Page:Popular Science Monthly Volume 53.djvu/606

586 period, the accumulations of such movements have raised the same superficial formations of the coastal plains of America to an altitude of one hundred feet near Cape Hatteras, to eight hundred feet farther south, to fifteen hundred feet in the neighboring mountain region, to only three hundred or four hundred feet in the Mississippi Valley; while the same coastal plain in Mexico has been elevated to seventeen hundred feet. Farther south, in the Tehuantepec Isthmus, the corresponding change of level has not been more than eight hundred feet. These movements do not bend, crumple, or break the strata, or appreciably affect their horizontality. The amount of elevation just mentioned was sufficient to cause the final separation of the Mexican Gulf and the Pacific Ocean, but inadequate to account for the high plateaus.

Another class of terrestrial movements gives rise to mountain folds, thrusts, and faults, with a general dislocation and disturbance of all the formations. Just how much of a plateau was elevated by these forces is not apparent, for the surface base levels of erosion still retain their older courses of drainage. Consequently, one is led to suspect that the great escarpments are not entirely due to denudation, but that a third class of earth movements has obtained, lifting the plateaus abruptly to a considerable proportion of their height (six thousand feet, more or less) above the inner margin of the coastal plain, without greater deformation of their surface than that of the coastal plains themselves. This third movement seems to be a sort of squeezing up of great segments of the earth's crust, without tilting its surface by more than a few feet per mile; consequently, it implies a great dislocation and slip or fault along the margin of the table-land, since modified upon the surface by the atmospheric denudation. The analysis of these complex movements is far from complete, and, although we do not fully understand them, this ignorance of causes does not affect the evidence of the very recent elevation of the plateaus.

—The old geological formations of the plateaus are mostly buried beneath secondary accumulations; so that the present physical surface features are largely due to the atmospheric agents which have been at work since about the end of the Cretaceous period, as out of the formations of that date a large portion of the base levels of erosion have been molded. These great base levels required a long time for their development, which was provided for during the greater portion of the subsequent Tertiary period, when the present table-lands were mostly low continental plains interrupted by mountain ridges. In the meanwhile, the present coastal plains (now rising to seventeen hundred feet) were submerged and were receiving the older Tertiary accumulations