When did the canyon form?
The Shivwits Plateau, just to the north of the Grand Canyon, contains gravels deposited by rivers flowing from the southwest. Soon after they were deposited, the gravels were covered by a lava flow dated at six million years old. The present westerly flow of the Colorado River cuts across that ancient drainage. This means that the canyon cannot be older than six million years. Its minimum age is established by another lava flow, dated at 3.8 million years old, that followed a portion of the existing river channel.