The greater intensity with which the rocks in the southwestern portion of the Ozark uplift have been folded than in the more northern portion and the varying degrees to which the beds have yielded to denudation have resulted in giving to its various parts different types of topography. This diversity has led to the recognition of several distinct divisions, such as the Shawnee Hills, at the extreme northeastern end of the uplift, where the rocks have been folded and the ridges cut across by the Mississippi; the St. François Mountains, in southeastern Missouri, composed of a large number of isolated hills and rising from 500 to 800 feet above the adjacent valleys; the Ozark plateau, in southwestern Missouri and northwestern Arkansas, the central part of which has a general elevation of 1,500 feet above the sea, and to one travelling over it seems a boundless and featureless plain underlaid by apparently horizontal but in reality gently westward dipping sheets of stratified rocks; the Boston Mountains, in central and western Arkansas, consisting of rugged irregular ridges and truncated summits with a general crest-line elevation of 1,000 feet
above the sea; and the Ouachita (pronounced Wichitaw) Mountains, formed of numerous rudely parallel upward folds of hard rock, which rise from 500 to 1,000 feet above the adjacent valleys and form a belt of unusually picturesque, forest-crowned hills, extending from Little Rock, Arkansas, westward into Indian Territory.
The study of the island-like Ozark region in the broad, ocean-like expanse of the prairies is far from being complete. Although topographically distinct and appearing as one of the minor units in the geology of the continent, geologists are inclined to the view that the Ozark uplift as above described should be considered as consisting of two independent but contiguous areas of upheaval, namely, the Ozark Hills, situated mainly in Missouri, and the Ouachita Hills, lying mainly in Arkansas and the Indian Territory. This Ozark-Ouachita region—by whatever name finally designated—is one with a long and varied, nay, even a poetic history. In writing of the Archean rocks of the Iron Mountain region, Missouri, Arthur Winslow states that they "are truly ancient elevations, older than any others in the State, older than the mountains of Arkansas, older than the Appalachians, older than the Rocky Mountains; if venerable be an attribute of great age, they certainly merit that appellation. For not only are all other rocks of Missouri youthful as compared with these, but there is a genetic relationship, and the former are in a sense descendants of the latter. For when the limestones and other sedimentary rocks were yet unformed these crystalline rocks must have existed as parts of a continental mass, and from the degradation of this continent resulted the materials of the later formed sedimentary rocks. The present granite and porphyry hills are but protruding parts of the remnant of this ancient continent which stood as islands above the ocean waters while the beds of limestone and sandstone were being formed about them, which rose with these beds when they were lifted from the waters, which now, rugged and weather-beaten, yet tempered by age and varied experience, rear themselves above the surrounding younger rocks and bid
fair still to live when the latter have yielded to the forces of degradation."
Besides its pleasing scenery, varied and abundant mineral resources, and health-giving springs, this oasis of hills amid the unvaried monotony of the grass-covered plains in the southern portion of the continental basin derives an additional attraction from its forest growths in which southern pines are mingled with oaks, hickories, walnuts, and other broad-leaved trees. The soil is generally productive, and great fields of corn and cotton may be seen side by side.
The Gulf Plains.—The Gulf plains include the western portion of Florida, and extend westward and southward about the borders of the Gulf of Mexico in a continuous belt from 50 to 60 to perhaps 100 miles wide, to where the Pacific mountains approach the coast in east-central Mexico. This low, gently seaward-sloping region, underlaid by soft horizontal strata, possesses a generally rich soil well adapted for the cultivation of cotton, corn, sugar-cane, and rice. In the low, hot country of eastern Mexico nearly all tropical fruits can be successfully raised. The most characteristic as well as the broadest portion of this productive belt is in the States of Mississippi and Louisiana, and extends northward with a gradually decreasing width to the mouth of the Ohio. This is the lower Mississippi basin, which owes its existence mainly to the deposits of silt laid down by the river after which it is named. Much of the land is really the delta of the "Father of Waters," over which that river spreads out in vast inundations each year.
The Gulf plains skirt the southern end of the Appalachian Mountains, and to the west of the Mississippi are bordered in part by the Ozark uplift. There are certain reasons for believing that these two regions of elevation, characterized by a similar geological structure, are portions of a single greatly disturbed belt, but are now separated by a broad area which has been depressed and deeply covered with comparatively recent sediments. But that this general view of the origin of the larger features in the
relief of the Gulf States can be accepted with entire confidence is questionable. True it is, however, that the delta region of the Mississippi has undergone many up and down movements, and that several successive sheets of sediment have been laid down upon it, but that the folds and crumplings characteristic of the southern Appalachians and of the Ozark uplift extend across the intervening space beneath the covering of horizontal rocks has not been demonstrated.
The Gulf plains throughout are less than 500 feet above the sea, and much of the Gulf margin and the similar tract which extends northward to the mouth of the Ohio has an elevation of less than 100 feet. The fringe of lowland bordering the Gulf and extending up the course of the Mississippi is generally swampy and contains numerous small water bodies which owe their existence to the cutting off of the beds of the river so as to form what are termed ox-bow lakes.
Previous to the settlement of the Gulf plains by Europeans and the clearing of much of the land for plantations it was clothed with such a dense growth of trees and vines as to be almost impenetrable. The southern pine there reaches its greatest perfection and is the basis of a great lumber industry, and oaks of several species, the wide-spreading white-trunked sycamore, the still more stately tulip-tree with its cup-like blossoms of yellow, the fragrant magnolia, the seemingly always aged cypress, the gum-tree, and many other species of arboreal vegetation also find most congenial conditions for their growth. The dwarf palmetto, which forms such a characteristic growth in Florida, extends northward in the Mississippi basin to the southern border of the Ozark uplift. Much of the luxuriant moss and lichen draped forest of the Gulf plains with all its primitive network of shrubs and vines still remains.