The Texas Prairies in East Central Texas, extending from Red River to the Coastal Plain, are plains areas that have developed on outcrops of calcareous clays or chalk. The Black Prairies area, occupying the Inner Lowland, which forms the interior portion of the Coastal Plain and which is underlain by limy clays, marls, and chalk beds of the Upper Cretaceous, is best known. The topography is of a round, rolling, or undulating nature, and the soils are generally deep except along the western margin of the Austin chalk, where erosion is particularly active. The Black Prairies comprise several belts, each of which is developed on the outcrops of the major formations composing the Upper Cretaceous section. For instance, the Austin chalk, or White Rock strip, not only forms a belt of slightly higher elevation in the Black Prairies, but also its western portion is rather strongly erosional, with characteristically shallow soils, and it has areas of considerable size with the chalk exposed at the surface. The shallow soil areas afforded better foundations for roads in the early days, particularly in contrast to the deep soil areas to the east and west of the Austin chalk, which are characterized by deep muds during much of the year. Since the 1880s the Black Prairies have constituted one of the outstanding cotton regions of the United States. With the coming of railroads into the area important commercial centers arose. The flat Coastal Prairies just interior from the Gulf Coast afford examples of two types of prairie soils. Most of the Beaumont clays belt is characterized by heavy black soils; the interior Lissie formation, although characteristically occupied by tall grasses, has light-colored soils. In the middle portion of the Central Dissected Belt of the Coastal Plain is a strip of country underlain by calcareous clays of Tertiary age. These include the Brenham-Schulenburg prairies, which form an important farming region. Although the surface is somewhat more deeply and more intricately dissected than is the case of the Black Prairies, this district has good black soils that form the basis for its prosperous farming operations. On either side of the Brenham-Schulenburg Prairies belt are woodlands, which occur on dissected areas underlain by non-limy sands and sandy clays. The Coastal Prairies region is an important oil-producing district. The Upper Gulf or Houston district, which extends into the timbered country along the southern margin of East Texas, is generally designated as the salt dome area. The ordinary salt domes are of the piercement type, which rise nearly to the surface; others are of the seep dome type, of which the Conroe oilfield is considered a good example.