Texas is large, very large. East to west it would reach from Washington DC past St. Louis, Missouri. North to south it reaches from Boise, Idaho to Phoenix, Arizona. It has room to house the smallest fifteen states with space left over for a couple more Delawares and a few Rhode Islands as well. The border towns of El Paso and Laredo are getting a lot of “wall” press right now but there are a dozen cities whose names you likely recognize: Dallas, Fort Worth, Brownsville, Houston, Waco, San Antonio, Lubbock, Amarillo, Austin, and Corpus Christi. Whether you know these cities from old cowboy movies, from Janis Joplin (Port Arthur actually), from college football, or from NASA space shots the names conjure up images of a modern industrial, oil-rich, state with huge expanses of open space, perhaps dotted with cattle. Cattle country as much a space shuttle country. Hot and windy country with surprisingly cold winters. There are horses and cattle and rustlers and cowboys that also decorate our mental images. But mostly the cities are cities and the country is wild.
Texas is all of the above and more – but I am going to show a bit of the nearly empty, spectacularly beautiful (geologically speaking), southwestern part of the state. Big Bend NP is over 800,000 acres of Chihuahuan Desert. There are well over 1000 arid-ground plants, 400+ bird species, 55_+ species of reptiles, and about 75 species of mammals that live (or pass through) this region. The geology is simply amazing and the Rio Grande River flows along the southern edge of the park for more than 1000 miles. The park was established in 1944 and gets about half a million visitors each year.
In reality the southern half of the river, southern from mid-way in the deepest channel that is, is actually in Mexico; the border is the center of that channel. The river is rather unimpressive throughout is length. It fluctuates greatly with rainfall but is usually a placid brown river flowing rather gently. Crossing it has never been much of a problem for area residents whether man or beast.
The area was inhabited by American “Indians” for several hundred years. The Chisos Indians were a nomadic nation of unknown derivation but with an Aztec-based language. They were followed by Apaches of several types. The best known may be the Mescalero Apaches. They were given that name by the Spaniards because they used the mescal (an agave) for food and drink. (Tequila is made from Blue Agave and mescal, the liquor, can be made from any of thirty different agave species; there are 200+ agaves species all together. Mescal is made in Oaxaca, Mexico mainly.) Anyway, the American army was located in these southwestern desert areas to protect the westward migrants in the decades after the Civil War. It was during that period that the trails were established that allowed people to reach California and when many of the towns and cities along the trails were established to service the travelers. Not many west-bound travelers traveled as far south as the Big Bend area and the fort to the north, about 100 miles, was abandoned in 1891. Fort Davis was home to the famous Buffalo Soldiers; these were the members of the Ninth U. S. Cavalry; an all black soldiering unit formed just after the Civil war ended. As the Indian Wars ceased and our westward expansion was by more northerly trails the fort lays deserted for almost 100 years before historic restoration began.
The geologic history is both amazing and vividly on display. There is obvious volcanism, stark inselberg towers (we may call then monadnocks), tuff layers, pyroclastic flows, dikes, and sedimentary layers; all laid out for the observer.
Because we (most of us anyway) cannot look at a scene and know its geologic history we are fortunate that the park service and others have provided a great many interpretive devices for us.
The mountain building that now forms only the base rock of the region occurred about 550 million years ago. This happened when the South American tectonic plate moved in on the North American plate and immense pressure squeezed up the edges of the plates, forming a mountain range from the uplifted edges of the plates. There was later a great period, 60 million years or so, where the US east of the Rockies and west of the Appalachians was under a great sea. Vast beds of ocean debris, largely calcium carbonate stuff, formed during this period and now makes up a goodly percentage of the rock in this part of Texas. The center of the US was covered in sea from about 120 million years about until about 65 million years ago.