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The drainage flows directly across the scarp line and has cut great canyons backward into the Edwards Plateau. The depth and precipitous character of these increase in the streams successively encountered as one goes westward. The portions of these streams lying within the area of the plateau before they cross the fault line have cut their channels approximately down to the level of the Rio Grande Plain. Their bocas in many places cut the scarp line into a series of tongue like salients projecting toward the plain, and the line is further diversified by plateau remnants in the form of outlying buttes. The position of the scarp is determined by a complex dislocation of the rocks, the Balcones fault, which will be described in a subsequent section (p. 258).

The Llano Estacado and the Edwards Plateau together constitute in Texas the Plateau of the Plains. This lies within the area inclosed by the Canadian on the north, the Pecos River on the west, the Balcones escarpment on the south and southeast, and an irregular line of scarps along the headwaters of the eastward flowing drainage of the Colorado, Brazos, and Red rivers of Texas. The general outline of this area is shown in the accompanying photograph of a model, P1. XXIII. It is over 500 miles in length and in places 280 miles broad. It is a vast quadrangular mesa, surrounded on all sides by descending escarpments. In its horizontal geologic structure and its relations to the surrounding region it may be broadly compared to a book laid upon a table and very slightly tilted or raised at one end. Its comonent strata are almost as parallel and regular as the leaves of the volume. While its central portion still presents a general level, its borders are cut by headwater erosion into a fringe of projecting drainage divides, accompanied by many remnantal buttes and mesas, showing the great erosion by which the plateau has been and is being gradually etched away. Its eastern margin in particular has been greatly reduced by this process.

The Llano Estacado and Edwards Plateau merge into each other along the central portion of the summit of the greater plateau, and there is no known line of demarcation between them. There is a great difference between the characters of their surface, their soils, and the underlying geologic strata, which collectively gives to each of the two regions a distinct character. The soil and underlying rocks of the Llano Estacado consist of unindurated loams, marls, and sands. Hardly a stone of building size can be found in its broad extent. The Edwards Plateau is in part a rough limestone country, resembling in some respects the western margin of the limestone country of the Grand "

Boca, a Spanish topographic term, used to indicate the mouth of a canyon valley where it debouches on a plain. Example, Boca del Agna, Jamaica, corrupted by the English into "Bog walk."