Twin Rocks Viewpoint

The Twin Rocks is a classic feature that showcases the process of differential erosion.

NPS / Chris Roundtree

The balancing act of Twin Rocks is the result of a process called differential erosion. Simply put, different layers of rock are breaking down at different speeds. Differential erosion also produces the hoodoos of Bryce Canyon National Park and the goblins of Goblin Valley State Park. The orange bottom half of Twin Rocks is the Moenkopi Formation. The white top half is the Shinarump Member of the Chinle Formation. A landscape of rivers and streams in a tropical climate created the Chinle Formation, about 220 million years old. Its Shinarump Member comprises gravel-filled rock formed by those ancient rivers. It is much tougher than the clay and silt of the Moenkopi below. The Shinarump protects the Moenkopi like a hard hat, while the Moenkopi erodes faster from the outside. This relationship has produced a large boulder balanced on a small, shrinking pedestal. Twin Rocks will someday collapse and join the other white boulders on the ground below them.