Wednesday, September 14, 2016

Geology of Natural Bridges, Calaveras County

In Calaveras County, on Parrot's Ferry Road between the towns of Columbia and Angel's Camp, is an impressive geologic formation, a set of "natural bridges" over Coyote Creek that are more like the area's caverns than they are like other natural bridges or arches of the California coast or desert southwest.  They are a little taste of the Mother Lode's karst topography - something not very typical of California.

Karst caverns form when acidic water erodes underlying rock composed of limestone or marble.  Limestone is a sedimentary rock composed of calcium carbonate fossils deposited at the bottom of a body of water, which over time solidifies into rock; marble is the metamorphic rock limestone is transformed into after undergoing the intense heat and pressure of natural geologic processes, such as subduction, that is great enough to recrystallize the rock.  As the softer rock is eroded, hidden cavern systems form below ground.  At times, roofs of these caverns collapse, revealing the structure beneath.  
Upper entrance to the lower cavern.

In the Mother Lode region of California, along the western edge of the Sierra Nevada, is a suite of rocks known as the Calaveras Complex.  The marble in this complex can be seen throughout this area (the town of Columbia has some great examples) and was formed long ago, something like 200 or 300 million years ago, when the oceanic plate subducted underneath the North American plate and ancient sea floor sediments were deposited and compressed against the edge of the continent.  Erosion and uplift have served to expose the marble we can see today.  
Deep pool at the downstream end of the second cavern.  The ceiling and walls are beautifully adorned. 
But note the spray painted letters at the left of the photo.  Please don't vandalize our precious natural places.
So, what does all of that have to do with Natural Bridges?  They are related to other karst formations in the sense that they are underlain by marble; however, from the bit of researching I did prior to our recent trip to Natural Bridges, these "caves" were formed a bit differently.

There are a couple slightly different and slightly vague explanations I read about regarding the formation of Natural Bridges. 

Downstream entrance to the main cavern.  Note the spring that still flows into the cave through the rocks.
But the consensus among geologists seems to be that they were formed by calcium carbonate-rich springs emerging from the side of the ravine that contains Coyote Creek.  The springs flowed from the walls of the ravine, and as the water evaporated, minerals were left behind.  These calcite deposits (or travertine) eventually built up to the point that they spanned the ravine and temporarily blocked the flow of the creek.  The water eroded a path through the bottom of the deposits, eventually carving a creek bed into the underlying marble. 

In Scenes of Wonder and Curiosity in California, an 1862 account of California by James M. Hutchings, an early promoter of Yosemite who came to California during the Gold Rush, Hutchings writes of Natural Bridges:

"The entire rock formation of the vicinity is limestone, and various are the conjectures relative to the first formation of these natural bridges or tunnels. Some believing them to have been formed by the rocky deposit contained in, and precipitated by, the water of countless springs, issuing from the banks of the creek, that, gradually accumulating and projecting, at length united the two sides, forming these great arched passages."
 
So, the idea that these caverns were formed by mineral deposits from natural springs has apparently been around for a while.
Lower (downstream) mouth of the main cavern, or "natural bridge"
The first bridge the hiker discovers after descending about one mile down a well-used trail is the main cave, the larger and more popular of the two.  The lower entrance is broad and low, the water just inside quite shallow.  But not far into the cave, the rocky creek bottom drops off into a deep pool.  (While swimming in the cave in September, after a long dry summer and several years of drought, I estimated the depth to be about seven feet, possibly more.)  To reach the upper entrance of the main cave, one can scramble up over the top and back down into the ravine and stay dry, or brave the frigid water and swim!  I prefer to swim, as long as it's summertime!  Inside the cavern are beautiful cave formations decorating the walls and ceiling, created as mineral-rich water has deposited calcite in a variety of shapes.  Unfortunately, many formations within easy reach have been vandalized in these caves.
Upper entrance to the second or lower cavern; note the especially deep channel to the right.
A second bridge or cave can be found downstream, though the trail is not well-defined and getting there may involve getting your feet wet.  This cave is smaller than the first, but contains a wealth of beautiful formations inside.  In the warm months, it is possible to wade in very shallow water far into the cave; but a deep pool (again, probably at least seven feet deep) blocks the lower entrance to the cave.
View downstream, looking out of the lower end of the second cave.
It's important to mention... please, please respect these marvelous places if you visit.  Pick up your trash (or someone else's), leave the cave formations as you found them, and don't write on the walls!


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