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change is in the nature of an almost imperceptible monoclinal fold, with a steeper grade on the east and a flattening profile to the west. This trivial difference of slope, which increases south as the Balcones fault zone develops, and which is reflected in the dip of the underlying strata, has an important effect upon the topography, drainage, and conditions of distribution of underground water.

The belts of Cretaceous prairie are, as a rule, of low relief, increasing in rugosity from east to west. The belts of faintest relief are on the east and those of more pronounced relief on the west, where they consist of flat-topped buttes, mesas, and widely terraced slopes and scarp lines.

The several belts also present contrasting minor variations of relief according to their structure. Those composed of the harder formations are usually flat in their coastward extension and are sometimes dissected into cut plains along their inland margin. Certain prairie belts in which the underlying formation consists of less consolidated terranes are eroded into a mammillary or hilly topography of the type known as rolling prairie.

The gently sloping dip plains of the various belts are accompanied by low interior-facing stratified escarpments, escarpment troughs, and drainage valleys. The escarpments are the outcrops or exposed edges of the beds of strata underlying the dip plains (see fig. 3). Escarpment troughs (parting valleys) are valleys occurring where the lower margin of a dip plain abuts against the base of the inward-facing escarpment of the succeeding plain.

While the dip plains incline coastward the accompanying escarpments always face in the opposite direction. There are two low inward-facing stratified escarpments near the interior borders of the Black and Grand prairies, which are specially conspicuous features of relief, constituting marked breaks in the otherwise monotonous surface of the adjacent areas. That of the Black Prairie, known as the White Rock scarp, extends through the plain Texas area southward from Sherman toward Austin. Its margin overlooks the narrow belt of the Eagle Ford Prairie. This feature is lacking in the Red River subdivision. The western and northern escarpment of the Grand Prairie-a more conspicuous feature-extends from the boundary of Arkansas due west through Indian Territory to the ninety-eighth meridian and thence south through Texas, in a much lobed and crenulated line, to the Colorado.