Along the picturesque Middle Fork of the San Joaquin River at 7,600 feet on the western slopes of the Sierra Nevada lies Devils Postpile National Monument. The 800-acre monument near the resort community of Mammoth Lakes was established in 1911 to preserve two natural features:
- the formation of columnar basalt known as Devils Postpile, and
- 101-foot Rainbow Falls
A Formation of Columnar Basalt
As basalt lava erupts from volcanic vents and cools, it shrinks and cracks. Sometimes vertical columns form. Well developed columns result from homogeneous lava cooling at a uniform rate. At Devils Postpile the rock columns have from three to seven sides. The top of the postpile shows the columns' geometry and the polishing and scratch marks of glacial ice. The postpile's sheer wall face is 60 feet high.
How it Happened
A volcanic event. Formation of Devils Postpile began when basalt lava erupted in the valley of the Middle Fork of the San Joaquin River. As lava flowed from the vent, it filled the valley near the postpile to a depth of 400 feet. Radiometric dating of rocks thought to correspond with this basalt (a dark gray, fine grained rock with feldspar crystals) suggests an age of less than 100,000 years.
Surface cracks formed when tensions caused by the shrinkage of the cooling lava were greater than the lava's strength. Each crack branched when it reached a critical length. Together with other cracks it formed a pattern on the surfaces of the flow. Ideal conditions allowed surface cracks to deepen and form long post-like columns.
A glacial event. Some 10,000 years ago a glacier flowed down the Middle Fork of the San Joaquin River and overrode the fractured mass of lava. The moving ice quarried away one side of the postpile, exposing a sheer wall of columns 60 feet high. Many fallen columns lie fragmented on the talus slope below.
A hike to the top of the postpile reveals a cross section of the columns. The glacially polished column tops, looking like floor tiles, show parallel striations where rocks frozen into glacial ice scraped across them.
At Rainbow Falls the Middle Fork of the San Joaquin River drops 101 feet over a cliff of the volcanic lavas andesite and rhyodacite. It is thought that after the last glacier melted, the river flowed downstream from Devils Postpile in channels about 1,500 feet west of its present course. Flowing in these older channels, it cut through the lava to granite, leaving a cliff of rhyodacite for its eastern bank. Then, some distance upstream, the waters were diverted eastward. The river left its bed to follow its present path until it returned to the old channel, by cascading down the cliff it had cut earlier. Thus Rainbow Falls was formed. A stairway and short trail lead to the bottom of the falls.
Soda Springs. Nearby mineral springs are evidence of recent local volcanic activity. The Soda Springs lie on a San Joaquin River gravel bar north of the postpile. Gases driven upward from hot areas deep in the Earth combine with groundwater to produce cold and highly carbonated mineralized springs. Iron in the water oxidizes on exposure to air and stains gravel a reddish brown.
The General park map handed out at the visitor center is available on the park's map webpage.For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.
A general photo album for this park can be found here. For information on other photo collections featuring National Park geology, please see the Image Sources page.
Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.
Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.
For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.
The NPS maintains a searchable data base of research needs that have been identified by parks.
A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.
NPS Geology and Soils PartnersAssociation of American State Geologists
Geological Society of America
Natural Resource Conservation Service - Soils
U.S. Geological Survey
Currently, we do not have a listing for any park-specific geology education programs or activities.
General information about the park's education and intrepretive programs is available on the park's education webpage.For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.