THE ORIGIN AND THE FORMS OF MOUNTAINS

A mountain defined.—As ordinarily understood, mountains are elevations upon the earth’s surface which rise above the general level of the country. Their summits need not be at great heights above the sea, but it is essential that they project above the average level of the surrounding country by at least a quarter of a mile. Lower elevations are described as hills. On the other hand, the elevation of a plateau like the “High Plains” of the western United States may be as much as a mile, but the vast expanse of nearly level surface precludes the use of the term “mountain.” The word is thus applied to a feature of the earth and not merely to an elevated tract.

In a collective sense, though more often in the plural form, the term is properly applied to groups of similar features which have a common origin in local uplift of the land. The origin of mountains used in this sense of mountain complexes is thus connected with some essentially local uplift of the earth’s surface. This may take place by the processes of folding and superincumbent fault displacement, by volcanic extravasations or ejections, or by a deeper seated and essentially hydrostatic elevation of rock beds over molten rock material.

The existing forms of mountains, as we are to see, are largely shaped by the erosional processes which are set in operation by the uplift itself, though often completed long subsequent to it.

The festoons of mountain arcs.—From our earliest studies of school geographies, we have become familiar with the arrangement of the more important mountains in long chains or systems. Comparatively few persons have given any further attention to the arrangement of the chains, though over large areas of the earth’s surface the distribution of mountain ranges is deeply significant. The map of Asia in particular presents a series of great sweeping arcs or crescents which are grouped as though hung upon the map in festoons with knots or vertexes to separate neighboring groups (Fig. 474, p. 438, and Fig. 472).

The significance of these mountain groupings in the evolution of the earth’s surface has been pointed out by the great Viennese geologist Suess, to whom we are indebted for focusing upon the plan of the earth an amount of attention which before had been largely given to the preparation of hypothetical sections of strata which were largely buried from sight beneath the earth’s surface. Broadly speaking, the mountain arcs may be said to be grouped about those shields of older rock which geological studies have shown to be the oldest land masses upon the globe. Within the northern hemisphere these original continents are represented by the areas of crystalline rock centered over Hudson Bay, the Baltic Sea, and an area in northeastern Siberia known to geologists as Angara Land. In our study of the figure of the earth (Chapter II) it was found that these shields represent the truncated angles of the rounded tetrahedral form toward which the planet is tending (Fig. 3, p. 12).

Theories of origin of the mountain arcs.—The mountain arcs, when studied in detail, are found to be composed of closely folded rock strata, the flexures of which are generally so overturned that their axial planes dip toward the center of the arc (Fig. 473). It was the view of Suess that these arcs are to be explained by a pushing outward of the rock strata from the center of the arc toward its periphery, thus causing a wrinkling of the surface strata and an overriding of the surrounding formations, which upon this hypothesis opposed a greater resistance to the sliding movement. The folding together of the strata due to the sliding naturally involves a very considerable diminution of the surface area presented by the strata (Fig. 22, p. 42). In the case of the Alpine chains it has been estimated that a flat land area, four hundred to eight hundred miles across, has by the folding process been reduced to a width of only about one hundred miles, or from a fourth to an eighth of its former width.

The weakness of Professor Suess’ theory lies in the fact that such compression as it implies is assumed to be due to an outward movement of the relatively small area of the earth’s outer shell which is included within the arc. It must be obvious that such a movement, being from a center toward three sides at once, would for this circumscribed area involve enormous proportionate reduction in superficial area of the strata and could only result in a hiatus near the center of the arc. No such gap is to be found, and one would, moreover, be difficult to account for upon any plausible hypothesis. On the other hand, the general contraction of the planet as a whole, involving as it does reduction of surface over large areas, is a well-recognized fact; and if it be true that the shields formed by the older continents are less subject to contraction than the remaining portions of the surface, it is easy to understand why the earth’s outer skin should be wrinkled by underfolding and thrusting about these continental margins. The contrast of this view with that of Professor Suess is expressed in the diagrams of Fig. 473.

We may illustrate this conception by a stretched sheet of rubber cloth such as is in common use by dentists, upon which a thin layer of hot Canada balsam has been spread. This substance congeals upon cooling to near-normal temperatures, and if a small local area of the balsam layer be chilled and the tension upon the rubber then released, the viscous balsam of the unchilled portion of the layer is thrown into wrinkles about the cooled and more resistant areas. These more resistant portions of the stratum may thus represent the ancient continental shields of our planet.

The Atlantic and Pacific coasts contrasted.—In his studies of mountain arcs in their relation to the plan of the earth, Professor Suess has shown how the arrangements of the mountain chains about the two larger oceans represent two strongly contrasted types. Whereas about the Pacific margin the mountain arcs are, as it were, strung in festoons which trend parallel to and are convex toward the coast, or else lie in fringing garlands of islands in the same attitude (Fig. 474); the mountain chains about the Atlantic become sharply truncated as they reach the coast, and thus indicate that the basin of this ocean has been produced by an inthrow or depression between great marginal displacements in some period subsequent to the formation of the mountains.

Thus the mountain folds of the Appalachian system are in Newfoundland cut off abruptly at the coast line, and the same beds, similarly truncated, are encountered again across the expanse of ocean in the folds at the coast of western Europe (Fig. 475). In discontinuous remnants this ancient mountain chain may be traced in an east and west direction across western and central Europe. We have thus here to do with a single mountain system which extends from central Europe to northern Alabama, out of which a great link has been taken by the subsequent sinking in of the basin of the Atlantic Ocean.

The block type of mountain.—The inclusion of most elevations in mountain chains and arcs is one of the most obvious facts to any one who has examined world atlases with this subject in mind. Such chains are almost invariably composed of folded rocks, thus indicating that erosion has removed great superincumbent masses of strata since the crustal compression produced the folds at considerable depths below the then surface.

There are, however, large elevated tracts upon the earth’s surface which are intersected by deep valleys, but where no arrangement of the elevated portions within chains or ranges is to be detected. In such cases the distribution of mountain and valley may bear a resemblance to a mosaic of disturbed parts which stand at different levels (Fig. 476).

Such block mountain districts are to be found in many parts of the earth’s surface, but notably within the Great Basin of the western United States, and in the land area which borders the Indian Ocean upon the west and northwest. In contrast with the mountain arcs, so strikingly exemplified by the continent of Asia as a whole, its extreme southwestern portion is made up of an alternation of plateau and rift valley separated from each other by great displacements. Though modified to some extent by erosion, the elevations seem generally to represent the displaced crust blocks which in mutual adjustments have been left at the highest levels (Fig. 477). The valley of the Jordan, with the mountains of Lebanon rising above it, is near the northern extremity of this faulted mountain region (Fig. 434, p. 404), while the Great Rift valley, crossing east Central Africa, and the many neighboring rifts to the east and west, are graven in lines so deep that an observer upon a neighboring planet might perhaps detect them.

It is not necessary in all cases to assume that the block mountains of a faulted district represent the blocks which in the adjustments were left the highest. Erosion in the course of time accomplishes marvels of transformation, and it may result that heavy masses of more resistant rock eventually project the highest, even though they may represent the downthrown blocks in the fault mosaic (Fig. 43, p. 60).

Where in addition to undergoing changes of level the earth blocks have been tilted, the features long since described from our western interior basin as “Basin Range structure” are developed. Here the upper surface of the disturbed earth blocks betrays the evidence of a definite tilt in some one direction (Fig. 478, and Fig. 431, p. 402).

Mountains of outflow or upheap.—An important type of mountain, generally described as volcanic, may be due either to the outflow of lava at the earth’s surface, or to accumulations of separated fragments of lava, first thrown into the air, and then deposited by gravity or admixed with water as volcanic mud. Such mountains, both before and after modification by erosion, assume the strikingly characteristic forms which have been fully discussed in Chapters IX and X. The dominant types are the lava dome and the puy, the cinder cone, and the more complex composite cone. Excepting only the surface produced by the few great fissure eruptions and the semivolcanic mesa type, the individual mountains of volcanic origin develop features with notably circular bases.

Domed mountains of uplift—laccolites.—At a considerable number of widely separated localities upon the earth’s surface, mountainous regions are encountered, the central areas or cores of which are composed of intrusive igneous rock such as granite, and about this core the sediments dip away in all directions as though they had once formed a continuous roof above it and had been forced into this dome by hydrostatic pressure of the once viscous material beneath (Fig. 152, p. 143, and Figs. 479 and 480). Examples of such domed mountains of uplift were first described by Gilbert from the Henry Mountains of Utah, but instances are furnished by many elevated tracts, especially within the western United States. Such mountains are known as laccolites, but when one margin at least of the igneous core corresponds to a displacement, the mountain is described as a bysmalite (Fig. 481).

When subjected to long-continued erosion, the generally fissured granitic core of the laccolite weathers in a wholly different manner from the bedded sediments which surround and still in part mount over it. The former usually presents a more or less jagged surface which contrasts sharply with the gently sloping tables of the latter (Fig. 479). About the high granite core of the mountain, the several strata of the uptilted formations present each a steep slope toward this higher land, and a gentler slope in the opposite direction. Such unsymmetrical ridges which surround the mountain area are often referred to as “hog backs” (plate 12 B). The arrangement of the strata in the hog backs thus presents an overlapping series like the shingles upon a roof, except that the overlapping is here from the bottom instead of the top, and the exposed ends thus face toward the crest. Unlike a shingle roof the hog backs do not shed the water which descends to them from the higher levels, but, on the contrary, they cause it to flow in troughs parallel to the base of the slope except where outlets are found through them.

Mountains carved from plateaus.—In the mountain types thus far discussed, the local uplifting of the land has itself developed features which in the aggregate may be referred to as mountains, even though the characters of the original surface are soon destroyed by erosive processes of one sort or the other. Erosive processes are, however, quite competent to produce mountain forms from a featureless plateau, and particularly through the incision by streams of running water, the best studied process of mountain sculpture (see Chapters XI-XIII). This process of throwing valleys about an elevated section of the earth’s surface, and so carving out mountains, is sometimes described as circumvallation; and if the term “mountain” be applied in its ordinary sense to describe an individual feature, it is clear that most mountains have been formed in this way.

To discuss the characteristic shapes of such mountains would be largely to review the contents of this book, and especially those portions which discuss the character profiles resulting from the action of each sculpturing or molding agent. The work of frost and other weathering agencies, of running water, of mountain and of continental glacier, would all have to be considered in order to evolve the history of each mountain.

In addition to discovering the agents which were chiefly responsible for the shaping of the mountain, we may, further, in many cases determine at what stage the work of one agent has been succeeded by that of another, and at least at what stage of its complete cycle of activity the latest agent is now at work.

The climatic conditions of the mountain sculpture.—Since the different geological agencies operate either in a different manner or with differences in vigor according to the varying climatic conditions, the mountains of arid regions may in most cases be readily differentiated from those of the more habitable humid sections of country. In broad lines these differences may be summed up in the greater prevalence of the curving line within the landscapes of humid districts. This may be largely ascribed to the influence of the mat of vegetation, which protects the rock surface from more rapid mechanical degeneration, and arrests the sliding movements within the already loosened rock dÉbris. In place of the reversed curves of the lines of beauty, so generally observed in the landscapes of well-watered regions, the desert lands present ever a repetition of the vertical cliff alternating with a sort of many gabled faÇade which is occasionally due to truncation of mountain spurs by the waves of former lakes, but far more often the outlines of dÉbris cones built up beneath each prominent joint of the cliff walls (Fig. 482).

The effect of the resistant stratum.—In a striking manner mountain landscapes may disclose the influence of the diversified rock materials and of the rock structures as well. After prolonged erosion there is likely to be little correspondence between the positions of the anticlinal folds and the crests of the higher mountains. Such mountains are, in fact, much more likely to rise over synclines than upon the site of anticlines. The traveler who enters the Alps by any of the several railways, or who journeys by steamer over the beautiful lake of Lucerne, has a most favorable opportunity to study the position of the rock folds in the mountain sections that are unrolled in succession before him. Rarely indeed will he find a definite anticline in correspondence with a mountain peak, for the layers which are most resistant have developed the peaks, and it is because the outer layers of the anticlines open by local tension (see Fig. 26, p. 45) that they were first cut away by erosion, so that the hard layers within the synclines are likely to constitute the peaks within the existing surface.

When, as sometimes happens, an older and likewise more resistant bed has been folded back upon younger and softer formations, an isolated remnant may be found “unrooted” to its base, upon which it appears as though floating within a billowy sea of the softer formations (Fig. 483).

The mark of the rift in the eroded mountains.—Applying the term “mountain” in its collective sense for a circumscribed area of uplifted crust, whether represented to-day by a folded or a faulted complex, a lava mass, or a granite dome; the period of uplift has marked the beginning of the activity of sculpturing agencies. By these the mass is pared down as it is shaped into a more or less intricate design of component and essentially repeating units. In the vernacular the word “mountain” is applied to these units into which the larger mountain mass is subdivided.

It has been one of the main objects of this work to point out that the peculiar shapes of these elementary mountains are each characteristic of the erosive agents which produced them, and that each surface has marks which may be recognized in those lines of profile which recur within the landscape—the character profiles. In the subdivision of the larger mass—the genetical mountain—to form the numerous smaller masses—the erosional or circumvallational mountains—there is disclosed a pattern of fractures which has guided the erosional agents in their incisional operations (see Chapter XVII). In high altitudes, where the action of frost is so potent in prying at the wider fractures, this subdivision of the mass may be revealed by the sculpturing of squared towers or battlements (Fig. 484).

For other examples in which the sculptured surface is largely the handiwork of a single erosional agent, as over vast areas in the Canadian wilderness, the revelation of the fracture design is no less apparent. Here a series of crystalline rocks underlie broad expanses of territory and are without noteworthy variations of hardness and almost bare of surface dÉbris. Sculptured beneath a mantling ice sheet, excavation has naturally been concentrated above the more widely gaping fissures of the joint-fault system, doubtless already marked out in the river network which the glacier overrode. The result has been a division of the surface into a series of low, oval ridges or hummocks, which over vast areas are repeated with monotonous regularity. Wherever the lower levels have been flooded, symmetrical low islands of nearly uniform elevation rise from the expanse of water and may be counted by thousands. Though the smaller islands have notably regular shore lines, the larger ones disclose their composition from smaller units by the breaking of their shores into similar bays spaced with regular intervals (Fig. 485, and Figs. 243 and 245, p. 229).

The ever repeating fracture design of the earth’s crust is not restricted to the mountain masses which it has broken up, and the unity of which it has done so much to conceal. It extends far outside the margin of these masses, and is in fact common to whole continents and perhaps even to the planet as a whole. The part played by this design of fractures in the control of the sculpture of landscapes it would be hard to overestimate. Through its influence the striking features molded by one agent have been merged in the contrasted shapes developed by another. It is the great outline blender in the creation of nature’s masterpieces of form and color. Thus the lines of this mysterious fracture network, though stamped in indelible characters upon our landscapes, are generally lost in the ensemble effect and may long remain undiscovered. Like a moss-grown inscription upon a slab of marble, though veiled, it may yet be deciphered; and if the veil be withdrawn, the runic characters are disclosed, and one of nature’s laws lies open before us.

Reading References for Chapter XXXI

Mountain arcs or festoons:—

Ed. Suess. The Face of the Earth, vol. 2, 1906, pp. 201-207; vol. 4, 1909, pp. 498-542.

Block mountains:—

G. K. Gilbert. Surveys West of the 100th Meridian (Wheeler), vol. 3, Geology, Washington, 1875, Pt. 1, pp. 19 et seq., 48.

J. W. Powell. Report on the Geology of the Eastern Portion of the Uinta Mountains and a Region of Country Adjacent thereto, U. S. Geol. and Geogr. Surv. Ter., II Div. Washington, 1876, pp. 218.

John W. Gregory. The Great Rift Valley. London, 1896, pp. 422.

Laccolites and bysmalites:—

G. K. Gilbert. Report on the Geology of the Henry Mountains, U. S. Geol. and Geogr. Surv. Ter., 1877, pp. 18-98.

Whitman Cross. The Laccolitic Mountain Groups of Colorado, Utah, and Arizona, 14th Ann. Rept. U. S. Geol. Surv., 1895, pp. 157-241, pls. 7-16.

W. H. Weed and L. V. Pirsson. Geology and Mineral Resources of the Judith Mountains of Montana, 18th Ann. Rept. U. S. Geol. Surv., Pt. iii, 1898, pp. 485-556, pl. 75.

W. H. Weed. Geology of the Little Belt Mountains, Montana, etc., 20th Ann. Rept. U. S. Geol. Surv., Pt. iii, 1900, pp. 387-400.

Vera de Derwies. Recherches gÉologiques et pÉtrographiques sur les loccolithes des environs de Piatigorsk (Caucase du Nord). Geneva, 1905, pp. 84, pls. 3.

R. A. Daly. The Mechanics of Igneous Intrusion, Am. Jour. Sci. (4), vol. 15, 1903, pp. 269-278; vol. 16, 1903, pp. 107-126.

Joseph Barrell. Geology of the Marysville Mining District, Montana. A study of Igneous Intrusion and Contact Metamorphism. Prof. Pap. 57, U. S. Geol. Surv., 1907, pp. 151-178.

Climatic condition in relation to land sculpture:—

C. E. Dutton. Tertiary History of the Grand Canyon District, Mon. 2, U. S. Geol. Surv., 1882, pp. 264, pls. 42.

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