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NIAGARA .) Many additional features associated with the glacial See also:period might be described, but space can be given to four only. In certain districts the subglacial till was not spread out in a smooth See also:plain, but accumulated in elliptical mounds, See also:loo or 200 ft. high, See also:half a mile or a mile See also:long, with axes parallel to the direction of the See also:ice See also:motion as indicated by striae on the underlying See also:rock See also:floor; these hills are known by the Irish name, drumlins, used for similar hills in See also:north-western See also:Ireland. The most remarkable See also:groups of drumlins occur in western New See also:York, where their number is estimated at over 600o, and in See also:southern See also:Wisconsin, where it is placed at 5000. They completely dominate the See also:topography of their districts. A curious See also:deposit of an impalpably See also:fine and unstratified silt, known by the See also:German name See also:loess, lies on the older See also:drift sheets near the larger See also:river courses of the upper See also:Mississippi See also:basin. It attains a thickness of 20 ft. or more near the See also:rivers and gradually fades away at a distance of ten or more See also:miles on either See also:side. It is of inexhaustible fertility, being in this as well as in other respects closely like the loess in See also:China and other parts of See also:Asia, as well as in See also:Germany. It contains See also:land shells, and hence cannot be attributed to marine or lacustrine submergence. The best explanation suggested for loess is that, during certain phases of the glacial period, it was carried as dust by the winds from the See also:flood plains of aggrading rivers, and slowly deposited on the neighbouring grass-covered plains. See also:South-western Wisconsin and parts of the adjacent states of See also:Illinois, See also:Iowa and See also:Minnesota are known as the "driftless See also:area," because, although bordered by drift sheets and moraines, it is See also:free from glacial deposits. It must therefore have been a sort of See also:oasis, when the ice sheets from the north advanced past it on the See also:east and See also:west and joined around its southern border. The See also:reason for this exemption from glaciation is the converse of that for the southward convexity of the morainic loops; for while they See also:mark the paths of greatest glacial advance along See also:lowland troughs (See also:lake basins), the driftless area is a See also:district protected from ice invasion by reason of the obstruction which the See also:highlands of See also:northern Wisconsin and See also:Michigan (See also:part of the See also:Superior oldland) offered to glacial advance. The course of the upper Mississippi river is largely consequent upon glacial deposits. Its See also:sources are in the morainic lakes in northern Minnesota; Lake Itasca being only one of many glacial lakes which See also:supply the headwater branches of the See also:great river. The drift deposits thereabouts are so heavy that the See also:present divides between the drainage basins of See also:Hudson See also:Bay, Lake Superior and the Gulf of See also:Mexico evidently stand in no very definite relation to the preglacial divides. The course of the Mississippi through Minnesota is largely guided by the See also:form of the drift See also:cover. Several rapids and the Falls of St See also:Anthony (determining the site of Minneapolis) are signs of immaturity, resulting from superposition through the drift on the under rock. Farther south, as far as the entrance of the See also:Ohio, the Mississippi follows a rock-walled valley 300 to 400 ft. deep, with a flood-plain 2 to 4 M. wide; this valley seems to represent the path of an enlarged See also:early-glacial Mississippi, when much precipitation that is to-See also:day discharged to Hudson Bay and the Gulf of St See also:Lawrence was delivered to the Gulf of Mexico, for the curves of the present river are of distinctly smaller See also:radius than the curves of the valley. Lake Pepin (3o M. below St See also:Paul), a picturesque expansion of the river across its flood-plain, is due to the aggradation of the valley floor where the Chippewa river, coming from the north-east, brought an overload of fluvio-glacial drift. Hence even the " See also:father of See also:waters," like so many other rivers in the Northern states. owes many of its features more or less directly to glacial See also:action. The fertility of the prairies is a natural consequence of their origin. During the See also:mechanical comminution of the till no vegetation was present to remove the minerals essential to plantgrowth, as is the See also:case in the soils of normally weathered and dissected peneplains, such as the Appalachian See also:piedmont, where the soils, though not exhausted by the primeval See also:forest cover, are by no means • so See also:rich as the till sheets of the prairies. Moreover, whatever the rocky understructure, the till See also:soil has been averaged by a thorough mechanical mixture of rock grindings; hence the prairies are continuously fertile for scores of miles together. The true prairies, when first explored, were covered with a rich growth of natural grass and See also:annual flowering See also:plants. To-day they are covered with farms. The cause of the treelessness has been much discussed. It does not seem to See also:lie in peculiarities of temperature or of precipitation; for trees thrive where they are properly planted on the prairies; every See also:town and See also:farm to-day has its avenues and groves of trees; but it should be noted that west of the Mississippi river increasing aridity becomes an important See also:factor, and is the See also:chief cause of the treelessness of the Great Plains (see below). The treelessness of the prairies cannot be due to insufficient See also:time for See also:tree invasion since glacial evacuation; for forests cover the rocky uplands of See also:Canada, which were occupied by ice for ages after the prairies were laid See also:bare. A more probable cause is found in the fineness of the See also:prairie soil, which is inimical to the growth of See also:young trees in competition with the See also:grasses and annual plants. Prairie fires, both of natural and artificial origin, are also a contributive cause; for young trees are exterminated by fires, but annual plants soon reappear. The Gulf Coastal Plain.—The westward See also:extension of the See also:Atlantic coastal plain around the Gulf of Mexico carries with it a repetition of certain features already described, and the addition of several new ones. As in the Atlantic coastal plain, it is only the See also:lower, seaward part of this region that deserves the name of plain, for there alone is the See also:surface unbroken by hills or valleys; the inner part, initially a plain by reason of its essentially See also:horizontal (gently seaward-sloping) structure, - has been converted by mature See also:dissection into an elaborate complex of hills and valleys, usually of increasing See also:altitude and See also:relief as one passes inland. The See also:special features of the Gulf Plain are the See also:peninsular extension of the plain in See also:Florida, the belted arrangement of relief and soils in See also:Alabama and in See also:Texas, and the Mississippi embayment or inland extension of the plain half-way up the course of the Mississippi river, with the Mississippi flood plain there included. A broad, See also:low crustal See also:arch extends southward at the junction of the Atlantic and Gulf coastal plains; the emerged half of the arch constitutes the visible lowland See also:peninsula of Florida; Florida. the submerged half extends westward under the shallow overlapping waters of the Gulf of Mexico. The northern part of the peninsula is composed largely of a weak See also:limestone; here much of the lowland drainage is underground, forming many sink-holes (See also:swallow-holes). Many small lakes in the lowland appear to owe their basins to the See also:solution of the limestones. Valuable phosphate deposits occur in certain districts. The southern part of the See also:state includes the " See also:Everglades " (q.v.), a large area of low, See also:flat, marshy land, overgrown with tall reedy grass, a veritable See also:wilderness; thus giving Florida an unenvied first See also:rank among the states in See also:marsh area. The eastern See also:coast is fringed by long-stretching See also:sand reefs, enclosing lagoons so narrow and continuous that they are popularly called " rivers." At the southern end of the peninsula is a See also:series of See also:coral islands, known as " keys "; they appear to be due to the forward growth of See also:corals and other See also:lime-secreting organisms towards the strong current of the Gulf Stream, by which their See also:food is supplied: the part of the peninsula composed of coral reefs is less than has been formerly supposed. The western coast has fewer and shorter off-See also:shore reefs; much of it is of minutely irregular outline, which seems to be determined less by the See also:work of the See also:sea than by the forward growth of See also:mangrove swamps in the shallow See also:salt See also:water. A typical example of a belted coastal plain is found in Alabama and the adjacent part of Mississippi. The plain is here about 15o m. wide. The basal formation is chiefly a weak limestone, which has been stripped from its See also:original Alabama. innermost extension and worn down to a flat inner lowland of rich See also:black soil, thus gaining the name of the " black See also:belt." The lowland is enclosed by an upland or See also:cuesta, known as Chunnenugga See also:Ridge, sustained by partly consolidated sandy strata ; the upland, however, is not continuous, and hence should be described as a " maturely dissected cuesta." It has a relatively rapid descent toward the inner lowland, and a very See also:gradual descent to the coast prairies, which become very low, flat and marshy before dipping under the Gulf waters, where they are generally fringed by off-shore reefs. The coastal plain extends 500 M. inland on the See also:axis of the Mississippi embayment. Its inner border affords admirable examples of topographical discordance where it sweeps north-westward square across the trend of the piedmont belt, the ridges and valleys, and the See also:plateau of the Appalachians, which are all terminated by dipping The gently beneath the unconformable cover of the coastal Mississippi plain strata. In the same way the western side of the em- Bmbaymeat.11ower south-eastern See also:sou s de th an of t See also:head dissected Ozark 1plateau of southern See also:Missouri and northern See also:Arkansas, which in many ways resembles the Appalachian plateau, and along the eastern end of the Massern ranges of the Ouachita See also:mountain See also:system in central Arkansas, which in See also:geological See also:history and topographical form present many analogies with the ridges and valleys of the Appalachians; and as the coastal plain turns westward to Texas it See also:borders the Arbuckle hills in See also:Oklahoma, a small analogue of the crystalline Appalachian belt. In the embayment of the coastal plain some low cuesta-like belts of hills with associated strips of lowlands suggest the features of a beltedcoastal plain ; the hillybeltordissected cuesta determined by the See also:Grand Gulf formation in western Mississippi is the most distinct. Important salt deposits occur in the coastal plain strata near the coast. The most striking feature of the embayment is the broad valley which the Mississippi has eroded across it. The lower Mississippi is the See also:truck in which three large rivers join; the chief figures (approximate only) regarding them are as follows: Drainage Area Percentage of (square miles). See also:Total See also:Discharge. Upper Mississippi 170,000 i8 Ohio 210,000 31 Missouri 530, 000 14 The small proportion of total water See also:volume supplied from the great Missouri basin is due to the See also:light precipitation in that region. The The Lower lower Mississippi receives no large tributary from the The Lower east, but two important ones come from the west; the Mississippi Arkansas drainage area being a little less than that River. of the Ohio, and the basin of the Red River of See also:Louisiana being about half as large. The great river thus constituted drains an area of about 1,250,000 sq. m., or about one-third of the See also:United States; and discharges 75,000 cub. yds. of water per second, or 785,190,000,000 cubic yds. per annum, which corresponds roughly to one See also:quarter of the total precipitation on its drainage basin. Its load of land See also:waste (see I. C. See also:Russell, Rivers of North See also:America) is as follows: In suspension . . 6,718,694,400 cub. ft. or 241 ft. deep over 1 sq. m. Swept along bottom 750,000,000 „ „ 26 „ 1 „ In solution . . 1,350,000,000 45 , 1 See also:Average annual removal of waste from entire basin, ,.-fib in. or 1 ft. in 4000 years. The head of the coastal plain embayment is near the junction of the Ohio and the Mississippi. Thence southward for 56o m. the great river flows through the semi-consolidated strata of the plain, in which it has eroded a valley, 40 or 50 M. wide, and 29,700 sq. M. in area, enclosed by bluffs one or two See also:hundred feet high in the northern part, generally decreasing to the southward, but with See also:local increase of height associated with a decrease in flood plain breadth on the eastern side where the Grand Gulf cuesta is traversed. This valley in the coastal plain, with the much narrower rock-walled valley of the upper river in the prairie states, is the true valley of the Mississippi river; but in popular phrase the "Mississippi Valley " is taken to include a large central part of the Mississippi drainage basin. The valley floor is covered with a flood plain of fine silt, having a southward slope of only half a See also:foot to a mile. The length of the river itself, from the Ohio mouth to the Gulf, is, owing to its windings, about 1o6o m.; its mean fall is about 3 in. in a mile. On See also:account of the rapid deposition of sediment near the See also:main channel at times of overflow, the flood plain, as is normally the case on mature valley floors, has a lateral slope of as much as 5, to, or even 12 ft. in the first mile from the river; but this soon decreases to a less amount. Hence at a See also:short distance from the river the flood plain is often swampy, unless its surface is there aggraded by the tributary streams: for this reason Louisiana, Arkansas and Mississippi rank next after Florida in swamp area. The great river receives an abundant load of silt from its tributaries, and takes up and See also:lays down silt from its own See also:bed and See also:banks with every See also:change of velocity. The swiftest current tends, by reason of centrifugal force, to follow the See also:outer side of every significant See also:curve in the channel; hence the See also:concave See also:bank, against which the rapid current sweeps, is worn away; thus any See also:chance irregularity is exaggerated, and in time a series of large serpentines or meanders is See also:developed, the most-symmetrical examples at present being those near See also:Greenville, See also:Miss. The growth of the meanders tends to give the river continually increasing length; but this tendency is See also:counter-acted by the sudden occurrence of cut-offs from time to time, so that a fairly See also:constant length is maintained. The floods of the Mississippi usually occur in See also:spring or summer: Owing to the great See also:size of the drainage basin, it seldom happens that the three upper tributaries are in flood at the same time; the coincident occurrence of floods in only two tributaries is of serious import in the lower river, which rises 30, 40, or occasionally 5o ft. Theabundant records by the Mississippi River See also:Commission and the United States See also:Weather See also:Bureau (by which accurate and extremely useful predictions of floods in the lower river course are made, on the basis of the observed rise in the tributaries) demonstrate a number of interesting features, of which the chief are as follows: the fall of the river is significantly steepened and its velocity is accelerated down stream from the point of highest rise; conversely, the fall and the velocity are both diminished up stream from the same point. The load of silt See also:borne down stream by the river finally, after many halts on the way, reaches the waters of the Gulf, where the decrease of velocity, aided by the salinity of the sea water, causes the formation of a remarkable See also:delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river). The See also:ordinary triangular form of deltas, due to the smoothing of the delta front by sea action, is here wanting, because of the weakness of sea action in comparison with the strength of the current in each of the four distributaries or " passes " into which the river divides near its mouth. (See MississlPPI RIVER.) After constriction from the Mississippi embayment to 250 m, in western Louisiana, the coastal plain continues south-westward with this breadth until it narrows to about 13o m. in The Texas southern Texas near the See also:crossing of the See also:Colorado river, coastal (of Texas) ; but it again widens to 30o m. at the plain. See also:national boundary as a See also:joint effect of embayment up the valley of the Rio Grande and of the seaward advance of this river's rounded delta front: these several changes take See also:place in a distance of about 500 tn., and hence include a region of over See also:ioo,000 sq. m.—less than half of the large state of Texas. A belted arrangement of reliefs and soils, resulting from See also:differential erosion on strata of unlike See also:composition and resistance, characterizes almost the entire area of the coastal plain. Most of the plain is treeless prairie, but the sandier belts are forested; two of them are known as " See also:cross timbers," because their trend is transverse to the See also:general course of the main consequent rivers. An inland extension from the coastal plain in north-central Texas leads to a large cuesta known as Grand Prairie (not structurally included in the coastal plain), upheld at altitudes of 1200 or 1300 ft. by a resistant Cretaceous limestone, which dips gently seaward; its scalloped inland-facing escarpment overlooks a denuded central prairie region of irregular structure and form; its See also:gentle coastward slope (16 ft. to a mile) is dissected by many branching consequent streams; in its southern part, as it approaches the Colorado river the cuesta is dissected into a belt of discontinuous hills. The western cross timbers follow a sandy belt along the inner See also:base of the ragged escarpment of Grand Prairie; the eastern cross timbers follow another sandy belt in the lowland between the eastern slope of Grand Prairie and the See also:pale western ,escarpment of the next eastward and lower Black Prairie cuesta. This cuesta is supported at an altitude of 700 ft. or less by a See also:chalk formation, which gives an infacing slope some 200 ft. in height, while its gently undulating or " See also:rolling " seaward slope (2 or 3 ft. in a mile), covered with marly strata and rich black soil, determines an important See also:cotton district. Then comes the East Texas See also:timber belt, broad in the north-east, narrowing to a point before reaching the Rio Grande, a low and thoroughly dissected cuesta of sandy See also:Eocene strata; and this is followed by the Coast Prairie, a very young plain, with a seaward slope of less than 2 ft. in a mile, its smooth surface interrupted only by the still more nearly level flood plains of the shallow, consequent river valleys. Near the Colorado river the dissected cuesta of the Grand Prairie passes southward, by a change to a more nearly horizontal structure, into the dissected See also:Edwards plateau (to be referred to again as part of the Great Plains), which terminates in a maturely dissected See also:fault scarp, 300 or 400 ft. in height, the northern boundary of the Rio Grande embayment. From the Colorado to the Rio Grande, the Black Prairie, the timber belt and the Coast Prairie See also:merge in a vast plain, little differentiated, overgrown with " chaparral " (See also:shrub-like trees, often thorny), widening eastward in the Rio Grande delta, and extending southward into Mexico. Although the Coast Prairie is a sea bottom of very See also:modern uplift, it appears already to have suffered a slight See also:movement of depression, for its small rivers all enter embayments; the larger rivers, however, seem to have counteracted the encroachment of the sea on the land by a sufficiently active delta See also:building, with a resulting forward growth of the land into the sea. The Mississippi has already been mentioned as rapidly building forward its digitate delta; the Rio Grande, next in size, has built its delta about 50 m. forward from the general coast-iine, but this river being much smaller than the Mississippi, its delta front is rounded by seashore agencies. In front of the Brazos and the Colorado, the largest of the Texan rivers, the coast-See also:line is very gently bowed forward, as if by delta growth, and the sea touches the mainland in a nearly straight shore line. Nearly all the See also:rest of the coast is fringed by off-shore reefs, built up by waves from the very shallow sea bottom; in virtue of weak tides, the reefs continue in long unbroken stretches between the few inlets. The Great Plains.—A broad stretch of See also:country underlaid by nearly horizontal strata extends westward from the 97th See also:meridian to the base of the Rocky Mountains, a distance of from 300 to 500 m., and northward from the Mexican boundary far into Canada. This is the See also:province of the Great Plains. Although the altitude of plains increases gradually from,600 or 1200 ft. on the east to 4000, 5000 or 6000 ft. near the mountains, the local relief is generally small; the sub-arid See also:climate excludes tree growth and opens far-reaching views. The plains are by no means a See also:simple unit; they are of diverse structure and of various stages of erosional development; they are occasionally interrupted by buttes and escarpments; they are frequently broken by valleys: yet on the whole a broadly extended surface of moderate relief so often prevails that the name, Great Plains, for the region as a whole is well deserved. The western boundary of the plains is usually well defined by the abrupt ascent of the mountains. The eastern boundary of the plains is more See also:climatic than topographic. The line of 20 in. of annual rainfall trends a little east of northward near the 97th meridian, and if a boundary must be See also:drawn where nature presents only a gradual transition, this rainfall line may be taken to See also:divide the drier plains from the moister prairies. The plains may be described in northern, intermediate, central and southern sections, in relation to certain See also:peculiar features. The northern See also:section of the Great Plains, north of See also:latitude 44°, including eastern See also:Montana, north-eastern See also:Wyoming and most of the Dakotas, is a moderately dissected peneplain, one of the best examples of its class. The strata here are Cretaceous or early See also:Tertiary, lying nearly horizontal. The surface is shown to be a plain of degradation by a gradual ascent here and there to the See also:crest of a ragged escarpment, the cuesta-remnant of a resistant stratum ; and by the presence of See also:lava-capped mesas and See also:dike-ridges, surmounting the general level by 500 ft. or more and manifestly demonstrating the widespread erosion of the surrounding plains. All these reliefs are more plentiful towards the mountains in central Montana. The peneplain is no longer in the See also:cycle of erosion that witnessed its See also:production; it appears to have suffered a regional See also:elevation, for the rivers—the upper Missouri and its branches—no longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level. A significant exception to the See also:rule of mature valleys occurs, however, in the case of the Missouri, the largest river, which is broken by several falls on hard sandstones about 50 M. east of the mountains. This peculiar feature is explained as the result of displacement of the river from a better graded preglacial valley by the See also:Pleistocene ice-See also:sheet, which here overspread the plains from the moderately elevated See also:Canadian high-lands far on the north-east, instead of from the much higher mountains near by on the west. The present altitude of the plains near the mountain base is 4000 ft. The northern plains are interrupted by several small mountain areas. The Black Hills, chiefly in western South Dakota, are the largest See also:group: they rise like a large See also:island from the sea, occupying an See also:oval area of about Too m. north-south by 50 M. east-west, reaching an altitude in Harney See also:Peak of 7216 ft., and an effective relief over the plains of 2000 or 3000 ft. This mountain See also:mass is of flat-arched, See also:dome-like structure, now well dissected by radiating consequent streams, so that the weaker uppermost strata have been eroded down to the level of the plains where their upturned edges are evenly truncated, and the next following harder strata have been sufficiently eroded to disclose the core of underlying crystalline rocks in about half of the domed area. In the intermediate section of the plains, between latitudes 44° and 42°, including southern South Dakota and northern See also:Nebraska, the erosion of certain large districts is peculiarly elaborate, giving rise to a minutely dissected form, known as " See also:bad lands," with a relief of a few hundred feet. This is due to several causes: first, the dry climate, which prevents the growth of a grassy See also:turf ; next, the fine texture of the Tertiary strata in the had land districts; and consequently the success with which every little rill, at times of See also:rain, carves its own little valley. Travel across the bad lands is very fatiguing because of the many small ascents and descents; and it is from this that their name, " mauvaises terres pour traverser," was given by the early See also:French voyageurs. The central section of the Great Plains, between latitudes 4'2 ° and 36°, occupying eastern Colorado and western See also:Kansas, is, briefly stated, for the most part a dissected fluviatile plain; that is, this section was once smoothly covered with a gently sloping plain of See also:gravel and sand that had been spread far forward on a broad denuded area as a piedmont deposit by the rivers which issued from the mountains; and since then it has been more or less dissected by the erosion of valleys. The central section of the plains thus presents a marked contrast to the northern section; for while the northern section owes its smoothness to the removal of local gravels and sands from a formerly uneven surface by the action of degrading rivers and their inflowing tributaries, the southern section owes its smoothness to the deposition of imported gravels and sands upon a previously uneven surface by the action of aggrading rivers and their outgoing distributaries. The two sections are also unlike in that residual eminences still here and there surmount the peneplain of the northern section, while the fluviatile plain of the central section completely buried the pre-existent relief. Exception to this statement must be made in the south-west, See also:close to the mountains in southern Colorado, where some lava-capped mesas (See also:Mesa de See also:Maya, See also:Raton Mesa) stand several thousand feet above the general plain level, and thus testify to the widespread erosion of this region before it was aggraded. The southern section of the Great Plains, between latitudes 351° and 291°, lies in eastern Texas and eastern New Mexico; like thecentral section it is for the most part a dissected fluviatile plain, but the lower lands which surround It on all sides place it in so strong relief that it stands up as a table-land, known from the time of Mexican occupation as the Llano Estacado. It See also:measures roughly 15o m. east-west and 400 M. north-south, but it is of very irregular outline, narrowing to the south. Its altitude is 5500 ft. at the highest western point, nearest the mountains whence its gravels were sup-plied; and thence it slopes south-eastward at a decreasing See also:rate, first about 12 ft., then about 7 ft. in a mile, to its eastern and southern borders, where it is 2000 ft. in altitude: like the High Plains farther north, it is extraordinarily smooth; it is very dry, except for occa• sional shallow and temporary water sheets after rains. The Llano is separated from the plains on the north by the mature consequent valley of the Canadian river, and from the mountains on the west by the broad and probably mature valley of the Pecos river. On the east it is strongly undercut by the retrogressive erosion of the head-waters of the Red, Brazos and Colorado rivers of Texas, and presents a ragged escarpment, 500 to Boo ft. high, overlooking the central denuded area of that state ; and there, between the Brazos and Colorado rivers, occurs a series of isolated outliers capped by a limestone which underlies both the Llano on the west and the Grand Prairies cuesta on the east. The southern and narrow part of the table-land, called the Edwards Plateau, is more dissected than the rest, and falls off to the south in a frayed-out fault scarp, as already mentioned, overlooking the coastal plain of the Rio Grande embayment. The central denuded area, east of the Llano, resembles the east-central section of the plains in exposing older rocks; between these two similar areas, in the space limited by the Canadian and Red rivers. rise the subdued forms of the See also:Wichita Mountains in Oklahoma, the westernmost member of the Ouachita system.
The Cordilleran Region.—From the western border of the Great Plains to the Pacific coast, there is a vast elevated area, occupied by mountains, plateaus and intermont plains. The intermont plains are at all altitudes from sea-level to 4000 ft.; the plateaus from 5000 to 10,000 ft.; and the mountains from 800o to 14,000 ft. The higher mountains are barren from the See also:cold of altitude; the timber line in Colorado stands at 11,000 to 12,000 ft.
The chief provinces of the Cordilleran region are: The Rocky Mountain system and its basins, from northern New Mexico north-See also: Then turning more to the north-west through Wyoming, the ranges decrease in breadth and height; in Montana their breadth is not more than 15o m., and only seven summits exceed 11,000 ft. (one reaching 12,834) As far north as the See also:gorge of the Missouri river in Montana, the Front range, facing the Great Plains, is a rather simple uplift, usually formed by upturning the flanking strata, less often by a fracture. Along the eastern side of the Front Range in Colorado most of the upturned stratified formations have been so well worn down that, except for a few low piedmont ridges, their even surface may now be included with that of the plains, and the crystalline core of the range is exposed almost to the mountain base. Here the streams that drain the higher areas descend to the plains through narrow canyons in the mountain border, impassable for ordinary roads and difficult of entrance even by See also:railways; a well-known example is the gorge of Clear See also:Creek east of the See also:Georgetown See also:mining district. The crystalline highlands thereabouts, at altitudes of 800o to 10,000 ft., are of so moderate a relief as to suggest that the mass had stood much lower in a former cycle of erosion and had then been worn down to rounded hills; and that since uplift to the present altitude the revived streams of the current cycle of erosion have not entrenched themselves deep enough to develop strong relief. This See also:idea is confirmed 8o m. farther south, where See also:Pike's Peak (14,108 ft.), a conspicuous landmark far out on the plains, has every See also:appearance of being a huge See also:monadnock, surmounting a rough peneplain of io,000 ft. in general elevation. The idea is still better confirmed farther north in Wyoming, where the See also:Laramie Range, flanked with upturned strata on the east and west, is for the most part a broad upland at altitudes of 7000 or 8000 ft., with no strong surmounting summits and as yet no deep carved valleys. Here the first of the Pacific railways See also:chose its pass. When the See also:summit is reached, the traveller is tempted to ask, " Where are the mountains?" so small is the relief of the upland surface. This low range turns westward in a curve through the See also:Rattlesnake Mountains towards the high See also:Wind River Mountains (Gannett Peak, 13,775 ft.), an anticlinal range within the See also:body of the mountain system, with flanking strata rising well on the slopes. Flanking strata are even better exhibited in the Bighorn Mountains, the front range of northern Wyoming, crescentic in outline and See also:convex to the north-east, like the Laramie Range, but much higher; here heavy sheets of limestone arch far up towards the range crest, and are deeply notched where consequent streams have cut down their See also:gorges. Farther north in Montana, beyond the gorge of the Missouri river, the structure of the Front Range is altogether different; it is here the carved residual of a great mass of moderately See also:bent Palaeozoic strata, overthrust eastward upon the Mesozoic strata of the plains; instead of exposing the See also:oldest rocks along the axis and the youngest rocks low down on the flanks, the younger rocks of the northern range follow its axis, and the oldest rocks outcrop along its eastern flanks, where they override the much younger strata of the plains; the harder strata, instead of lapping on the mountain flanks in great slab-like masses, as in the Bighorns, form out-facing scarps, which See also:retreat into the mountain interior where they are cut down by outflowing streams. The structure of the inner ranges is so variable as to elude simple description; but mention should be made of the Uinta range of broad anticlinal structure in north-east Utah, with east-west trend, as if corresponding to the east-west Rattlesnake Mountains, already named. The Wasatch Range, trending north-south in central Utah, is peculiar in possessing large east-west folds, which_ are seen in cross-section in the dissected western See also:face of the range, because the whole mass is there squarely cut off by a great north-south fault with down-throw to the Basin Range province, the fault face being elaborately carved.
Volcanic action has been restricted in the Rocky Mountains proper. West See also:Spanish Peak (q ,62o ft.), in the Front Range of southern Colorado, may be mentioned as a fine example of a deeply dissected See also:volcano, originally of greater height, with many unusually strong radiating dike-ridges near its denuded flanks. In north-western Wyoming there are extensive and heavy lava sheets, uplifted and dissected, and crowned with a few dissected volcanoes. It is in association with this See also: In central Colorado and Wyoming, where the mountains are higher and the Pleistocene glaciers were larger, the valley heads were hollowed out in well-formed cirques, often holding small lakes; and the mountain valleys were enlarged into U-shaped troughs as far down as the ice reached, with See also:hanging lateral valleys on the way. Different stages of See also:cirque development, with accompanying transformation of mountain shape, are finely illustrated in several ranges around the headwaters of the Arkansas river in central Colorado, where the highest summit of the Rocky Mountains is found (Mt Massive, 14,424 ft., in the Sawatch range) ; and perhaps even better in the Bighorn range of Wyoming. In this central region, however, it is only by way of exception that the cirques were so far enlarged by retrogressive glacial erosionas to sharpen the preglacial dome-like summits into acute peaks; and in no case did glacial action here extend down to the plains at the eastern base of the mountains; but the widened, trough-like glaciated valleys frequently descend to the level of the elevated intermont basins, where moraines were deployed forward on the basin floor. The finest examples of this See also:kind are the moraines about See also:Jackson Lake on the basin floor east of the Teton Range (Grand Teton, 13,747 ft.), a superb north-south range which lies close to the meridional boundary line between Wyoming and See also:Idaho. Farther north in Montana, in spite of a decrease of height, there are to-day a few small glaciers with snowfields of See also:good size; and here the effects of sculpture by the much larger Pleistocene glaciers are seen in forms of almost alpine strength. The intermont basins which so strongly characterize the Rocky Mountain system are areas which have been less uplifted than the enclosing ranges, and have therefore usually become the depositories of waste from the surrounding mountains. Some of the most important basins may be mentioned. See also:San Luis " Valley " is an oval basin about 6o m. long near the southern end of the mountain system in New Mexico and Colorado; its level, treeless floor, at an altitude of 7000 ft.. is as yet hardly trenched by the Rio Grande, which escapes through an impassable See also:canyon south-. ward on its way to the Gulf of Mexico. The much smaller basin of the upper Arkansas river in Colorado is well known because the Royal Gorge, a very narrow cleft by which the river escapes through the Front Range to the plains, is followed by a railroad at river-level. South Park, directly west of Pike's Peak, is one of the highest basins (nearly 10,000 ft.), and gains its name from the scattered, park-like growth of large See also:pine trees; it is drained chiefly by the South See also:Platte river (Missouri-Mississippi system), through a deep gorge in the dissected mass of the plateau-like Front Range. The Laramie Plains and the See also:Green river basin, essentially a single structural basic. between the east-west ranges of Rattlesnake Mountains on the north and the Uinta Range on the south, measuring roughly 26o m. east-west by ioo m. north-south, is the largest intermont basin; it is well known from being traversed through its greatest length by the See also:Union Pacific railway. Its eastern part is drained north-eastward through a gorge that separates the Laramie and Rattlesnake (Front) ranges by the North Platte river to the Missouri-Mississippi; its western part, where the basin floor is much dissected, often assuming a bad-land expression, is drained south-ward by the Green river, through a deep canyon in the Uinta Range to the Colorado river and then to the Pacific. The Bighorn basin has a moderately dissected floor, drained north-eastward by Bighorn river through a deep canyon in the range of the same name to the Missouri. Several smaller basins occur in Montana, all somewhat dissected and drained through narrow gorges and canyons by members of the Missouri system.
The Plateau province, next west of the southern Rocky Mountains, is characterized for the most part by large-textured forms, developed on a great thickness of nearly horizontal Palaeozoic, The Plateau Mesozoic and Tertiary formations, and by a dry climate. province.
The province was uplifted and divided into great blocks
by faults or monoclinal flexures and thus exposed to long-lasting denudation in a See also:mid-Tertiary cycle of erosion ; and then broadly elevated again, with renewed movement on some of the fault lines; thus was introduced in See also:late Tertiary time the current cycle of erosion in which the deep canyons of the region have been trenched. The results of the first cycle of erosion are seen in the widespread exposure of the resistant Carboniferous limestone as a broad See also:platform in the south-western area of greater uplift through central See also:Arizona, where the higher formations were worn away; and in the development of a series of huge, south-facing, retreating escarpments of irregular outline on the edges of the higher formations farther north. Each escarpment stands forth where a resistant formation overlies a weaker one; each escarpment is separated from the next higher one by a broad step of weaker strata. A wonderful series of these forms occurs in southern Utah, where in passing northward from the Carboniferous platform one ascends in See also:succession the See also:Vermilion Cliffs (Triassic sandstones), the See also: Thus the Virgin river, a northern branch of the Colorado, has cut a See also:vertical slit, See also:i000 ft. deep, hardly wider at the See also:top than at the bottom, in the heavy Triassic sandstones of southern Utah ; but the most famous example is the Grand Canyon (q.v.) of Arizona, eroded by the Colorado river across the uplifted platform of Carboniferous limestone.
During the current cycle of erosion, several of the faults, whose scarps had been worn away in the previous cycle, have been brought to light again as topographic features by the removal of the weak strata along one side of the fault line, leaving the harder strata on the other side in relief ; such scarps are known as " fault-line scarps," in distinction from the original " fault scarps." They are peculiar in having their altitude dependent on the See also:depth of revived erosion, instead of the amount of faulting, and they are sometimes " topographically reversed," in that the revived scarp overlooks a lowland worn on a weak formation in the upheaved fault-See also:block. Another consequence of revived erosion is seen in the occurrence of great landslides, where the removal of weak (See also:Permian) clays has sapped the face of the Vermilion Cliffs (Triassic See also:sandstone), so that huge slices of the cliff face have slid down and forward a mile or two, all shattered into a confused tumult of forms for a See also:score or more of miles along the cliff base.
Volcanic features occur in abundance in the Plateau province. Some of the high plateaus in the north are capped with remnants of heavy lava flows of early eruption. A group of large volcanoes occurs on the limestone platform south of the Grand Canyon, culminating in Mt San Francisco (12,794 ft.), a moderately dissected See also:cone, and associated with many more See also:recent smaller cones and fresh-looking lava flows. Mt See also: Some of the flows are still so young as to preserve their scoriaceous surface; here the " shore-line " of the lava contours evenly around the spurs and enters, bay-like, into the valleys of the enclosing mountains, occasionally isolating an outlying mass. Other parts of the lava flood are much older and have been more or less deformed and eroded. Thus the uplifted, dislocated and dissected lava sheets of the Yellowstone National Park in the Rocky Mountains on the east (about the headwaters of the Snake river) are associated with the older lavas.of the Columbian plains. The Columbia river has entrenched itself in a canyon-like valley around the northern and western side of the lava plains ; Snake river has cut a deeper canyon farther south-east where the plains are higher and has disclosed the many lava sheets which build up the plains, occasionally revealing a buried mountain in which the superposed river has cut an even narrower canyon. One of the most remarkable features of this province is seen in the temporary course taken by the Columbia river across the plains, while its canyon was obstructed by Pleistocene glaciers that came from the Cascade Mountains on the north-west. The river followed the 'temporary course long enough to See also:erode a deep gorge, known as Grande Coulee," along part of its length. The lava plains are treeless and for the most part too dry for See also:agriculture; but they support many See also:cattle and horses. Along parts of their eastern border, where the rainfall is a little increased by the approach of the See also:westerly winds to the Rocky Mountains, there is a belt of very deep, impalpably fine soil, supposed to be a dust deposit brought from the drier parts of the plains farther west; excellent crops of See also:wheat are here raised. The large province of the Basin ranges, an arid region throughout, even though it reaches the sea in southern California, involves some The Basler novel problems in its description. It is characterized The B by numerous disconnected mountain ranges trending Range. north and south, from 30 to loo in. in length, the higher Provi ranges reaching altitudes of 8000 or 10,000 ft., separated by broad, intermont desert plains or basins at altitudes varying from sea-level (or a little less) in the south-west, to 4000 or 5000 ft. farther inland. Many of the intermont plains—these chiefly in the north—appear to be heavily aggraded with mountain waste; while others—these chiefly in the south—are rock-floored and thinly veneered with See also:alluvium. The origin of these forms is still in discussion; but the following See also:interpretation is well supported. The ranges are primarily the result of faulting and uplifting of large blocks of the See also:earth's crust. The structure of the region previous to faulting was dependent on long antecedent processes of See also:accumulation and deformation and the surface of the region then was dependent on the amount of erosion suffered in the prefaulting cycle. When. the region was broken into fault blocks and the blocks were uplifted and tilted, the back slope of each block was a part of the previously eroded surface and the face of the block was a surface of fracture; the present form of the higher blocks is more or less affected by erosion since faulting, while many of the lower blocks have been buried under the waste of the higher ones. In the north, where dislocations have invaded the field of the horizontal Columbian lavas, as in south-eastern Oregon and north-eastern California, the blocks are monoclinal in structure as well as in attitude; here the amount of dissection is relatively moderate, for some of the fault faces are described as ravined but not yet deeply dissected ; hence these dislocations appear to be of recent date. In western Utah and through most of Nevada many of the blocks exhibit deformed structures, involving folds and faults of relatively ancient (Jurassic) date; so ancient that the moms. tains then formed by the folding were worn down to the lowland stage of old age before the block-faulting occurred. When this old-mountain lowland was broken into blocks and the blocks were tilted, their attitude, but not their structure, was monoclinal ; and in this new attitude they have been so maturely re-dissected in the new cycle of erosion upon which they have now entered as to have gained elaborately carved forms in which the initial form of the uplifted blocks can hardly be perceived; yet at least some of them still retain along one side the highly significant feature of a relatively' simple base-line, transecting hard and soft structures alike, and thus indicating the faulted margin of a tilted block. Here the less uplifted blocks are now heavily aggraded with waste from the dissected ranges: the waste takes the form of huge alluvial fans, formed chiefly by occasional See also:boulder-bearing floods from the mountains; each See also:fan heads in a See also:ravine at the mountain base, and becomes laterally confluent with adjacent fans as it stretches several miles forward with decreasing slope and increasing fineness of material. In the southern part of the Basin Range province the ranges are well dissected and some of the intermont depressions have rock floors with gentle, centripetal slopes; hence it is suggested that the time since the last dislocation in this part of the province is relatively remote; that erosion in the current cycle has here advanced much farther than in the central or northern parts of the province; and that, either by outwash to the sea or by exportation of wind-borne dust, the depressions--perhaps aggraded for a time in the earlier stages of the cycle—have now been so deeply worn down as to degrade the lower and weaker parts of the tilted blocks to an evenly sloping surface, leaving the higher and harder parts still in relief as residual ranges. If this be true, the southern district will furnish a good See also:illustration of an advanced stage of the cycle of arid erosion, in which the exportation of waste from enclosed depressions by the wind has played an important part. In such case the washing of the centripetal slopes of the depressions by occasional " sheet-floods " (widespreading sheets of turbid See also:running water, supplied by heavy short-lived rains) has been efficient in keeping the rock floor at even grade toward a central basin, where the finest waste is collected while waiting to be removed by the winds. Only a small part of the Basin Range province is drained to the sea. A few intermont areas in the north-west part of the province have outlet westward by See also:Klamath river through the Cascade range and by See also:Pitt river (upper part of the See also:Sacramento) through the Sierra Nevada: a few basins in the south-east have outlet by the Rio Grande to the Gulf of Mexico; a much larger but still narrow medial area is drained south-westward by the Colorado to the head of the Gulf of California, where this large and very turbid river has formed an extensive delta, north of which the former head of the gulf is now cut off from the sea and laid bare by evaporation as a plain below sea-level. It is here that an See also:irrigation project, involving the diversion of some of the river water to the low plain, led to disaster in 1904, when the flooded river washed away the See also:canal See also:gates at the intake and overflowed the plain, drowning the newly established farms, compelling a railway to shift its track, and forming a lake (Salton Sea) which would require years of evaporation to remove (see COLORADO RIVER). Many streams descend from the ravines only to See also:wither away on the desert basin floors before uniting in a See also:trunk river along the axis of a depression; others succeed in uniting in the See also:winter See also:season, when evaporation is much reduced, and then their trunk flows for a few score miles, only to disappear by " sinking" (evaporating) farther on. A few of the large streams may, when in flood, spr.ead out in a temporary shallow sheet qn a dead level of See also:clay, or See also:playa, in a basin centre, but the sheet of water vanishes in the warm season and the stream shrinks far up its course, the absolutely barren clay floor of the playa, impassable when wet, becomes See also:firm enough for crossing when dry. One of the south-western basins, with its floor below sea-level, has a plain of salt in its centre. A few of the basins are occupied by lakes without outlet, of which Great Salt Lake (q.v.), in north-west Utah, is the largest. Several smaller lakes occur in the basins of western Nevada, next east of the Sierra Nevada. Additional information and CommentsThere are no comments yet for this article.
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