Chapter 10

Fig. 177.An Unroofed Anticline

As folding goes on so slowly, it is never left to form surface features unmodified by the action of other agencies. An anticlinal fold is attacked by erosion as soon as it begins to rise above the original level, and the higher it is uplifted, and the stronger are its slopes, the faster is it worn away. Even while rising, a young upfold is often thus unroofed, and instead of appearing as a long, Smooth, boat-shaped ridge, it commonly has had opened along the rocks of the axis, when these are weak, a valley which is overlooked by the infacing escarpments of the hard layers of the sides of the fold (Fig. 177). Under long-continued erosion, anticlines may bedegraded to valleys, while the synclines of the same system may be left in relief as ridges (Fig. 167).

Folded mountains.The vastness of the forces which wrinkle the crust is best realized in the presence of some lofty mountain range. All mountains, indeed, are not the result of folding. Some, as we shall see, are due to upwarps or to fractures of the crust; some are piles of volcanic material; some are swellings caused by the intrusion of molten matter beneath the surface; some are the relicts left after the long denudation of high plateaus.

Fig. 178.Mountain Peaks carved in Folded Strata, Rocky Mountains, Montana

But most of the mountain ranges of the earth, and some of the greatest, such as the Alps and the Himalayas, were originally mountains of folding. The earth’s crust has wrinkled into a fold; or into a series of folds, forming a series of parallel ridges and intervening valleys; or a number of folds have been mashed together into a vast upswelling of the crust, in which the layers have been so crumpled and twisted, overturned andcrushed, that it is exceedingly difficult to make out the original structure.

The close and intricate folds seen in great mountain ranges were formed, as we have seen, deep below the surface, within the zone of folding. Hence they may never have found expression in any individual surface features. As the result of these deformations deep under ground the surface was broadly lifted to mountain height, and the crumpled and twisted mountain structures are now to be seen only because erosion has swept away the heavy cover of surface rocks under whose load they were developed.

Fig. 179.Section of a Portion of the Alps

When the structure of mountains has been deciphered it is possible to estimate roughly the amount of horizontal compression which the region has suffered. If the strata of the folds of the Alps were smoothed out, they would occupy a belt seventy-four miles wider than that to which they have been compressed, or twice their present width. A section across the Appalachian folds in Pennsylvania shows a compression to about two thirds the original width; the belt has been shortened thirty-five miles in every hundred.Considering the thickness of their strata, the compression which mountains have undergone accounts fully for their height, with enough to spare for all that has been lost by denudation.The Appalachian folds involve strata thirty thousand feet in thickness. Assuming that the folded strata rested on an unyielding foundation, and that what was lost in width was gained in height, what elevation would the range have reached had not denudation worn it as it rose?

Considering the thickness of their strata, the compression which mountains have undergone accounts fully for their height, with enough to spare for all that has been lost by denudation.

The Appalachian folds involve strata thirty thousand feet in thickness. Assuming that the folded strata rested on an unyielding foundation, and that what was lost in width was gained in height, what elevation would the range have reached had not denudation worn it as it rose?

The life history of mountains.While the disturbance and uplift of mountain masses are due to deformation, their sculpture into ridges and peaks, valleys and deep ravines, and all the forms which meet the eye in mountain scenery, excepting in the very youngest ranges, is due solely to erosion. We may therefore classify mountains according to the degree to which they have been dissected. The Juras are an example of the stage of early youth, in which the anticlines still persist as ridges and the synclines coincide with the valleys; this they owe as much to the slight height of their uplift as to the recency of its date (Fig. 180).

Fig. 180.Section of a Portion of the Jura Mountains

The Alps were upheaved at various times (p. 399), the last uplift being later than the uplift of the Juras, but to so much greater height that erosion has already advanced them well on towards maturity. The mountain mass has been cut to the core, revealing strange contortions of strata which could never have found expression at the surface. Sharp peaks, knife-edged crests, deep valleys with ungraded slopes subject to frequent landslides, are all features of Alpine scenery typical of a mountain range at this stage in its life history. They represent the survival of the hardest rocks and the strongest structures, and the destruction of the weaker in their long struggle for existence against the agents of erosion. Although miles of rock have been removed from such ranges as the Alps, we need not suppose that they ever stood much, if any, higher than at present. All this vast denudation may easily have been accomplished while their slow upheaval was going on; in several mountain ranges we have evidence that elevation has not yet ceased.

Fig. 181.Young Mountains, Rocky Mountains of Canada

Under long denudation mountains are subdued to the forms characteristic of old age. The lofty peaks and jagged crests of their earlier life are smoothed down to low domes and rounded crests. The southern Appalachians and portions of the Hartz Mountains in Germany (Fig. 182) are examples of mountains which have reached this stage.

Fig. 182.Subdued Mountains, the Hartz Mountains, Germany

There are numerous regions of upland and plains in which the rocks are found to have the same structure that we have seen in folded mountains; they are tilted, crumpled, and overturned, and have clearly suffered intense compression. We may infer that their folds were once lifted to the height of mountains and have since been wasted to low-lying lands. Such a section as that ofFigure 67illustrates how ancient mountains may be leveled to their roots, and represents the final stage to which even the Alps and the Himalayas must sometime arrive. Mountains, perhaps of Alpine height, once stood about Lake Superior; a lofty range once extended from New England and New Jersey southwestward to Georgia along the Piedmont belt. In our study of historic geology we shall see more clearly howshort is the life of mountains as the earth counts time, and how great ranges have been lifted, worn away, and again upheaved into a new cycle of erosion.

The sedimentary history of folded mountains.We may mention here some of the conditions which have commonly been antecedent to great foldings of the crust.

1. Mountain ranges are made of belts of enormously and exceptionally thick sediments. The strata of the Appalachians are thirty thousand feet thick, while the same formations thin out to five thousand feet in the Mississippi valley. The folds of the Wasatch Mountains involve strata thirty thousand feet thick, which thin to two thousand feet in the region of the Plains.

2. The sedimentary strata of which mountains are made are for the most part the shallow-water deposits of continental deltas. Mountain ranges have been upfolded along the margins of continents.

3. Shallow-water deposits of the immense thickness found in mountain ranges can be laid only in a gradually sinking area. A profound subsidence, often to be reckoned in tens of thousands of feet, precedes the upfolding of a mountain range.

Thus the history of mountains of folding is as follows: For long ages the sea bottom off the coast of a continent slowly subsides, and the great trough, as fast as it forms, is filled with sediments, which at last come to be many thousands of feet thick. The downward movement finally ceases. A slow but resistless pressure sets in, and gradually, and with a long series of many intermittent movements, the vast mass of accumulated sediments is crumpled and uplifted into a mountain range.

Fractures and Dislocations of the Crust

Considering the immense stresses to which the rocks of the crust are subjected, it is not surprising to find that they often yield by fracture, like brittle bodies, instead of by folding andflowing, like plastic solids. Whether rocks bend or break depends on the character and condition of the rocks, the load of overlying rocks which they bear, and the amount of the force and the slowness with which it is applied.

Joints.At the surface, where their load is least, we find rocks universally broken into blocks of greater or less size by partings known as joints. Under this name are included many division planes caused by cooling and drying; but it is now generally believed that the larger and more regular joints, especially those which run parallel to the dip and strike of the strata, are fractures due to up-and-down movements and foldings and twistings of the rocks.

Fig. 183.Joints utilized by a River in widening its Valley, Iowa

Joints are used to great advantage in quarrying, and we have seen how they are utilized by the weather in breaking up rock masses, by rivers in widening their valleys, by the sea in driving back its cliffs, by glaciers in plucking their beds, and how they are enlarged in soluble rocks to form natural passageways for underground waters. The ends of the parted strata match alongboth sides of joint planes; in. joints there has been little or no displacement of the broken rocks.

Fig. 184.A Normal Fault

Faults.InFigure 184the rocks have been both broken and dislocated along the planeff´. One side must have been moved up or down past the other. Such a dislocation is called a fault. The amount of the displacement, as measured by the vertical distance between the ends of a parted layer, is thethrow(cd). The angle (ff´v) which the fault plane makes with the vertical is thehade. InFigure 184the right side has gone down relatively to the left; the right is the side of the downthrow, while the left is the side of the upthrow. Where the fault plane is not vertical the surfaces on the two sides may be distinguished as thehanging wall(that on the right ofFigure 184) and thefoot wall(that on the left of the same figure). Faults differ in throw from a fraction of an inch to many thousands of feet.

Slickensides.If we examine the walls of a fault, we may find further evidence of movement in the fact that the surfaces are polished and grooved by the enormous friction which they have suffered as they have ground one upon the other. These appearances, called slickensides, have sometimes been mistaken for the results of glacial action.

Normal faults.Faults are of two kinds,—normal faults and thrust faults. Normal faults, of whichFigure 184is an example, hade to the downthrow; the hanging wall has gone down. The total length of the strata has been increased by the displacement. It seems that the strata have been stretched and broken, and that the blocks have readjusted themselves under the action of gravity as they settled.

Thrust faults.Thrust faults hade to the upthrow; the hanging wall has gone up. Clearly such faults, where the strata occupy less space than before, are due to lateral thrust. Foldsand thrust faults are closely associated. Under lateral pressure strata may fold to a certain point and then tear apart and fault along the surface of least resistance. Under immense pressure strata also break by shear without folding. Thus, inFigure 185, the rigid earth block under lateral thrust has found it easier to break along the fault plane than to fold. Where such faults are nearly horizontal they are distinguished asthrust planes.

Fig. 185.A Thrust Fault

In all thrust faults one mass has been pushed over another, so as to bring the underlying and older strata upon younger beds; and when the fault planes are nearly horizontal, and especially when the rocks have been broken into many slices which have slidden far one upon another, the true succession of strata is extremely hard to decipher.

In the Selkirk Mountains of Canada the basement rocks of the region have been driven east for seven miles on a thrust plane, over rocks which originally lay thousands of feet above them.Along the western Appalachians, from Virginia to Georgia, the mountain folds are broken by more than fifteen parallel thrust planes, running from northeast to southwest, along which the older strata have been pushed westward over the younger. The longest continuous fault has been traced three hundred and seventy-five miles, and the greatest horizontal displacement has been estimated at not less than eleven miles.

In the Selkirk Mountains of Canada the basement rocks of the region have been driven east for seven miles on a thrust plane, over rocks which originally lay thousands of feet above them.

Along the western Appalachians, from Virginia to Georgia, the mountain folds are broken by more than fifteen parallel thrust planes, running from northeast to southwest, along which the older strata have been pushed westward over the younger. The longest continuous fault has been traced three hundred and seventy-five miles, and the greatest horizontal displacement has been estimated at not less than eleven miles.

Crush breccia.Rocks often do not fault with a clean and simple fracture, but along a zone, sometimes several yards in width, in which they are broken to fragments. It may occur also that strata which as a whole yield to lateral thrust by folding include beds of brittle rocks, such as thin-layered limestones, which are crushed to pieces by the strain. In either case the fragments when recemented by percolating waters form a rock known as acrush breccia(pronouncedbretcha)(Fig. 186).

Fig. 186.Breccia

Breccia is a term applied to any rock formed of cementedangularfragments. This rock may be made by the consolidation of volcanic cinders, of angular waste at the foot of cliffs, or of fragments of coral torn by the waves from coral reefs, as well as of strata crushed by crustal movements.

Surface Features due to Dislocations

Fault scarps.A fault of recent date may be marked at surface by a scarp, because the face of the upthrown block has not yet been worn to the level of the downthrow side.

After the upthrown block has been worn down to this level, differential erosion produces fault scarps wherever weak rocks and resistant rocks are brought in contact along the fault plane; and the harder rocks, whether on the upthrow or the downthrow side, emerge in a line of cliffs. Where a fault is so old that no abrupt scarps appear, its general course is sometimes marked by the line of division between highland and lowland or hill and plain. Great faults have sometimes brought ancient crystalline rocks in contact with weaker and younger sedimentary rocks, and long after erosion has destroyed all fault scarps the harder crystallines rise in an upland of rugged or mountainous country which meets the lowland along the line of faulting.

Fig. 187.A Concealed FaultThis fault may be inferred from the changes in strata in passing along the strike, as frombtoa´and fromctob´

The vast majority of faults give rise to no surface features. The faulted region may be old enough to have been baseleveled, or the rocks on both sides of the line of dislocation may be alike in their resistance to erosion and therefore have been worn down to a common slope. The fault may be entirely concealed by the mantle of waste, and in such cases it can be inferred from abrupt changes in the character or the strike and dip of the strata where they may outcrop near it (Fig. 187).

Fig. 188.East-West Section across the Broken Plateau north of the Grand Canyon of the Colorado River, Arizona

The plateau trenched by the Grand Canyon of the Colorado River exhibits a series of magnificent fault scarps whose general course is from north to south, marking the edges of the great crust blocks into which the country has been broken. The highest part of the plateau is a crust block ninety miles long and thirty-five miles in maximum width, which has been hoisted to nine thousand three hundred feet above, sea level. On the east it descends four thousand feet by a monoclinal fold, which passes into a fault towards the north. On the west it breaks down by a succession of terraces faced by fault scarps. The throw of these faults varies from seven hundred feet to more than a mile. The escarpments, however, are due in a large degree to the erosion of weaker rock on the downthrow side.

Fig. 189.The Fault separating the Highlands and the Lowlands, Scotland

The Highlands of Scotland (Fig. 189) meet the Lowlands on the south with a bold front of rugged hills along a line of dislocation which runsacross the country from sea to sea. On the one side are hills of ancient crystalline rocks whose crumpled structures prove that they are but the roots of once lofty mountains; on the other lies a lowland of sandstone and other stratified rocks formed from the waste of those long-vanished mountain ranges. Remnants of sandstone occur in places on the north of the great fault, and are here seen to rest on the worn and fairly even surface of the crystallines. We may infer that these ancient mountains were reduced along their margins to low plains, which were slowly lowered beneath the sea to receive a cover of sedimentary rocks. Still later came an uplift and dislocation. On the one side erosion has since stripped off the sandstones for the most part, but the hard crystalline rocks yet stand in bold relief. On the other side the weak sedimentary rocks have been worn down to lowlands.

Rift valleys.In a broken region undergoing uplift or the unequal settling which may follow, a slice inclosed between two fissures may sink below the level of the crust blocks on either side, thus forming a linear depression known as a rift valley, or valley of fracture.

Fig. 190.Section from the Mountains of Palestine to the Mountains of Moab across the Dead Seaa, ancient schists;b, Carboniferous strata;c,d, ande, Cretaceous strata

One of the most striking examples of this rare type of valley is the long trough which runs straight from the Lebanon Mountains of Syria on the north to the Red Sea on the south, and whose central portion is occupied by the Jordan valley and the Dead Sea. The plateau which it gashes has been lifted more than three thousand feet above sea level, and the bottom of the trough reaches a depth of two thousand six hundred feet below that level in parts of the Dead Sea. South of the Dead Sea the floor of the trough rises somewhat above sea level, and in the Gulf of Akabah again sinks below it. This uneven floor could be accounted for either by the profound warping ofa valley of erosion or by the unequal depression of the floor of a rift valley. But that the trough is a true valley of fracture is proved by the fact that on either side it is bounded by fault scarps and monoclinal folds. The keystone of the arch has subsided. Many geologists believe that the Jordan-Akabah trough, the long narrow basin of the Red Sea, and the chain of down-faulted valleys which in Africa extends from the strait of Bab-el-Mandeb as far south as Lake Nyassa—valleys which contain more than thirty lakes—belong to a single system of dislocation.Should you expect the lateral valleys of a rift valley at the time of its formation to enter it as hanging valleys or at a common level?

Should you expect the lateral valleys of a rift valley at the time of its formation to enter it as hanging valleys or at a common level?

Block mountains.Dislocations take place on so grand a scale that by the upheaval of blocks of the earth’s crust or the down- faulting of the blocks about one which is relatively stationary, mountains known as block mountains are produced. A tilted crust block may present a steep slope on the side upheaved and a more gentle descent on the side depressed.

Fig. 191.Block Mountains, Southern Oregon

The Basin ranges.The plateaus of the United States bounded by the Rocky Mountains on the east, and on the west by the ranges which front the Pacific, have been profoundly fractured and faulted. The system of great fissures by which they are broken extends north and south, and the long, narrow, tilted crust blocks intercepted between the fissures give rise to the numerous north-south ranges of the region. Some of the tilted blocks, as those of southern Oregon, are as yet but moderately carved by erosion, and shallow lakes lie on the waste that has been washed into the depressions between them (Fig. 191). We may therefore conclude that their displacement is somewhat recent. Others, as those of Nevada, are so old that they have been deeply dissected; their original form has been destroyed by erosion, and the intermontane depressions are occupied by wide plains of waste.

Dislocations and river valleys.Before geologists had proved that rivers can by their own unaided efforts cut deep canyons, itwas common to consider any narrow gorge as a gaping fissure of the crust. This crude view has long since been set aside. A map of the plateaus of northern Arizona shows how independent of the immense faults of the region is the course of the Colorado River. In the Alps the tunnels on the Saint Gotthard railway pass six times beneath the gorge of the Reuss, but at no point do the rocks show the slightest trace of a fault.

Fig. 192.Fault crossing Valley in Japan

Rate of dislocation.So far as human experience goes, the earth movements which we have just studied, some of which have produced deep-sunk valleys and lofty mountain ranges, and faults whose throw is to be measured in thousands of feet, are slow and gradual. They are not accomplished by a single paroxysmal effort, but by slow creep and a series of slight slips continued for vast lengths of time.

In the Aspen mining district in Colorado faulting is now going on at a comparatively rapid rate. Although no sudden slips take place, the creep of the rock along certain planes of faulting gradually bends outof shape the square-set timbers in horizontal drifts and has closed some vertical shafts by shifting the upper portion across the lower. Along one of the faults of this region it is estimated that there has been a movement of at least four hundred feet since the Glacial epoch. More conspicuous are the instances of active faulting by means of sudden slips. In 1891 there occurred along an old fault plane in Japan a slip which produced an earth rent traced for fifty miles (Fig. 192). The country on one side was depressed in places twenty feet below that on the other, and also shifted as much as thirteen feet horizontally in the direction of the fault line.In 1872 a slip occurred for forty miles on the great line of dislocation which runs along the eastern base of the Sierra Nevada Mountains. In the Owens valley, California, the throw amounted to twenty-five feet in places, with a horizontal movement along the fault line of as much as eighteen feet. Both this slip and that in Japan just mentioned caused severe earthquakes.

In 1872 a slip occurred for forty miles on the great line of dislocation which runs along the eastern base of the Sierra Nevada Mountains. In the Owens valley, California, the throw amounted to twenty-five feet in places, with a horizontal movement along the fault line of as much as eighteen feet. Both this slip and that in Japan just mentioned caused severe earthquakes.

For the sake of clearness we have described oscillations, foldings, and fractures of the crust as separate processes, each giving rise to its own peculiar surface features, but in nature earth movements are by no means so simple,—they are often implicated with one another: folds pass into faults; in a deformed region certain rocks have bent, while others under the same strain, but under different conditions of plasticity and load, have broken; folded mountains have been worn to their roots, and the peneplains to which they have been denuded have been upwarped to mountain height and afterwards dissected,—as in the case of the Allegheny ridges, the southern Carpathians, and other ranges, —or, as in the case of the Sierra Nevada Mountains, have been broken and uplifted as mountains of fracture.

Draw the following diagrams, being careful to show the direction in which the faulted blocks have moved, by the position of the two parts of some well-defined layer of limestone, sandstone, or shale, which occurs on each side of the fault plane, as inFigure 184.

Fig. 193

1. A normal fault with a hade of 15°, the original fault scarp remaining.2. A normal fault with a hade of 50°, the original fault scarp worn away, showing cliffs caused by harder strata on the downthrow side.3. A thrust fault with a hade of 30°, showing cliffs due to harder strata outcropping on the downthrow.4. A thrust fault with a hade of 80°, with surface baseleveled.5. In a region of normal faults a coal mine is being worked along the seam of coalAB(Fig. 193). AtBit is found broken by a fault f which hades towardA. To find the seam again, should you advise tunneling up or down fromB?6. In a vertical shaft of a coal mine the same bed of coal is pierced twice at different levels because of a fault. Draw a diagram to show whether the fault is normal or a thrust.

1. A normal fault with a hade of 15°, the original fault scarp remaining.

2. A normal fault with a hade of 50°, the original fault scarp worn away, showing cliffs caused by harder strata on the downthrow side.

3. A thrust fault with a hade of 30°, showing cliffs due to harder strata outcropping on the downthrow.

4. A thrust fault with a hade of 80°, with surface baseleveled.

5. In a region of normal faults a coal mine is being worked along the seam of coalAB(Fig. 193). AtBit is found broken by a fault f which hades towardA. To find the seam again, should you advise tunneling up or down fromB?

6. In a vertical shaft of a coal mine the same bed of coal is pierced twice at different levels because of a fault. Draw a diagram to show whether the fault is normal or a thrust.

Fig. 194.Ridges to be explained by Faulting

7. Copy the diagram inFigure 194, showing how the two ridges may be accounted for by a single resistant stratum dislocated by a fault. Is the fault astrike fault, i.e. one running parallel with the strike of the strata, or adip fault, one running parallel with the direction of the dip?

Fig. 195.Earth Block of Tilted Strata, with Included Seam of Coalcc

8. Draw a diagram of the block inFigure 195as it would appear if dislocated along the planeefgby a normal fault whose throw equals one fourth the height of the block. Is the fault a strike or a dip fault?Draw a second diagram showing the same block after denudation has worn it down below the center of the upthrown side. Note that the outcrop of the coal seam is now deceptively repeated. This exercise may be done in blocks of wood instead of drawings.

Fig. 196.AandB. Repeated Outcrops of Same Strata

9. Draw diagrams showing by dotted lines the conditions both ofAandB,Figure 196, after deformation had given the strata their present attitude.

Fig. 197.A Block Mountain

10. What is the attitude of the strata of this earth block,Figure 197? What has taken place along the planebaf? When did the dislocation occur compared with the folding of the strata? With the erosion of the valleys on the right-hand side of the mountain? With the deposition of the sedimentsefg? Do you find any remnants of the original surfacebafproduced by the dislocation? From the left-hand side of the mountain infer what was the relief of the region before the dislocation. Give the complete history recorded in the diagram from the deposition of the strata to the present.

Fig. 198.A Faulted Lava Flowaa´

Fig. 199.Measurement of the Thickness of Inclined Strata

11. Which is the older fault, inFigure 198,forf´? When did the lava flow occur? How long a time elapsed between the formation of the two faults as measured in the work done in the interval? How long a time since the formation of the later fault?12. Measure by the scale the thicknessbcof the coal-bearing strata outcropping fromatobinFigure 199. On any convenient scale draw a similar section of strata with a dip of 30° outcropping along a horizontal line normal to the strike one thousand feet in length, and measure the thickness of the strata by the scale employed. The thickness may also be calculated by trigonometry.

12. Measure by the scale the thicknessbcof the coal-bearing strata outcropping fromatobinFigure 199. On any convenient scale draw a similar section of strata with a dip of 30° outcropping along a horizontal line normal to the strike one thousand feet in length, and measure the thickness of the strata by the scale employed. The thickness may also be calculated by trigonometry.

Fig. 200.Unconformity between Parallel Strata

Fig. 201.Unconformity between Non-parallel Strata

Unconformity

Strata deposited one upon, another in an unbroken succession are said to beconformable. But the continuous deposition of strata is often interrupted by movements of the earth’s crust, Old sea floors are lifted to form land and are again depressed beneath the sea to receive a cover of sediments only after an interval during which they were carved by subaërial erosion. An erosion surface which thus parts older from younger strata is known as anunconformity, and the strata above it aresaid to beunconformablewith the rocks below, or to rest unconformably upon them. An unconformity thus records movements of the crust and a consequent break in the deposition of the strata. It denotes a period of land erosion of greater or less length, which may sometimes be roughly measured by the stage in the erosion cycle which the land surface had attained before its burial. Unconformable strata may beparallel, as inFigure 200, where the record includes the deposition of strataa, their emergence, the erosion of the land surfacess, a submergence and the deposit of the stratab, and lastly, emergence and the erosion of the present surfaces´s´.

Fig. 202.Carboniferous Limestone resting unconformably on Early Silurian Slates, Yorkshire, England

Often the earth movements to which the uplift or depression was due involved tilting or folding of the earlier strata, so that the strata are now nonparallel as well as unconformable. InFigure 201, for example, the record includes deposition, uplift, andtiltingofa; erosion, depression, the deposit ofb; and finally the uplift which has brought the rocks to open air and permitted the dissection by which the unconformity is revealed.

From this section we infer that during early Silurian times the area was sea, and thick sea muds were laid upon it. These were later altered to hard slates by pressure and upfolded into mountains. During the laterSilurian and the Devonian the area was land and suffered vast denudation. In the Carboniferous period it was lowered beneath the sea and received a cover of limestone.

Fig. 203.Diagram Illustrating how the Age of Mountains is determined

Fig. 204.Section of Mountain Range showing repeated Upliftsa, strata whose folding formed a mountain range;uu, baseleveled surface produced by long denudation of the mountains;b, tilted strata resting unconformably ona;c, horizontal strata parted frombby the unconformityu´u´. The first uplift of the range preceded the period of time whenbwas deposited. The second uplift, to which the present mountains owe their height, was later than this period but earlier than the period when stratacwere laid

Fig. 204.Section of Mountain Range showing repeated Uplifts

a, strata whose folding formed a mountain range;uu, baseleveled surface produced by long denudation of the mountains;b, tilted strata resting unconformably ona;c, horizontal strata parted frombby the unconformityu´u´. The first uplift of the range preceded the period of time whenbwas deposited. The second uplift, to which the present mountains owe their height, was later than this period but earlier than the period when stratacwere laid

The age of mountains.It is largely by means of unconformities that we read the history of mountain making and other deformations and movements of the crust. InFigure 203, for example, the deformation which upfolded the range of mountains took place after the deposit of the series of strataaof which the mountains are composed, and before the deposit of the stratified rocksb, which rest unconformably onaand have not shared their uplift.

Most great mountain ranges, like the Sierra Nevada and the Alps, mark lines of weakness along which the earth’s crust has yielded again and again during the long ages of geological time. The strata deposited at various times about their flanks have been infolded by later crumplings with the original mountain mass, and have been repeatedly crushed, inverted, faulted, intruded with igneous rocks, and denuded. The structure of great mountain ranges thus becomes exceedingly complex and difficult to read. A comparatively simple case of repeated uplift is shown inFigure 204. In the section of a portion of the Alps shown inFigure 179a far more complicated history may be deciphered.

Fig. 205.Unconformity showing Buried Valleyslm, limestone;sh, shale;r,r´, andr´´, river silts filling eroded valleys in the limestone. The upper surface of the limestone is evidently a land surface developed by erosion. The valleys which trench it are narrow and steep-sided; hence the land surface had not reached maturity. The sands and muds, now hardened to firm rock, which fill these valleys,r,r´, andr´´, contain no relics of the sea, but Instead the remains of land animals and plants. They are river deposits, and we may infer that owing to a subsidence the young rivers ceased to degrade their channels and slowly filled their gorges with sands and silts. The overlying shale records a further depression which brought the lanes below the level of the sea. A section similar to this is to be seen in the coal mines of Bernissant, Belgium, where a gorge twice as deep as that of Niagara was discovered within whose ancient river deposits were found entombed the skeletons of more than a score of the huge reptiles characteristic of the age when the gorge was cut and filled

Fig. 205.Unconformity showing Buried Valleys

lm, limestone;sh, shale;r,r´, andr´´, river silts filling eroded valleys in the limestone. The upper surface of the limestone is evidently a land surface developed by erosion. The valleys which trench it are narrow and steep-sided; hence the land surface had not reached maturity. The sands and muds, now hardened to firm rock, which fill these valleys,r,r´, andr´´, contain no relics of the sea, but Instead the remains of land animals and plants. They are river deposits, and we may infer that owing to a subsidence the young rivers ceased to degrade their channels and slowly filled their gorges with sands and silts. The overlying shale records a further depression which brought the lanes below the level of the sea. A section similar to this is to be seen in the coal mines of Bernissant, Belgium, where a gorge twice as deep as that of Niagara was discovered within whose ancient river deposits were found entombed the skeletons of more than a score of the huge reptiles characteristic of the age when the gorge was cut and filled

Fig. 206.Unconformity showing Buried Mountains, Scotlandgn, ancient crystalline rocks;ss, marine sandstones. The surfacebbof the ancient crystalline rocks is mountainous, with peaks rising to a height of as much as three thousand feet. It is one of the most ancient land surfaces on the planet and is covered unconformably with pre-Cambrian sandstones thousands of feet in thickness, in which the Torridonian Mountains of Scotland have been carved. What has been the history of the region since the mountainous surfacebbwas produced by erosion?

Fig. 206.Unconformity showing Buried Mountains, Scotland

gn, ancient crystalline rocks;ss, marine sandstones. The surfacebbof the ancient crystalline rocks is mountainous, with peaks rising to a height of as much as three thousand feet. It is one of the most ancient land surfaces on the planet and is covered unconformably with pre-Cambrian sandstones thousands of feet in thickness, in which the Torridonian Mountains of Scotland have been carved. What has been the history of the region since the mountainous surfacebbwas produced by erosion?

Unconformities in the Colorado Canyon, Arizona.How geological history may be read in unconformities is further illustrated in Figures207and208. The dark crystalline rocksaat the bottom of the canyon are among the most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstonesb, whose total thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip. Bothaandbrise to a common levelnn´and upon them rest the horizontal sea-laid stratac, in which the upper portion of the canyon has been cut.Note that the crystalline rocks a have been crumpled and crushed. Comparing their structure with that of folded mountains, what do you infer as to their relief after their deformation? To which surface were they first worn down,mm´ornm? Describe and account for the surfacemm´. How does it differ from the surface of the crystalline rocks seen in the Torridonian Mountains (Fig. 206), and why? This surfacemm´is one of the oldest land surfaces of which any vestige remains. It is a bit of fossil geography buried from view since the earliest geological ages and recently brought to light by the erosion of the canyon.

Note that the crystalline rocks a have been crumpled and crushed. Comparing their structure with that of folded mountains, what do you infer as to their relief after their deformation? To which surface were they first worn down,mm´ornm? Describe and account for the surfacemm´. How does it differ from the surface of the crystalline rocks seen in the Torridonian Mountains (Fig. 206), and why? This surfacemm´is one of the oldest land surfaces of which any vestige remains. It is a bit of fossil geography buried from view since the earliest geological ages and recently brought to light by the erosion of the canyon.

Fig. 207.Diagram of Wall of the ColoradoCanyon, Arizona, showing Unconformities

Fig. 208.View of the North Wall of the Grand Canyon of the Colorado River, Arizona, showing the Unconformities illustrated inFigure 207

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