Chapter 8

Fig. 137.A Stack, Scotland

Fig. 138.Wave-Cut Islands, ScotlandHow far did the land once extend?

Coves.Where zones of soft or closely jointed rock outcrop along a shore, or where minor water courses conic down to the sea and aid in erosion, the shore is worn back in curved reëntrants called coves; while the more resistant rocks on either hand are left projecting as headlands (Fig. 139). After coves are cut back a short distance by the waves, the headlands come to protect them, as with breakwaters, and prevent their indefinite retreat. The shore takes a curve of equilibrium, along which the hard rock of the exposed headland and the weak rock of the protected cove wear back at an equal rate.

Fig. 139.Coves formed in Softer StrataS,S; while the Harder StrataH,H, are left as Headlands

Rate of recession.The rate at which a shore recedes depends on several factors. In soft or incoherent rocks exposed to violent storms the retreat is so rapid as to be easily measured. The coast of Yorkshire, England, whose cliffs are cut in glacial drift, loses seven feet a year on the average, and since the Norman conquest a strip a mile wide, with farmsteads and villages and historic seaports, has been devoured by the sea. The sandy south shore of Martha’sVineyard wears back three feet a year. But hard rocks retreat so slowly that their recession has seldom been measured by the records of history.

Fig. 140.A Pebble Beach, Cape Ann, Massachusetts

Shore Drift

Bowlder and pebble beaches.About as fast as formed the waste of the sea cliff is swept both along the shore and out to sea. The road of waste along shore is thebeach. We may also define the beach as the exposed edge of the sheet of sediment formed by the carriage of land waste out to sea. At the foot of sea cliffs, where the waves are pounding hardest, one commonly finds the rock bench strewn on its inner margin with large stones, dislodged by the waves and by the weather and some-what worn on their corners and edges. From thisbowlder beachthe smaller fragments of waste from the cliff and the fragments into which the bowlders are at last broken drift on to more sheltered places and there accumulate in apebble beach, made of pebbles well rounded by the wear which they have suffered. Such beaches form a mill whose raw material is constantlysupplied by the cliff. The breakers of storms set it in motion to a depth of several feet, grinding the pebbles together with a clatter to be heard above the roar of the surf. In such a rock crusher the life of a pebble is short. Where ships have stranded on our Atlantic coast with cargoes of hard-burned brick or of coal, a year of time and a drift of five miles along the shore have proved enough to wear brick and coal to powder. At no great distance from their source, therefore, pebble beaches give place to beaches of sand, which occupy the more sheltered reaches of the shore.

Sand beaches.The angular sand grains of various minerals into which pebbles are broken by the waves are ground together under the beating surf and rounded, and those of the softer minerals are crushed to powder. The process, however, is a slow one, and if we study these sand grains under a lens we may be surprised to see that, though their corners and edges have been blunted, they are yet far from the spherical form of the pebbles from which they were derived. The grains are small, and in water they have lost about half their weight in air; the blows which they strike one another are therefore weak. Besides, each grain of sand of the wet beach is protected by a cushion of water from the blows of its neighbors.

The shape and size of these grains and the relative proportion of grains of the softer minerals which still remain give a rough measure of the distance in space and time which they have traveled from their source. The sand of many beaches, derived from the rocks of adjacent cliffs or brought in by torrential streams from neighboring highlands, is dark with grains of a number of minerals softer than quartz. The white sand of other beaches, as those of the east coast of Florida, is almost wholly composed of quartz grains; for in its long travel down the Atlantic coast the weaker minerals have been worn to powder and the hardest alone survive.

How does the absence of cleavage in quartz affect the durability of quartz sand?

How shore drift migrates.It is under the action of waves and currents that shore drift migrates slowly along a coast. Where waves strike a coast obliquely they drive the waste before them little by little along the shore. Thus on a north-south coast, where the predominant storms are from the northeast, there will be a migration of shore drift southwards.

All shores are swept also by currents produced by winds and tides. These are usually far too gentle to transport of themselves the coarse materials of which beaches are made. But while the wave stirs the grains of sand and gravel, and for a moment lifts them from the bottom, the current carries them a step forward on their way. The current cannot lift and the wave cannot carry, but together the two transport the waste along the shore. The road of shore drift is therefore the zone of the breaking waves.

Fig. 141.A Bay Bar, Lake Ontario

The bay-head beach.As the waste derived from the wear of waves and that brought in by streams is trailed along a coast it assumes, under varying conditions, a number of distinct forms. When swept into the head of a sheltered bay it constitutes the bay-head beach. By the highest storm waves the beach is often built higher than the ground immediately behind it, and forms a dam inclosing a shallow pond or marsh.

The bay bar.As the stream of shore drift reaches the mouth of a bay of some size it often occurs that, instead of turning in, it sets directly across toward the opposite headland. The wasteis carried out from shore into the deeper waters of the bay mouth; where it is no longer supported by the breaking waves, and sinks to the bottom. The dump is gradually built to the surface as a stubby spur, pointing across the bay, and as it reaches the zone of wave action current and wave can now combine to carry shore drift along it, depositing their load continually at the point of the spur. An embankment is thus constructed in much the same manner as a railway fill, which, while it is building, serves as a roadway along which the dirt from an adjacent cut is carted to be dumped at the end. When the embankment is completed it bridges the bay with a highway along which shore drift now moves without interruption, and becomes a bay bar.

Fig. 142.A Hook, Lake Michigan

Incomplete bay bars.Under certain conditions the sea cannot carry out its intention to bridge a bay. Rivers discharging in bays demand open way to the ocean. Strong tidal currents also are able to keep open channels scoured by their ebb and flow. In such cases the most that land waste can do is to build spits and shoals, narrowing and shoaling the channel as much as possible. Incomplete bay bars sometimes have their points recurved by currents setting at right angles to the stream of shore drift and are then classified ashooks(Fig. 142).

Fig. 143.Cross Section of Sand Reefsr, and Lagoon;sl, Sea Level

Sand reefs.On low coasts where shallow water extends some distance out, the highway of shore drift lies along a low, narrow ridge, termed the sand reef, separated from the land by a narrow stretch of shallow water called thelagoon(Fig. 143). At intervals the reef is held openbyinlets,—gaps through which the tide flows and ebbs, and by which the water of streams finds way to the sea.

Fig. 144.Sand Reef and Lagoon, Texas

No finer example of this kind of shore line is to be found in the world than the coast of Texas. From near the mouth of the Rio Grande a continuous sand reef draws its even curve for a hundred miles to Corpus Christi Pass, and the reefs are but seldom interrupted by inlets as far north as Galveston Harbor. On this coast the tides are variable and exceptionally weak, being less than one foot in height, while the amount of waste swept along the shore is large. The lagoon is extremely shallow, and much of it is a mud flat too shoal for even small boats. On the coast of New Jersey strong tides are able to keep open inlets at intervals of from two to twenty miles in spite of a heavy alongshore drift.

Sand reefs are formed where the water is so shallow near shore that storm waves cannot run in it and therefore break some distance out from land. Where storm waves first drag bottom they erode and deepen the sea floor, and sweep in sediment as far as the line where they break. Here, where they lose their force, they drop their load and beat up the ridge which is known as the sand reef when it reaches the surface.

Shores of Elevation and Depression

Our studies have already brought to our notice two distinct forms of strand lines,—one the high, rocky coast cut back to cliffs by the attack of the waves, and the other the low, sandy coast where the waves break usually upon the sand reef. To understand the origin of these two types we must know thatthe meeting place of sea and land is determined primarily by movements of the earth’s crust. Where a coast land emerges the—shore line moves seaward; where it is being submerged the shore line advances on the land.

Shores of elevation.The retreat of the sea, either because of a local uplift of the land or for any other reason, such as the lowering of any portion of ocean bottom, lays bare the inner margin of the sea floor. Where the sea floor has long received the waste of the land it has been built up to a smooth, subaqueous plain, gently shelving from the land. Since the new shore line is drawn across this even surface it is simple and regular, and is bordered on the one side by shallow water gradually deepening seaward, and on the other by low land composed of material which has not yet thoroughly consolidated to firm rock. A sand reef is soon beaten up by the waves, and for some time conditions will favor its growth. The loss of sand driven into the lagoon beyond, and of that ground to powder by the surf and carried out to sea, is more than made up by the stream of alongshore drift, and especially by the drag of sediments to the reef by the waves as they deepen the sea floor on its seaward side.

Meanwhile the lagoon gradually fills with waste from the reef and from the land. It is invaded by various grasses and reeds which have learned to grow in salt and brackish water; the marsh, laid bare only at low tide, is built above high tide by wind drift and vegetable deposits, and becomes a meadow, soldering the sand reef to the mainland.

While the lagoon has been filling, the waves have been so deepening the sea floor off the sand reef that at last they are able to attack it vigorously. They now wear it back, and, driving the shore line across the lagoon or meadow, cut a line of low cliffs on the mainland. Such a shore is that of Gascony in southwestern France,—a low, straight, sandy shore, bordered by dunes and unprotected by reefs from the attack of the waves of the Bay of Biscay.

Fig. 145.Map of New Jersey, with that Portion of the State one Hundred Feet and more above Sea Level shadedDescribe the coast line which the state would have if depressed one hundred feet. Compare it with the present coastline

We may say, then, that on shores of elevation the presence of sand reefs and lagoons indicates the stage of youth, while the absence of these features and the vigorous and unimpeded attack by the sea upon the mainland indicate the stage of maturity. Where much waste is brought in by rivers the maturity of such a coast may be long delayed. The waste from the land keeps the sea shallow offshore and constantly renews the sand reef. The energy of the waves is consumed in handling shore drift, and no energy is left for an effective attack upon the land. Indeed, with an excessive amount of waste brought down by streams the land may be built out and encroach temporarily upon the sea; and not until long denudation has lowered the land, and thus decreased the amount of waste from it, may the waves be able to cut through the sand reef and thus the coast reach maturity.

Shores of Depression

Where a coastal region is undergoing submergence the shore line moves landward. The horizontal plane of the sea now intersects an old land surface roughened by subaërial denudation. The shore line is irregular and indented in proportion to the relief of the land and the amount of the submergence which the land has suffered. It follows up partially submerged valleys, forming bays, and bends round the divides, leaving them to project as promontories and peninsulas. The outlines of shores of depression are as varied as are the forms of the land partially submerged. We give a few typical illustrations.

Fig. 146.Chesapeake BayDraw a sketch of this area before its depression

The characteristics of the coast of Maine are due chiefly to the fact that a mountainous region of hard rocks, once worn to a peneplain, and after a subsequent elevation deeply dissected by north-south valleys, has subsided, the depression amounting on its southern margin to as much as six hundred feet below sea level. Drowned valleys penetrate the land in long, narrow bays, and rugged divides project in long, narrow land arms prolonged seaward by islands representing the high portions of their extremities. Of this exceedingly ragged shore there are said to be two thousand miles from the New Brunswick boundary as far west as Portland,—a straight-line distance of but two hundred miles. Since the time of its greatest depression the land is known to have risen some three hundred feet; for the bays have been shortened, and the waste with which their floors were strewn is now in part laid bare as clay plains about the bay heads and in narrow selvages about the peninsulas and islands.The coast of Dalmatia, on the Adriatic Sea, is characterized by long land arms and chains of long and narrow islands, all parallel to the trend of the coast. A region of parallel mountain ranges has been depressed, and the longitudinal valleys which lie between them are occupied by arms of the sea.Chesapeake Bay is a branching bay due to the depression of an ancient coastal plain which, after having emerged from the sea, was channeled with broad, shallow valleys. The sea has invaded the valley of the trunk stream and those of its tributaries, forming a shallow bay whose many branches are all directed toward its axis (Fig. 146).Hudson Bay, and the North, the Baltic, and the Yellow seas are examples where the sinking of the land has brought the sea in over low plains of large extent, thus deeply indenting the continental out-line. The rise of a few hundred feet would restore these submerged plains to the land.

The coast of Dalmatia, on the Adriatic Sea, is characterized by long land arms and chains of long and narrow islands, all parallel to the trend of the coast. A region of parallel mountain ranges has been depressed, and the longitudinal valleys which lie between them are occupied by arms of the sea.

Chesapeake Bay is a branching bay due to the depression of an ancient coastal plain which, after having emerged from the sea, was channeled with broad, shallow valleys. The sea has invaded the valley of the trunk stream and those of its tributaries, forming a shallow bay whose many branches are all directed toward its axis (Fig. 146).

Hudson Bay, and the North, the Baltic, and the Yellow seas are examples where the sinking of the land has brought the sea in over low plains of large extent, thus deeply indenting the continental out-line. The rise of a few hundred feet would restore these submerged plains to the land.

The cycle of shores of depression.In itsinfantile stagethe outline of a shore of depression depends almost wholly on the previous relief of the land, and but little on erosion by the sea. Sea cliffs and narrow benches appear where headlands and outlying islands have been nipped by the waves. As yet, little shore waste has been formed. The coast of Maine is an example of this stage.

Inearly youthall promontories have been strongly cliffed, and under a vigorous attack of the sea the shore of open bays may be cut back also. Sea stacks and rocky islets, caves and coves, make the shore minutely ragged. The irregularity of the coast, due to depression, is for a while increased by differential wave wear on harder and softer rocks. The rock bench is still narrow. Shore waste, though being produced in large amounts, is for the most part swept into deeper water and buried out of sight. Examples of this stage are the east coast of Scotland and the California coast near San Francisco.

Later youthis characterized by a large accumulation of shore waste. The rock bench has been cut back so that it now furnishes a good roadway for shore drift. The stream ofalongshore drift grows larger and larger, filling the heads of the smaller bays with beaches, building spits and hooks, and tying islands with sand bars to the mainland. It bridges the larger bays with bay bars, while their length is being reduced as their inclosing promontories are cut back by the waves. Thus there comes to be a straight, continuous, and easy road, no longer interrupted by headlands and bays, for the transportation of waste alongshore. The Baltic coast of Germany is in this stage.

Fig. 147.Portion of the Northwest Coast of France

All this while streams have been busy filling with delta deposits the bays into which they empty. By these steps a coast gradually advances tomaturity, the stage when the irregularities due to depression have been effaced, when outlying islands formed by subsidence have been planed away, and when the shore line has been driven back behind the former bay heads. The sea now attacks the land most effectively along a continuous and fairly straight line of cliffs. Although the first effect of wave wear was to increase the irregularities of the shore, it sooner or later rectifies it, making it simple and smooth. The northwest coast of France is often cited as an example of a coast which has reached this stage of development (Fig. 147).

In theold ageof coasts the rock bench is cut back so far that the waves can no longer exert their full effect upon the shore. Their energy is dissipated in moving shore drift hither and thither and in abrading the bench when they drag bottomupon it. Little by little the bench is deepened by tidal currents and the drag of waves; but this process is so slow that meanwhile the sea cliffs melt down under the weather, and the bench becomes a broad shoal where waves and tides gradually work over the waste from the land to greater fineness and sweep it out to sea.

Fig. 148.The South Shore of Martha’s VineyardThe land is shaded. To what class of coasts does this belong? What stage has it reached, and by what process? What changes will take place in the future?

Fig. 148.The South Shore of Martha’s Vineyard

The land is shaded. To what class of coasts does this belong? What stage has it reached, and by what process? What changes will take place in the future?

Plains of marine abrasion.While subaërial denudation reduces the land to baselevel, the sea is sawing its edges towave base, i.e. the lowest limit of the wave’s effective wear. The widened rock bench forms when uplifted a plain of marine abrasion, which like the peneplain bevels across strata regardless of their various inclinations and various degrees of hardness.

How may a plain of marine abrasion be expected to differ from a peneplain in its mantle of waste?

Compared with subaërial denudation, marine abrasion is a comparatively feeble agent. At the rate of five feet per century—a higher rate than obtains on the youthful rocky, coast of Britain—it would require more than ten million years to parea strip one hundred miles wide from the margin of a continent, a time sufficient, at the rate at which the Mississippi valley is now being worn away, for subaërial denudation to lower the lands of the globe to the level of the sea.

Slow submergence favors the cutting of a wide rock bench. The water continually deepens upon the bench; storm waves can therefore always ride in to the base of the cliffs and attack them with full force; shore waste cannot impede the onset of the waves, for it is continually washed out in deeper water below wave base.

Basal conglomerates.As the sea marches across the land during a slow submergence, the platform is covered with sheets of sea-laid sediments. Lowest of these is a conglomerate,—the bowlder and pebble beach, widened indefinitely by the retreat of the cliffs at whose base it was formed, and preserved by the finer deposits laid upon it in the constantly deepening water as the land subsides. Such basal conglomerates are not uncommon among the ancient rocks of the land, and we may know them by their rounded pebbles and larger stones, composed of the same kind of rock as that of the abraded and evened surface on which they lie.

CHAPTER VIII

OFFSHORE AND DEEP-SEA DEPOSITS

The alongshore deposits which we have now studied are the exposed edge of a vast subaqueous sheet of waste which borders the continents and extends often for as much as two or three hundred miles from land. Soundings show that offshore deposits are laid in belts parallel to the coast, the coarsest materials lying nearest to the land and the finest farthest out. The pebbles and gravel and the clean, coarse sand of beaches give place to broad stretches of sand, which grows finer and finer until it is succeeded by sheets of mud. Clearly there is an offshore movement of waste by which it is sorted, the coarser being sooner dropped and the finer being carried farther out.

Offshore Deposits

The débris torn by waves from rocky shores is far less in amount than the waste of the land brought down to the sea by rivers, being only one thirty-third as great, according to a conservative estimate. Both mingle alongshore in all the forms of beach and bar that have been described, and both are together slowly carried out to sea. On the shelving ocean floor waste is agitated by various movements of the unquiet water,—by the undertow (an outward‑running bottom current near the shore), by the ebb and flow of tides, by ocean currents where they approach the land, and by waves and ground swells, whose effects are sometimes felt to a depth of six hundred feet. By all these means the waste is slowly washed to and fro, and as it is thus ground finer and finer and its soluble parts are moreand more dissolved, it drifts farther and farther out from land. It is by no steady and rapid movement that waste is swept from the shore to its final resting place. Day after day and century after century the grains of sand and particles of mud are shifted to and fro, winnowed and spread in layers, which are destroyed and rebuilt again and again before they are buried safe from further disturbance.

These processes which are hidden from the eye are among the most important of those with which our science has to do; for it is they which have given shape to by far the largest part of the stratified rocks of which the land is made.

The continental delta.This fitting term has been recently suggested for the sheet of waste slowly accumulating along the borders of the continents. Within a narrow belt, which rarely exceeds two or three hundred miles, except near the mouths of muddy rivers such as the Amazon and Congo, nearly all the waste of the continent, whether worn from its surface by the weather, by streams, by glaciers, or by the wind, or from its edge by the chafing of the waves, comes at last to its final resting place. The agencies which spread the material of the continental delta grow more and more feeble as they pass into deeper and more quiet water away from shore. Coarse materials are therefore soon dropped along narrow belts near land. Gravels and coarse sands lie in thick, wedge-shaped masses which thin out seaward rapidly and give place to sheets of finer sand.

Sea muds.Outermost of the sediments derived from the waste of the continents is a wide belt of mud; for fine clays settle so slowly, even in sea water,—whose saltness causes them to sink much faster than they would in fresh water,—that they are wafted far before they reach a bottom where they may remain undisturbed. Muds are also found near shore, carpeting the floors of estuaries, and among stretches of sandy deposits in hollows where the more quiet water has permitted the finer silt to rest.

Sea muds are commonly bluish and consolidate to bluish shales; the red coloring matter brought from land waste—iron oxide—is altered to other iron compounds by decomposing organic matter in the presence of sea water. Yellow and red muds occur where the amount of iron oxide in the silt brought down to the sea by rivers is too great to be reduced, or decomposed, by the organic matter present.

Green muds and green sand owe their color to certain chemical changes which take place where waste from the land accumulates on the sea floor with extreme slowness. A greenish mineral calledglauconite—a silicate of iron and alumina—is then formed. Such deposits, known asgreen sand, are now in process of making in several patches off the Atlantic coast, and are found on the coastal plain of New Jersey among the offshore deposits of earlier geological ages.

Organic deposits.Living creatures swarm along the shore and on the shallows out from land as nowhere else in the ocean. Seaweed often mantles the rock of the sea cliff between the levels of high and low tide, protecting it to some degree from the blows of waves. On the rock bench each little pool left by the ebbing tide is an aquarium abounding in the lowly forms of marine life. Below low-tide level occur beds of molluscous shells, such as the oyster, with countless numbers of other humble organisms. Their harder parts—the shells of mollusks, the white framework of corals, the carapaces of crabs and other crustaceans, the shells of sea urchins, the bones and teeth of fishes—are gradually buried within the accumulating sheets of sediment, either whole or, far more often, broken into fragments by the waves.

By means of these organic remains each layer of beach deposits and those of the continental delta may contain a record of the life of the time when it was laid. Such a record has been made ever since living creatures with hard parts appeared upon the globe. We shall find it sealed away in the stratifiedrocks of the continents,— parts of ancient sea deposits now raised to form the dry land. Thus we have in the traces of living creatures found in the rocks, i.e. in fossils, a history of the progress of life upon the planet.

Fig. 149.Coquina, Florida

Molluscous shell deposits.The forms of marine life of importance in rock making thrive best in clear water, where little sediment is being laid, and where at the same time the depth is not so great as to deprive them of needed light, heat, and of sufficient oxygen absorbed by sea water from the air. In such clear and comparatively shallow water there often grow countless myriads of animals, such as mollusks and corals, whose shells and skeletons of carbonate of lime gradually accumulate in beds of limestone.

A shell limestone made of broken fragments cemented together is sometimes calledcoquina, a local term applied to such beds recently uplifted from the sea along the coast of Florida (Fig. 149).Oöliticlimestone (ōon, an egg;lithos, a stone) is so named from the likeness of the tiny spherules which compose it to the roe of fish. Corals and shells have been pounded by the waves to calcareous sand, and each grain has been covered with successive concentric coatings of lime carbonate deposited about it from solution.

A shell limestone made of broken fragments cemented together is sometimes calledcoquina, a local term applied to such beds recently uplifted from the sea along the coast of Florida (Fig. 149).

Oöliticlimestone (ōon, an egg;lithos, a stone) is so named from the likeness of the tiny spherules which compose it to the roe of fish. Corals and shells have been pounded by the waves to calcareous sand, and each grain has been covered with successive concentric coatings of lime carbonate deposited about it from solution.

The impalpable powder to which calcareous sand is ground by the waves settles at some distance from shore in deeper and quieter water as a limy silt, and hardens into a dense, fine-grained limestone in which perhaps no trace of fossil is found to suggest the fact that it is of organic origin.

From Florida Keys there extends south to the trough of Florida Straits a limestone bank covered by from five hundred and forty to eighteen hundred feet of water. The rocky bottom consists of limestone now slowly building from the accumulation of the remains of mollusks, small corals, sea urchins, worms with calcareous tubes, and lime-secreting seaweed, which live upon its surface.

Where sponges and other silica-secreting organisms abound on limestone banks, silica forms part of the accumulated deposit, either in its original condition, as, for example, the spicules of sponges, or gathered into concretions and layers of flint.

Where considerable mud is being deposited along with carbonate of lime there is in process of making a clayey limestone or a limy shale; where considerable sand, a sandy limestone or a limy sandstone.

Consolidation of offshore deposits.We cannot doubt that all these loose sediments of the sea floor are being slowly consolidated to solid rock. They are soaked with water which carries in solution lime carbonate and other cementing substances. These cements are deposited between the fragments of shells and corals, the grains of sand and the particles of mud, binding them together into firm rock. Where sediments have accumulated to great thickness the lower portions tend also to consolidate under the weight of the overlying beds. Except in the case of limestones, recent sea deposits uplifted to form land are seldom so well cemented as are the older strata, which have long been acted upon by underground waters deep below the surface within the zone of cementation, and have been exposed to view by great erosion.

Fig. 150.Ripple Marks on Layers of Ancient Sandstone, Wisconsin

Ripple marks, sun cracks, etc.The pulse of waves and tidal currents agitates the loose material of offshore deposits, throwing it into fine parallel ridges called ripple marks. One may see this beautiful ribbing imprinted on beach sands uncovered by the outgoing tide, and it is also produced where the water is of considerable depth. While the tide is out the surface of shore deposits may be marked by the footprints of birds and other animals, or by the raindrops of a passing shower (Fig. 153). The mud of flats, thus exposed to the sun and dried, cracks in a characteristic way (Figs.151and152). Such markings may be covered over with a thin layer of sediment at the next flood tide and sealed away as a lasting record of the manner and place in which the strata were laid. InFigure 150we have an illustration of a very ancient ripple-marked sand consolidated to hard stone, uplifted and set on edge by movements of the earth’s crust, and exposed to open air after long erosion.

Fig. 151.Sun Cracks

Stratification.For the most part the sheet of sea-laid waste is hidden from our sight. Where its edge is exposed along the shore we may see the surface markings which have just beennoticed. Soundings also, and the observations made in shallow waters by divers, tell something of its surface; but to learn more of its structures we must study those ancient sediments which have been lifted from the sea and dissected by subaërial agencies. From them we ascertain that sea deposits are stratified. They lie in distinct layers which often differ from one another in thickness, in size of particles, and perhaps in color. They are parted by bedding planes, each of which represents either a change in material or a pause during which deposition ceased and the material of one layer had time to settle and become somewhat consolidated before the material of the next was laid upon it. Stratification is thus due to intermittently acting forces, such as the agitation of the water during storms, the flow and ebb of the tide, and the shifting channels of tidal currents. Off the mouths of rivers, stratification is also caused by the coarser and more abundant material brought down at time of floods being laid on the finer silt which is discharged during ordinary stages.

Fig. 152.The Under Side of a Layer deposited upon a Sun-Cracked Surface, showing Casts of the Cracks

Fig. 153.Rain Prints

How stratified deposits are built up is well illustrated in the flats which border estuaries, such as the Bay of Fundy. Each advance of the tide spreads a film of mud, which dries and hardens in the air during low water before another film is laid upon it by the next incoming tidal flood. In this way the flats have been covered by a clay which splits into leaves as thin as sheets of paper.

It is in fine material, such as clays and shales and limestones, that the thinnest and most uniform layers, as well as those of widest extent, occur. On the other hand, coarse materials are commonly laid in thick beds, which soon thin out seaward and give place to deposits of finer stuff. In a general way strata are laid in well-nigh horizontal sheets, for the surface on which they are laid is generally of very gentle inclination. Each stratum, however, is lenticular, or lenslike, in form, having an area where it is thickest, and thinning out thence to its edges, where it is overlapped by strata similar in shape.

Fig. 154.Cross Bedding in Sandstone, England

Cross bedding.There is an apparent exception to this rule where strata whose upper and lower surfaces may be about horizontal are made up of layers inclined at angles which may be as high as the angle of repose. In this case each stratum grew by the addition along its edge of successive layers of sediment, precisely as does a sand bar in a river, the sand being pushed continuously over the edge and coming to rest on a sloping surface. Shoals built by strong and shifting tidal currents often show successive strata in which the cross bedding is inclined in different directions.

Thickness of sea deposits.Remembering the vast amount of material denuded from the land and deposited offshore, we should expect that with the lapse of time sea deposits would have grown to an enormous thickness. It is a suggestive fact that, as a rule, the profile of the ocean bed is that of a soup plate,—a basin surrounded by a flaring rim. On thecontinental shelf, as the rim is called, the water is seldom more than six hundred feet in depth at the outer edge, and shallows gradually towards shore. Along the eastern coast of the United States the continental shelf is from fifty to one hundred and more miles in width; on the Pacific coast it is much narrower. So far as it is due to upbuilding, a wide continental shelf, such as that of the Atlantic coast, implies a massive continental delta thousands of feet in thickness. The coastal plain of the Atlantic states may be regarded as the emerged inner margin of this shelf, and borings made along the coast probe it to the depth of as much as three thousand feet without finding the bottom of ancient offshore deposits. Continental shelves may also be due in part to a submergence of the outer margin of a continental plateau and to marine abrasion.

Deposition of sediments and subsidence.The stratified rocks of the land show in many places ancient sediments which reach a thickness which is measured in miles, and which are yet the product of well-nigh continuous deposition. Such strata may prove by their fossils and by their composition and structure that they were all laid offshore in shallow water. We must infer that, during the vast length of time recorded by the enormous pile, the floor of the sea along the coast was slowly sinking, and that the trough was constantly being filled, foot by foot, as fast as it was depressed. Such gradual, quiet movements of the earth’s crust not only modify the outline of coasts, as we have seen, but are of far greater geological importance in that they permit the making of immense deposits of stratified rock.

A slow subsidence continued during long time is recorded also in the succession of the various kinds of rock that come to be deposited in the same area. As the sea transgresses the land, i.e. encroaches upon it, any given part of the sea bottom is brought farther and farther from the shore. The basal conglomerate formed by bowlder and pebble beaches comes to be covered with sheets of sand, and these with layers of mud as the sea becomes deeper and the shore more remote; while deposits of limestone are made when at last no waste is brought to the place from the now distant land, and the water is left clear for the growth of mollusks and other lime-secreting organisms.

Fig. 155.Succession of Deposits recording a Transgressing Seac, conglomerate;ss, sandstone;sh, shale;lm, limestone

Rate of deposition.As deposition in the sea corresponds to denudation on the land, we are able to make a general estimate of the rate at which the former process is going on. Leaving out of account the soluble matter removed, the Mississippi is lowering its basin at the rate of one foot in five thousand years, and we may assume this as the average rate at which the earth’s land surface of fifty-seven million square miles is now being denuded by the removal of its mechanical waste. But sediments from the land are spread within a zone but two or three hundred miles in width along the margin of the continents, a line one hundred thousand miles long. As the area of deposition—about twenty-five million square miles—is about one half the area of denudation, the average rate of deposition must be twice the average rate of denudation, i.e. about one foot in twenty-five hundred years. If some deposits are made much more rapidly than this, others are made much more slowly. Ifthey were laid no faster than the present average rate, the strata of ancient sea deposits exposed in a quarry fifty feet deep represent a lapse of at least one hundred and twenty-five thousand years, and those of a formation five hundred feet thick required for their accumulation one million two hundred and fifty thousand years.

Fig. 156.Thick Offshore Deposits of Coarse Waste recording the Presence of a Young Mountain Range near Shore

The sedimentary record and the denudation cycle.We have seen that the successive stages in a cycle of denudation, such as that by which a land mass of lofty mountains is worn to low plains, are marked each by its own peculiar land forms, and that the forms of the earlier stages are more or less completely effaced as the cycle draws toward an end. Far more lasting records of each stage are left in the sedimentary deposits of the continental delta. Thus, in the youth of such a land mass as we have mentioned, torrential streams flowing down the steep mountain sides deliver to the adjacent sea their heavy loads of coarse waste, and thick offshore deposits of sand and gravel (Fig. 156) record the high elevation of the bordering land. As the land is worn to lower levels, the amount and coarseness of the waste brought to the sea diminishes, until the sluggish streams carry only a fine silt which settles on the ocean floor near to land in wide sheets of mud which harden into shale. At last, in the old age of the region (Fig. 157), its low plains contribute little to the sea except the soluble elements of the rocks, and in the clear waters near the land lime-secreting organisms flourish and their remains accumulate in beds of limestone. When long-weathered lands mantled with deep, well-oxidized waste are uplifted by a gradual movement of the earth’s crust, and themantle is rapidly stripped off by the revived streams, the uprise is recorded in wide deposits of red and yellow clays and sands upon the adjacent ocean floor.

Where the waste brought in is more than the waves can easily distribute, as off the mouths of turbid rivers which drain highlands near the sea, deposits are little winnowed, and are laid in rapidly alternating, shaly sandstones and sandy shales.

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