During the Paleozoic Era, between 570 and 245 million years ago, the Big Bend area was covered by ocean, which has left layers of gravel, sand and clay. It wasn't until the end of this period, around 300 million years ago, that a collision between the continents of North America, South America and Africa caused the area of Big Bend to rise above sea level.
Early Cretaceous Period
During the early Cretaceous period, around 135 million years ago, the area was still covered by ocean and was home to tiny organisms like mollusks and ammonites. The remains of these organisms can be found among the sandstone and limestone deposits.
Late Cretaceous Period
The ocean levels receded throughout the Cretaceous period so that by the late Cretaceous period the area was a swampland with vegetation, clay and also dinosaur fossils such as the Texas Pterosaur.
During the Cenozoic period, the oceans receded to where they are now and mammals began to appear. Around 35 million years ago, several large volcanic eruptions left the Big Bend region with volcanic ash and other debris that can be seen in-between layers of sediment, around the Chisos Mountains
The landscape around Big Bend continues to be shaped today through rainfall, which sweeps sand and silt away to lower elevations and the impressive Rio Grande River, which continues to carve through the canyons around Big Bend.