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. One example is Abiathar Peak (pronounced uh-BI-uh-ther) of the Absaroka Range. 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.

More recently, around 45 million years ago, volcanoes erupted repeatedly near here, sending material called tephra across the landscape and covering the older rocks. (See how this fits into the Geological Time Line.) The Absaroka volcanoes were very high with steep sides, covered with snow and glaciers. Erupting material mixed with snow and ice to form debris and mudflows that filled valleys. Interlayered with the mudflows were air-fall tephra deposits and pyroclastic flows, which are hot gases mixed with tephra that roared down the volcanoes’ sides with incredible speed.

All of these materials formed different colored layers. Now, as we look at these mountains of the Absaroka Range, erosion has exposed the edges of the various colored layers.

CREDITS: The photo of Abiathar Peak taken by Niklas Dellby in July 2003 can be seen on page 202 of Yellowstone Treasures, updated fifth edition. The photo of Pyramid Peak is by Bruno Giletti.

IN THE GUIDEBOOK: Learn much more about the park’s varied geological history in “The Stories in Yellowstone’s Rocks,” written by geologists Bruno Giletti and Jo-Ann Sherwin, pages 302–18 of Yellowstone Treasures.

Copyright Janet Chapple. All Rights Reserved. Revised May 1, 2018.