TXTexas Geography

The tallest mountain in Texas is a fossil reef

Guadalupe Peak, 8,751 feet, and the sheer prow of El Capitan below it are not volcanic and not folded granite. They are limestone built by sponges, algae and lime mud along the rim of a shallow tropical sea about 260 million years ago. The Capitan Reef ringed the Delaware Basin, was buried under thousands of feet of salt and gypsum when the sea evaporated, and was later lifted and stripped bare. The same reef surfaces again across the New Mexico line, where groundwater hollowed it out into Carlsbad Caverns, and the basin it enclosed is the Permian Basin that still produces most of the state's oil. From that summit down to the Gulf shoreline at sea level is the full 8,751 feet of Texan relief.

Why the big cities sit in a line

Draw a line through Del Rio, San Antonio, New Braunfels, San Marcos, Austin and Waco and you have traced the Balcones Escarpment, a zone of faults that slipped roughly fifteen to twenty million years ago and dropped the coastal plain against the edge of the Edwards Plateau. The faulting matters because it intercepts the Edwards Aquifer. Water moving through the honeycombed limestone hits the fault and comes out as enormous springs, Comal at New Braunfels, San Marcos Springs, Barton Springs in Austin, and towns grew on each of them. The escarpment also shoves humid Gulf air up several hundred feet in a few miles, which is why the belt is nicknamed Flash Flood Alley and holds some of the most extreme short duration rainfall totals recorded in North America.

The tableland with almost no rivers

North and west of the escarpment the Great Plains arrive as the Llano Estacado, a tableland of roughly 30,000 square miles spilling into eastern New Mexico. It is bounded on the east and north by the Caprock Escarpment, a cliff up to 1,000 feet high that you can see coming for twenty miles, and it is so flat and so poorly drained that rain does not run off. Instead it collects in tens of thousands of shallow clay floored basins called playas, most only a few acres across, that hold water for days and then vanish. Irrigation there draws on the Ogallala Aquifer rather than on surface flow. Where the Prairie Dog Town Fork of the Red River has managed to breach the caprock it opened Palo Duro Canyon, about 120 miles long and up to 800 feet deep, the second largest canyon in the country.

Boundaries that were argued in court

The Rio Grande forms about 1,250 miles of international boundary, more than any other state has with another country, and in the Big Bend it turns hard northeast through the Santa Elena, Mariscal and Boquillas canyons where the walls run 1,500 feet above the water. A river makes a poor boundary because it moves. Near El Paso the channel shifted enough after 1864 to strand several hundred acres, and the resulting Chamizal dispute took until the 1963 convention and a concrete relocated channel to settle. On the Oklahoma side, the Red River boundary sits at the south bank rather than midstream, a distinction that produced Supreme Court litigation once oil was found in the riverbed. Texas also lost Greer County, the wedge between the north and south forks of the Red, when the court awarded it to Oklahoma Territory in 1896.

The Panhandle is bounded by pure numbers: the 100th meridian on the east, the 103rd on the west and the parallel of 36 degrees 30 minutes on the north, the same latitude the Missouri Compromise used. The nineteenth century survey of the 103rd came out a few miles off the true meridian, and the marks on the ground were allowed to stand rather than the mathematics. Along the coast, 367 miles of shoreline is fronted almost continuously by barrier islands, the longest of which, Padre Island, runs about 113 miles.

From sixty inches of rain to nine

Texas has the steepest west to east moisture gradient of any state. Orange, in the far southeast corner, averages close to sixty inches of rain a year. El Paso, at the far western tip, averages under nine. As a rule of thumb annual rainfall drops about an inch for every fifteen miles you travel west, which is why the vegetation changes from bald cypress swamp to post oak savanna to shortgrass prairie to creosote desert along a single highway. That gradient also explains the state's lake map: nearly every large body of water in Texas was built by a dam, and Caddo Lake on the Louisiana line is the one substantial natural exception, ponded up behind the Great Raft, a centuries old log jam on the Red River.

The record high is 120 degrees, set at Seymour on 12 August 1936 and matched at Monahans on 28 June 1994. The record low is 23 below zero, at Tulia in 1899 and Seminole in 1933. El Paso and Hudspeth counties keep Mountain Time while the rest of the state runs on Central, and El Paso is closer by road to San Diego than to Beaumont.

Geography of the states around Texas

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