64

The drainage of the Cretaceous prairies is of two general types:

• (1) Through-flowing rivers-the Red, Brazos, and Colorado, which enter the region from the west and cross it -and
• (2) a system of less copious locally developed autogenous streams.

The through-flowing rivers, descending from the Central Province, cross the Black and Grand prairies in deeply indented grooves or valleys cut far below the general upland level, and are established upon a lower profile and have a more flattened gradient than the streams of the second class mentioned. They are practically great canals passing across the region without drawing much lateral drainage directly from it.

The Trinity, between Brazos and Red rivers, and the Paluxy, Leon, and San Gabriel, between the Brazos and the Colorado, are rivers of the second class. These are developed upon upland stretches of plain which separate the stream valleys of the older class of streams. (See Pl. XI.) The many ramifying branches of this second group gather all the upland drainage of the prairies between older rivers, taking it even from the very scarps of the valleys of the greater through-flowing rivers.

The trunks of these secondary rivers are primarily autogenous slope streams which were originally established upon higher surfaces, now stripped away; they have maintained their location by inheritance as the general region was degraded by erosion. These rivers mostly rise close to the western margin of the Grand Prairie plain, but by rapid descent of their streamways soon become so deeply indented that their paths are much lower than the regional surface of the upland prairies, which are flat-topped divides between them. Their permanent water is largely derived from the structural drainage of the sands of the Cretaceous beds. Some of them, like the Leon and San Gabriel, are also largely reenforced as they cross the Balcones fault zone, at the border of the Black and Grand prairies, by springs rising under hydrostatic pressure through fissures.

The laterals and sublaterals of these streams do the principal work of gradation and are of a peculiar type. The principal laterals tend to parallel the escarpment troughs in a north-south direction at right angles to the main trunks. The sublaterals flowing into the laterals gather their drainage on the east by short ravines from the inward-facing escarpments and from the west by long drains developed on the surface of the dip plains, as shown in Pl. XI.

The main trunks of the streams of the second class are usually interrupted in flow, the water running in deep pools for some stretches of their courses and again disappearing in dry stony channels. The