Geologic Trip, Mammoth Area
The Devils Postpile is the most spectacular and best-known example of columnar jointing in California. The postpile is exposed on the south side of the steep river valley formed by the Middle Fork of the San Joaquin River, about three miles west of Mammoth Mountain. The basalt that forms the postpile came from nearby vents on the valley floor about 100,000 years ago, during the Tahoe glacial episode. After leaving the vents, the lava flowed down the river valley about 2.5 miles. At that point it encountered an obstruction, perhaps a glacial moraine, and the lava ponded in back of the obstruction until it reached a thickness of 400 feet - much thicker than most basalt flows. The thick mass of lava then cooled very slowly and formed the columns as contraction joints while cooling. The Middle Fork of the San Joaquin River then cut through and removed much of the lava flow, leaving these large columns along the south side of the valley. A short path leads to the top of the Devils Postpile where the glacier polished the top of the basalt columns.
This photograph shows the top of the Devils Postpile lava flow. The tops of the basalt columns have been polished and striated by the Pleistocene glaciers that flowed down the valley of the Middle Fork of the San Joaquin River. The striations indicate the direction of movement of the glacier.
Geologic Background: Mammoth Area
Geologic Trips: Devils Postpile
NPS: Devils Postpile NM
1. As the lava cooled, it shrank and small contraction cracks began to form.
2. After the cracks extended about ten inches, they branched to form an angle of about 120º. This provided the maximum stress relief.
3. After each new crack extended another ten inches, it branched again, forming six-sided columns.