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Devils Postpile National Park Geologic Story

Renewed Uplift and Stream: Incision

About 25 million years ago, this lowland area began to be uplifted and tilted toward the southwest, a construction which would eventually lead to the present Sierra Nevada. As the rate and amount of southwest tilt increased, the gradients of streams flowing southwest also increased and the faster flowing streams cut deeper and deeper canyons. At that time the steep eastern Sierra Nevada escarpment had not yet formed. The main trunk of the San Joaquin River then had its headwaters perhaps as far east as Nevada, flowing in a channel that crossed what is now the Sierran crest north of Minaret Summit. Within the growing mountain range the specific course of the river was controlled by differing hardness of the bedrock as well as by the tilt of the range. For example, the highly resistant metamorphic rocks of the Ritter Range proved a great obstacle to the southwestward flow of the river and forced it to flow southeastward and then south through the Devils Postpile area. South of the Postpile the river finally was able to swing southwest across the less resistant granite at the south end of the Ritter Range.

San Joaquin River Drainage
The Sierra Nevada rose and tilted along a long normal fault that follows the western edge of the Owens Valley.

About 3 million years ago the river channel north of Minaret Summit was filled by lava flows- reddish- colored, layered rocks visible on the slope above the road descending to Agnew Meadows. This blocked the former channel and isolated the present San Joaquin drainage basin from the area east of the crest. Also about this time the Long Valley-Mono Lake area began to lag behind as the present range continued to rise and the growth of the east-facing Sierra Nevada escarpment further blocked any renewal of a trans-Sierra San Joaquin River. The Sierra Nevada continues to rise today.

During the Pleistocene epoch, or ice age, glaciers, fed by heavy snowpacks, descended slowly through the old stream-cut canyons, deepening and widening them. Ice accumulated on the east slope of the Ritter Range and flowed down the valley of the Middle Fork of the San Joaquin River several times, shaping the valley to near its present form. These glaciers, carving cirques back into the highly jointed, but resistant, rocks of the Ritter Range sculptured the magnificent Minarets and Matterhorn-like peaks of the range crest.

Continue to Rocks of Devils Postpile Formed, See the Contents or Return to the Beginning

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.


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