Nugget 12: How Did These Mountains Get Striped?

On both sides of the Northeast Entrance Road in Yellowstone, you can see mountains that look like a slice of layer cake. They are geological evidence of some of the processes that have shaped the area over the past 350 million years. In that long-ago era, a quiet sea covered everything here. Remains of tiny marine organisms accumulated at the bottom of undisturbed water. In time, and as additional sediments pressed down from above, these remains became cemented together to form limestones.

Abiathar Peak (pronounced uh-BI-uh-ther) of the Absaroka Range (left)

More recently, around 50 million years ago, volcanoes erupted repeatedly near here, sending layers of lava across the landscape and covering the older rocks. What we see today are some of the flows but also a lot of debris from giant mud slides that carried down the volcanic sands, gravels, and boulders together to create these mixtures, called conglomerates. Now, as we look at these mountains of the Absaroka Range, erosion has exposed the edges of the various colored layers.

Pyramid Mountain (right) has quaking aspens at its base.

CREDITS: The text and both photos on this page are by Bruno Giletti.

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 Learn much more about the park's varied geological history in "The Stories in Yellowstone's Rocks," pages 294–307.

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