LOWER PLAINS. The area north of the Edwards Plateau and extending from the Caprock escarpment of the High Plains to the western boundary of the Grand Prairiesqv is an undulating to rolling terrain known by such names as Lower Plains, North Central Plains, Osage Plains, and Rolling Plains. Elevations range from 2,200 feet on the western edges of the region to 1,000 feet in the east. The variegated topography and outcrops of diverse geologic materials led Robert T. Hill to refer to the area as the Central Denuded Region. Northward, this region extends across much of western Oklahoma and terminates in south central Kansas. In Texas it comprises the Western Cross Timbers (see CROSS TIMBERS) and the Red Beds Plains, roughly between the Colorado and Red rivers on the north and south and between the Caprock on the west and a line paralleling the eastern boundaries of Montague, Wise, Parker, and Hood counties on the east. The Western Cross Timbers are underlain by the Trinity sands of Lower Cretaceous, or Comanchean age, at the east and at the west by the hard rocks, predominantly sandstones, of the Pennsylvania and the Wichita beds of the early Permian. The Red Beds Plains are underlain by outcrops of the Permian and Triassic, the geologic materials of which are dominantly clays with interbedded harder layers of sandstones and gypsum. The region is crossed by several streams, all of which have their origin in the High Plains. At a former time in geologic history it was a plain, the floor of which was composed of the same type of thick and hard limestones of Comanchean age that now are exposed at the surface of the Edwards Plateau and that also extended under much of the area of the High Plains. In relatively recent geologic time these limestone layers were removed in a vast denudation, which has left only a few remnants of a former smooth surface in the flat-topped mountains that occur throughout the southern portion of the Lower Plains. These eminences, which were landmarks for the early explorers and travelers across the region, include Comanche Peak, Round Mountain, Santa Anna Mountain, Flat Top Mountain, and others. The elongated Callahan Divide area lying between the drainages of the Colorado and the Brazos rivers is another one of these erosional remnants. Once the hard Cretaceous limestones were cut through, denudation proceeded rapidly, unroofing the diverse geologic materials and structural features that now comprise the surface of the area. Something of the diversity of the geologic materials has been indicated. The strata of both the Pennsylvanian and Permian beds dip westward, extending beneath the High Plains, as they continue in the subsurface across the Permian basin. On the other hand, the geologically younger Cretaceous beds dip gently eastward.
In the Red Beds country, the combination of beds of clays interstratified with thin but harder layers of sandstone and gypsum, all dipping westward, upon being denuded, gives a stairstep pattern to the topography through the recurrence of more or less parallel cuestas extending across the landscape. Each hard layer forms the brow of an east-facing escarpment; the back slope-a structural plain developed on the dip of the hard layer-extends beneath the next escarpment, and so on, with the elevation rising westward, step by step over these escarpments, until the Caprock of the High Plains is reached. The dip plain lying between any two successive escarpments is what has been left by the removal of the interbedded clays, but some clays remain on the plain, giving it a constructional surface. The characteristic vegetation of the Western Cross Timbers country is a sparse woodland, comprising hardwood species. Areas with outcrops of limestone have a characteristic mesquite shrub-short grass vegetation that provides excellent grazing facilities. The Trinity sands section of the Western Cross Timbers has deep sandy soils where they have not been eroded. In contrast, the hard rock Pennsylvanian outcrops form an erosional upland country with shallow soils. The Red Bed Plains country is typically expressed in the belts underlain by Permian outcrops, especially the belt underlain by the Clear Fork group. Outcrops of the Clear Fork extend from the plains about San Angelo discontinuously (being broken by the Callahan Divide) to the belt of constructional plains that extends from Abilene to Red River and beyond into southwestern Oklahoma. These plains, underlain by soft clays, are covered by a mantle of wind-blown silts except where the latter have been eroded in the vicinity of larger streams. Thus the outcrops of the Clear Fork are characterized by a constructional surface that makes these plains a miniature High Plains country. On the smooth interstream areas, this mantle of silts is not only relatively thick but is also physiographically stable, and in such environmental conditions the typical soil of the region-the black earth, or Chernozem-has developed. Such areas are important for farming purposes. Outcrops of the Albany formation form a typical stairstep topography, with a somewhat broken surface such as can be seen in the vicinity of Coleman. The Albany formation consists of a succession of limestones and interbedded marly clays, the limestones form the steeper escarpment faces, and the clays on the westward, dipped back slopes of the cuestas form a constructional surface characterized by a cover of black earth or Chernozem soils.
The Double Mountain group of Permian formations outcropping in the western portion of the Lower Plains and beyond the Clear Fork outcrops forms what Hill designated the Gypsum Plains, because of the prominence of gypsum exposures in this belt. In general, the Gypsum Plains have a rolling topography, part of which is erosional, but most of the slopes are covered with a rather deep mantle of windblown silty materials. The entire belt is characterized by erosion of the deep gully type, which has developed many veritable gorges. Scattered throughout the Gypsum Plains belt are smooth, flattish areas of variable extent; such areas have a deep mantle of wind-blown silts, and the topography is sufficiently stable as to allow the formation of soils of the black earth type. The largest one of these areas is the Roscoe District. These areas, being smooth and having excellent soils, are good farm lands; they comprise smaller editions of the typical High Plains environment. The geologic matrix of erosional areas in the Clear Fork and Double Mountain belts consists of fine textured materials, which, where not stabilized by the natural vegetation, are readily susceptible to erosion. These conditions account for the vast amounts of sediments carried by such streams as the Colorado, the Brazos, and the Red rivers, especially during the "red rises" typical of these streams in early summer.