Summary of Glacial and Volcanic History
The intertwining of glacial and volcanic events in the valley of the Middle Fork of the San Joaquin River is reasonably clear, but some events must be based upon inference derived from data obtained elsewhere in the Sierra Nevada. From our present knowledge we can summarize the glacial and volcanic history of the Devils Postpile region as follows (listed in order of decreasing age):
- The oldest volcanic rock in the vicinity of the Monument is believed to be the basalt now exposed at The Buttresses, although its exact age is not known.
- The valley of the Middle Fork of the San Joaquin River was glaciated, probably more than once, prior to about 760,000 years ago.
- Deposition of the tuff near Reds Meadow about 760,000 years ago was concurrent with extensive volcanic activity that created the Long Valley caldera and produced the Bishop Tuff.
- Stream erosion, perhaps aided by another glaciation, removed much of the tuff from the central part of the valley.
- Rhyodacite lava erupted from vents just downstream from the present site of the Postpile and flowed southward beyond Rainbow Falls.
- Andesite lava erupted near Mammoth Pass and cascaded into the Middle Fork valley.
- Less than 100,000 years ago basalt now exposed at the Devils Postpile erupted north of Pumice Flat and flowed out to cover, at the very least, several square miles of the valley floor.
- Stream and glacial erosion again removed much of the accumulated volcanic rock from the Middle Fork valley. The last glaciation, which ended about 10,000 years ago, produced the Polish and striations visible on the top of the Devils Postpile.
- The Red Cones and their associated lava flow were formed sometime after the last glacier vanished from the valley.
- Pumice erupted from the Inyo and Mono Craters and covers the area as a surface deposit.