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PART VIII
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See also:PART VIII .-PHYSIOGRAPHICAL See also:GEOLOGY This See also:department of See also:geological inquiry investigates the origin and See also:history of the See also:present topographical features of the See also:land. As these features must obviously be related to those of earlier See also:time which are recorded in the rocks of the See also:earth's crust, they cannot be satisfactorily studied until at least the See also:main outlines of the history of these rocks have been traced. Hence physiographical See also:research comes appropriately after the other branches of the See also:science have been considered. From the stratigraphy of the terrestrial crust we learn that by far the largest part of the See also:area of dry land is built up of marine formations; and therefore that the present land is not an aboriginal portion of the earth's See also:surface, but has been overspread by the See also:sea in which its rocks were mainly accumulated. We further discover that this submergence of the land did not happen once only, but again and again in past ages and in all parts of the See also:world. Yet although the terrestrial areas varied much from See also:age to age in their extent and in their See also:distribution, being at one time more See also:continental, at another more insular, there is See also:reason to believe that these successive diminutions and expansions have on the whole been effected within, or not far outside, the limits of the existing continents. There is no See also:evidence that any portion of the present land ever See also:lay under the deeper parts of the ocean. The abysmal deposits of the ocean-See also:floor have no true representatives among the sedimentary formations anywhere visible on the land. Nor, on the other See also:hand, can it be shown that any part of the existing ocean abysses ever See also:rose above sea-level into dry land. Hence geologists have See also:drawn the inference that the ocean basins have probably been always where they now are; and that although the continental areas have often been narrowed by submergence and by denudation, there has probably seldom or never been a See also:complete The Geological See also:Record or See also:Order of See also:Succession of the Stratified ,Formations of the Earth's Crust. See also:Europe. See also:North See also:America. . c-... Historic, up to the present time. Similar to the See also:European 'de-
r P. Prehistoric, comprising deposits of the velopment, but with scantier
t -.a E See also:Iron, See also:Bronze, and later See also: o Wanting in See also:Britain; well See also:developed in Represented in the Eastern
France, S. E. Europe and Italy; divis- States by a marine series
ible into the following groups in (See also:Yorktown or Chesapeake,
descending order: (s) Pontian; (2) Chipola and Chattahoochee
Sarmatian; (3) Tortonian; (4) See also:Hel- groups), and in the interior
vetian; (5) Langhian (Burdigalian). by the lacustrine Loup See also:Fork
(See also:Nebraska), Deep River, and
See also: France and Belgium; fresh-water formations, See also:corn- Spardacian and Thanetian groups. prising, in descending order, the Uinta, Bridger, See also:Wind River, Wasatch, Torrejon, and Puerco groups. On the Pacific side the marine Tejon series of Oregon and See also:California. Daman—wanting in Britain; uppermost On the Atlantic border both See also:limestone of See also:Denmark. marine strata and others See also:con- Senonian —Upper See also:Chalk with Flints of taming a terrestrial See also:flora re- See also:England; Atari= and Emscherian present the Cretaceous series stages on the European See also:continent. of formations. Turonian—See also:Middle Chalk with few In the interior there is also a flints, and comprising the Angoumian commingling of marine with and Ligerian stages. lacustrine deposits. At the Cenomanian—Lower Chalk and Chalk See also:top lies the See also:Laramie or Lig- sfarl. nitic series with an abundant See also:Albian=Upper See also:Greensand and See also:Gault. terrestrial flora, passing down See also:Aptian — Lower Greensand; Marls and into the lacustrine and limestones of See also:Provence, &c. brackish-water See also:Montana Urgonian (Barremian)—Atherfield clay; series. Of older date, the massive Hippurite limestones of See also:Colorado series contains, an See also:southern France. abundant marine See also:fauna, yet Neocomian—See also:Weald clay and See also:Hastings includes also some See also:coal-seams. See also:sand; Hauterivian and Valanginian The Niobrara marls and See also:lime- sub-stages of See also:Switzerland and France. stones are likewise of marine origin, but the lower members of the series (See also:Benton and Dakota) show another See also:great See also:representation of fresh-water sedimentation with lignites and coals. In California a vast succession of marine deposits (Shasta- Chico) represents the Cre- taceous See also:system; and in western See also:British N. America coal-seams also occur. See also:Purbeckian—Purbeck beds; Mfinder Representatives of the Middle ,. Mergel; largely present in See also:West- and lower See also:Jurassic forma- o phalia. tions have been found in "-' See also:Portlandian—See also:Portland group of Eng- California and Oregon, and land, represented in S. France by the farther north among the See also:Arctic . thick Tithonian limestones. islands. Kimmeridgian—Kimmeridge Clay of Strata containing Lower Turns- England; Virgulian and Plerocerian sic marine fossils appear in groups of N. Prance; represented by See also:Wyoming and Dakota; and thick limestones in the Mediterranean above them come the Atlanto- ba,See also:sin. sauces and Baptanodon beds, Europe. North America. o t; See also:Corallian-See also:Coral Rag, Coralline Oolite; which have yielded so large a ti Sequanian stages of the Continent, variety of deinosaurs and other comprising the sub-stages of Astartian vertebrates, and especially the and Rauracian. remains of a number of genera See also:Oxfordian—See also:Oxford Clay; Argovian and of small mammals. Neuvizyan stages. Calloviain—Kellaways Rock, Divesian sub-See also:stage of N. France. Batlioaian—series of English strata from See also:Cornbrash down to See also:Fuller's Earth. See also:Bajocian—Inferior Oolite of England. Liassic—divisible into (s) Upper See also:Lias or Toarcian, (2) Middle Lias, See also:Marl- stone or Charmouthian, (3) Lower Lias of Sineinurian and Hettangian. ° J In See also:Germany and western Europe this In New See also:York, See also:Connecticut, New •g l division represents the deposits of See also:Brunswick, and Nova See also:Scotia inland seas or lagoons, and is divisible a series of red See also:sandstone into the following stages in descending (See also:Newark series) contains land- order; See also:plants and labyrinthodonts order; (s) See also:Rhaetic, (2) See also:Keuper, (3) like the See also:lagoon type of central See also:Muschelkalk, (4) See also:Bunter. In the and western Europe. On the eastern See also:Alps and the Mediterranean Pacific slope, however, marine basin the contemporaneous sediment- equivalents occur, represent- ary formations are those of open clear See also:ing the pelagic type of south- sea, in which a thickness of many eastern Europe. thousand feet of strata was accumu- la ted. v T huringian—Zechstein, Magnesian To this division of the geologi- a Limestone; named from its develop- cal record the Upper Barren ment in Thuringia; well represented See also:Measures of the coal-See also:fields of also in See also:Saxony, See also:Bavaria and Bohemia. See also:Pennsylvania, See also:Prince See also:Edward Saxonian—Rothliegendes Group; Red See also:Island, Nova Scotia and Sandstones, &c. New Brunswick have been Autuni an—where the strata present the assigned. lagoon facies, well displayed at See also:Autun Farther south in See also:Kansas, See also:Texas, in France; where the marine type is and Nebraska the representa- predominant, as in See also:Russia, the group lives of the division have an has been termed Artinskian. abundant marine fauna. v Stephanian or Uralian—represented in Upper productive Coal- p Russia by marine formations, and in measures. U central and western Europe by numer- Lower Barren measures. ous small basins containing a See also:peculiar Lower productive Coal- flora and in some places a great variety measures. of See also:insects. See also:Pottsville See also:conglomerate. .. W e s t p h a l i a n or Moscovian—Coal- Mauch Chunk shales; lime-
measures, Millstone Grit. stones of See also:Chester, St See also: Group. ehtes See also:Oriskany Sandstone. w i t h c p aspis, See also:Pier- aspis, &c. See also:Ludlow Group. Lower Helderberg Group. Upper 'I See also:Wenlock Water-Lime. See also:Llandovery" See also:Niagara Shale and Limestone. See also:Clinton Group. See also:Medina a (Caradoc or See also:Bala Group. See also:Cincinnati Group. See also:mica Lower Trenton (Ordovician) Sl Arenig Chazy Arenig Calciferous Aren U Upper or Olenus series—Tremadoc slates Upper or See also:Potsdam series with andLingulaFlags. Olenus and Dicelocephalus Middle or Paradoxides series—Mene- fauna. vian Group. Middle or See also:Acadian series with Lower or Olenellus series—See also:Llanberis Paradoxides fauna. and See also:Harlech Group, and Olenellus- Lower or Georgian.series with See also:zone. Olenellus fauna o In See also:Scotland, underneath the See also:Cambrian In See also:Canada and the Lake Olenellus group, lies unconformably See also:Superior region of the See also:United a See also:mass of red sandstone and con- States a vast succession of glomerate (See also:Torridonian) 8000 or so,000 rocks of Pre-Cambrian age ft . thick, which rests with a strong has been grouped into the unconformability on a series of coarse following subdivisions in de- B gneisses and See also:schists (Lewisian). A scending order: (r) Keweena- o thick series of slates and phyllites lies wan, lying unconformably on E below the See also:oldest Palaeozoic rocks in . (2) Animikie, separated by a central. Europe, with coarse gneisses strong unconformability from below. (3) Upper Huronian, (t) Lower V Huronian with an unconform- able See also:base, (5) Goutchiching, (6) Laurentian. In the eastern part of Canada, Newfound- land, &c., and also in Mon- See also:tana, estat sedimentary formations See also:low Cambr an s one have been found to contain some obscure organisms. disappearance of land. The fact that the sedimentary formations of each successive geological period consist to so large an extent of mechanically formed See also:terrigenous detritus, affords See also:good evidence of the coexistence of tracts of land as well as of extensive denudation. From these See also:general considerations we proceed to inquire how the existing topographical features of the land arose. Obviously the co-operation of the two great geological agencies of hypogene and epigene See also:energy, which have been at See also:work from the beginning of our globe's decipherable history, must have been the cause to which these features are to be assigned; and the task of the geologist is to ascertain, if possible, the part that has been taken by each. There is a natural tendency to see in a stupendous piece of scenery, such as a deep See also:ravine, a range of hills, a See also:line of precipice or a See also:chain of mountains, evidence only of subterranean convulsion; and before the subject was taken up as a See also:matter of strict scientific See also:induction, an See also:appeal to former cataclysms was considered a sufficient See also:solution of the problems presented by such features of landscape. The rise of the See also:modern Huttonian school, however, led to a more careful examination of these problems. The important See also:share taken by erosion in the determination of the present features of landscape was then recognized, while a fuller appreciation of the relative parts played by the hypogene and epigene causes has gradually been reached. r. The study of the progress of denudation at the present time has led to the conclusion that even if the See also:rate of See also:waste were not more rapid than it is to-day, it would yet suffice in a comparatively brief geological period to reduce the dry land to below the sea-level. But not only would the area of the land be diminished by denudation, it could hardly fail to be more or less involved in those widespread movements of subsidence, during which the thick sedimentary formations of the crust appear to have been accumulated. It is thus See also:manifest that there must have been from time to time during the history of our globe upward movements of the crust, whereby the See also:balance between land and sea was redressed. Proofs of such movements have been abundantly preserved among the stratified formations. We there learn that the uplifts have usually followed each other at See also:long intervals between which subsidence prevailed, and thus that there has been a prolonged oscillation of the crust over the great continental areas of the earth's surface. An examination of that surface leads to the recognition of two great types of upheaval. In the one, the sea-floor, with all its thick accumulations of sediment, has been carried upwards, sometimes for several thousand feet, so equably that the strata retain their See also:original flatness with hardly any sensible disturbance for hundreds of square See also:miles. In the other type the solid crust has been plicated, corrugated and dislocated, especially along particular lines, and has attained its most stupendous disruption in lofty chains of mountains. Between these two phases of uplift many intermediate stages have been developed, according to the direction and intensity of the subterranean force and the varying nature and disposition of the rocks of the crust. (a) Where the uplift has extended over wide spaces, without appreciable deformation of the crust, the See also:flat strata have given rise to low plains, or if the amount of uprise has been great enough, to high plains, plateaux or tablelands. The plains of Russia, for example, See also:lie for the most part on such tracts of equably uplifted strata. The great plains of the western interior of the United States See also:form a great See also:plateau or tableland, 5000 or 6000 ft. above the sea, and many thousands of square miles in extent, on which the Rocky Mountains have been ridged up. (b) It is in a great See also:mountain-chain that the complicated structures developed during disturbances of the earth's crush can best be studied (see Parts IV. and V. of this See also:article), and where the See also:influence of these structures on the See also:topography of the surface is most effectively displayed. Such a chain may be the result of one See also:colossal disturbance; but those of high geological antiquity usually furnish proofs of successive uplifts with more or less intervening denudation. Formed along lines of continental displacement in the crust, they have again and again givenrelief from the See also:strain of See also:compression by fresh crumpling, fracture and uprise. The See also:chief See also:guide in tracing these successive stages of growth is supplied by unconformability. If, for example, a mountain-range consists of upraised See also:Silurian rocks, upon the upturned and denuded edges of which the Carboniferous Lime-stone lies transgressively, it is clear that its original upheaval must have taken place in the period of geological time represented by the See also:interval between the Silurian and the Carboniferous Limestone formations. If, as the range is followed along its course, the Carboniferous Limestone is found to be also highly inclined and covered unconformably by the Upper Coal-measures, a second uplift of that, portion of the ground can be proved to have taken place between the time of the Limestone-and that of the Upper Coal-measures. By this See also:simple and obvious See also:kind of evidence the relative ages of different mountain-chains may be compared. In most great chains, however, the rocks have been so intensely crumpled, and even inverted, that much labour may be required before their true relations can be deter-See also:mined. The Alps furnish an instructive example of the long series of revolutions through which a great mountain-system may have passed before reaching its present development. The first beginnings of the chain may have been upraised before the oldest Palaeozoic formations were laid down. There are at least traces of land and See also:shore-lines in the Carboniferous period. Subsequent submergences and uplifts appear to have occurred during the Mesozoic periods. There is evidence that thereafter the whole region sank deep under the sea, in which the older See also:Tertiary sediments were accumulated, and which seems to have spread right across the See also:heart of the Old World. But after the deposition of the See also:Eocene formations came the gigantic disruptions whereby all the rocks of the Alpine region were folded over each other, crushed, corrugated, fractured and displaced, some of their older, portions, including the fundamental gneisses and schists, being squeezed up, torn off, and pushed horizontally for many miles over the younger rocks. But this upheaval, though the most momentous, was not the last which the chain has undergone, for at a later See also:epoch in Tertiary time renewed disturbance gave rise to a further series of ruptures and plications. The chain thus successively upheaved has been continuously exposed to denudation and has consequently lost much of its original height. That it has been See also:left in a See also:state of instability is indicated- by the frequent earthquakes of the Alpine region, which doubtless arise from the sudden snapping of rocks under intense strain. A distinct type of mountain due to See also:direct hypogene See also:action is to be seen in a See also:volcano. It has been already pointed out (Part IV. See also:sect. I) that at the vents which maintain a communication between the molten magma of the earth's interior and the surface, eruptions take place whereby quantities of See also:lava and fragmentary materials are heaped See also:round each orifice of See also:discharge. A typical volcanic mountain takes the form of a perfect See also:cone, but as it grows in See also:size and its main. vent is choked, while the sides of the cone are unable to withstand the force of the explosions or the pressure of the ascending See also:column of lava, eruptions take place laterally, and numerous parasitic cones arise on the flanks of the See also:parent mountain. Where lava flows out from long fissures, it may See also:pile up vast sheets of rock, and See also:bury the surrounding See also:country under several thousand feet of solid stone, covering many hundreds of square miles. In this way volcanic tablelands have been formed which, attacked by the denuding forces, are gradually trenched by valleys and ravines, until the original level surface of the lava-See also: Side by side with the various upheavals and subsidences, there has been a continuous removal of materials from the land, and an equally persistent See also:deposit of these materials under water, with the consequent growth of new rocks. Denudation has been aptly compared to a See also:process of sculpturing wherein, while each of the implements employed by nature, like a See also:special kind of graving See also:tool, produces its own characteristic impress on the land, they all combine harmoniously towards the achievement of their one See also:common task. Hence the present contours of the land depend partly on the original configuration of the ground, and the influence it may have had in guiding the operations of the erosive agents, partly on the vigour with which these agents perform their work, and partly on the varying structure and See also:powers of resistance possessed by the rocks on which the erosion is carried on. Where a new See also:tract of land has been raised out of the sea by such an energetic See also:movement as See also:broke up the crust and produced the complicated structure and tumultuous See also:external forms of a great mountain chain, the influence of the hypogene forces on the topography attains its highest development. But even the youngest existing chain has suffered so greatly from denudation that the aspect which it presented at the time of its uplift can only be dimly perceived. No more striking See also:illustration of this feature can be found than that supplied by the Alps, nor one where the geotectonic structures have been so fully studied in detail. On the See also:outer flanks of these mountains the See also:longitudinal ridges and valleys of the See also:Jura correspond with lines of anticline and syncline. Yet though the dominant topographical elements of the region have obviously been produced by the plication of the stratified formations, each See also:ridge has suffered so large an amount of erosion that the younger rocks have been removed from its See also:crest where the older members of the series are now exposed to view, while on every slope proofs may be seen of extensive denudation. If from these long See also:wave-like undulations of the ground, where the relations between the disposition of the rocks below and the forms of the surface are so clearly traceable, the observer proceeds inwards to the main chain, he finds that the plications and displacements of the various formations assume an increasingly complicated character; and that although proofs of great denudation continue to abound, it becomes increasingly difficult to form any satisfactory conjecture as to the shape of the ground when the upheaval ended or any reliable estimate of the amount of material which has since then been removed. Along the 'central heights the mountains lift themselves towards the See also:sky like the See also:storm-swept crests of vast earth-billows. The whole aspect of the ground suggests intense commotion, and the impression thus given is often much intensified by the See also:twisted and.crumpled strata, visible from a long distance, on the crags and crests. On this broken-up surface the various agents ofdenudation have been ceaselessly engaged since it emerged from the sea. They have excavated valleys, sometimes along depressions provided for them by the subterranean disturbances, sometimes down the slopes of the disrupted blocks of ground. So powerful has been this erosion that valleys cut out along lines of anticline, which were natural ridges, have sometimes become more important than those in lines of syncline, which were structurally depressions. The same subaerial forces have eroded lake-basins, dug out corries or cirques, notched the ridges, splintered the crests and furrowed the slopes, leaving no part of the original surface of the uplifted chain unmodified. It has often been noted with surprise that features of underground structure which, it might have been confidently anticipated, should have exercised a marked influence on the topography of the surface have not been able to resist the levelling action of the denuding agents, and do not now affect the surface at all. This result is conspicuously seen in coal-fields where the strata are abundantly traversed by faults. These dislocations, having sometimes a displacement of several See also:hundred feet, might have been expected to break up the surface into a network of cliffs and plains; yet in general they do not modify the level character of the ground above. One of the most remarkable faults in Europe is the great thrust which See also:bounds the southern edge of the Belgian coal-field and brings the Devonian rocks above the Coal-measures. It can be traced across Belgium into the Boulonnais, and may not improbably run beneath the Secondary and Tertiary rocks of the south of England. It is crossed by the valleys of the See also:Meuse and other northerly-flowing streams. Yet so indistinctly is it marked in the Meuse valley that no one would suspect its existence from any peculiarity in the general form of the ground, and even an experienced geologist, until he had learned the structure of the See also:district, would scarcely detect any See also:fault at all. Where faults have influenced the superficial topography, it is usually by giving rise to a hollow along which the subaerial agents and especially See also:running water can See also:act effectively. Such a hollow may be eventually widened and deepened into a valley. On See also:bare crags and crests, lines of fault are See also:apt to be marked by notches or clefts, and they thus help to produce the pinnacles and serrated outlines of these exposed uplands. It was cogently enforced by See also:Hutton and Playf air, and independently by See also:Lamarck, that no co-operation of underground agency is needed to produce such topography as may be seen in a great part of the world, but that if a tract of sea-floor were upraised into a wide See also:plain, the fall of rain and the circulation of water over its surface would in the end carve out such a system of hills and valleys as may be seen on the dry land now. No such plain would be a dead-level. It would have inequalities on its surface which would serve as channels to guide the drainage from the first showers of rain. And these channels would be slowly widened and deepened until they would become ravines and valleys, while the ground between them would be left projecting as ridges and hills. Nor would the erosion of such a system of water-courses require a long series of geological periods for its accomplishment. From measurements and estimates of the amount of erosion now taking place in the basin of the Mississippi river it has been computed that valleys 800 ft. deep might be carved out in less than a million years. In the vast tablelands of Colorado and other western regions of the United States an impressive picture is presented of the results of See also:mere subaerial erosion on undisturbed and nearly level strata. Systems of stream-courses and valleys, river See also:gorges unexampled elsewhere in the world for See also:depth and length, vast winding lines of escarpment, like ranges of sea-cliffs, terraced slopes rising from plateau to plateau, huge buttresses and solitary stacks See also:standing like islands out of the plains, great mountain-masses towering into picturesque peaks and pinnacles cleft by innumerable gullies, yet everywhere marked by the parallel bars of the See also:horizontal strata out of which they have been carved—these are the orderly symmetrical characteristics of a country where the scenery is due entirely to the action of subaerial agents on the one hand and the varying resistance of perfectly See also:regular stratified rocks on the other. The details of the See also:sculpture of the land have mainly depended on the nature of the materials on which nature's erosive tools have been employed. The See also:joints by which all rocks are traversed have been especially serviceable as dominant lines down which the rain has filtered, up which the springs have risen and into which the See also:frost wedges have been driven. On the high bare scarps of a lofty mountain the inner structure of the mass is laid open, and there the system of joints even more than faults is seen to have determined the lines of crest, the See also:vertical walls of cliff and precipice, the forms of See also:buttress and See also:recess, the position of cleft and chasm, the outline of See also:spire and See also:pinnacle. On the lower slopes, even under the See also:tapestry of verdure which nature delights to hang where she can over her naked rocks, we may detect the same pervading influence of the joints upon the forms assumed by ravines and crags. Each kind of stone, too, gives rise to its own characteristic form of scenery. Massive crystalline rocks, such as See also:granite, break up along their joints and often decay into sand or earth along their exposed surfaces, giving rise to rugged crags with long See also:talus slopes at their base. The stratified rocks besides splitting at their joints are especially distinguished by parallel ledges, cornices and recesses, produced by the irregular decay of their component strata, so that they often assume curiously architectural types of scenery. But besides this See also:family feature they display many See also:minor varieties of aspect according to their lithological See also:composition. A range of sandstone hills, for example, presents a marked contrast to one of limestone, and a line of chalk See also:downs to the escarpments formed by alternating bands of harder and softer clays and shales. It may suffice here merely to allude to a few of the more important parts of the topography of the land in their relation to physiographical geology. A true mountain-chain, viewed from the geological side, is a mass of high ground which owes its prominence to a ridging-up of the earth's crust, and the intense plication and rupture of the rocks of which it is composed. But ranges of hills almost mountainous in their bulk may be formed by the See also:gradual erosion of valleys out of a mass of original high ground, such as a high plateau or tableland. Eminences which have been isolated by denudation from the main mass of the formations of which they originally formed part are known as " outliers " or " hills of circumdenudation." Tablelands, as already pointed out, may be produced either by the upheaval of tracts of horizontal strata from the sea-floor into land; or by the uprise of plains of denudation, where rocks of various composition, structure and age have been levelled down to near or below the level of the sea by the co-operation of the various erosive agents. Most of the great tablelands of the globe are platforms of little-disturbed strata which have been upraised bodily to a considerable See also:elevation. No sooner, however, are they placed in that position than they are attacked by running water, and begin to he hollowed out into systems of valleys. As the valleys sink, the platforms between them grow into narrower and more definite ridges, until eventually the level tableland is converted into a complicated network of hills and valleys, wherein, nevertheless, the See also: Where the influence of rain and frost has been slight, and the streams, supplied from distant See also:sources, have had sufficient declivity, deep, narrow, precipitous ravines or gorges have been excavated. The canyons of the arid region of the Colorado are a magnificent example of this result. Where, on the other hand, See also:ordinary atmospheric action has been more rapid, the sides of the river channels have been attacked, and open sloping glens and valleys have been hollowed out. A See also:gorge or See also:defile is usually due to the action of a See also:waterfall, which, beginning with some abrupt declivity or precipice in the course of the river when it first commenced to flow, or caused by some hard rock See also:crossing the channel, has eaten its way backward.
Lakes have been already referred to, and their modes of origin have been mentioned. As they are continually being filled up with the detritus washed into them from the surrounding regions they cannot be of any great geological antiquity, unless where by some unknown process their basins are from time to time widened and deepened.
In the general subaerial denudation of a country, innumerable minor features are worked out as the structure of the rocks controls the operations of the eroding agents. Thus, among comparatively undisturbed strata, a hard bed resting upon others of a softer kind is apt to form along its outcrop a line of cliff or escarpment. Though a long range of such cliffs resembles a See also:coast that has been worn by the sea, it may be entirely due to mere atmospheric waste. Again, the more resisting portions of a rock may be seen projecting as crags or knolls. An igneous mass will stand out as a bold See also: The torrents See also:tear out deep gullies from the sides of the declivities. Corries or cirques are scooped out on the one hand and naked precipices are left on the other. The harder bands of rock project as massive ribs down the slopes, shoot up into prominent aiguilles, or help to give to the summits the notched saw-like outlines they so often present.
The materials worn from the surface of the higher are spread out over the lower grounds. The streams as they descend begin to drop their See also:freight of sediment when, by the lessening of their declivity, their carrying See also:power is diminished. The great plains of the earth's surface are due to this deposit of See also:gravel, sand and See also:loam. They are thus monuments at once of the destructive and reproductive processes which have been in progress unceasingly since the first land rose above the sea and the first shower of rain See also:fell. Every pebble and particle of their See also:soil, once part of the distant mountains, has travelled slowly and fitfully to lower levels. Again and again have these materials been shifted, ever moving downward and sea-See also: (A. Additional information and CommentsThere are no comments yet for this article.
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