The Grand Canyon is a paradise for geologists, or scientists who study the history and shape of Earth.
One way they do that is by examining rocks. They can read rocks like a history book. Each part tells a story.
Erosion has exposed rock layers that go a mile up the canyon walls. Strata are layers of rock. Each layer formed in a different time period. Every time, the ocean moved in, covered the land, and then moved out.
