Devils Postpile view. Photo by Wymond W. Eckhardt, NPS.
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. 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.
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.
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.