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Read Ebook: Devils Postpile National Monument California [1984] by United States National Park Service
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Devils Postpile NATIONAL MONUMENT o CALIFORNIA
DEVILS POSTPILE NATIONAL MONUMENT
Along the picturesque Middle Fork of the San Joaquin River at 2,300 meters on the western slopes of the Sierra Nevada lies Devils Postpile National Monument. The 324-hectare monument near the resort community of Mammoth Lakes includes one dominant feature: a formation of columnar basalt known as Devils Postpile. Another important feature in the area is Rainbow Falls. Pumice is the dominant rock-type encountered, but basalt, andesite, rhyodacite, and granite lie just under the surface and crop out in many places. Plants and animals are typical of lodgepole pine and red fir forests. To see the features of the monument one must walk.
DEVILS POSTPILE
The 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 122 meters . Recent radiometric dating of rocks thought to correlate with basalt of Devils Postpile suggest 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 strength of the lava itself. Each crack branched when obtaining a critical length and together with other cracks formed a pattern on the surfaces of the flow. Ideal conditions allowed surface cracks to deepen, resulting in the formation of long columns.
Approximately 10,000 years ago, glaciers flowed down the Middle Fork of the San Joaquin River. This moving ice easily overrode the fractured mass of lava and quarried away one side of the Postpile, exposing a sheer wall of columns 18 meters high. Many fallen columns now lie fragmented on the talus slope below.
A hike to the top of the Postpile reveals not only a cross section of the posts, but the most interesting effect of the ice--polished tops of the basalt columns. Here, the column ends are exposed like a tiled floor and exhibit parallel striations where the glacier dragged rocks across them.
Even though Devils Postpile is among the world's finest examples of columnar-jointed basalt, it is not unique. Giant's Causeway in Ireland and Fingal's Cave in Scotland are similar formations.
SODA SPRINGS
Nearby mineral springs are evidence of recent local volcanic activity. The Soda Springs lie on a gravel bar in the San Joaquin River just north of the Postpile. Here, gases driven upward from hot areas deep in the earth combine with ground water to produce cold and highly carbonated mineralized springs. The iron present in Soda Springs water oxidizes on exposure to the atmosphere and stains river gravel a reddish brown.
SAFETY
For your safety, remember that there are hazards in the mountains that usually do not exist in the city. Stay on designated trails; these are the safest places to hike. Use caution when viewing features from near cliff or gorge edges. Footing can be hazardous. WARNING: Bears inhabit the monument. Proper food storage is required by Federal law.
RAINBOW FALLS
At Rainbow Falls the Middle Fork of the San Joaquin River drops 31 meters over an andesite and rhyodacite cliff. It is thought that after the last glacier melted, the river flowed downstream from Devils Postpile in channels about 457 meters west of its present course. Flowing in these older channels, it cut through the rhyodacite lava down 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 earlier cut. Thus Rainbow Falls was formed. A stairway and short trail lead to the bottom of the falls, where numerous flowers and grasses form an enchanting garden. Cars may be driven to within 1.6 kilometers of the falls via the road to Reds Meadow.
ABOUT YOUR VISIT
National Park Service U.S. DEPARTMENT OF THE INTERIOR
GPO 1984--421-613/317 Reprint 1984
Transcriber's Notes
--Silently corrected a few typos.
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