The ancestral river that carved Santa Elena, Mariscal, and Boquillas Canyons through bedrock was the Rio Conchos. (The present Rio Conchos contributes most of the water flowing through today’s Big Bend. It flows into the Rio Grande just upstream of the park.) When the ancestral river hit the limestone mountain uplifts, it had no alternative but to cut its way through. Steep-walled, narrow canyons resulted. Santa Elena Canyon is cut through the Mesa de Anguila. Mariscal Canyon severs its namesake mountains. Boquillas Canyon, the longest, cuts through the massive limestone Sierra del Carmen. You can see how steep these canyons are by taking a river trip (see page 122) or hiking park trails (see page 119) to the river or to canyon rims. The region was once much higher in elevation than it is today, but erosion has taken its toll. Mountains and mesas are landscape formations whose rock erodes more slowly than surrounding materials do. Castle-like peaks and high, sharp-rimmed mesas stand as weathered monuments to earlier times when elevations were higher. Such stranded vestiges of geologic eras punctuate the stark Chihuahuan Desert landscape with eerie architecture. Astronauts have used Big Bend terrain to simulate moonscapes. The sequence of geologic diagrams shows how the Big Bend canyons formed and what their future would be if slow processes of erosion continue. [Illustration: ① Faulting uplifts bedrock to form the mountain mass.] [Illustration: ② Streams erode the mountains and begin to deposit sediments in the valley.] [Illustration: ③ Streams continue depositing valley sediments eroded from the mountain mass.] [Illustration: ④ Streams have now cut clear through the sharply eroded mountains and formed steep-walled canyons.] [Illustration: ⑤ All but isolated mesa remnants of the mountains have eroded and weathered away. The riverbed rests in the deep layer of sediments.] [Illustration: ⑥ In a future stage the bedrock mountain uplift might be entirely eroded away to become a deep layer of sediments.] When you think that the Earth is perhaps 4.5 billion years old, that complex organisms have existed for no more than about 700 million years, that the oldest rocks exposed in the park are 300 million years old, and that fossils of backboned animals in the park cover a time span of 70 million years, you can realize how fragmentary the fossil record really is. You get few glimpses of the relatively recent past, but these are astonishing.