The Elements of Geology Part 11

BLOWHOLES AND SEA ARCHES. As a sea cave is drilled back into the rock, it may encounter a joint or crevice opened to the surface by percolating water. The shock of the waves soon enlarges this to a blowhole, which one may find on the breezy upland, perhaps a hundred yards and more back from the cliff's edge. In quiet weather the blowhole is a deep well; in storm it plays a fountain as the waves drive through the long tunnel below and spout their spray high in air in successive jets. As the roof of the cave thus breaks down in the rear, there may remain in front for a while a sea arch, similar to the natural bridges of land caverns.

STACKS AND WAVE-CUT ISLANDS. As the sea drives its tunnels and open drifts into the cliff, it breaks through behind the intervening portions and leaves them isolated as stacks, much as monuments are detached from inland escarpments by the weather; and as the sea cliff retreats, these remnant ma.s.ses may be left behind as rocky islets. Thus the rock bench is often set with stacks, islets in all stages of destruction, and sunken reefs, all wrecks of the land testifying to its retreat before the incessant attack of the waves.

COVES. Where zones of soft or closely jointed rock outcrop along a sh.o.r.e, or where minor water courses conic down to the sea and aid in erosion, the sh.o.r.e is worn back in curved reentrants 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 sh.o.r.e 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.

RATE OF RECESSION. The rate at which a sh.o.r.e 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 Yorks.h.i.+re, 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 sh.o.r.e of Martha's Vineyard wears back three feet a year. But hard rocks retreat so slowly that their recession has seldom been measured by the records of history.

Sh.o.r.e DRIFT

BOWLDER AND PEBBLE BEACHES. About as fast as formed the waste of the sea cliff is swept both along the sh.o.r.e and out to sea. The road of waste along sh.o.r.e is the BEACH. 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 this BOWLDER BEACH the 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 acc.u.mulate in a PEBBLE BEACH, made of pebbles well rounded by the wear which they have suffered. Such beaches form a mill whose raw material is constantly supplied 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 s.h.i.+ps 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 sh.o.r.e 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 sh.o.r.e.

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 weiglit 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 cus.h.i.+on 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 s.p.a.ce 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 Sh.o.r.e DRIFT MIGRATES. It is under the action of waves and currents that sh.o.r.e drift migrates slowly along a coast. Where waves strike a coast obliquely they drive the waste before them little by little along the sh.o.r.e. Thus on a north-south coast, where the predominant storms are from the northeast, there will be a migration of sh.o.r.e drift southwards.

All sh.o.r.es are swept also by currents produced by winds and tides.

These are usually far too gentle to transport of themselves the coa.r.s.e 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 sh.o.r.e. The road of sh.o.r.e drift is therefore the zone of the breaking waves.

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 a.s.sumes, under varying conditions, a number of distinct forms.

When swept into the head of a sheltered bay it const.i.tutes 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 sh.o.r.e 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 waste is carried out from sh.o.r.e 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 sh.o.r.e 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 sh.o.r.e drift now moves without interruption, and becomes a bay bar.

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 sh.o.r.e drift and are then cla.s.sified as HOOKS (Fig. 142).

SAND REEFS. On low coasts where shallow water extends some distance out, the highway of sh.o.r.e drift lies along a low, narrow ridge, termed the sand reef, separated from the land by a narrow stretch of shallow water called the LAGOON. At intervals the reef is held open by INLETS,--gaps through which the tide flows and ebbs, and by which the water of streams finds way to the sea.

No finer example of this kind of sh.o.r.e 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 Pa.s.s, 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 sh.o.r.e 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 alongsh.o.r.e drift.

Sand reefs are formed where the water is so shallow near sh.o.r.e 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.

Sh.o.r.eS 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 that the meeting place of sea and land is determined primarily by movements of the earth's crust. Where a coast land emerges the--sh.o.r.e line moves seaward; where it is being submerged the sh.o.r.e line advances on the land.

Sh.o.r.eS 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 sh.o.r.e 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 alongsh.o.r.e 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 gra.s.ses 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 sh.o.r.e line across the lagoon or meadow, cut a line of low cliffs on the mainland. Such a sh.o.r.e is that of Gascony in southwestern France,--a low, straight, sandy sh.o.r.e, bordered by dunes and unprotected by reefs from the attack of the waves of the Bay of Biscay.

We may say, then, that on sh.o.r.es 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 offsh.o.r.e and constantly renews the sand reef. The energy of the waves is consumed in handling sh.o.r.e 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.

Sh.o.r.eS OF DEPRESSION

Where a coastal region is undergoing submergence the sh.o.r.e line moves landward. The horizontal plane of the sea now intersects an old land surface roughened by subaerial denudation. The sh.o.r.e 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 sh.o.r.es of depression are as varied as are the forms of the land partially submerged. We give a few typical ill.u.s.trations.

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 sh.o.r.e 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 CYCLE OF Sh.o.r.eS OF DEPRESSION. In its infantile stage the outline of a sh.o.r.e 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 sh.o.r.e waste has been formed. The coast of Maine is an example of this stage.

In early youth all promontories have been strongly cliffed, and under a vigorous attack of the sea the sh.o.r.e of open bays may be cut back also. Sea stacks and rocky islets, caves and coves, make the sh.o.r.e 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. Sh.o.r.e 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 youth is characterized by a large acc.u.mulation of sh.o.r.e waste. The rock bench has been cut back so that it now furnishes a good roadway for sh.o.r.e drift. The stream of alongsh.o.r.e 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 alongsh.o.r.e.

The Baltic coast of Germany is in this stage.

All this while streams have been busy filling with delta deposits the bays into which they empty. By these steps a coast gradually advances to MATURITY, the stage when the irregularities due to depression have been effaced, when outlying islands formed by subsidence have been planed away, and when the sh.o.r.e 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 sh.o.r.e, it sooner or later rectifies it, making it simple and smooth. Northwestern France may be cited as an upland plain, dissected and depressed, whose coast has reached maturity.

In the OLD AGE of coasts the rock bench is cut back so far that the waves can no longer exert their full effect upon the sh.o.r.e.

Their energy is dissipated in moving sh.o.r.e drift hither and thither and in abrading the bench when they drag bottom upon 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.

PLAINS OF MARINE ABRASION. While subaerial denudation reduces the land to baselevel, the sea is sawing its edges to WAVE 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 subaerial 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 pare a 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 subaerial 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; sh.o.r.e waste cannot impede the onset of the waves, for it is continually washed out in deeper water below wave base.

BASAL CONGOLMERATES. 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

OFFSh.o.r.e AND DEEP-SEA DEPOSITS

The alongsh.o.r.e 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 offsh.o.r.e deposits are laid in belts parallel to the coast, the coa.r.s.est materials lying nearest to the land and the finest farthest out. The pebbles and gravel and the clean, coa.r.s.e 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 offsh.o.r.e movement of waste by which it is sorted, the coa.r.s.er being sooner dropped and the finer being carried farther out.

OFFSh.o.r.e DEPOSITS