• Twin Sisters

    City Of Rocks

    National Reserve Idaho

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Spires along the Geological Interpretive Trail
Spires along the Geological Interpretive Trail
Photo by Kris hawkins

Did You Know?

Twin Sisters granite spires against a summer sky.

The Twin Sisters in City of Rocks National Reserve are made up of different rocks. The darker sister is made of granite that is 2.5 billion years old and is some of the oldest rock in the Lower 48 states. The lighter sister is made of granite from a younger formation that is 25 million years old.