Chapter 5

Fig. 60.Successive Stages,A,B,C, andD, in Valley-Widening by PlanationDescribe valleyA. What changes have taken place inB,C, andD? Do the river bends remain stationary or move up or down valley? With what effect on the projecting spurs of the valley sides? Draw diagrams showing a still later stage thanD

Fig. 60.Successive Stages,A,B,C, andD, in Valley-Widening by Planation

Describe valleyA. What changes have taken place inB,C, andD? Do the river bends remain stationary or move up or down valley? With what effect on the projecting spurs of the valley sides? Draw diagrams showing a still later stage thanD

Lateral erosion.On reaching grade a river ceases to scour its bed, and it does not again begin to do so until some changein load or volume enables it to find grade at a lower level. On the other hand, a stream erodes its banks at all stages in its history, and with graded rivers this process, called lateral erosion, orplanation, is specially important. The current of a stream follows the outer side of all curves or bends in the channel, and on this side it excavates its bed the deepest and continually wears and saps its banks. On the inner side deposition takes place in the more shallow and slower-moving water. The inner bank of bends is thus built out while the outer bank is worn away. By swinging its curves against the valley sides a graded river continually cuts a wider and wider floor. TheV-valley of youth is thus changed by planation to a flat-floored valley with flaring sides which gradually become subdued by the weather to gentle slopes. While widening their valleys streams maintain a constant width of channel, so that a wide-floored valley does not signify that it ever was occupied by a river of equal width.

The gradient.The gradients of graded rivers differ widely. A large river with a light load reaches grade on a faint slope, while a smaller stream heavily burdened with waste requires a steep slope to give it velocity sufficient to move the load.

The Platte, a graded river of Nebraska with its headwaters in the Rocky Mountains, is enfeebled by the semi-arid climate of the Great Plains and surcharged with the waste brought down both by its branches in the mountains and by those whose tracks lie over the soft rocks of the plains. It is compelled to maintain a gradient of eight feet to the mile in western Nebraska. The Ohio reaches grade with a slope of less than four inches to the mile from Cincinnati to its mouth, and the powerful Mississippi washes along its load with a fall of but three inches per mile from Cairo to the Gulf.Other things being equal, which of graded streams will have the steeper gradient, a trunk stream or its tributaries? a stream supplied with gravel or one with silt?Other factors remaining the same, what changes would occur if the Platte should increase in volume? What changes would occur if the load should be increased in amount or in coarseness?

Other things being equal, which of graded streams will have the steeper gradient, a trunk stream or its tributaries? a stream supplied with gravel or one with silt?

Other factors remaining the same, what changes would occur if the Platte should increase in volume? What changes would occur if the load should be increased in amount or in coarseness?

Fig. 61.Successive Cross Sections of a Region as it advances from Infancya, to Old Agee

The old age of rivers.As rivers pass their prime, as denudation lowers the relief of the region, less waste and finer is washed over the gentler slopes of the lowering hills. With smaller loads to carry, the rivers now deepen their valleys and find grade with fainter declivities nearer the level of the sea. This limit of the level of the sea beneath which they cannot erode is known asbaselevel.[1]As streams grow old they approach more and more closely to baselevel, although they are never able to attain it. Some slight slope is needed that water may flow and waste be transported over the land. Meanwhile the relief of the land has ever lessened. The master streams and their main tributaries now wander with sluggish currents over the broad valley floors which they have planed away; while under the erosion of their innumerable branches and the wear of the weather the divides everywhere are lowered and subdued to more and more gentle slopes. Mountains and high plateaus are thus reduced to rolling hills, and at last to plains, surmounted only by such hills as may still be unreduced to the common level, because of the harder rocks of which they are composed or because of their distance from the main erosion channels. Such regions of faint relief, worn down to near base level by subaërial agencies, are known aspeneplains(almost plains). Any residual masses which rise above them are calledmonadnocks, from the name of a conical peak of New Hampshire which overlooks the now uplifted peneplain of southern New England.

[1]The term “baselevel” is also used to designate the close approximation to sea level to which streams are able to subdue the land.

In its old age a region becomes mantled with thick sheets of fine and weathered waste, slowly moving over the faint slopes toward the water ways and unbroken by ledges of bare rock. In other words, the waste mantle also is now graded, and as waterfalls have been effaced in the river beds, so now any ledges in the wide streams of waste are worn away and covered beneath smooth slopes of fine soil. Ground water stands high and may exude in areas of swamp. In youth the land mass was roughhewn and cut deep by stream erosion. In old age the faint reliefs of the land dissolve away, chiefly under the action of the weather, beneath their cloak of waste.

Fig. 62.Peneplain surrounded by Monadnocks, Piedmont Belt, VirginiaFrom Davis’Elementary Physical Geography

The cycle of erosion.The successive stages through which a land mass passes while it is being leveled to the sea constitute together a cycle of erosion. Each stage of the cycle from infancy to old age leaves, as we have seen, its characteristic records in the forms sculptured on the land, such as the shapes of valleys and the contours of hills and plains. The geologist is thus able to determine by the land forms of any region the stage in the erosion cycle to which it now belongs, and knowing what are the earlier stages of the cycle, to read something of the geological history of the region.

Interrupted cycles.So long a time is needed to reduce a land mass to baselevel that the process is seldom if ever completed during a single uninterrupted cycle of erosion. Of all the various interruptions which may occur the most important are gradual movements of the earth’s crust, by which a region is either depressed or elevated relative to sea level.

Fig. 63.Young Inner Gorge in Wide Older Valley, Alaska

Thedepressionof a region hastens its old age by decreasing the gradient of streams, by destroying their power to excavate their beds and carry their loads to a degree corresponding to the amount of the depression, and by lessening the amount of work they have to do. The slackened river currents deposit their waste in Hood plains which increase in height as the subsidence continues. The lower courses of the rivers are invaded by the sea and become estuaries, while the lower tributaries are cut off from the trunk stream.

Elevation, on the other hand, increases the activity of all agencies of weathering, erosion, and transportation, restores the region to its youth, and inaugurates a new cycle of erosion. Streams are given a steeper gradient, greater velocity, and increased energy to carry their loads and wear their beds.They cut through the alluvium of their flood plains, leaving it on either bank as successive terraces, and intrench themselves in the underlying rock. In their older and wider valleys they cut narrow, steep-walled inner gorges, in which they flow swiftly over rocky floors, broken here and there by falls and rapids where a harder layer of rock has been discovered. Winding streams on plains may thus incise their meanders in solid rock as the plains are gradually uplifted. Streams which are thus restored to their youth are said to berevived.

Fig. 64.Incised Meanders of Oneota River, Iowa

As streams cut deeper and the valley slopes are steepened, the mantle of waste of the region undergoing elevation is set in more rapid movement. It is now removed particle by particle faster than it forms. As the waste mantle thins, weathering attacks the rocks of the region more energetically until an equilibrium is reached again; the rocks waste rapidly and their waste is as rapidly removed.

Dissected peneplains.When a rise of the land brings one cycle to an end and begins another, the characteristic land forms of each cycle are found together and the topography of the region is composite until the second cycle is so far advanced that the land forms of the first cycle are entirely destroyed. The contrast between the land surfaces of the later and the earlier cycles ismost striking when the earlier had advanced to age and the later is still in youth. Thus many peneplains which have been elevated and dissected have been recognized by the remnants of their ancient erosion surfaces, and the length of time which has elapsed since their uplift has been measured by the stage to which the new cycle has advanced.

Fig. 65.Describe the valley of streama. Is it young or old? How does the valley ofbdiffer from that ofa? Compare as to form and age the inner valley ofbwith the outer valley and with the valley ofa. Account for the inner valley. Why does it not extend to the upper portion of the course ofb? Will it ever do so? Draw longitudinal profile ofb, showing the different gradient of upper and lower portions of its course not here seen. As the inner valley of tributarycextends headward it may invade the valley ofabefore the inner valley ofahas worked upstream to the area seen in the diagram. With what results?

Fig. 65.

Describe the valley of streama. Is it young or old? How does the valley ofbdiffer from that ofa? Compare as to form and age the inner valley ofbwith the outer valley and with the valley ofa. Account for the inner valley. Why does it not extend to the upper portion of the course ofb? Will it ever do so? Draw longitudinal profile ofb, showing the different gradient of upper and lower portions of its course not here seen. As the inner valley of tributarycextends headward it may invade the valley ofabefore the inner valley ofahas worked upstream to the area seen in the diagram. With what results?

The piedmont belt.As an example of an ancient peneplain uplifted and dissected we may cite the Piedmont Belt, a broad upland lying between the Appalachian Mountains and the Atlantic coastal plain. The surface of the Piedmont is gently rolling. The divides, which are often smooth areas of considerable width, rise to a common plane, and from them one sees in every direction an even sky line except where in places some lone hill or ridge may lift itself above the general level (Fig. 62). The surface is an ancient one, for the mantle of residual waste lies deep upon it, soils are reddened by long oxidation, and the rocks are rotted to a depth of scores of feet.At present, however, the waste mantle is not forming so rapidly as it is being removed. The streams of the upland are actively engaged in its destruction. They flow swiftly in narrow, rock- walled valleys over rocky beds. This contrast between the young streams and the agedsurface which they are now so vigorously dissecting can only be explained by the theory that the region once stood lower than at present and has recently been upraised. If now we imagine the valleys refilled with the waste which the streams have swept away, and the upland lowered, we restore the Piedmont region to the condition in which it stood before its uplift and dissection,—a gently rolling plain, surmounted here and there by isolated hills and ridges.

At present, however, the waste mantle is not forming so rapidly as it is being removed. The streams of the upland are actively engaged in its destruction. They flow swiftly in narrow, rock- walled valleys over rocky beds. This contrast between the young streams and the agedsurface which they are now so vigorously dissecting can only be explained by the theory that the region once stood lower than at present and has recently been upraised. If now we imagine the valleys refilled with the waste which the streams have swept away, and the upland lowered, we restore the Piedmont region to the condition in which it stood before its uplift and dissection,—a gently rolling plain, surmounted here and there by isolated hills and ridges.

Fig. 66.Dissected Peneplain of Southern New England

The surface of the ancient Piedmont plain, as it may be restored from the remnants of it found on the divides, is not in accordance with the structures of the country rocks. Where these are exposed to view they are seen to be far from horizontal. On the walls of river gorges they dip steeply and in various directions and the streams flow over their upturned edges. As shown inFigure 67, the rocks of the Piedmont have been folded and broken and tilted.

Fig. 67.Section in Piedmont BeltM, a monadnock

It is not reasonable to believe that when the rocks of the Piedmont were thus folded and otherwise deformed the surface of the region was a plain. The upturned layers have not always stopped abruptly at the even surface of the Piedmont plain which now cuts across them. They are the bases of great folds and tilted blocks which must once have risenhigh in air. The complex and disorderly structures of the Piedmont rocks are those seen in great mountain ranges, and there is every reason to believe that these rocks after their deformation rose to mountain height.

Fig. 68.The area of the Laurentian Peneplain (shaded)

The ancient Piedmont plain cuts across these upturned rocks as independently of their structure as the even surface of the sawed stump of some great tree is independent of the direction of its fibers. Hence the Piedmont plain as it was before its uplift was not a coastal plain formed of strata spread in horizontal sheets beneath the sea and then uplifted; nor was it a structural plain, due to the resistance to erosion of some hard, flat-lying layer of rock. Even surfaces developed on rocks of discordant structure, such as the Piedmont shows, are produced by long denudation, and we may consider the Piedmont as a peneplain formed by the wearing down of mountain ranges, and recently uplifted.

The Laurentian peneplain.This is the name given to a denuded surface on very ancient rocks which extends from theArctic Ocean to the St. Lawrence River and Lake Superior, with small areas also in northern Wisconsin and New York. Throughout thisU-shaped area, which incloses Hudson Bay within its arms, the country rocks have the complicated and contorted structures which characterize mountain ranges (seeFig. 179, P. 211). But the surface of the area is by no means mountainous. The sky line when viewed from the divides is unbroken by mountain peaks or rugged hills. The surface of the arm west of Hudson Bay is gently undulating and that of the eastern arm has been roughened to low-rolling hills and dissected in places by such deep river gorges as those of the Ottawa and Saguenay. This immense area may be regarded as an ancient peneplain truncating the bases of long-vanished mountains and dissected after elevation.

In the examples cited the uplift has been a broad one and to comparatively little height. Where peneplains have been uplifted to great height and have since been well dissected, and where they have been upfolded and broken and uptilted, their recognition becomes more difficult. Yet recent observers have found evidences of ancient lowland surfaces of erosion on the summits of the Allegheny ridges, the Cascade Mountains (Fig. 69), and the western slope of the Sierra Nevadas.

Fig. 69.View in the Cascade Mountains, WashingtonThe general level to which these ridges rise may be accounted for by the uplift and dissection of a once low-lying peneplain

The southern Appalachian region.We have here an example of an area the latter part of whose geological history may be deciphered by means of its land forms. The generalized section ofFigure 70, which passes from west to east across a portion of the region in eastern Tennessee, shows on the west a part of the broad Cumberland plateau. On the east is a roughened upland platform, from which rise in the distance the peaks of the Great Smoky Mountains. The plateau, consisting of strata but little changed from their original flat-lying attitude, and the platform, developed on rocks of disordered structure made crystalline by heat and pressure, both stand at the common level of the line AB. They are separated by the Appalachian valley, forty miles wide, cut in strata which have been folded and broken into long narrow blocks. The valley is traversed lengthwise by long, low ridges, the outcropping edges of the harder strata, which rise to about the same level,—that of the linecd. Between these ridges stretch valley lowlands at the levelefexcavated in the weaker rocks, while somewhat below them lie the channels of the present streams now busily engaged in deepening their beds.

The valley lowlands.Were they planed by graded or ungraded streams? Have the present streams reached grade? Why did the streams cease widening the floors of the valley lowlands? How long since? When will they begin anew the work of lateral planation? What effect will this have on the ridges if the present cycle of erosion continues long uninterrupted?

Fig. 70.Generalized Section of the Southern Appalachian Region in Eastern Tennessee

The ridges of the Appalachian valley.Why do they stand above the valley lowlands? Why do their summits lie in about the same plane? Refilling the valleys intervening between these ridges with the material removed by the streams, what is the nature of the surface thus restored? Does this surface cd accord with the rock structures on which it has been developed? How may it have been made? At what height did the land stand then, compared with its present height? What elevations stood above the surface cd? Why? What name may you use to designate them? How does the length of time needed to develop the surface cd compare with that needed to develop the valley lowlands?The platform and plateau.Why do they stand at a common levelab? Of what surface may they be remnants? Is it accordant with the rock structure? How was it produced? What unconsumed masses overlooked it? Did the rocks of the Appalachian valley stand above this surface when it was produced? Did they then stand below it? Compare the time needed to develop this surface with that needed to developcd. Which surface is the older?How many cycles of erosion are represented here? Give the erosion history of the region by cycles, beginning with the oldest, the work done in each and the work left undone, what brought each cycle to a close, and how long relatively it continued.

The platform and plateau.Why do they stand at a common levelab? Of what surface may they be remnants? Is it accordant with the rock structure? How was it produced? What unconsumed masses overlooked it? Did the rocks of the Appalachian valley stand above this surface when it was produced? Did they then stand below it? Compare the time needed to develop this surface with that needed to developcd. Which surface is the older?

How many cycles of erosion are represented here? Give the erosion history of the region by cycles, beginning with the oldest, the work done in each and the work left undone, what brought each cycle to a close, and how long relatively it continued.

CHAPTER IV

RIVER DEPOSITS

The characteristic features of river deposits and the forms which they assume may be treated under three heads: (1) valley deposits, (2) basin deposits, and (3) deltas.

Valley Deposits

Flood plains.The deposits which streams build along their courses at times of flood are known as flood plains. A swift current then sweeps along the channel, while a shallow sheet of water moves slowly over the flood plain, spreading upon it a thin layer of sediment. It has been estimated that each inundation of the Nile leaves a layer of fertilizing silt three hundredths of an inch thick over the flood plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the flat rock floor of a valley which is being widened by the lateral erosion of a graded stream (Fig. 60). Flood-plain deposits of great thickness may be built by aggrading rivers even in valleys whose rock floors have never been thus widened (Fig. 368).

Fig. 71.Cross Section of a Flood Plain

A cross section of a flood plain (Fig. 71) shows that it is highest next the river, sloping gradually thence to the valley sides. These wide natural embankments are due to the fact that the river deposit is heavier near the bank, where the velocity of the silt-laden channel current is first checked by contact with the slower-moving overflow.

Fig. 72.Waste-filled Valley and Braided Channels of the Upper Mississippi

Thus banked off from the stream, the outer portions of a flood plain are often ill-drained and swampy, and here vegetal deposits, such as peat, may be interbedded with river silts.

A map of a wide flood plain, such as that of the Mississippi or the Missouri (Fig. 77), shows that the courses of the tributaries on entering it are deflected downstream. Why?

The aggrading streams by which flood plains are constructed gradually build their immediate banks and beds to higher and higher levels, and therefore find it easy at times of great floods to break their natural embankments and take new courses over the plain. In this way they aggrade each portion of it in turn by means of their shifting channels.

Braided channels.A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and builds itself a newway which reunites down valley with the other channels. Thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel.

Fig. 73.Terraced Valley of River in Central Asia

Fig. 74.Terraces carved in Alluvial DepositsWhich is older, the rock floor of the valley or the river deposits which fill it? What are the relative ages of terracesa,b,c, ande? It will be noted that the remnants of the higher flood plains have not been swept away by the meandering river, as it swung from side to side of the valley at lower levels, because they have been defended by ledges of hard rock in the projecting spurs of the initial valley. The stream has encountered such defending ledges at the point markedd

Fig. 75.River Terraces of Rock covered with Alluviumc, recent flood plain of the river. To what processes is it due? Account for the alluvium ataandband on the opposite side of the valley at the same levels. Which is the older? Account for the flat rock floors on which these deposits of alluvium rest. Give the entire history which may be read in the section

Terraces.While aggrading streams thus tend to shift their channels, degrading streams, on the contrary, become more and more deeply intrenched in their valleys. It often occurs that a stream, after having built a flood plain, ceases to aggrade its bed because of a lessened load or for other reasons, such as an uplift of the region, and begins instead to degrade it. It leaves the original flood plain out of reach of even the highest floods. When again it reaches grade at a lower level it produces a new flood plain by lateral erosion in the older deposits, remnants of which stand as terraces on one or both sides of the valley. In this way a valley may be lined with a succession of terraces at different levels, each level representing an abandoned flood plain.

Fig. 76.Development of a MeanderThe dotted line ina,b, andcshows the stage preceding that indicated by the unbroken line

Meanders.Valleys aggraded with fine waste form well-nigh level plains over which streams wind from side to side of a direct course in symmetric bends known as meanders, from the name of a winding river of Asia Minor. The giant Mississippi has developed meanders with a radius of one and one half miles, but a little creek may display on its meadow as perfect curves only a rod or so in radius. On the flood plain of either river or creek we may find examples of the successive stages in the development of the meander, from its beginning in the slight initial bend sufficient to deflect thecurrent against the outer side. Eroding here and depositing on the inner side of the bend, it gradually reaches first the open bend (Fig. 76,a) whose width and length are not far from equal, and later that of the horseshoe meander (Fig. 76,b) whose diameter transverse to the course of the stream is much greater than that parallel with it. Little by little the neck of land projecting into the bend is narrowed, until at last it is cut through and a “cut-off” is established. The old channel is now silted up at both ends and becomes a crescentic lagoon (Fig. 76,c), or oxbow lake, which fills gradually to an arc-shaped shallow depression.

Fig. 77.Map of a portion of the Flood Plain of the Missouri RiverEach small square represents one square mile. How wide is the flood plain of the Missouri? How wide is the flood plain of the Big Sioux? Why is the latter river deflected down valley on entering the flood plain of the master stream? How do the meanders of the two rivers compare in size? How does the width of each flood plain compare with the width of the belt occupied by the meanders of the river? Do you find traces of any former channels?

Flood plains characteristic of mature rivers.On reaching grade a stream planes a flat floor for its continually wideningvalley. Ever cutting on the outer bank of its curves, it deposits on the inner bank scroll-like flood-plain patches (Fig 60). For a while the valley bluffs do not give its growing meanders room to develop to their normal size, but as planation goes on, the bluffs are driven back to the full width of the meander belt and still later to a width which gives room for broad stretches of flood plain on either side (Fig. 77).

Usually a river first attains grade near its mouth, and here first sinks its bed to near baselevel. Extending its graded course upstream by cutting away barrier after barrier, it comes to have a widened and mature valley over its lower course, while its young headwaters are still busily eroding their beds. Its ungraded branches may thus bring down to its lower course more waste than it is competent to carry on to the sea, and here it aggrades its bed and builds a flood plain in order to gain a steeper gradient and velocity enough to transport its load.

As maturity is past and the relief of the land is lessened, a smaller and smaller load of waste is delivered to the river. It now has energy to spare and again degrades its valley, excavating its former flood plains and leaving them in terraces on either side, and at last in its old age sweeping them away.

Fig. 78.Alluvial Cones, Wyoming

Alluvial cones and fans.In hilly and mountainous countries one often sees on a valley side a conical or fan-shaped deposit of waste at the mouth of a lateral stream. The cause is obvious:the young branch has not been able as yet to wear its bed to accordant level with the already deepened valley of the master stream. It therefore builds its bed to grade at the point of juncture by depositing here its load of waste,—a load too heavy to be carried along the more gentle profile of the trunk valley.

Fig. 79.Tributaries and Distributaries of aFan-Building Stream

Where rivers descend from a mountainous region upon the plain they may build alluvial fans of exceedingly gentle slope. Thus the rivers of the western side of the Sierra Nevada Mountains have spread fans with a radius of as much as forty miles and a slope too slight to be detected without instruments, where they leave the rock-cut canyons in the mountains and descend upon the broad central valley of California.

As a river flows over its fan it commonly divides into a branchwork of shifting channels calleddistributaries, since they lead off the water from the main stream. In this way each part of the fan is aggraded and its symmetric form is preserved.

Piedmont plains.Mountain streams may build their confluent fans into widespread piedmont (foot of the mountain) alluvial plains. These are especially characteristic of arid lands, where the streams wither as they flow out upon the thirsty lowlands and are therefore compelled to lay down a large portion of theirload. In humid climates mountain-born streams are usually competent to carry their loads of waste on to the sea, and have energy to spare to cut the lower mountain slopes into foothills. In arid regions foothills are commonly absent and the ranges rise, as from pedestals, above broad, sloping plains of stream-laid waste.

Fig. 80.Section from the Rocky Mountains EastwardRiver deposits dotted

The High Plains.The rivers which flow eastward from the Rocky Mountains have united their fans in a continuous sheet of waste which stretches forward from the base of the mountains for hundreds of miles and in places is five hundred feet thick (Fig. 80). That the deposit was made in ancient times on land and not in the sea is proved by the remains which it contains of land animals and plants of species now extinct. That it was laid by rivers and not by fresh-water lakes is shown by its structure. Wide stretches of flat-lying, clays and sands are interrupted by long, narrow belts of gravel which mark the channels of the ancient streams. Gravels, and sands are often cross bedded, and their well worn pebbles may be identified with the rocks of the mountains. After building this sheet of waste the streams ceased to aggrade and began the work of destruction. Large uneroded remnants, their surfaces flat as a floor, remain as the High Plains of western Kansas and Nebraska.

River deposits in subsiding troughs.To a geologist the most important river deposits are those which gather in areas of gradual subsidence; they are often of vast extent and immense thickness, and such deposits of past geological ages have not infrequently been preserved, with all their records of the times in which they were built, by being carried below the level of the sea, to be brought to light by a later uplift. On the other hand, river deposits which remain above baselevels of erosion are swept away comparatively soon.

The Great Valley Of Californiais a monotonously level plain of great fertility, four hundred miles in length and fifty miles in average width, built of waste swept down by streams from the mountain ranges which inclose it,—the Sierra Nevada on the east and the Coast Range on the west. On the waste slopes at the foot of the bordering hills coarse gravels and even bowlders are left, while over the interior the slow-flowing streams at times of flood spread wide sheets of silt. Organic deposits are now forming by the decay of vegetation in swampy tule (reed) lands and in shallow lakes which occupy depressions left by the aggrading streams.Deep borings show that this great trough is filled to a depth of at least two thousand feet below sea level with recent unconsolidated sands and silts containing logs of wood and fresh- water shells. These are land deposits, and the absence of any marine deposits among them proves that the region has not been invaded by the sea since the accumulation began. It has therefore been slowly subsiding and its streams, although continually carried below grade, have yet been able to aggrade the surface as rapidly as the region sank, and have maintained it, as at present, slightly above sea level.The Indo-Gangetic Plain, spread by the Brahmaputra, the Ganges, and the Indus river systems, stretches for sixteen hundred miles along the southern base of the Himalaya Mountains and occupies an area of three hundred thousand square miles (Fig. 342). It consists of the flood plains of the master streams and the confluent fans of the tributaries which issue from the mountains on the north. Large areas are subject to overflow each season of flood, and still larger tracts mark abandoned flood plains below which the rivers have now cut their beds. The plain is built of far- stretching beds of clay, penetrated by streaks of sand, and also of gravel near the mountains. Beds of impure peat occur in it, and it contains fresh-water shells and the bones of land animals of species now living in northern India. At Lucknow an artesian well was sunk to one thousand feet below sea level without reaching the bottom of these river-laid sands and silts, proving a slow subsidence with which the aggrading rivers have kept pace.

Deep borings show that this great trough is filled to a depth of at least two thousand feet below sea level with recent unconsolidated sands and silts containing logs of wood and fresh- water shells. These are land deposits, and the absence of any marine deposits among them proves that the region has not been invaded by the sea since the accumulation began. It has therefore been slowly subsiding and its streams, although continually carried below grade, have yet been able to aggrade the surface as rapidly as the region sank, and have maintained it, as at present, slightly above sea level.

The Indo-Gangetic Plain, spread by the Brahmaputra, the Ganges, and the Indus river systems, stretches for sixteen hundred miles along the southern base of the Himalaya Mountains and occupies an area of three hundred thousand square miles (Fig. 342). It consists of the flood plains of the master streams and the confluent fans of the tributaries which issue from the mountains on the north. Large areas are subject to overflow each season of flood, and still larger tracts mark abandoned flood plains below which the rivers have now cut their beds. The plain is built of far- stretching beds of clay, penetrated by streaks of sand, and also of gravel near the mountains. Beds of impure peat occur in it, and it contains fresh-water shells and the bones of land animals of species now living in northern India. At Lucknow an artesian well was sunk to one thousand feet below sea level without reaching the bottom of these river-laid sands and silts, proving a slow subsidence with which the aggrading rivers have kept pace.

Warped valleys.It is not necessary that an area should sink below sea level in order to be filled with stream-swept waste. High valleys among growing mountain ranges may suffer warping, or may be blockaded by rising mountain foldsathwart them. Where the deformation is rapid enough, the river may be ponded and the valley filled with lake-laid sediments. Even when the river is able to maintain its right of way it may yet have its declivity so lessened that it is compelled to aggrade its course continually, filling the valley with river deposits which may grow to an enormous thickness.

Behind the outer ranges of the Himalaya Mountains lie several waste-filled valleys, the largest of which are Kashmir and Nepal, the former being an alluvial plain about as large as the state of Delaware. The rivers which drain these plains have already cut down their outlet gorges sufficiently to begin the task of the removal of the broad accumulations which they have brought in from the surrounding mountains. Their present flood plains lie as much as some hundreds of feet below wide alluvial terraces which mark their former levels. Indeed, the horizontal beds of the Hundes Valley have been trenched to the depth of nearly three thousand feet by the Sutlej River. These deposits are recent or subrecent, for there have been found at various levels the remains of land plants and land and fresh-water shells, and in some the bones of such animals as the hyena and the goat, of species or of genera now living. Such soft deposits cannot be expected to endure through any considerable length of future time the rapid erosion to which their great height above the level of the sea will subject them.

Fig. 81.Cross Section of Aggraded Valley, showing Structure of River Deposits

Characteristics of river deposits.The examples just cited teach clearly the characteristic features of extensive river deposits. These deposits consist of broad, flat-lying sheets of clay and fine sand left by the overflow at time of flood, and traversed here and there by long, narrow strips of coarse, cross-bedded sands and gravels thrown down by the swifter currents of the shifting channels. Occasional beds of muck mark the sites of shallow lakelets or fresh-water swamps. The various strata also contain some remains of the countless myriads of animals and plants which live upon the surface ofthe plain as it is in process of building. River shells such as the mussel, land shells such as those of snails, the bones of fishes and of such land animals as suffer drowning at times of flood or are mired in swampy places, logs of wood, and the stems and leaves of plants are examples of the variety of the remains of land and fresh-water organisms which are entombed in river deposits and sealed away as a record of the life of the time, and as proof that the deposits were laid by streams and not beneath the sea.

Basin Deposits

Deposits in dry basins.On desert areas without outlet to the sea, as on the Great Basin of the United States and the deserts of central Asia, stream-swept waste accumulates indefinitely. The rivers of the surrounding mountains, fed by the rains and melting snows of these comparatively moist elevations, dry and soak away as they come down upon the arid plains. They are compelled to lay aside their entire load of waste eroded from the mountain valleys, in fans which grow to enormous size, reaching in some instances thousands of feet in thickness.

The monotonous levels of Turkestan include vast alluvial tracts now in process of building by the floods of the frequently shifting channels of the Oxus and other rivers of the region. For about seven hundred miles from its mouth in Aral Lake the Oxus receives no tributaries, since even the larger branches of its system are lost in a network of distributaries and choked with desert sands before they reach their master stream. These aggrading rivers, which have channels but no valleys, spread their muddy floods—which in the case of the Oxus sometimes equal the average volume of the Mississippi—far and wide over the plain, washing the bases of the desert dunes.

Playas.In arid interior basins the central depressions may be occupied by playas,—plains of fine mud washed forward from the margins. In the wet season the playa is covered with a thin sheet of muddy water, a playa lake, supplied usually bysome stream at flood. In the dry season the lake evaporates, the river which fed it retreats, and there is left to view a hard, smooth, level floor of sun-baked and sun-cracked yellow clay utterly devoid of vegetation.

In the Black Rock desert of Nevada a playa lake spreads over an area fifty miles long and twenty miles wide. In summer it disappears; the Quinn River, which feeds it, shrinks back one hundred miles toward its source, leaving an absolutely barren floor of clay, level as the sea.

Lake deposits.Regarding lakes as parts of river systems, we may now notice the characteristic features of the deposits in lake basins. Soundings in lakes of considerable size and depth show that their bottoms are being covered with tine clays. Sand and gravel are found along; their margins, being brought in by streams and worn by waves from the shore, but there are no tidal or other strong currents to sweep coarse waste out from shore to any considerable distance. Where fine clays are now found on the land in even, horizontal layers containing the remains of fresh-water animals and plants, uncut by channels tilled with cross-bedded gravels and sands and bordered by beach deposits of coarse waste, we may safely infer the existence of ancient lakes.

Marl.Marl is a soft, whitish deposit of carbonate of lime, mingled often with more or less of clay, accumulated in small lakes whose feeding springs are charged with carbonate of lime and into which little waste is washed from the land. Such lakelets are not infrequent on the surface of the younger drift sheets of Michigan and northern Indiana, where their beds of marl—sometimes as much as forty feet thick—are utilized in the manufacture of Portland cement. The deposit results from the decay of certain aquatic plants which secrete lime carbonate from the water, from the decomposition of the calcareous shells of tiny mollusks which live in countless numbers on the lake floor, and in some cases apparently from chemical precipitation.

Peat.We have seen how lakelets are extinguished by the decaying remains of the vegetation which they support. Asection of such a fossil lake shows that below the growing mosses and other plants of the surface of the bog lies a spongy mass composed of dead vegetable tissue, which passes downward gradually intopeat,—a dense, dark brown carbonaceous deposit in which, to the unaided eye, little or no trace of vegetable structure remains. When dried, peat forms a fuel of some value and is used either cut into slabs and dried or pressed into bricks by machinery.

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