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NIAGARA

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Originally appearing in Volume V27, Page 623 of the 1911 Encyclopedia Britannica.
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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 .

Phoenix-squares

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:ward, including all the mountains from the front ranges bordering on the plains to the Uinta and Wasatch ranges in See also:Utah; the Pacific ranges including the Sierra See also:Nevada of See also:California, the Cascade range of See also:Oregon and See also:Washington, and the Coast range along the Pacific nearly to the southern end of California; and a great inter-mediate area, including in the north the Columbian lava plains and in the south the large province of the Basin ranges, which extends into Mexico and widens from the centre southward, so as to meet the Great Plains in eastern New Mexico, and to extend to the.Pacific coast in southern California . There is also a province of plateaus between the central part of the Basin ranges and the southern part of the Rocky Mountains . An important geological characteristic of most of the Cordilteran region is that the Carboniferous strata, which in western See also:Europe and the eastern United States contain many See also:coal seams, are represented in the western United States by a marine limestone; and that the important unconformity which in Europe and the eastern United States separates the Palaeozoic and Mesozoic eras does not occur in the western United States, where the formations over a great area follow in conformable sequence from early Palaeozoic through the Mesozoic . The Rocky Mountains begin in northern Mexico, where the axial crystalline rocks rise to 12,000 ft. between the horizontal structures of the plains on the east and the plateaus on the west . The Rocky The upturned stratified formations wrap around the mountains. flanks of the range, with ridges and valleys formed on their eroded edges and drained southward by the Pecos river to the Rio Grande and the Gulf of Mexico . The mountains rapidly grow wider and higher northward, by taking on new complications of structure and by including large basins between the axes of uplift, until in northern Colorado and Utah a complex of ranges has a breadth of 300 m., and in Colorado alone there are 40 summits over 14,000 ft. in altitude, though none rises to 14,500 .

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:field of See also: