Geology

There are many canyons formed by the world's rivers, but there's only one Grand Canyon. Carved by the mighty Colorado River over the course of 17 million years, the canyon is one of the seven wonders of the natural world -- more than a mile deep, 277 miles long, and up to 18 miles wide. The Colorado River you see today is relatively placid by historic standards, tamed by a series of dams erected in the past century, especially the Glen Canyon Dam that forms Lake Powell. For millennia, however, this raging, sediment-rich river scraped its way through 1.5 billion years worth of geological history. Frequent flooding and a steep grade helped the river cut deeply into the earth, while the arid climate helped prevent the formation of a broad river valley. The result is the Grand Canyon's distinctively stark landscape of naked, eroded rock. The canyon contains almost 40 different rock layers, which scientists have been studying for more than 150 years. Of course, that's a fleeting moment in time compared to the age of the rocks themselves. The oldest rocks in the canyon are the two-billion-year-old Vishnu Schist, found at the bottom of the canyon's inner gorge, while the youngest is the Kaibab limestone on the rim -- a mere 230 million years old.