Chapter 9

Fig. 157.Offshore Deposits recording Old Age of the Adjacent Landss, sandstone;sh, shale;lm, limestone

Where the highlands are of igneous rock, such as granite, and mechanical disintegration is going on more rapidly than chemical decay, these conditions are recorded in the nature of the deposits laid offshore. The waste swept in by streams contains much feldspar and other minerals softer and more soluble than quartz, and where the waves have little opportunity to wear and winnow it, it comes to rest in beds of sandstone in which grains of feldspar and other soft minerals are abundant. Such feldspathic sandstones are known asarkose.

On the other hand, where the waste supplied to the sea comes chiefly from wide, sandy, coastal plains, there are deposited off- shore clean sandstones of well-worn grains of quartz alone. In such coastal plains the waste of the land is stored for ages. Again and again they are abandoned and invaded by the sea as from time to time the land slowly emerges and is again submerged. Their deposits are long exposed to the weather, and sorted over by the streams, and winnowed and worked over again and again by the waves. In the course of long ages such deposits thus become thoroughly sorted, and the grains of all minerals softer than quartz are ground to mud.

Deep-Sea Oozes and Clays

Fig. 158.Globigerina Oozeunder the Microscope

Globigerina ooze.Beyond the reach of waste from the land the bottom of the deep sea is carpeted for the most part with either chalky ooze or a fine red clay. The surface waters of the warm seas swarm with minute and lowly animals belonging to the order of theForaminifera, which secrete shells of carbonate of lime. At death these tiny white shells fall through the sea water like snowflakes in the air, and, slowly dissolving, seem to melt quite away before they can reach depths greater than about three miles. Near shore they reach bottom, but are masked by the rapid deposit of waste derived from the land. At intermediate depths they mantle the ocean floor with a white, soft lime deposit known asGlobigerina ooze, from a genus of the Foraminifera which contributes largely to its formation.

Red clay.Below depths of from fifteen to eighteen thousand feet the ocean bottom is sheeted with red or chocolate colored clay. It is the insoluble residue of seashells, of the débris of submarine volcanic eruptions, of volcanic dust wafted by the winds, and of pieces of pumice drifted by ocean currents far from the volcanoes from which they were hurled. The red clay builds up with such inconceivable slowness that the teeth of sharks and the hard ear bones of whales may be dredged in large numbers from the deep ocean bed, where they have lain unburied for thousands of years; and an appreciable part of the clay is also formed by the dust of meteorites consumed in the atmosphere,—a dust which falls everywhere on sea and land, but which elsewhere is wholly masked by other deposits.

The dark, cold abysses of the ocean are far less affected by change than any other portion of the surface of the lithosphere. These vast, silent plains of ooze lie far below the reach of storms. They know no succession of summer and winter, or of night and day. A mantle of deep and quiet water protects them from the agents of erosion which continually attack, furrow, and destroy the surface of the land. While the land is the area of erosion, the sea is the area of deposition. The sheets of sediment which are slowly spread there tend to efface any inequalities, and to form a smooth and featureless subaqueous plain.

With few exceptions, the stratified rocks of the land are proved by their fossils and composition to have been laid in the sea; but in the same way they are proved to be offshore, shallow-water deposits, akin to those now making on continental shelves. Deep- sea deposits are absent from the rocks of the land, and we may therefore infer that the deep sea has never held sway where the continents now are,—that the continents have ever been, as now, the elevated portions of the lithosphere, and that the deep seas of the present have ever been its most depressed portions.

The Reef-Building Corals

In warm seas the most conspicuous of rock-making organisms are the corals known as the reef builders. Floating in a boat over a coral reef, as, for example, off the south coast of Florida or among the Bahamas, one looks down through clear water on thickets of branching coral shrubs perhaps as much as eight feet high, and hemispherical masses three or four feet thick, all abloom with countless minute flowerlike coral polyps, gorgeous in their colors of yellow, orange, green, and red. In structure each tiny polyp is little more than a fleshy sac whose mouth is surrounded with petal-like tentacles, or feelers. From the sea water the polyps secrete calcium carbonate and build it up into the stony framework which supports their colonies. Boringmollusks, worms, and sponges perforate and honeycomb this framework even while its surface is covered with myriads of living polyps. It is thus easily broken by the waves, and white fragments of coral trees strew the ground beneath. Brilliantly colored fishes live in these coral groves, and countless mollusks, sea urchins, and other forms of marine life make here their home. With the débris from all these sources the reef is constantly built up until it rises to low-tide level. Higher than this the corals cannot grow, since they are killed by a few hours’ exposure to the air.

Fig. 159.Patch of Growing Corals exposed at an Exceptionally Low Tide, Great Barrier Reef, Australia

When the reef has risen to wave base, the waves abrade it on the windward side and pile to leeward coral blocks torn from their foundation, filling the interstices with finer fragments. Thus they heap up along the reef low, narrow islands (Fig. 160).

Reef building is a comparatively rapid progress. It has been estimated that off Florida a reef could be built up to the surface from a depth of fifty feet in about fifteen hundred years.

Fig. 160.Wave-Built Island on Coral Reefr, reef;sl, sea level

Coral limestones.Limestones of various kinds are due to the reef builders. The reef rock is made of corals in place and broken fragments of all sizes, cemented together with calcium carbonate from solution by infiltrating waters. On the island beaches coral sand is forming oolitic limestone, and the white coral mud with which the sea is milky for miles about the reef in times of storm settles and concretes into a compact limestone of finest grain. Corals have been among the most important limestone builders of the sea ever since they made their appearance in the early geological ages.

The areas on which coral limestone is now forming are large. The Great Barrier Reef of Australia, which lies off the north-eastern coast, is twelve hundred and fifty miles long, and has a width of from ten to ninety miles. Most of the islands of the tropics are either skirted with coral reefs or are themselves of coral formation.

Conditions of coral growth.Reef-building corals cannot live except in clear salt water less, as a rule, than one hundred and fifty feet in depth, with a winter temperature not lower than 68° F. An important condition also is an abundant food supply, and this is best secured in the path of the warm oceanic currents.

Coral reefs may be grouped in three classes,—fringing reefs, barrier reefs, and atolls.

Fringing reefs.These take their name from the fact that they are attached as narrow fringes to the shore. An example is the reef which forms a selvage about a mile wide along the northeastern coast of Cuba. The outer margin, indicated by the line of white surf, where the corals are in vigorous growth, rises from about forty feet of water.Between this and the shore lies a stretch of shoal across which one can wade at low water, composed of coral sand with here and there a clump of growing coral.

Barrier reefs.Reefs separated from the shore by a ship channel of quiet water, often several miles in width and sometimes as much as three hundred feet in depth, are known as barrier reefs. The seaward face rises abruptly from water too deep for coral growth. Low islands are cast up by the waves upon the reef, and inlets give place for the ebb and flow of the tides. Along the west coast of the island of New Caledonia a barrier reef extends for four hundred miles, and for a length of many leagues seldom approaches within eight miles of the shore.

Atolls.These are ring-shaped or irregular coral islands, or island-studded reefs, inclosing a central lagoon. The narrow zone of land, like the rim of a great bowl sunken to the water’s edge, rises hardly more than twenty feet at most above the sea, and is covered with a forest of trees such as the cocoanut, whose seeds can be drifted to it uninjured from long distances. The white beach of coral sand leads down to the growing reef, on whose outer margin the surf is constantly breaking. The sea face of the reef falls off abruptly, often to depths of thousands of feet, while the lagoon varies in depth from a few feet to one hundred and fifty or two hundred, and exceptionally measures as much as three hundred and fifty feet.

Theories of coral reefs.Fringing reefs require no explanation, since the depth of water about them is not greater than that at which coral can grow; but barrier reefs and atolls, which may rise from depths too great for coral growth demand a theory of their origin.

Fig. 161.Diagram illustrating the Subsidence Theory of Coral Reefs

Darwin’s theory holds that barrier reefs and atolls are formed from fringing reefs bysubsidence. The rate of sinking cannot be greater than that of the upbuilding of the reef, since otherwise the corals would be carried below their depth and drowned.The process is illustrated inFigure 161, where v represents a volcanic island in mid ocean undergoing slow depression, andssthe sea level before the sinking began, when the island was surrounded by a fringing reef. As the island slowly sinks, the reef builds up with equal pace. It rears its seaward face more steep than the island slope, and thus the intervening space between the sinking, narrowing land and the outer margin of the reef constantly widens. In this intervening space the corals are more or less smothered with silt from the outer reef and from the land, and are also deprived in large measure of the needful supply of food and oxygen by the vigorous growth of the corals on the outer rim. The outer rim thus becomes a barrier reef and the inner belt of retarded growth is deepened by subsidence to a ship channel,s´s´representing sea level at this time. The final stage, where the island has been carried completely beneath the sea and overgrown by the contracting reef, whose outer ring now forms an atoll, is represented bys´´s´´.

Fig. 162.Barrier Reef formed without Subsidencea, zone of coral growth;f, former fringing reef;t, talus;b, barrier reef

In very many instances, however, atolls and barrier reefs may be explained without subsidence. Thus a barrier reef may be formed by the seaward growth of a fringing reef upon the talus of its sea face. InFigure 162,fis a fringing reef whose outer wall rises from about one hundred and fifty feet, the lower limit of the reef-building species. At the foot of this submarine cliff a talus of fallen blocks t accumulates, and as it reaches the zoneof coral growth becomes the foundation on which the reef is steadily extended seaward. As the reef widens, the polyps of the circumference flourish, while those of the inner belt are retarded in their growth and at last perish. The coral rock of the inner belt is now dissolved by sea water and scoured out by tidal currents until it gives place to a gradually deepening ship channel, while the outer margin is left as a barrier reef.

Fig. 163.Section of Atoll on a Shoal which has been built up to near the Surface by Organic Deposits upon a Submarine Volcanic Peakv, volcano;f, foraminiferal deposits;m, molluscous shell deposits;c, coral reef;sl, sea level

In much the same way atolls may be built on any shoal which lies within the zone of coral growth. Such shoals may be produced when volcanic islands are leveled by waves and ocean currents, and when submarine plateaus, ridges, and peaks are built up by various organic agencies, such as molluscous and foraminiferal shell deposits (Fig. 163). The reef-building corals, whose eggs are drifted widely over the tropic seas by ocean currents, colonize such submarine foundations wherever the conditions are favorable for their growth. As the reef approaches the surface the corals of the inner area are smothered by silt and starved, and their Submarine Volcanic Peak hard parts are dissolved and scoured away; while those of the circumference, with abundant food supply, nourish and build the ring of the atoll. Atolls may be produced also by the backward drift of sand from either end of a crescentic coral reef or island, the spits uniting in the quiet water of the lee to inclose a lagoon. In the Maldive Archipelagoall gradations between crescent-shaped islets and complete atoll rings have been observed.

In a number of instances where coral reefs have been raised by movements of the earth’s crust, the reef formation is found to be a thin veneer built upon a foundation of other deposits. Thus Christmas Island, in the Indian Ocean, is a volcanic pile rising eleven hundred feet above sea level and fifteen thousand five hundred feet above the bottom of the sea. The summit is a plateau surrounded by a rim of hills of reef formation, which represent the ring of islets of an ancient atoll. Beneath the reef are thick beds of limestone, composed largely of the remains of foraminifers, which cover the lavas and fragmental materials of the old submarine volcano.Among the ancient sediments which now form the stratified rocks of the land there occur many thin reef deposits, but none are known of the immense thickness which modern reefs are supposed to reach according to the theory of subsidence.Barrier and fringing reefs are commonly interrupted off the mouths of rivers. Why?

Among the ancient sediments which now form the stratified rocks of the land there occur many thin reef deposits, but none are known of the immense thickness which modern reefs are supposed to reach according to the theory of subsidence.

Barrier and fringing reefs are commonly interrupted off the mouths of rivers. Why?

Summary.We have seen that the ocean bed is the goal to which the waste of the rocks of the land at last arrives. Their soluble parts, dissolved by underground waters and carried to the sea by rivers, are largely built up by living creatures into vast sheets of limestone. The less soluble portions—the waste brought in by streams and the waste of the shore—form the muds and sands of continental deltas. All of these sea deposits consolidate and harden, and the coherent rocks of the land are thus reconstructed on the ocean floor. But the destination is not a final one. The stratified rocks of the land are for the most part ancient deposits of the sea, which have been lifted above sea level; and we may believe that the sediments now being laid offshore are the “dust of continents to be,” and will some time emerge to form additions to the land. We are now to study the movements of the earth’s crust which restore the sediments of the sea to the light of day, and to whose beneficence we owe the habitable lands of the present.

PART II

INTERNAL GEOLOGICAL AGENCIES

CHAPTER IX

MOVEMENTS OF THE EARTH’S CRUST

The geological agencies which we have so far studied—weathering, streams, underground waters, glaciers, winds, and the ocean—all work upon the earth from without, and all are set in motion by an energy external to the earth, namely, the radiant energy of the sun. All, too, have a common tendency to reduce the inequalities of the earth’s surface by leveling the lands and strewing their waste beneath the sea.

But despite the unceasing efforts of these external agencies, they have not destroyed the continents, which still rear their broad plains and great plateaus and mountain ranges above the sea. Either, then, the earth is very young and the agents of denudation have not yet had time to do their work, or they have been opposed successfully by other forces.

We enter now upon a department of our science which treats of forces which work upon the earth from within, and increase the inequalities of its surface. It is they which uplift and recreate the lands which the agents of denudation are continually destroying; it is they which deepen the ocean bed and thus withdraw its waters from the shores. At times also these forces have aided in the destruction of the lands by gradually lowering them and bringing in the sea. Under the action of forces resident within the earth the crust slowly rises or sinks; fromtime to time it has been folded and broken; while vast quantities of molten rock have been pressed up into it from beneath and outpoured upon its surface. We shall take up these phenomena in the following chapters, which treat of upheavals and depressions of the crust, foldings and fractures of the crust, earthquakes, volcanoes, the interior conditions of the earth, mineral veins, and metamorphism.

Oscillations of the Crust

Of the various movements of the crust due to internal agencies we will consider first those called oscillations, which lift or depress large areas so slowly that a long time is needed to produce perceptible changes of level, and which leave the strata in nearly their original horizontal attitude. These movements are most conspicuous along coasts, where they can be referred to the datum plane of sea level; we will therefore take our first illustrations from rising and sinking shores.

New Jersey.Along the coasts of New Jersey one may find awash at high tide ancient shell heaps, the remains of tribal feasts of aborigines. Meadows and old forest grounds, with the stumps still standing, are now overflowed by the sea, and fragments of their turf and wood are brought to shore by waves. Assuming that the sea level remains constant, it is clear that the New Jersey coast is now gradually sinking. The rate of submergence has been estimated at about two feet per century.On the other hand, the wide coastal plain of New Jersey is made of stratified sands and clays, which, as their marine fossils show, were outspread beneath the sea. Their present position above sea level proves that the land now subsiding emerged in the recent past.

On the other hand, the wide coastal plain of New Jersey is made of stratified sands and clays, which, as their marine fossils show, were outspread beneath the sea. Their present position above sea level proves that the land now subsiding emerged in the recent past.

The coast of New Jersey is an example of the slow and tranquil oscillations of the earth’s unstable crust now in progress along many shores. Some are emerging from the sea, some are sinking beneath it; and no part of the land seems to have been exempt from these changes in the past.

Evidences of changes of level.Taking the surface of the sea as a level of reference, we may accept as proofs of relative upheaval whatever is now found in place above sea level and could have been formed only at or beneath it, and as proofs of relative subsidence whatever is now found beneath the sea and could only have been formed above it.

Thus old strand lines with sea cliffs, wave-cut rock benches, and beaches of wave-worn pebbles or sand, are striking proofs of recent emergence to the amount of their present height above tide. No less conclusive is the presence of sea-laid rocks which we may find in the neighboring quarry or outcrop, although it may have been long ages since they were lifted from the sea to form part of the dry land.

Among common proofs of subsidence are roads and buildings and other works of man, and vegetal growths and deposits, such as forest grounds and peat beds, now submerged beneath the sea. In the deltas of many large rivers, such as the Po, the Nile, the Ganges, and the Mississippi, buried soils prove subsidences of hundreds of feet; and in several cases, as in the Mississippi delta, the depression seems to be now in progress.

Other proofs of the same movement are drowned land forms which are modeled only in open air. Since rivers cannot cut their valleys farther below the baselevel of the sea than the depths of their channels,drowned valleysare among the plainest proofs of depression. To this class belong Narragansett, Delaware, Chesapeake, Mobile, and San Francisco bays, and many other similar drowned valleys along the coasts of the United States. Less conspicuous are thesubmarine channelswhich, as soundings show, extend from the mouths of a number of rivers some distance out to sea. Such is the submerged channel which reaches from New York Bay southeast to the edge of the continental shelf, and which is supposed to have been cut by the Hudson River when this part of the shelf was a coastal plain.

Warping.In a region undergoing changes of level the rate of movement commonly varies in different parts. Portions of an area may be rising or sinking, while adjacent portions are stationary or moving in the opposite direction. In this way a land surface becomeswarped. Thus, while Nova Scotia and New Brunswick are now rising from the level of the sea, Prince Edward Island and Cape Breton Island are sinking, and the sea now flows over the site of the famous old town of Louisburg destroyed in 1758.

Since the close of the glacial epoch the coasts of Newfoundland and Labrador have risen hundreds of feet, but the rate of emergence has not been uniform. The old strand line, which stands at five hundred and seventy-five feet above tide at St. John’s, Newfoundland, declines to two hundred and fifty feet near the northern point of Labrador (Fig. 164).

Fig. 164.Warped Strand Line from St. John’s, Newfoundland, to Nachvak, Labrador

The Great Lakesis now under-going perceptible warping. Rivers enter the lakes from the south and west with sluggish currents and deep channels resembling the estuaries of drowned rivers; while those that enter from opposite directions are swift and shallow. At the western end of Lake Erie are found submerged caves containing stalactites, and old meadows and forest grounds are now under water. It is thus seen that the water of the lakes is rising along their southwestern shores, while from their north-eastern shores it is being withdrawn. The region of the Great Lakes is therefore warping; it is rising in the northeast as compared with the southwest.From old bench marks and records of lake levels it has been estimated thatthe rate of warpingamounts to five inches a century for every one hundred miles. It is calculated that the water of Lake Michigan is rising at Chicago at the rate of nine or ten inches per century. The divide at this point between the tributaries of the Mississippi and Lake Michigan is but eight feet above the mean stage of the lake. If thecanting of the region continues at its present rate, in a thousand years the waters of the lake will here overflow the divide. In three thousand five hundred years all the lakes except Ontario will discharge by this outlet, via the Illinois and Mississippi rivers, into the Gulf of Mexico. The present outlet by the Niagara River will be left dry, and the divide between the St. Lawrence and the Mississippi systems will have shifted from Chicago to the vicinity of Buffalo.

From old bench marks and records of lake levels it has been estimated thatthe rate of warpingamounts to five inches a century for every one hundred miles. It is calculated that the water of Lake Michigan is rising at Chicago at the rate of nine or ten inches per century. The divide at this point between the tributaries of the Mississippi and Lake Michigan is but eight feet above the mean stage of the lake. If thecanting of the region continues at its present rate, in a thousand years the waters of the lake will here overflow the divide. In three thousand five hundred years all the lakes except Ontario will discharge by this outlet, via the Illinois and Mississippi rivers, into the Gulf of Mexico. The present outlet by the Niagara River will be left dry, and the divide between the St. Lawrence and the Mississippi systems will have shifted from Chicago to the vicinity of Buffalo.

Physiographic effects of oscillations.We have already mentioned several of the most important effects of movements of elevation and depression, such as their effects on rivers, the mantle of waste (pp.85,86), and the forms of coasts (p. 166). Movements of elevation—including uplifts by folding and fracture of the crust to be noticed later— are the necessary conditions for erosion by whatever agent. They determine the various agencies which are to be chiefly concerned m the wear of any land,—whether streams or glaciers, weathering or the wind,—and the degree of their efficiency. The lands must be uplifted before they can be eroded, and since they must be eroded before their waste can be deposited, movements of elevation are a prerequisite condition for sedimentation also. Subsidence is a necessary condition for deposits of great thickness, such as those of the Great Valley of California and the Indo-Gangetic plain (p. 101), the Mississippi delta (p. 109), and the still more important formations of the continental delta in gradually sinking troughs (p. 183). It is not too much to say that the character and thickness of each formation of the stratified rocks depend primarily on these crustal movements.

Along the Baltic coast of Sweden, bench marks show that the sea is withdrawing from the land at a rate which at the north amounts to between three and four feet per century; Towards the south the rate decreases. South of Stockholm, until recent years, the sea has gained upon the land, and here in several seaboard towns streets by the shore are still submerged. The rate of oscillation increases also from the coast inland. On the other hand, along the German coast of the Baltic the only historic fluctuations of sea level are those which may be accounted for by variations due to changes in rainfall. In 1730 Celsius explained the changes of level of the Swedish coast as due to a lowering of the Baltic instead of to an elevation of the land. Are the facts just stated consistent with his theory?

Fig. 165.Old Strand Lines, Tadousac, Quebec

At the little town of Tadousac—where the Saguenay River empties into the St. Lawrence—there are terraces of old sea beaches, some almost as fresh as recent railway fills, the highest standing two hundred and thirty feet above the river (Fig. 165). Here the Saguenay is eight hundred and forty feet in depth, and the tide ebbs and flows far up its stream. Was its channel cut to this depth by the river when the land was at its present height? What oscillations are here recorded, and to what amount?

Fig. 166.Diagram showing Ruins of Temple, North of NaplesC, ancient sea cliff;m, marble pillars, dotted where bored by mollusks;sl, sea level

A few miles north of Naples, Italy, the ruins of an ancient Roman temple lie by the edge of the sea, on a narrow plain which is overlooked in the rear by an old sea cliff (Fig. 166). Three marble pillars are still standing. For eleven feet above their bases these columns are uninjured, for to this height they were protected by an accumulation of volcanic ashes; but from eleven to nineteen feet they are closely pitted with the holes of boring marine mollusks. From these facts trace the history of the oscillations of the region.

Fig. 167.Section in a Region of Folded Rocks

Foldings of the Crust

The oscillations which we have just described leave the strata not far from their original horizontal attitude.Figure 167represents a region in which movements of a very different naturehave taken place. Here, on either side of the valleyv, we find outcrops of layers tilted at high angles. Sections along the ridgershow that it is composed of layers which slant inward from either side. In places the outcropping strata stand nearly on edge, and on the right of the valley they are quite overturned; a shaleshhas come to overlie a limestonelmalthough the shale is the older rock, whose original position was beneath the limestone.

Fig. 168.Dip and Strike

It is not reasonable to suppose that these rocks were deposited in the attitude in which we find them now; we must believe that, like other stratified rocks, they were outspread in nearly level sheets upon the ocean floor. Since that time they must have been deformed. Layers of solid rock several miles in thickness have been crumpled and folded like soft wax in the hand, and a vast denudation has worn away the upper portions of the folds, in part represented in our section by dotted lines.

Dip and strike.In districts where the strata have been disturbed it is desirable to record their attitude. This is most easily done by taking the angle at which the strata are inclined and the compass direction in which they slant. It is also convenient to record the direction in which the outcrop of the strata trends across the country.

Fig. 169.An Anticline, Maryland

The inclination of a bed of rocks to the horizon is itsdip(Fig. 168). The amount of the dip is the angle made with ahorizontal plane. The dip of a horizontal layer is zero, and that of a vertical layer is 90°. The direction of the dip is taken with the compass. Thus a geologist’s notebook in describing the attitude of outcropping strata contains many such entries as these: dip 32° north, or dip 8° south 20° west,—meaning in the latter case that the amount of the dip is 8° and the direction of the dip bears 20° west of south.

The line of intersection of a layer with the horizontal plane is thestrike. The strike always runs at right angles to the dip.

Dip and strike may be illustrated by a book set aslant on a shelf. The dip is the acute angle made with the shelf by the side of the book, while the strike is represented by a line running along the book’s upper edge. If the dip is north or south, the strike runs east and west.

Fig. 170.Folded Strata, Coast of EnglandA syncline in the center, with an anticline on either side

Folded structures.An upfold, in which the strata dip away from a line drawn along the crest and called the axis of the fold, is known as ananticline(Fig. 169). A downfold, where the strata dip from either side toward the axis of the trough, iscalled asyncline(Fig. 170). There is sometimes seen a downward bend in horizontal or gently inclined strata, by which they descend to a lower level. Such a single flexure is amonocline(Fig. 171).

Fig. 171.A Monocline

Degrees of folding.Folds vary in degree from broad, low swells, which can hardly be detected, to the most highly contorted and complicated structures. Insymmetricfolds (Figs.169and180) the dips of the rocks on each side the axis of the fold are equal. Inunsymmetricalfolds one limb is steeper than the other, as in the anticline inFigure 167. Inoverturnedfolds (Figs.167and172) one limb is inclined beyond the perpendicular.Fan foldshave been so pinched that the original anticlines are left broader at the top than at the bottom (Fig. 173).

Fig. 172.Overturned Fold, Vermont

In folds where the compression has been great the layers are often found thickened at the crest and thinned along the limbs (174). Where strong rocks such as heavy limestones are folded together withweak rocks such as shales, the strong rocks are often bent into great simple folds, while the weak rocks are minutely crumpled.

Fig. 173.Fan Folds, the Alps

Systems of folds.As a rule, folds occur in systems. Over the Appalachian mountain belt, for example, extending from northeastern Pennsylvania to northern Alabama and Georgia, the earth’s crust has been thrown into a series of parallel folds whose axes run from northeast to southwest (Fig. 175). In Pennsylvania one may count a score or more of these earth waves,— some but from ten to twenty miles in length, and some extending as much as two hundred miles before they die away. On the eastern part of this belt the folds are steeper and more numerous than on the western side.

Fig. 174.Folds with Layers thickened at the Crest and thinned along the Limbs

Cause and conditions of folding.The sections which we have studied suggest that rocks are folded by lateral pressure. While a single, simple fold might be produced by a heave, a series of folds, including overturns, fan folds, and folds thickened on their crests at the expense of their limbs, could only be made in one way,—by pressure from the side. Experiment has reproduced all forms of folds by subjecting to lateral thrust layers of plastic material such as wax.

Vast as the force must have been which could fold the solid rocks of the crust as one may crumple the leaves of a magazine in the fingers, it is only under certain conditions that it could have produced the results which we see. Rocks are brittle, and it is only when under aheavy loadand bygreat pressure slowly applied, that they can thus be foldedand bent instead of being crushed to pieces. Under these conditions, experiments prove that not only metals such as steel, but also brittle rocks such as marble, can be deformed and molded and made to flow like plastic clay.

Fig. 175.Relief Map of the Northern Appalachian RegionFrom Bingham’sGeographic Influences in American History

Zone of flow, zone of flow and fracture, and zone of fracture.We may believe that at depths which must be reckoned in tens of thousands of feet the load of overlying rocks is so great that rocks of all kinds yield by folding to lateral pressure, and flow instead of breaking. Indeed, at such profound depths and under such inconceivable weight no cavity can form, and any fractures would be healed at once by the welding of grain to grain. At less depths there exists a zone where soft rocks fold and flow under stress, and hard rocks are fractured; while at and near the surface hard and soft rocks alike yield by fracture to strong pressure.

Structures developed in Compressed Rocks

Deformed rocks show the effects of the stresses to which they have yielded, not only in the immense folds into which they have been thrown but in their smallest parts as well. A hand specimen of slate, or even a particle under the microscope, may show plications similar in form and origin to the foldings which have produced ranges of mountains. A tiny flake of mica in the rocks of the Alps may be puckered by the same resistless forces which have folded miles of solid rock to form that lofty range.

Slaty cleavage.Rocks which have yielded to pressure often split easily in a certain direction across the bedding planes. This cleavage is known as slaty cleavage, since it is most perfectly developed in fine-grained, homogeneous rocks, such as slates, which cleave to the thin, smooth-surfaced plates with which we are familiar in the slates used in roofing and for ciphering and blackboards. In coarse-grained rocks, pressure develops more distant partings which separate the rocks into blocks.

Slaty cleavage cannot be due to lamination, since it commonly crosses bedding planes at an angle, while these planes have been often well-nigh or quite obliterated. Examining slate witha microscope, we find that its cleavage is due to the grain of the rock. Its particles are flattened and lie with their broad faces in parallel planes, along which the rock naturally splits more easily than in any other direction. The irregular grains of the mud which has been altered to slate have been squeezed flat by a pressure exerted at right angles to the plane of cleavage. Cleavage is found only in folded rocks, and, as we may see inFigure 176, the strike of the cleavage runs parallel to the strike of the strata and the axis of the folds. The dip of the cleavage is generally steep, hence the pressure was nearly horizontal. The pressure which has acted at right angles to the cleavage, and to which it is due, is the same lateral pressure which has thrown the strata into folds.

Fig. 176.Slaty Cleavage

We find additional proof that slates have undergone compression at right angles to their cleavage in the fact that any inclusions in them, such as nodules and fossils, have been squeezed out of shape and have their long diameters lying in the planes of cleavage.That pressure is competent to cause cleavage is shown by experiment. Homogeneous material of fine grain, such as beeswax, when subjected to heavy pressure cleaves at right angles to the direction of the compressing force.

That pressure is competent to cause cleavage is shown by experiment. Homogeneous material of fine grain, such as beeswax, when subjected to heavy pressure cleaves at right angles to the direction of the compressing force.

Rate of folding.All the facts known with regard to rock deformation agree that it is a secular process, taking place so slowly that, like the deepening of valleys by erosion, it escapes the notice of the inhabitants of the region. It is only under stresses slowly applied that rocks bend without breaking. The folds of some of the highest mountains have risen so gradually that strong, well-intrenched rivers which had the right of way across the region were able to hold to their courses, and asa circular saw cuts its way through the log which is steadily driven against it, so these rivers sawed their gorges through the fold as fast as it rose beneath them. Streams which thus maintain the course which they had antecedent to a deformation of the region are known asantecedentstreams. Examples of such are the Sutlej and other rivers of India, whose valleys trench the outer ranges of the Himalayas and whose earlier river deposits have been upturned by the rising ridges. On the other hand, mountain crests are usually divides, parting the head waters of different drainage systems. In these cases the original streams of the region have been broken or destroyed by the uplift of the mountain mass across their paths.

On the whole, which have worked more rapidly, processes of deformation or of denudation?

Land Forms due to Folding

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