Devils Postpile National Monument

Along the picturesque Middle Fork of the San Joaquin River at 7,600 feet on the western slopes of theSierra Nevada lies Devils Postpile National Monument. The 800-acre monument near the resortcommunity of Mammoth Lakes was established in 1911 to preserve two natural features, the formation ofcolumnar basalt known as Devils Postpile and 101-foot Rainbow Falls. To see these features one mustwalk. Although pumice is the dominant rock-type encountered, basalt, andesite, rhyodacite, and granitecrop out in many places. Native plants and animals are typical of lodgepole pine and red fir forests.

While Devils Postpile ranks among the world's finest examples of columnar jointed basalt, it is notunique. Giant's Causeway in Ireland and Fingal's Cave in Scotland are similar formations.

How It Happened
Formation of Devils Postpile began when basalt lava erupted in the valley of the Middle Fork of the SanJoaquin 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 correlate with this basalt-a dark gray, fine-grained rock withfeldspar 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 thelava's strength. Each crack branched when it reached a critical length. Together with other cracks itformed a pattern on the surfaces of the flow. Ideal conditions allowed surface cracks to deepen and formlong post-like columns.

Some 10,000 years ago a glacier flowed down the Middle Fork of the San Joaquin River and overrode thefractured mass of lava. The moving ice quarried away one side of the postpile, exposing a sheer wall ofcolumns 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 columntops, looking like floor tiles, show parallel striations where rocks frozen into glacial ice scraped acrossthem.

Rainbow Falls
At Rainbow Falls the Middle Fork of the San Joaquin River drops 101 feet over a cliff of the volcaniclavas andesite and rhyodacite. It is thought that after the last glacier melted, the river flowed downstreamfrom Devils Postpile in channels about 1,500 feet west of its present course. Flowing in these olderchannels, it cut through the lava to granite, leaving a cliff of rhyodacite for its eastern bank. Then, somedistance upstream, the waters were diverted eastward. The river left its bed to follow its present path untilit returned to the old channel, by cascading down the cliff it had cut earlier. Thus Rainbow Falls wasformed. 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 SodaSprings lie on a San Joaquin River gravel bar north of the postpile. Gases driven upward from hot areasdeep 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.

Visiting the Monument
To reach the monument 10 miles west from U.S. 395 on S.R. 203 to Minaret Summit and then 7 miles onpaved narrow mountain road. It is closed in winter. Park rangers assist visitors, conduct interpretiveprograms, and enforce regulations.

Shuttle bus: From late June to early September, day-use visitors to the Devils Postpile RedsMeadow area must ride the shuttle bus between 7:30 a.m. and 5:30 p.m. Buy tickets and board buses atMammoth Mountain Inn.

Accommodations and services: Meals, lodging, groceries, gasoline, horses, and other facilitiesand services are available at Mammoth Lakes or nearby Reds Meadow.

Camping: A 21-site campground is maintained near the ranger station from about July 1 toOctober 15, depending on the weather.

There is hiking south on the Rainbow Falls Trail and west on King Creek Trail. Tripsmay be made north or south on the John Muir and Pacific Crest trails. The monument has several shortloop trails.

John Muir Trail : Named for conservationist John Muir, this 211-mile trail links YosemiteNational Park with Sequoia and Kings Canyon National Parks. It crosses Devils Postpile NationalMonument with access at Rainbow Falls Trailhead and the ranger station.

Fishing and Hunting
Fishing is permitted in the monument with a California angling license forpersons 16 years of age or older. Hunting is prohibited.

Pets are permitted only if kept under direct physical control at all times.

Bicycle riding is permitted on roads but prohibited on trails or crosscountry.

Safety & Park Regulations
Obtain the latest park regulations and safety tips from Monument personnel before your visit!

Published: 29 Apr 2002 | Last Updated: 13 Sep 2011
Details mentioned in this article were accurate at the time of publication


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