The Grand Canyon is a paradise for geologists, or scientists who study the history and structure of Earth.
One way geologists do this is by examining rocks, reading their colors and textures like a history book. Each level tells a different story.
Erosion has exposed a vertical mile of strata along the canyon walls. Strata are layers of rock. Each layer was formed in a different time period, in which the ocean moved in, covered the land, and then retreated.