Texas's major physiographic divisions are the Gulf Coastal Plain in the east and southeast; the North Central Plains, covering most of central Texas; the Great Plains, extending from west-central Texas up into the panhandle; and the mountainous trans-Pecos area in the extreme west.
Within the Gulf Coastal Plain are the Piney Woods, an extension of western Louisiana that introduces into East Texas for about 125 mi (200 km), and the Post Oak Belt, a flat region of mixed soil that gives way to the rolling prairie of the Blackland Belt, the state's most densely populated region. The Balcones Escarpment (so-called by the Spanish because its sharp profile suggests a balcony), a geological fault line running from the Rio Grande near Del Rio across central Texas, separates the Gulf Coastal Plain and Rio Grande Plain from the North Central Plains and south-central Hill Country, and in so doing, divides East Texas from West Texas, watered Texas from dry Texas, and (culturally speaking) the Old South from the burgeoning West.
The North Central Plains extend from the Blackland Belt to the Cap Rock Escarpment, a natural boundary carved by erosion to heights of nearly 1,000 ft (300 m) in some places. Much of this plains region is rolling prairie, but the dude ranches of the Hill Country and the mineral-rich Burnet-Llano Basin are also found here. West of the Cap Rock Escarpment are the Great Plains, stretching north–south from the Panhandle Plains to the Edwards Plateau, just north of the Balcones Escarpment. Along the western edge of the panhandle and extending into New Mexico is the Llano Estacado (Staked Plains), an extension of the High Plains lying east of the base of the Rocky Mountains.
The trans-Pecos region, between the Pecos River and the Rio Grande, contains the highest point in the state: Guadalupe Peak, with an altitude of 8,749 ft (2,668 m), part of the Guadalupe Range extending southward from New Mexico into western Texas for about 20 mi (32 km). Also in the trans-Pecos region is the Diablo Plateau, which has no runoff to the sea and holds its scant water in lakes that often evaporate entirely. Farther south are the Davis Mountains, with a number of peaks rising above 7,000 ft (2,100 m), and Big Bend country (surrounded on three sides by the Rio Grande), whose canyons sometimes reach depths of nearly 2,000 ft (600 m). The Chisos Mountains, also exceeding 7,000 ft (2,100 m) at some points stand just north and west of the Rio Grande.
For its vast expanse, Texas boasts few natural lakes. Caddo Lake, which lies in Texas and Louisiana, is the state's largest natural lake, though its present length of 20 mi (32 km) includes waters added by dam construction in Louisiana. Two artificial reservoirs—Amistad (shared with Mexico), near Del Rio, and Toledo Bend (shared with Louisiana) on the Sabine River—have respective storage capacities exceeding 3 million and 4 million acre-ft, and the Sam Rayburn Reservoir (covering 179 sq mi/464 sq km) has a capacity of 2.9 million acre-ft. All together, the state contains close to 200 major reservoirs, eight of which can store more than 1 million acre-ft of water. From the air, Texas looks as well watered as Minnesota, but the lakes are artificial, and much of the soil is dry.
One reason Texas has so many reservoirs is that it is blessed with a number of major river systems, although none is navigable for more than 50 mi (80 km) inland. Starting from the west, the Rio Grande, a majestic stream in some places but a trickling trough in others, imparts life to the Texas desert and serves as the international boundary with Mexico. Its total length of 1,896 mi (3,051 km), including segments in Colorado and New Mexico, makes the Rio Grande the nation's 2nd-longest river, exceeded only by the Missouri-Mississippi river system. The Colorado River is the longest river wholly within the state, extending about 600 mi (970 km) on its journey across central and southeastern Texas to the Gulf of Mexico. Other important rivers include the Nueces, in whose brushy valley the range cattle industry began; the San Antonio, which stems from springs within the present city limits and flows, like most Texas rivers, to the Gulf of Mexico; the Brazos, which rises in New Mexico and stretches diagonally for about 840 mi (1,350 km) across Texas; the Trinity, which serves Fort Worth and Dallas; the San Jacinto, a short river but one of the most heavily trafficked in North America, overlapping the Houston Ship Channel, which connects the Port of Houston with the Gulf; the Neches, which makes an ocean port out of Beaumont; the Sabine, which has the largest water discharge (6,800,000 acre-ft) at its mouth of any Texas river; the Red, forming part of the northern boundary; and the Canadian, which crosses the Texas panhandle from New Mexico to Oklahoma, bringing moisture to the cattle raisers and wheat growers of that region. In all, Texas has about 3,700 identifiable streams, many of which dry up in the summer and flood during periods of rainfall.
Because of its extensive outcroppings of limestone, extending westward from the Balcones Escarpment, Texas contains a maze of caverns. Among the better-known caves are Longhorn Cavern in Burnet County; Wonder Cave, near San Marcos; the Caverns of Sonora, at Sonora; and Jack Pit Cave, in Menard County, which, with 19,000 ft (5,800 m) of passages, is the most extensive cave yet mapped in the state.
About 1 billion years ago, shallow seas covered much of Texas. After the seas receded, the land dropped gradually over millions of years, leaving a thick sediment that was then compressed into a long mountain range called the Ouachita Fold Belt. The sea was eventually restricted to a zone in West Texas called the Permian Basin, a giant evaporation pan holding gypsum and salt deposits hundreds of feet deep. As the mountain chain across central Texas eroded and the land continued to subside, the Rocky Mountains were uplifted, leaving deep cuts in Big Bend country and creating the Llano Estacado. The Gulf of Mexico subsided rapidly, depositing sediment accumulations several thousand feet deep, while salt domes formed over vast petroleum and sulfur deposits. All this geologic activity also deposited quicksilver in the Terlingua section of the Big Bend, built up the Horseshoe Atoll (a buried reef in west-central Texas that is the largest limestone reservoir in the nation), created uranium deposits in southern Texas, and preserved the oil-bearing Jurassic rocks of the northeast.