The Colorado River is young by geologic standards — only five or six million years. But the channel it cuts through the Colorado Plateau exposes rocks that are much older. At the bottom of the Grand Canyon, river runners can spot ancient bedrock that formed nearly two billion years ago. In that vast span of time, the Earth experienced five mass extinctions. But the signs of those cataclysms are mostly hidden.
Take the Earth’s most famous extinction event: the asteroid that killed the dinosaurs. It struck on the Yucatán Peninsula in Mexico 66 million years ago and scattered debris all over the planet. Temperatures spiked and forests on the Colorado Plateau burst into flame. Dust blocked out the sun, leaving the world in darkness. When the dust settled, the world had changed. Ferns covered the Colorado Plateau where the forests used to be, and small mammals replaced the dinosaurs...