The Colorado River in Grand Canyon National Park has cut a vertical path through rock that dates from the Precambrian period (nearly two billion years ago) to the Permian (250 million years ago). As presented in this video segment adapted from NOVA, each of the exposed rock layers, or strata, tells the story of another time period. Through careful study of these layers, scientists can describe how the area's climate has changed, how this affected the environment, and how some of the previous inhabitants looked and behaved.