Patterns often reveal forces otherwise invisible to us. As you cross the bridge, look to your right for a small light-colored rock embedded in the dark basalt rock. This is a xenolith (zee-no-lith), a rock fragment foreign to the body of rock in which it occurs. When magma rose to the surface, it brought up pieces of limestone from 700 to 1,000 feet below us.

Uplift and erosion have exposed this rock (known as Kaibab limestone) elsewhere: at the rims of Grand Canyon and nearby Walnut Canyon and in cliffs at Wupatki National Monument.

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Meghann M. Vance, Northern Arizona University Anthropology Laboratories

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