Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V27, Page 623 of the 1911 Encyclopedia Britannica.
Spread the word: it!
NIAGARA.) Many additional features associated with the glacial 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 plain, but accumulated in elliptical mounds, loo or 200 ft. high, half a mile or a mile long, with axes parallel to the direction of the ice motion as indicated by striae on the underlying rock floor; these hills are known by the Irish name, drumlins, used for similar hills in north-western Ireland. The most remarkable groups of drumlins occur in western New York, where their number is estimated at over 600o, and in southern Wisconsin, where it is placed at 5000. They completely dominate the topography of their districts. A curious deposit of an impalpably fine and unstratified silt, known by the German name loess, lies on the older drift sheets near the larger river courses of the upper Mississippi basin. It attains a thickness of 20 ft. or more near the rivers and gradually fades away at a distance of ten or more miles on either side. It is of inexhaustible fertility, being in this as well as in other respects closely like the loess in China and other parts of Asia, as well as in Germany. It contains 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 flood plains of aggrading rivers, and slowly deposited on the neighbouring grass-covered plains. South-western Wisconsin and parts of the adjacent states of Illinois, Iowa and Minnesota are known as the "driftless area," because, although bordered by drift sheets and moraines, it is free from glacial deposits. It must therefore have been a sort of oasis, when the ice sheets from the north advanced past it on the east and west and joined around its southern border. The reason for this exemption from glaciation is the converse of that for the southward convexity of the morainic loops; for while they mark the paths of greatest glacial advance along lowland troughs (lake basins), the driftless area is a district protected from ice invasion by reason of the obstruction which the highlands of northern Wisconsin and Michigan (part of the Superior oldland) offered to glacial advance. The course of the upper Mississippi river is largely consequent upon glacial deposits. Its sources are in the morainic lakes in northern Minnesota; Lake Itasca being only one of many glacial lakes which supply the headwater branches of the great river. The drift deposits thereabouts are so heavy that the present divides between the drainage basins of Hudson Bay, Lake Superior and the Gulf of Mexico evidently stand in no very definite relation to the preglacial divides. The course of the Mississippi through Minnesota is largely guided by the form of the drift cover. Several rapids and the Falls of St 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 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 early-glacial Mississippi, when much precipitation that is to-day discharged to Hudson Bay and the Gulf of St Lawrence was delivered to the Gulf of Mexico, for the curves of the present river are of distinctly smaller radius than the curves of the valley. Lake Pepin (3o M. below St 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 " father of waters," like so many other rivers in the Northern states. owes many of its features more or less directly to glacial action. The fertility of the prairies is a natural consequence of their origin. During the mechanical comminution of the till no vegetation was present to remove the minerals essential to plantgrowth, as is the case in the soils of normally weathered and dissected peneplains, such as the Appalachian piedmont, where the soils, though not exhausted by the primeval forest cover, are by no means • so rich as the till sheets of the prairies. Moreover, whatever the rocky understructure, the till 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 annual flowering plants. To-day they are covered with farms. The cause of the treelessness has been much discussed. It does not seem to lie in peculiarities of temperature or of precipitation; for trees thrive where they are properly planted on the prairies; every town and 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 factor, and is the chief cause of the treelessness of the Great Plains (see below). The treelessness of the prairies cannot be due to insufficient time for tree invasion since glacial evacuation; for forests cover the rocky uplands of Canada, which were occupied by ice for ages after the prairies were laid bare. A more probable cause is found in the fineness of the prairie soil, which is inimical to the growth of young trees in competition with the 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 extension of the 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 lower, seaward part of this region that deserves the name of plain, for there alone is the surface unbroken by hills or valleys; the inner part, initially a plain by reason of its essentially horizontal (gently seaward-sloping) structure, - has been converted by mature dissection into an elaborate complex of hills and valleys, usually of increasing altitude and relief as one passes inland. The special features of the Gulf Plain are the peninsular extension of the plain in Florida, the belted arrangement of relief and soils in Alabama and in 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, low crustal arch extends southward at the junction of the Atlantic and Gulf coastal plains; the emerged half of the arch constitutes the visible lowland 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 limestone; here much of the lowland drainage is underground, forming many sink-holes (swallow-holes). Many small lakes in the lowland appear to owe their basins to the solution of the limestones. Valuable phosphate deposits occur in certain districts. The southern part of the state includes the " Everglades " (q.v.), a large area of low, flat, marshy land, overgrown with tall reedy grass, a veritable wilderness; thus giving Florida an unenvied first rank among the states in marsh area. The eastern coast is fringed by long-stretching sand reefs, enclosing lagoons so narrow and continuous that they are popularly called " rivers." At the southern end of the peninsula is a series of coral islands, known as " keys "; they appear to be due to the forward growth of corals and other lime-secreting organisms towards the strong current of the Gulf Stream, by which their 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-shore reefs; much of it is of minutely irregular outline, which seems to be determined less by the work of the sea than by the forward growth of mangrove swamps in the shallow salt 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 original Alabama. innermost extension and worn down to a flat inner lowland of rich black soil, thus gaining the name of the " black belt." The lowland is enclosed by an upland or cuesta, known as Chunnenugga 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 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 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 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 sou s de th an of t head dissected Ozark 1plateau of southern Missouri and northern Arkansas, which in many ways resembles the Appalachian plateau, and along the eastern end of the Massern ranges of the Ouachita mountain system in central Arkansas, which in geological 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 borders the Arbuckle hills in 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 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 truck in which three large rivers join; the chief figures (approximate only) regarding them are as follows: Drainage Area Percentage of (square miles). Total Discharge. Upper Mississippi 170,000 i8 Ohio 210,000 31 Missouri 530, 000 14 The small proportion of total water volume supplied from the great Missouri basin is due to the 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 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 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 quarter of the total precipitation on its drainage basin. Its load of land waste (see I. C. Russell, Rivers of North 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 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 hundred feet high in the northern part, generally decreasing to the southward, but with 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 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 account of the rapid deposition of sediment near the 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 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 lays down silt from its own bed and banks with every change of velocity. The swiftest current tends, by reason of centrifugal force, to follow the outer side of every significant curve in the channel; hence the concave bank, against which the rapid current sweeps, is worn away; thus any chance irregularity is exaggerated, and in time a series of large serpentines or meanders is developed, the most-symmetrical examples at present being those near Greenville, Miss. The growth of the meanders tends to give the river continually increasing length; but this tendency is counter-acted by the sudden occurrence of cut-offs from time to time, so that a fairly constant length is maintained. The floods of the Mississippi usually occur in spring or summer: Owing to the great 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 Commission and the United States Weather 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 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 delta, leaving less aggraded areas as shallow lakes (Lake Pontchartrain on the east, and Grand Lake on the west of the river). The 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 crossing of the Colorado river, coastal (of Texas) ; but it again widens to 30o m. at the plain. national boundary as a 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 place in a distance of about 500 tn., and hence include a region of over ioo,000 sq. m.—less than half of the large state of Texas. A belted arrangement of reliefs and soils, resulting from differential erosion on strata of unlike 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 " cross timbers," because their trend is transverse to the 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 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 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 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 chalk formation, which gives an infacing slope some 200 ft. in height, while its gently undulating or " rolling " seaward slope (2 or 3 ft. in a mile), covered with marly strata and rich black soil, determines an important cotton district. Then comes the East Texas timber belt, broad in the north-east, narrowing to a point before reaching the Rio Grande, a low and thoroughly dissected cuesta of sandy 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 Edwards plateau (to be referred to again as part of the Great Plains), which terminates in a maturely dissected 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 merge in a vast plain, little differentiated, overgrown with " chaparral " (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 modern uplift, it appears already to have suffered a slight 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 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-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 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 country underlaid by nearly horizontal strata extends westward from the 97th 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 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 climate excludes tree growth and opens far-reaching views. The plains are by no means a 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 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 drawn where nature presents only a gradual transition, this rainfall line may be taken to divide the drier plains from the moister prairies. The plains may be described in northern, intermediate, central and southern sections, in relation to certain peculiar features. The northern section of the Great Plains, north of latitude 44°, including eastern Montana, north-eastern 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 Tertiary, lying nearly horizontal. The surface is shown to be a plain of degradation by a gradual ascent here and there to the crest of a ragged escarpment, the cuesta-remnant of a resistant stratum ; and by the presence of lava-capped mesas and 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 cycle of erosion that witnessed its production; it appears to have suffered a regional 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 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 Pleistocene ice-sheet, which here overspread the plains from the moderately elevated 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 group: they rise like a large island from the sea, occupying an oval area of about Too m. north-south by 50 M. east-west, reaching an altitude in Harney Peak of 7216 ft., and an effective relief over the plains of 2000 or 3000 ft. This mountain mass is of flat-arched, 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 Nebraska, the erosion of certain large districts is peculiarly elaborate, giving rise to a minutely dissected form, known as " 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 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 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 French voyageurs. The central section of the Great Plains, between latitudes 4'2 ° and 36°, occupying eastern Colorado and western 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 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, close to the mountains in southern Colorado, where some lava-capped mesas (Mesa de Maya, 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 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 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 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 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-ward, including all the mountains from the front ranges bordering on the plains to the Uinta and Wasatch ranges in Utah; the Pacific ranges including the Sierra Nevada of California, the Cascade range of Oregon and 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 Europe and the eastern United States contain many 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 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 railways; a well-known example is the gorge of Clear Creek east of the Georgetown 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 idea is confirmed 8o m. farther south, where Pike's Peak (14,108 ft.), a conspicuous landmark far out on the plains, has every appearance of being a huge monadnock, surmounting a rough peneplain of io,000 ft. in general elevation. The idea is still better confirmed farther north in Wyoming, where the 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 chose its pass. When the 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 Rattlesnake Mountains towards the high Wind River Mountains (Gannett Peak, 13,775 ft.), an anticlinal range within the 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 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 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 bent Palaeozoic strata, overthrust eastward upon the Mesozoic strata of the plains; instead of exposing the 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 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 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 Spanish Peak (q ,62o ft.), in the Front Range of southern Colorado, may be mentioned as a fine example of a deeply dissected 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 field of extinct volcanic activity that a remark-able group of geysers and hot springs has been developed, from which the Yellowstone river, a branch of the Missouri, flows north-eastward, and the Snake river, a branch of the Columbia, flows south-westward. The geyser district is held as a national domain, the Yellowstone Park. Travellers whose idea of picturesqueness is based upon the abnormally sharpened peaks of the ice-sculptured Alps are disappointed with the scenery of the central and southern ranges of the Rocky Mountains. It is true that many of these ranges are characterized by the rounded tops and the rather evenly slanting, waste-covered slopes which normally result from the long-continued action of the ordinary agencies of erosion; that they bear little snow in summer and are practically wanting in glaciers; that forests are often scanty on the middle and lower slopes, the more so because of devastation by fires; and that the general impression of great altitude is much weakened because the mountains are seen from a base which itself is 5000 or 6000 ft. above sea-level. Nevertheless the mountains are of especial interest to the physiographer who wishes to make a comparative study of land forms as affected by normal and by glacial sculpture, in order to give due attention to " process " as well as to " structure and stage " in the analysis and description of mountain topography. A journey along the range from south to north reveals most strikingly a gradual increase-in the share of sculpture due to Pleistocene glaciers. In New Mexico, if glaciers were formed at all in the high valleys, they were so small as not greatly to modify the more normal forms. 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 hanging lateral valleys on the way. Different stages of 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 kind are the moraines about 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 Idaho. Farther north in Montana, in spite of a decrease of height, there are to-day a few small glaciers with snowfields of 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. 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 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 pine trees; it is drained chiefly by the South Platte river (Missouri-Mississippi system), through a deep gorge in the dissected mass of the plateau-like Front Range. The Laramie Plains and the 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 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 mid-Tertiary cycle of erosion ; and then broadly elevated again, with renewed movement on some of the fault lines; thus was introduced in 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 platform in the south-western area of greater uplift through central 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 succession the Vermilion Cliffs (Triassic sandstones), the White Cliffs (Jurassic sandstones, of remarkably cross-bedded structure, interpreted the dunes of an ancient desert), and finally the Pink Cliffs (Eocene strata of fluviatile and lacustrine origin) of the high, forested plateaus. Associated with these irregular escarpments are occasional rectilinear ridges, the work of extensive erosion on monoclinal structures, of whick Echo Cliffs, east of the Painted Desert (so called from its many-coloured sandstones and clays), is a good example. With the renewal of uplift by which the earlier cycle of erosion was interrupted and the present cycle introduced, inequalities of surface due to renewed faulting were again introduced ; these still appear as cliffs, of more nearly rectilinear front than the retreating escarpments formed in the previous cycle. These cliffs are peculiar in gradually passing from one formation to another, and in having a height dependent on the displacement of the fault rather than on the structures in the fault face; they are already somewhat battered and dissected by erosion. The most important line of cliffs of this class is associated with the western and southern boundary of the plateau province, where it was uplifted from the lower ground. The few rivers of the region must have reached the quiescence of old ale in the earlier cycle, but were revived by uplift to a vigorous youth in the current cycle; and it is to this newly introduced cycle of physiographic evolution that the deep canyons of the Plateau province are due. Thus the Virgin river, a northern branch of the Colorado, has cut a vertical slit, i000 ft. deep, hardly wider at the 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 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-block. Another consequence of revived erosion is seen in the occurrence of great landslides, where the removal of weak (Permian) clays has sapped the face of the Vermilion Cliffs (Triassic 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 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 cone, and associated with many more recent smaller cones and fresh-looking lava flows. Mt Taylor in western New Mexico is of similar age, but here dissection seems to have advanced farther, probably because of the weaker nature of the underlying rocks, with the result of removing the smaller cones and exposing many lava conduits or pipes in the form of volcanic necks or buttes. The Henry Mountains in south-western Utah are peculiar in owing their relief to the doming or blistering up of the plateau strata by the underground intrusion of large bodies or " cisterns " (laccolites) of lava, now more or less exposed by erosion. The lava plains of the Columbia basin are among the most extensive volcanic outpourings in the world. They cover 200,000 sq. m. or more in south-eastern Washington, eastern Oregon and south-western Idaho, and are known to be 4000 ft. deep in some river gorges. The lava completely buries the pre-existent land forms over most of its extent. The earlier supposition that these vast lava flows came chiefly from fissure eruptions has been made doubtful by the later discovery of flat-sloping volcanic cones from which much lava seems to have been poured out in a very liquid state. 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 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 agriculture; but they support many cattle and horses. Along parts of their eastern border, where the rainfall is a little increased by the approach of the 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 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 alluvium. The origin of these forms is still in discussion; but the following interpretation is well supported. The ranges are primarily the result of faulting and uplifting of large blocks of the earth's crust. The structure of the region previous to faulting was dependent on long antecedent processes of 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 boulder-bearing floods from the mountains; each fan heads in a 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 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 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 Klamath river through the Cascade range and by Pitt river (upper part of the 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 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 canal 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 wither away on the desert basin floors before uniting in a trunk river along the axis of a depression; others succeed in uniting in the winter 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 clay, or 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 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. During Pleistocene times all these lacustrine basins were occupied by lakes of much greater depth and larger size; the outlines of the eastern (Lake Bonneville) and the western (Lake Lahontan) water bodies are well recorded by shore lines
End of Article: NIAGARA

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.