<P> The Great Plains include the Llano Estacado, the Panhandle, Edwards Plateau, Toyah Basin, and the Llano Uplift . It is bordered on the east by the Caprock Escarpment in the panhandle and by the Balcones Fault to the southeast . Cities in this region include Midland and Odessa, Lubbock, and Amarillo . The Hill Country is a popular name for the area of hills along the Balcones Escarpment and is a transitional area between the Great Plains and the Gulf Coastal Plains . With about 15 to 31 inches (380 to 790 mm) annual rainfall, the southern end of the Great Plains are gently rolling plains of shrub and grassland, and home to the dramatic Caprock Canyons and Palo Duro Canyon state parks . The largest concentration of playa lakes in the world (nearly 22,000) is on the Southern High Plains of Texas and Eastern New Mexico . </P> <P> Texas' blackland prairies were some of the first areas farmed in Texas . Highly expansive clays with characteristic dark coloration, called the Houston Black series, occur on about 1.5 million acres (6,000 km2) extending from north of Dallas south to San Antonio . The Professional Soil Scientists Association of Texas has recommended to the State Legislature that the Houston Black series be designated the State soil . The series was established in 1902 . National Parks in this area are the Lyndon B. Johnson National Historical Park and the San Antonio Missions National Historical Park . </P> <P> The Trans - Pecos Natural Region has less than 12 inches (300 mm) annual rainfall . The most complex Natural Region, it includes Sand Hills, the Stockton Plateau, desert valleys, wooded mountain slopes and desert grasslands . The Basin and Range Province is in West Texas, west of the Pecos River, beginning with the Davis Mountains on the east and the Rio Grande to its west and south . The Trans - Pecos region is the only part of Texas regarded as mountainous and includes seven named peaks in elevation greater than 8,000 feet (2,400 m). This region includes sand hills, desert valleys, wooded mountain slopes and desert grasslands . The vegetation diversity includes at least 268 grass species and 447 species of woody plants . National Parks include the Amistad National Recreation Area, Big Bend National Park, Chamizal National Memorial, Fort Davis National Historic Site, Guadalupe Mountains National Park, and the Rio Grande Wild and Scenic River . This area is part of the Chihuahuan Desert . </P> <P> Texas is mostly sedimentary rocks, with East Texas underlain by a Cretaceous and younger sequence of sediments, the trace of ancient shorelines east and south until the active continental margin of the Gulf of Mexico is met . This sequence is built atop the subsided crest of the Appalachian Mountains--Ouachita Mountains--Marathon Mountains zone of Pennsylvanian continental collision, which collapsed when rifting in Jurassic time opened the Gulf of Mexico . West from this orogenic crest, which is buried beneath the Dallas--Waco--Austin--San Antonio trend, the sediments are Permian and Triassic in age . Oil is found in the Cretaceous sediments in the east, the Permian sediments in the west, and along the Gulf coast and out on the Texas continental shelf . A few exposures of Precambrian igneous and metamorphic rocks are found in the central and western parts of the state, and Oligocene volcanic rocks are found in far west Texas, in the Big Bend area . A blanket of Miocene sediments known as the Ogallala formation in the western high plains region is an important aquifer . Texas has no active or dormant volcanoes and few earthquakes, being situated far from an active plate tectonic boundary . The Big Bend area is the most seismically active; however, the area is sparsely populated and suffers minimal damages and injuries, and no known fatalities have been attributed to a Texas earthquake . </P>

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