Natural Bridges California State Park is a day use park only. There is not much parking along the one-lane road but there are pit toilets. The bridges are about ¾ of a mile down the trail. The first one has picnic benches and is busy on hot days. This cache should be considered dangerous during spring runoff and not attempted following rains or when there is the potential for flash floods
The bridges of Natural Bridges California State Park formed in a marble layer in the Calaveras Formation. The rocks of this formation were deposited in a shallow sea off the west coast of North America during the Carboniferous and Permean (though its age is not well defined so there are a variety of age estimates close to this range.). At this time there was a subduction zone along the coast, so the ocean was subducted under the continent, these rocks were transported across the sea to be attached to the side of the continent. They later got metamorphosized during the late Jurassic. Following a couple of mountain building and erosional cycles, few rocks remain in the Sierra Nevadas from the same time period as the Calvareas Formation. Most are in the western foothills of the Sierra in a band called the Western Metamorphic Belt or as isolated pockets scattered throughout the mountain range referred to as Roof Pendants (See Sequoia Roof Pendant and The Dinkey Creek Roof Pendant ).
The bridges are very recent features that formed through the dissolution of the marble by slightly acidic precipitation. As rain falls through the air, it reacts with carbon dioxide along with sulfur oxides, nitrogen, and other natural compounds to form weak acids. As these acids percolated through carbonate rocks, the water slowly dissolves the stone. Weak points along bedding planes or fractures dissolve fastest forming cracks. Once the ground water dissolves the cracks to a critical width, the flow of water becomes turbulent increasing the rate that the marble is dissolved. Even more water is then channeled though these widened cracks eventually creating the caverns.
Here, the one of the weak points must have gone under a more solid area allowing the creek to tunnel its way under the stronger material. Continued chemical erosion lowered the floor of the tunnel leaving the roof out of the flow of the creek.
Even so, some water (snow melt and rain water) percolates down vertically through the marble of the roof . As this water flows across the cave walls and drops from ceilings a variety of speleothems (dripstone features) began to form. The water that percolated through the marble contains a small amount of dissolved carbonate from being slightly acidic (see the description of that above). If the water drips from the roof, walls, cracks, and floor slowly enough some of the water evaporates and redeposits some carbonate. Depending upon how the water falls determines the type of speleothem that is made.
Logging requirements:
Send me a note with :
- The text "GC3RY4Q Natural Bridges California State Park" on the first line
- The number of people in your group (put in the log as well).
- On the left side of the entrance about 20 feet into the tunnel determine if the water flow from the roof and walls is eroding a larger conduit or building up a speleothem?
- Find another location that is doing the opposite and describe where that is in the tunnel.
- Is possible to go through the tunnel without getting soaked?
The following sources were used to generate this cache:
- http://www.mindat.org/loc-16293.html
- Hill, Mary. 2006. Geology of the Sierra Nevada. University of California Press. Berkeley.
- Geologic units in Calaveras county, California. USGS. http://mrdata.usgs.gov/geology/state/fips-unit.php?code=f06009 Page Last modified: 10:03 on 08-May-2012