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Arch Rock – Joshua Tree NP EarthCache

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Geocache Description:

This natural arch is located along a well posted geologic nature trail that starts in the White Tank Campground. The trail ranges from loose sand to hard rock with steps. There are great signs posted along the trail with additional geologic information about the area.

This natural arch formed in the White Tank Granite. This igneous rock is thought to have pushed its way up into the surrounding Pinto Gneiss between 135 to 155 million years ago creating a large pluton (see other earthcache). This pluton cooled slowly allowing the relatively large crystals of quartz, feldspar, and biotie to form.

Through a variety of erosional processes, an arch was formed in the granite. Jointing (see Joints of the Wonderland of Rocks) created lines of weakness in the massive granite pluton. Water dissolved the rock surface and seeped into small cracks where it froze and expanded causing the small cracks to get larger. By chance, the result of the erosion created the arch.

Some of the other features that the park service marked with signs include the granite mineralogy, the contact between the Pinto Gneiss and White Tank Granite, the formation of the unique rock piles of the area, evidence of soil erosion, prominent dikes, and cavernous weathering.

Logging requirements:
Send me a note with :

  1. The text "GCZ5Y7 Arch Rock – Joshua Tree NP" on the first line
  2. The number of people in your group.
  3. Examine the general direction of the cracks (joints) in the rock. Send me a note describing the relationship between the direction of the cracks and the shape of the arch.
The above information was compiled from the following sources:

  • USGS, Joshua Tree National Park: A Photographic Tour Featuring Park Geology,, Last modified Mon 3/18/2003

Placement approved by the Joshua Tree National Park

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