DEVILS POSTPILE NATIONAL MONUMENT
FIELD TRIP STOP – CLASSIC EXAMPLE OF COLUMNAR JOINTING
LOCATION: Devils Postpile N.M. is located near the eastern border of the central Sierra Nevada Mountains of California. The road leading to the Monument (SR 203) is off of Route 395 between Bridgeport and Bishop, CA. Drive 10 miles westward along 203 to the Mammoth Mountain Ski area where you must board a shuttle for the 8 mile trip to the Monument. (Under certain circumstances, accommodations can be made for those with disabilities. Check the Monuments website.) The monument is closed usually between October and June due to snowpack.
GEOLOGIC FEATURES: Columnar Jointing; Lava Flow; Talus Slope; Glacial Striations
DESCRIPTION: Devils Postpile is one of the finest examples of Columnar Jointing found anywhere.
It is now thought that lava (basaltic) rose along the eastern border of the Sierra Nevada Mountains in the Upper Cenozoic, about 80-100,000 years ago (earlier than previous estimates of 630,000 years) and as such is not directly related to the formation of the Sierra Nevadas.
Horizontal shrinkage in the homogeneous lava caused vertical jointing due to contraction of the cooling lava leading to the formation of columns. Columns are 2-3 ft in diameter, 50-60 ft long, and commonly 6-sided (though 4-7 sides are not uncommon).
The top of the flow had been abraded by Upper Pleistocene glacial activity (Wisconsin Advance ending about 11,000 years ago). Evidence of this is the presence of glacial striations (polished grooves showing direction of glacial movement). The same glacial advance is thought to have exposed the wall of columns seen at present.
Many columns have weathered away and fallen or have been released by earthquakes to form a talus slope.
STUDENT QUESTIONS:
(1) How are modern day mudcracks related to the formation of columnar jointing observed at Devils Postpile?
(2) Mineral crystals that compose the lava are not visible to the unaided eye. Why?
(3) CHALLENGE: How are Glacial Striations similar to Slickensides? How might one be able to give the actual direction of glacial movement from the striations left by the glacier?
SELECTED REFERENCES:
-National Park Service. Geology at Devils Postpile National Monument. Accessed on Aug. 4, 2023 from https://www.nps.gov/depo/learn/nature/geology.htm
-Harris, David V. and Eugene P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments. John Wiley and Sons, New York, 464 pp.
PHOTOS:
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Figure 1 - Devils Postpile showing vertical columns and talus slope at its base. Columns have fallen due to weatherng and earthquake activity.
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Figure 2 - Devils Postpile.
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Figure 3 - Note that not all columns are vertical. On the left side of the photo, columns are nearly horizontal. It is thought that columns form perpendicular to the cooling surface; so, if a cooling surface is vertical, horizontal columns will form.
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Figure 4 - Fine-grained (apanitic) texture with crystals not visible to the unaided eye.
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Figure 5 - Close-up of Talus Slope with polygonal columns.
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Figure 6 - Photo of Glacial Striations found atop the polygonal columns. Photo is taken from the Devils Postpile National Park Service brochure (updated 2014) provided at the Park.