Fig. 13.—The larger physiographic divisions of North America.
Each of the four physiographic provinces briefly described above is in a conspicuous manner elongated in a north and south direction. The mountains, valleys, and plateaus, as well as the controlling lines of structure in the rocks below the surface, throughout the main body of the continent coincide in direction more or less nearly with the parallels of longitude. At the south, however, and crossing the trend of each of the provinces named above, is the:
Fifth, or Caribbean province, which includes the West Indies, the southern part of Mexico, and all of Central
America. In this province are the Antillean Mountains, now mostly submerged, the principal axes of which trend east and west.
With this brief outline of the larger physical divisions of North America in mind, let us endeavour to become acquainted with the leading characteristics of each of the provinces as they exist to-day, and at the same time learn something of their long and varied histories.
COASTAL PLAINS AND PLATEAUS
The Coastal Plains.—From New York to Key West and thence about the borders of the Gulf of Mexico to the neighbourhood of Vera Cruz, the border of the present land area of the continent is formed by a low plain, from 30 to 50 miles broad in New Jersey, but increasing in width southward to Georgia and Florida, where its somewhat indefinite inland margin is more than 100 miles from the sea, and reaching its greatest development in the delta of the Mississippi. Extending southward about the west coast of the Gulf, it forms the low, featureless eastern border of Texas, about 50 miles broad, and passes into Mexico, but gradually narrows as the Pacific Mountains approach the coast, and ends in the vicinity of Vera Cruz.
The Atlantic and Gulf coastal plain everywhere slopes gently seaward, and on its landward margin has an elevation in general of from 200 to 300 feet. The character of the material of which the coastal plain is composed, the fossils contained in it, as well as its geographical features, show that it is a continuation of the continental shelf, and was formed at a time when the border of the continent was more deeply submerged than at present. Minor oscillations of the earth's crust have time and again allowed the sea to extend inland, only to be forced to recede when the land again rose. Each invasion of the sea left a sheet of soft sediment over the portion of the land that was submerged. These oscillations are still in progress, as is indicated by the fact that along the New Jersey coast a downward movement at the rate of about 2 feet per century is
taking place. A similar depression of the land is also thought to be in progress along the south Atlantic coast and in the delta of the Mississippi. The Atlantic coastal plain has its most characteristic development in South Carolina, and is roughly divisible according to its topography and soil into several belts parallel with the shore-line. At the same time it is transversely divided into strips by the several rivers which flow across it and by the many branches of these rivers originating on the plain itself.
The junction of the portion of the gently sloping border of the continent now above sea-level, with the submerged portion, is characterized by the presence of a belt of swamps, in part marine marshes where the salt water ebbs and flows, and in part fresh-water morasses in which the drainage is obstructed largely by decaying vegetation. Inland from the coastal swamps the surface becomes higher, is for the most part well drained, and when not too sandy furnishes rich agricultural lands. The Atlantic plain as a whole thus has three principal divisions: a submerged portion, a marsh portion, and a subaerial portion. During past ages the position of each of these belts migrated, owing to movements in the earth's crust, but their succession in reference to each other has been the same since the Tertiary period.