Innumerable gentle warpings have affected nearly every portion of the surface of the globe at nearly all stages of its history. Not only during the periods of great movements were there countless minor and gentler movements, but at times of relative quiescence there were slow swellings and saggings of the surface of the lithosphere. They sometimes affected small areas and sometimes large ones, and they were sometimes of upward phase and sometimes of downward. They were the immediate agencies in locating and controlling the deposition of stratified rocks, though they rest back on the great movements for their working conditions. Very slow sinkings of sea-borders have permitted deposition to go on in shallow water for long periods without being interrupted by the local filling of the sea. Very slow swellings of land tracts, relative or absolute, have permitted erosion to supply material for such sedimentation for long periods without exhausting the sources. Very slow upward warpings in one region and downward warpings in another have shifted the borders of the land and sea, and with them the areas of erosion and deposition. Thus have arisen overlaps and unconformities of strata and diversities in their distribution from stage to stage. Such movements may have amounted to a few inches, or a few feet, or a few fathoms per century. Downward movements have sometimes affected a considerable section of a continent, letting in a shallow epicontinental sea upon it, such, for example, as the North Sea upon the northwestern border of the continent of Europe, and Hudson Bay upon the northeastern part of North America. Similar movements seem to have extended the seas even more widely upon the surface of the land in times past, as attested by the great transgressions of the ocean-borders and the great epicontinental spread of strata. Notwithstanding their great breadths, the epicontinental seas were generally shallow. Similar gentle warpings of upward phase rescued the bottoms of shallow seas from submersion, and inaugurated erosion; or they bowed base-leveled lands upward, and rejuvenated their streams and inaugurated a new cycle of denudation. Often they connected continents previously separated by shallow straits, and thus inaugurated inter-continental migrations of land life, while they stopped inter-oceanic migration.
The gentleness and frequency of these movements is attested by the character of the sediments and by their relations to one another, as will be seen in the study of the sedimentary series.
Reciprocal features.—These minor warpings show a notable tendency to be reciprocal. If one area is bowed up, another near by is bowed down. If the continents settle, the oceans rise on their borders. If the land is cut down, the sea is filled up. There is an important phase of this deserving especial note. Certain tracts have been slowly bowed upwards into long land swells, the streams being rejuvenated and degradation hastened. Adjacent tracts have been slowly bowed downwards into long parallel troughs which received the wash from the adjacent swells, and thus became tracts of exceptional sedimentation. Such a tract of parallel swell and sag, if our interpretation be correct, developed along the Atlantic border of North America in the Paleozoic era. By the slow upward warping of the swells, the feeding-grounds of the streams were maintained, and the sags were filled about as fast as they sank. Thus a great depth of sediment was laid down in the course of an era measured by millions of years. So in other regions, especially near the borders of the continents, there have been similar reciprocal movements, giving at once feeding-grounds for the streams and lodgment-grounds for the sediments, side by side in parallel belts. It is a common view that these belts of deep sedimentation were the forerunners of mountain formation, and that they determined the formation of the mountains. In view of the grounds for doubting the efficiency of so superficial an agency in mountain formation, which will appear as we go on, it may be well to hold this view in abeyance, and to dwell on the reciprocal nature of the action, in which the upward bowing that gave the feeding-grounds is as vital a factor as the sagging that accommodated the sedimentation. It is important to recognize that in so far as the crust was weak enough to yield to these gentler forces, it was not strong enough to accumulate the great stresses necessary to form mountain ranges, and further, that in so far as the stresses were eased by the gentle warping, they could not be accumulated for the later work of mountain-folding. It is nevertheless probable that the conditions which located the gentle swelling and sagging also located the mountain-folding.
The Great Periodic Movements.
Mountain-forming movements.—Along certain tracts, usually near the borders of the continents, and at certain times, usually separated by long intervals, the crust was folded into gigantic wrinkles, and these constitute the chief type of mountains, though not the only type. The characteristic force in this folding was lateral thrust. The strata were not only arched, but often closely folded, and sometimes intensely crumpled. In extreme cases, like the Alps, the folds flared out above, giving overturn dips and reversed strata, as illustrated in the chapter on Structural Geology, [pp. 501–511]. In these cases there was an upward as well as a horizontal movement, for the folds themselves were lifted; but the horizontal thrust so much preponderated, and was so much the more remarkable, that the upward movement was overshadowed. It is well to note, however, that these mountain ranges are crumpled outward and not inward, as might be expected if they resulted simply from the shrinkage of the under side of a thin shell. The folds are sometimes nearly upright and symmetrical, and sometimes inclined and asymmetrical, as illustrated in the chapter referred to. Where the folds lean, the inference has been drawn that the active thrust came from the side of the gentler slope, the folds being pushed over toward the resisting side, and this seems to be commonly true. The original attitude of the beds, however, has much to do with the character of the folds.[240] By a slight change in the mode of thrust, sheets of paper may be so pushed as to lean forward or backward at pleasure. The leaning of the folds seems, therefore, a doubtful criterion for determining the direction of the active movement. Mountains of the thrust type usually consist of a series of folds nearly parallel to each other, the whole forming an anticlinorium.
Fig. 449.—The great Eurasian mountain tract. Jones Relief Globe. (Photo. by R. T. Chamberlin.)
Distribution of folded ranges.—The prevailing location of this class of mountains is so generally near the borders of the continents that the relation is probably significant. Dana[241] long ago called attention to the fact that the greatest mountain ranges stand opposite the greatest ocean-basins, and he connected the elevation of the one with the depression of the other. One of the most notable exceptions to this relation is the complex system of southern Europe, from the Pyrenees to the Caucasus, and another is the Altai and connected ranges ([Fig. 449]). The Urals and not a few minor ranges are also exceptions. It is probably better to regard the crumpled tracts as lying on the borders of great segments of the earth that acted essentially as units, and to regard the relationship to the sea as a coincidence that is only in part causal.[242]
Plateau-forming movements.—Another leading phase of crustal movement is the settling or rising of great blocks of the crust, as though by vertical rather than horizontal force. The western plateau of North America and the great plateau of Thibet are gigantic examples. The American plateau embraces numerous blocks which, while they have been elevated together, are individually tilted in their own fashion. At the surface, they are separated by fault-planes, but below, some of them, and perhaps most of them, pass into flexures. Most of these flexures are of the monoclinal type ([p. 516]), which dynamically means much the same as a fault; but some of them may be of the compressive type, without inconsistency with vertical fault-relief above. Research has not yet covered thoroughly any great plateau, and knowledge of this class of movements is less complete than that of folding by lateral thrust, and it has a less ample place in the literature of the subject. The plateau-forming movements are, however, much more massive than the mountain-folding movements, and stand next in magnitude to the continent-forming movements. Plateaus may be regarded as smaller platforms superposed on the continental platforms.
In the ocean-basins, there appear to be raised platforms of the plateau type, and there are remarkable “deeps” that have the aspect of anti-plateaus.