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The mountain’s namesake granite was used to build Seattle’s iconic Smith Tower and is coveted by climbers. But its peak is made of a much older rock.
There is Index granite all over the Skykomish Valley. From the walls of the Index quarry – known by rock climbers all over the world for its grippy quality – to the towering peak of Mount Index, signs of the granite are everywhere you look in the valley.
However, the top third of Mount Index itself isn’t granite at all, but a much older and more mysterious igneous rock that formed more than 140 million years ago at the bottom of the ocean.
Nick Zentner travels all over the valley taking a closer look at the famous granite.
Support for Nick on the Rocks is provided by Pacific Science Center.
