About 1 billion years ago, shallow seas covered much of Texas. After the seas receded, the land dropped gradually over millions of years. The sea eventually receded to an area in West Texas called the Permian Basin, and as it evaporated deposits of gypsum and salt hundreds of feet deep were left behind. As the Gulf of Mexico subsided, salt domes formed over vast petroleum and sulfur deposits. All this geologic activity also deposited quicksilver in the Terlingua area, 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 rocks of the northeast.
Texas’s has four major geographic areas: 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.
Stretching inland from the Gulf Coast, the Gulf Coastal Plains make up about two-fifths of the state’s land area and include the Post Oak Belt, a flat region of mixed soil that gives way to the rolling prairie of the Blackland Belt. The Gulf Coastal Plains end at a geological fault line called the Balcones Escarpment (named by the Spanish because its shape suggests a balcony). The Balcones Escarpment separates the Gulf Coastal Plain from the North Central Plains and south-central Hill Country, and divides East Texas from West Texas.
Much of the North Central Plains region is rolling prairie, with dude ranches in the Hill Country and the mineral-rich Burnet-Llano Basin. To the west are the Great Plains, stretching north–south from the Panhandle Plains to just north of the Balcones Escarpment.
To the west edge of the North Central Plains lies the High Plains country, and to the south lies the Trans-Pecos region. The trans-Pecos, 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). 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).
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.
For its vast expanse, Texas has few natural lakes. Caddo Lake, which is partly in Louisiana, is the state’s largest natural lake. However, Texas contains close to 200 major reservoirs, many of which were created to store water against drought. 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 has several major river systems.
The most important river in Texas is the Rio Grande. It extends some 1,900 miles (3,060 km) from its source in the Rocky Mountains of Colorado to the Gulf of Mexico, and its Texas segment forms the border between the United States and Mexico. Other major rivers traversing the state are the Brazos (at 1,280 miles [2,060 km], the longest river solely in Texas), as well as the Sabine, Trinity, and Red rivers, the last of which forms a large portion of the border between Texas and Oklahoma.
Just about every kind of weather has been known to occur in Texas – sometimes all at the same time. January temperatures in the Rio Grande valley can reach well into the 90s F (about 32 °C), while blizzards have blocked highways in the Panhandle section of the state during the same month. Brownsville, at the mouth of the Rio Grande, has had no measurable snowfall during all the years that records have been kept, but Vega, in the panhandle, averages 23 in (58 cm) of snowfall per year. Generally, the Gulf Coast has a maritime climate, while inland has a continental climate. The Balcones Escarpment is the main dividing line between the two zones. Texas has two basic seasons—a hot summer that may last from April through October, and a winter that starts in November and usually lasts until March. The Gulf Coast area around Houston has average annual temperatures in the low 70s F (about 21 °C) and precipitation of some 45 inches (1,100 mm), whereas the Panhandle averages in the low 60s F (about 16 °C) with less than 20 inches (500 mm) of rain. The driest region is the Trans-Pecos, and the wettest is the southeast.
Record temperatures range from –23°F (–31°C) at Seminole, on 8 February 1933, to 120°F (49°C) at Seymour in north-central Texas on 12 August 1936. On the High Plains country of West Texas, sandstorms are common. Heavy rainfall and flash floods are common in some areas. Many wide, flat riverbeds in the region remain dry most of the year, but they can become sluiceways for flash floods. Alvin, in Brazoria County on the Gulf Coast, had 43 in (109 cm) of rain on 25–26 July 1979, a national record for the most rainfall during a 24-hour period.
