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 :
- The text "GCZ5Y7 Arch Rock – Joshua Tree NP" on the first
line
- The number of people in your group.
- 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,
http://3dparks.wr.usgs.gov/joshuatree/index2.htm, Last modified Mon
3/18/2003
Placement approved by the Joshua Tree National Park
