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City of Rocks Geology - Twin Sisters

A cache by tgundy Send Message to Owner Message this owner
Hidden : 11/05/2007
1 out of 5
1 out of 5

Size: Size:   not chosen (not chosen)

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Geocache Description:

Part of the "City of Rocks Geology" series



These rocks are called the Twin Sisters.


Make your way to the coordinates listed above and face west to view the Twin Sisters. 

To get credit for this cache you must do the following:

1. Upload a photo taken from the listed coordinates of your GPSr

with the Twin Sisters sign and the Twin Sisters in the background.

2. Send me the answer to the question “What is the name of the younger pluton.”


The City of Rocks contains some of the oldest rock in North America. Some of the granite is 2.5 billion years old and other granite is 25 million years old. The Twin Sisters are not really sisters at all, but more like mother and daughter.  Can you guess which one is older?  The darker one on the left (south) is 2.5 billion years old and the lighter one on the right (north) is 25 million years old.  They cooled underground at different times but were both brought to the surface together through erosion and uplift.  The older rock is part of the Green Creek Complex which includes the granite and surrounding metamorphic rock. It is some of the oldest rock in the United States.  The younger granite is part of a pluton, which just means a large body of granite. You need to find the name of this pluton to get credit for the cache.  The Park Service is conducting a study on the erosion rates of rock climbers. So that they can compare climbing routes to non-climbing routes they have made Twin Sisters off-limits to climbing.  The Twin Sisters were a major landmark for the pioneers traveling the California Trail in the mid-1800s.


While you are here you might want to check out my other City of Rocks Geology caches.  Good sources of information for City of Rocks geology are the Visitor Center in Almo, the official park map, and climbing guides.


Additional Hints (No hints available.)

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