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Devils Postpile National Park Geologic Story

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Devils Postpile view
Devils Postpile view. Photo by Wymond W. Eckhardt, NPS.
Origin of the Sierra Nevada
spacer image About 150 million years ago, in the Jurassic period of geologic time, the sediments that had been deposited in the sea and later compressed into sedimentary rocks began to be lifted above sea level. Then began a time of explosive volcanic eruptions that spewed forth vast quantities of lava and volcanic ash, deeply burying the sedimentary rocks. The layered sedimentary and volcanic rocks were then deformed and, in many places, folded intricately, changing or metamorphosing them into tougher, very resistant metamorphic rocks. Examples of these metamorphic rocks can be seen at Minaret Summit and along the road descending to Agnew Meadow.
Layers of sedimentary and volcanic deposits were compressed into rocks
Layers of sedimentary and volcanic deposits were compressed into rocks. These rock layers were then strongly squeezed and folded. Erosion caused more resistant layers to stand out as ridges. Streams carved valleys in softer layers.

spacer image Concurrent with the folding and deformation of the layered rocks, molten rock bodies were intruded into these rocks under conditions of great heat and high pressure deep below the Earth’s surface; some of this molten rock, or magma, reached the surface to form new volcanoes. This intrusive activity ended about 80 million years ago, in the Cretaceous period. The molten rock cooled slowly and in time solidified into the granite core of the Sierra Nevada.
spacer image The episode of deformation, intrusion, and volcanic eruption ended in the formation of a mountain range that can be considered an ancestral Sierra. Erosion followed, and by the end of Cretaceous time, about 60 million years ago, most of the older volcanic and metamorphic rocks had been worn away, exposing the granite core of the range. The area had a low relief compared to the mountains of today and posed little obstacle to streams draining westward from the interior of the continent.

San Joaquin River Drainage
San Joaquin River Drainage - More resistant rocks caused the river to flow south through the monument before turning west toward the Great Valley.

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This site is a cooperative endeavor of the
US Geological Survey Western Region Geologic Mapping Team
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This page was last updated on 9/7/00


Material in this site is adapted from a pamphlet, Devils Postpile Story, by N. King Huber, USGS, and Wymond W. Eckhardt, NPS. It is published by Sequoia Natural History Association, Sequoia Natural History Association, HCR-89, PO Box 10, Three Rivers, CA 93271-9792, Telephone (559) 565-3759