Texas
The winding Rio Grande marks the southern border of Big Bend National Park and the dividing line between the United States and Mexico.
Everything is bigger in Texas. So it should come as no surprise that Texas’s biggest national park—Big Bend, on the southern border with Mexico—is home to some of the most complicated geology in North America. The park includes both the Chisos Mountains and a portion of the Chihuahuan Desert, as well as a significant stretch of the Rio Grande. From space, the Rio Grande provides a convenient visual marker of the park’s southern boundary. The river also forms the international line between the United States and Mexico.
Between 500 and 300 million years ago, a deep ocean trough extended from what is now Oklahoma and Arkansas southward into Texas and Mexico. For longer than 200 million years, sediments collected in this trough, piling thousands of feet thick. Around 300 million years ago, these layers of shale and sandstone were compressed and uplifted into the ancestral Ouachita Mountains, the highly eroded roots of which are still present today in Big Bend National Park.
That story may seem simple enough, but 200 million years of sedimentation makes for some thick layers of rock. Several later episodes of uplift, volcanism, and erosion have resulted in an impressively convoluted and colorful geologic map. The sparse vegetation of the Chihuahuan Desert ecosystem makes a perfect backdrop for studying the many complex layers of strata displayed throughout the Big Bend region.
One of the most prominent features of Big Bend is the Rio Grande, which flows for 2000 miles from the southern Rocky Mountains in south central Colorado through New Mexico, into Texas, where it forms the boundary with Mexico for more than 1000 miles before emptying into the Gulf of Mexico. About a quarter of this international boundary flows through Big Bend National Park, including the sharp turn from the southeast to the northeast that gives the park its name.
The rocks of Big Bend may date back 500 million years, but the Rio Grande is much younger. In fact, it’s the youngest major river system in North America, having achieved its path to the Gulf of Mexico only within the past 2 million years. Once its route was established, the river served as a conduit for transporting sediments out of the highly eroded basins in Big Bend; it continues to carry one of the highest sediment loads of any major river.
The Rio Grande is still working out its winding route to the sea. Occasionally, the upstream water demands on the river for municipal, irrigation, and industrial uses combine with seasonal drought conditions to slow the flow of water to the point that the Rio Grande begins dumping its sediment load just shy of the Gulf of Mexico. The sediments then form a large sandbar at the mouth of the river and dam the water’s flow into the Gulf. Dredging can help restore movement, but more sustainable water use practices on both the U.S. and Mexican sides of the Rio Grande will be needed to reestablish a robust flow of the river to the Gulf.
Try to spot Big Bend on flights to San Antonio or El Paso, Texas.
