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USGS Geology in the Parks

Devils Postpile National Park Geologic Story

Rainbow Falls

Rainbow Falls
Rainbow Falls- Middle Fork San Joaquin River, 101 feet high.

At Rainbow Falls the Middle Fork of the San Joaquin River drops 101 feet over a cliff of volcanic rock that is somewhat different from the rock making up the Devils Postpile. Although the rock is locally massive or poorly jointed, as that near the top of the falls, it is generally characterized by nearly horizontal, thinly-spaced joints giving the rock a platy appearance. This rock is the rhyodacite of Rainbow Falls. While there is glacial polish on the top of the massive volcanic rock and the cliff at the falls is similar in appearance to glacially sculptured lee slopes, the cliff was not formed by ice.

After the last glacier melted, the Middle Fork of the San Joaquin River flowed downstream from the Devils Postpile in a channel close to The Buttresses and about 1,500 feet west of its present channel. The old channel can still be traced northward from its junction with the present river at the first bend downstream from Rainbow Falls. Another old channel is nearby, but it is not cut as deeply and, therefore, was probably not occupied as long.

When the Middle Fork flowed in these older channels, it cut through the lava flows down to granite, leaving a cliff for its eastern bank. Then, some distance upstream, its waters were diverted eastward. The river left its bed to follow its present path until it returned to its old channel by cascading down the cliff it had cut earlier, thus forming the predecessor of today's Rainbow Falls but more than 500 feet west of the present falls.

As the Middle Fork cascaded over its former bank, its water eroded into the softer platy rock at the base and undercut the harder massive rock above, causing it to cave in. The cliff has thus retreated 500 feet or more upstream to the present location of Rainbow Falls. Normally, streams tend to erode channels with uniform gradients, bevelling off and eventually eliminating irregularities such as waterfalls. Rainbow Falls is an exception, however, because of the two, unequally hard, horizontal rock layers in its cliff. It is like Niagara Falls in this respect, for Niagara has a similarly hard rock layer at the top of its cliff and is being undercut in the same fashion.

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Material in this site is adapted from a pamphlet, Devils Postpile Story, by N. King Huber, USGS, and Wymond W. Eckhardt, NPS. It is published by Sequoia Natural History Association, Sequoia Natural History Association, HCR-89, PO Box 10, Three Rivers, CA 93271-9792, Telephone (559) 565-3759.

 

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