rivers are spread out in thin sheets on the surface of the heavier salt water, and thus exposed to exceptional evaporation, or where there is very exceptional agitation and aëration.[175]

Gypsum appears to be deposited in quantity only in the closed basins of arid regions where concentration reaches an advanced state.

Since normal sea-water is far from saturation with common salt, the latter is precipitated only in lagoons, closed seas, or other situations favorable to great concentration. This is usually achieved only in notably arid regions, and in basins that receive little or no drainage from the land.

Deposits of salt usually, therefore, signify highly arid conditions, and where they occur over wide ranges in latitude and longitude, as in certain periods of the past, unusual aridity is inferred. Where confined to limited areas, their climatic significance is less, for topographic conditions may determine local aridity. The total area where salt is now being precipitated is small, though on the whole the present is probably to be regarded as a rather arid period of the earth’s history. On the other hand, ancient deposits of salt preserved in the sedimentary strata show that the area of salt deposition has been much more considerable than now at one time and another in the earth’s history. The salt and gypsum deposits of the past seem, therefore, to tell an interesting tale of the climates of the past.

The magnesium salts are among the last to be thrown down as the sea-water is evaporated, and they most commonly take the form of sulphates and chlorides. They often form double salts with potassium, a relatively small and soluble constituent of sea-water. In the artificial evaporation of salt water to obtain common salt, the process is usually stopped before the saturation-point for the magnesium salts is reached, and the residue, the “mother-liquor,” or “bittern,” is drawn off to prevent these “bitter” salts from mixing with the common salt. The magnesium salts are among the last to be precipitated, not only because they are readily soluble, but because their quantity is small; yet in the original rock from which all the sea-salts came, there is at least as much magnesium as sodium, while in the sea there is about five times as much sodium as magnesium. Just what becomes of the remaining magnesium is not yet well understood. It has a notable disposition to form double salts with some other constituent, as noted above. In the earlier marine strata, dolomite, that is, limestone composed partly or wholly of the double carbonate of lime and magnesia, (CaMg)CO₃, abounds. This appears to have been formed by a gradual substitution of molecules of magnesium for those of calcium, but just how and when and why it was done has not been fully worked out. It appears to be a case where the saline matter of the sea made its contribution to the sedimentary deposits by chemical reaction upon them, rather than by precipitation because of saturation.

The relatively small amount of potash in the sea-water is probably due to its disposition to remain united with the clays and earths of the mantle rock and of the shaley deposits.

To some extent the salts in solution act directly on the earthy matter brought down into the sea by rivers, but where sedimentation is rapid, as it often is in shallow water, this action is limited and obscure. In the main, the ocean-waters protect the sediments from weathering and similar changes, except as organic matter buried with them induces change.

While the lime deposits are by far the greatest of the chemical and organic deposits of the sea, plants and animals also secrete notable quantities of silica. Silica deposits of organic origin are relatively much more important in the deep sea than in shallow water, and will be mentioned in that connection.

=Limestone.=—Something concerning the origin of limestone has already been given in the preceding paragraphs, but because of the importance of this formation, it may be added by way of summary that shallow seas free, or nearly free, from terrigenous sediment, and abounding in lime-secreting life, furnish the conditions for nearly pure deposits of limestone, and that most of the limestone within the areas of the present continents appears to have originated under such conditions. The common notion that limestone is normally a deep-water formation is a serious error. Although limestones are formed in deep as well as in shallow waters, by far the more important classes of lime-secreting organisms are photobathic, i.e. are limited to the depths to which light penetrates. In the shallow waters, these plants and animals are in part free and in part attached. Within the areas of deep water they are free and at the surface, and their remains drop to the bottom, if not sooner dissolved. But few forms live on the deep, dark, cold bottoms of abysmal depths. Clear waters, free from abundant terrigenous sediments and abounding in lime-secreting life, rather than deep waters, are, therefore, the most favorable conditions for the origin of limestone.

The purely chemical deposits of limestone are probably all of shallow-water origin. Once made, they are subject to solution, redeposition, and other mutations like other deposits. As a result, they often lose many of their original characteristics, but enough usually remain to tell the story of their origin.

_Deep-sea Deposits._

=Contrasted with shallow-water deposits.=—The deep-sea deposits cover the ocean-bottom below the 100-fathom line. Their area is considerably more than half the earth’s surface. The characteristic deposits are muds, organic oozes, and clays, which in their physical characteristics are remarkably uniform. In regions of floating ice, greater diversity is introduced from the varied nature of the materials which the ice transports, but gravels and sands, comparable to those of shallow water, are rarely found. “Tides, currents, and waves produce some mechanical effects at the upper limits of the deep-sea region, but on the whole there is an absence of the phenomena of erosion, and mechanical action would appear to be absent except in the case of submarine eruptions. The depth is too great for sunlight to penetrate, and vegetable life is limited to the upper zone. Animal life is present in the same zone and on the bottom, but absent or nearly so in the middle depths. The temperature (at the bottom) is below 40° Fahr. throughout the larger part of the area, and if subject to variation with latitude or change of season, these changes affect only the depths immediately beyond the 100-fathom line. Throughout the whole region there is a very uniform set of conditions. In the shallow-water and littoral zones, owing to the rapid accumulation and the mechanical effects of transportation and erosion, the effects of chemical modification are not very apparent in the deposits; but in deep-sea deposits, in consequence of the less rapid rate of accumulation, absence of transport, the nature and small size of the particles, many evident chemical reactions have taken place, resulting in the formation _in situ_ of glauconite, phosphatic and manganese nodules, zeolites, and other secondary products.”[1] With increasing depth and distance from the shore, the character of the deposits undergoes a change. There is less and less material derived directly from the land, and more “amorphous matter arising from the ultimate decomposition of minerals and rocks, and accompanied, in all moderate depths, by an increase [relative] of the remains of pelagic organisms. We thus pass insensibly from those deep-sea deposits of a terrestrial origin, which we call ‘terrigenous,’ to those deep-sea deposits denominated ‘pelagic,’ in which the remains of calcareous and siliceous organisms, clays and other substances of secondary origin play the principal rôle.”[176]

The following table[177] shows the relations of the various groups of marine deposits.

1. Deep-sea deposits beyond { Red clay } I. Pelagic deposits
100 fathoms { Radiolarian ooze } formed in deep
{ Diatom ooze } water removed
{ Globigerina ooze } from land.
{ Pteropod ooze }
{
{ Blue mud }
{ Red mud }
{ Green mud }
{ Volcanic mud } II. Terrigenous
{ Coral mud } deposits formed
} in deep and
2. Shallow-water deposits } Sands, gravels, } shallow water,
between low-water mark } muds, etc. } mostly close
and 100 fathoms } } to land.
}
3. Littoral deposits between } Sands, gravels, }
high- and low-water marks } muds, etc. }

=Sources.=—The pelagic deposits are made up in part of materials of organic origin, and in part of materials of inorganic origin. The inorganic materials may be of mechanical or chemical origin. Mechanical pelagic deposits originate in various ways. They may come (1) from the land by the ordinary processes of gradation, (2) from volcanic vents, or (3) from extra-terrestrial sources. Chemical deposits may be formed (1) _in situ_ by the chemical interaction of substances in the sea-water on materials of organic and inorganic origin, and (2) by direct precipitation from the sea-water.

=Mechanical inorganic deposits.=—The terrigenous materials which reach the deep sea are, as a rule, only the finest products of land decay, and are carried out by movements of water or by the winds. They are not commonly recognized in the dredgings more than 200 miles from the shore, but opposite the mouths of great rivers they extend much farther,—1000 miles in the case of the Amazon. They are especially abundant on the slopes of the continental shelves. Here occur the _blue_, _green_, and _red muds_, with which are associated volcanic and coral muds. The color of these various muds is dependent in part on the changes which they have undergone since their deposition. The green muds usually contain enough glauconite to give them their color, and are most commonly found off bold coasts where sedimentation is not rapid. The blue muds indicate lack of oxidation, or perhaps deoxidation. Red muds are not common, though they have been found in some situations. In general, these deposits are analogous to certain shales, marls, etc., found within the continents.

Though coarse materials derived from the land are occasionally found in the deep-sea deposits, their presence must be looked upon as in some sense accidental. Occasional pebbles, or even bowlders, are carried out into the ocean entangled in the roots of floating trees. Within limits, too, icebergs have carried out land débris, though it is probable that transportation by this means has been exaggerated. The amount which icebergs might carry, if fully loaded, is far greater than the amount which they do carry.

Of the identifiable inorganic materials in the deep sea, the most abundant are of volcanic origin, and among these the most common is pumice, which is frequently so light that it floats readily until it becomes water-logged. Pieces of pumice brought up by the Challenger and thoroughly dried were found to float for months in sea-water before settling even through the depth of water contained in the vessel in which the experiment was performed.[178] The next most abundant substance of volcanic origin in pelagic deposits is _volcanic glass_. This ranges from pieces of the size of a walnut down to the smallest fragments, which often serve as centers for concretions. _Lapilli_ (cinders) and _volcanic ash_ also are abundant in parts of the deep sea. The distribution of these volcanic products is essentially universal, though by no means uniform. Some of them are probably from submarine volcanoes.

The study of the deep sea deposits has revealed the presence of many nodules and grains which are believed to be of extra-terrestrial origin. Many of them are magnetic.[179] The dust of countless meteors which enter the atmosphere daily settles on land and sea alike, and enters into the sediment of the bottom of the latter. It is probably no more abundant in deep water than in shallow, but it is relatively more important, since other sedimentation is more meager. The number of meteorites which enter the atmosphere daily has been estimated at from 15,000,000 to 20,000,000.[180] If on the average the meteorites weigh ten grains each, probably a rather high estimate, the total amount of extra-terrestrial matter reaching the earth yearly would be 5,000 to 7,000 tons, and something like three-fourths of this must, on the average, fall into the sea. But even at this rate it would take some fifty billion years to cover the sea-bottom with a layer one foot in thickness.

=Organic constituents of pelagic deposits.=—With increasing distance from shores, and especially with increasing depth of water, terrigenous deposits become less and less abundant, and sediments derived from pelagic life increase in relative importance. Beyond the upper part of the outer slopes of the continental shelves, the pelagic deposits are largely made up of shells and skeletons of marine organisms which live in the surface-waters. Pelagic molluscs, foraminifera, and algæ secrete shells of lime carbonate, while diatoms and radiolarians secrete shells of silica. When the organisms die, they sink to the bottom with their shells, and these mineral matters of organic origin are mingled with the volcanic products which are universal over the sea-floor. Pelagic deposits of organic origin are named according to their characteristic constituents. Thus there are pteropod oozes, globigerina oozes, diatom oozes, radiolarian oozes, etc.[181] It is not to be understood that these oozes are made up exclusively of the shells which give them their names. Diatom ooze is an ooze in which diatom shells are abundant, not an ooze made up wholly of diatom shells; and globigerina ooze is an ooze in which globigerina shells are abundant, though in many cases they do not make up even the bulk of the matter. While samples of these various oozes might be selected which are thoroughly distinct from one another, there are all gradations between them, since pelagic life does not recognize boundary-lines.

It is a significant fact that with increasing depth the proportion of lime carbonate in the ooze decreases. Thus in tropical regions remote from land where the depths are less than 600 fathoms, the carbonate of lime of the shells of pelagic organisms may constitute 80% or 90% of the deposit. With the same surface conditions, but with increasing depth, the percentage of lime carbonate decreases, until at 2000 fathoms it is less than 60%; at 2400 fathoms, 30%, and at 2600 fathoms, 10%. Beyond this depth there are usually no more than traces of carbonate of lime. The data at hand show that the percentage of lime carbonate falls off below 2200 fathoms more rapidly than at lesser depths.

When the percentage of lime carbonate becomes very low, the calcareous oozes grade off into the red clay with which the sea-floor below 2400 to 2600 fathoms is covered.

=Chemical deposits.=—The chemical deposits of the deep sea are chiefly the alteration products of sediments which reach the sea-bottom by mechanical means. All sediment deposited in the sea undergoes more or less chemical change, but it is only when the change is very considerable that the product is referred to this class. Where sedimentation is rapid and the sediment coarse, the chemical change is relatively slight; but where the sedimentation is slow and the sediment fine, the chemical change is relatively great; for the longer exposure to the sea-water and the greater proportion of surface exposed to attack, both favor change. Both the area and the mass of sea-bottom sediment radically changed in this way are large, but most of the deposit does not correspond to any formation known on the land.

The red clay already referred to belongs to this class of deposits. Its origin has been the subject of much discussion. It contains much volcanic débris, various concretions, bones of mammals, zeolitic crystals, and extra-terrestrial spherules, and doubtless the insoluble products of the shells of pelagic life; but it is still a mooted question how far the clay itself is the product of decomposed shells, and how far the altered product of pulverized pumice, volcanic ash, dust, etc. Pelagic life does not seem to be less abundant at the surface where the water is deep than where it is shallow, and it would appear that the shells must sink in such situations as elsewhere. If the lime carbonate of globigerina ooze be removed by dilute acid, the inorganic residue is similar to the red clay in the ocean-bottom. This suggests that owing to the more complete solution in the very deep water, the lime carbonate of the shells has been dissolved, leaving the red clay as a residuum. The more complete solution at the bottom might be the result either of the greater pressure, or of a greater percentage of CO₂ in the water due to emanations from the sea-floor, or to both; but the suddenness of the transition from oozes to red clay, with increasing depth, does not seem to be fully explained by these assumptions. The study of the dredgings has inclined the students of these materials to the conclusion that volcanic materials, rather than shells, are the principal source of the red clay.[182] The volcanic materials are thought to have accumulated slowly and to have been long exposed to the action of sea-water. The various nodules and crystals in the clay are believed to be secondary products, the materials for which were derived from the decomposition of the same materials. Eolian dust may be a notable constituent of the red clay.

Various specific products of chemical change may be briefly referred to. The decomposition of certain mineral particles, such as feldspar, gives rise to _kaolin_, and kaolin is a very considerable constituent of most of the clayey deposits of the ocean-bottom. The kaolinization of feldspar may take place both on land and in the sea. _Manganiferous_ deposits are widespread in the ocean-bottom, occurring both as coatings on grains of mechanical sediments, shells, etc., and as concretions ranging in sizes from minute particles to nodules an inch or more in diameter. The concretions are sometimes approximately spheroidal, but often botryoidal. These manganiferous nodules are believed to have arisen from the decay of fragments of volcanic rocks. In their decay, the manganese and iron are believed to have been first changed to carbonates, and subsequently to oxides. After manganese oxide, iron oxide and silica are by far the most abundant constituents, but many other substances enter into their composition in minor quantities.

Another substance somewhat widely distributed in the sea-bed, though by no means universal, is _glauconite_, a complex silicate of alumina, iron, potassium, etc. Glauconite is, on the whole, most abundant along the edges of the continental shelves, though it is by no means universal in this position. It is not commonly found in deep water, nor very near the shore, but approximately at the “mud-line.” The glauconite grains begin to form, as a rule, in tiny shells, chiefly the shells of foraminifera. After filling the shell, the shell itself may disappear, while the glauconite goes on accumulating around the core already formed, until the grain attains considerable size. Glauconite is believed to be an alteration product of certain sorts of mechanical sediment, the change being effected under the influence of the decaying organic matter in the shells.[183] It does not occur where sedimentation is rapid, and its formation appears to be favored by considerable changes of temperature. Glauconite deposits occur on the land and are commonly known as _green sand marl_. Glauconite also occurs sparingly in many other sedimentary rocks.