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The relief features of the Greater Texas region range from vast stretches of apparently level country, like the Llano Estacado and Coast Prairie, presenting no visible breaks in their plain-like extent, to the rugged mountains of the Trans-Pecos Province, marked in places along the Rio Grande by abrupt canyons and declivities.

This relief, as a whole, may be classified as that of the mountains and that of the plains. The altitudes of the region vary from sea level to 13.000 feet (see Pl. II).

Broader features of the relief.- In a broad sense the Greater Texas region consists of a vast and diversified plain bordered on the west and north by mountains. That portion between the eastern front of the Cordilleras and the sea may be primarily conceived as an elongated plain. This plain inclines gently from the Cordilleras toward the sea. The inclination from the foot of the Cordilleras to the Gulf is generally in an easterly direction, but there are slight variations of direction. The area having this general inclination may be specifically called the Regional Coastward Slope, and its variation in gradient and direction, as will be explained later, has an important relation to the physiographic history. Except in the extreme northwest corner of Texas, where the Great Plains continue north unbroken, and on the east, where the Coastal Plain borders the sea or continues into Louisiana, the Regional Coastward Slope is terminated by the Cordilleran and the Ouachitan mountain systems, which extend at approximately right angles to each other, diverging so as to inclose the plain in a triangle having its wider base toward the sea. The plain is rudely comparable to a wide, low stairway, leading upward from the sea to the Cordilleras, in which the various subdivisions of the plain represent the treads, local escarpments the risers, and the limiting mountains the balustrades. This analogy can not be carried far, for great irregularities occur in the width and tread of the steps, and the wear and tear of time has scarred and disfigured their relief, etched valleys where the drainage depressions have crossed the plains, and lowered the mountain walls. Some escarpment steps in exceptional instances face westward, or upstairs, while other subdivisions of the plain succeed one another without any well-defined feature of relief. Furthermore, the structure of the inclosing mountain systems is of two entirely different types and periods of architecture. The formations which underlie some of the plains are crumpled up in the Cordilleras and are deposited against the Ouachitas. Hence, parts of the plains are older than the Rocky Mountains and younger than the Ouachita uplifts.