This footage was filmed from an airplane flying over the Grand Canyon in Arizona. The Colorado River, seen at the bottom, has formed this magnificent canyon by gradual erosion over the past nine million years. Changes in the course of the river have led to specific land formations such as the bluffs seen to the left. The layers, or strata, of sedimentary rocks are clearly visible, with the oldest at the bottom. By analyzing fossils from each stratum, we have learned much about changes in the diversity of life forms over time. The idea that we could attribute the formation of various land forms, such as canyons, to gradual mechanisms that still operate today, such as erosion by rivers, was introduced by geologists James Hutton and Charles Lyell in the late 18th and early 19th centuries. Credit: Courtesy of U.S. Geological Survey.