Big Bend National Monument: Hard and Historical
Lying in a great, down-dropped trough, the park features the typical basin and range topography of low flat basins surrounding sharply uplifted, isolated mountain ranges. The landscape is split by faults that have allowed some chunks to drop to form basins while adjacent land has risen to form eroded mountain ranges, all caused by the ongoing thrusts, stretches, uplifts, and drops that dominate the geology all the way from Mexico's Sierra Madre to the Sierra Nevadas, Cascades, and Rocky Mountains. The Rio Grande, the only major river system in a great expanse of New Mexico and Texas, runs down the bottom of one of the rifts caused by that settling between fault systems and its path defines the heart of the Chihuahuan Desert.
The oldest rocks in Big Bend National Monument date back some 500 million years to the Paleozoic era and consist of limestone and sandstone deposited in the bottom of what was then a vast, shallow, warm ocean or inland sea. At that time, almost no plants or animals had colonized the land so all the fossils bear witness to life's origins in the oceans. The billions of years of Earth's history recorded in the rocks before this period were swallowed up by uplift and erosion in this area and throughout most of the Chihuahuan Desert. Some 500 million years ago, the surface of Earth underwent a prolonged period of thrusting, buckling, and uplift, causing the creation of great mountain chains followed by a long period of erosion. This removed most of those older limestones and sandstones, leaving only telltale traces from their fossil clues left behind.
During most of the Mesozoic period between 240 million and 63 million years ago, the region was uplifted high above sea level. At this time, most of the continents were joined into a single supercontinent, which provided the warm, reliable conditions in which the dinosaurs emerged to dominate life on land. Dinosaur tracks and fossils bear witness to their presence here, including fossils of pterodactyls with 50-foot (12-m) wingspans, the largest creatures ever to take to the air. The rocks in Big Bend demonstrate that for a period in the Cretaceous a shallow sea engulfed much of what became North America, leaving telltale layers of limestone. That sea eventually receded as the continents rose and the addition of land to the west cut off the region from the oceans.
The current landscape began to take place in the Cenozoic period, the Age of Mammals, which started some 65 million years ago as the dinosaurs died out and shifts in the Earth's crust caused the breakup of the supercontinent. The rise of the Rocky Mountains and the Sierra Madre and the emergence of the Basin and Range Province some 30 million years later established both the current drainage including the Rio Grande and the rain shadows that would eventually produce North America's deserts. The Rio Grande originates in the Rocky Mountains and runs to the Gulf of Mexico, mostly meandering along a broad plain, but sometimes cutting deep gorges.