rocks

That first storm in October was our dress rehearsal for winter storms on the coast.  At least it came early in the season; gave us lessons on what we still need to do to be prepared; but left the community largely unharmed.  Since June we have been looking at smoke plumes, first from the east with the Indians Fire; then from the north with the Basin Complex Fire; and finally from the south with the Chalk Fire.  What a relief to walk down the road and see the puddles from the rain.  No complaining. 

 

Now that we aren’t looking for plumes, we’re looking for rocks….on the road!  While the highway crews are doing a phenomenal job keeping the debris off the Highway, there are still a few stray rocks that seem to like to fall between midnight and five in the morning.  But we want to tell you a different tale about rocks.

 

Earlier this year, before the fires, a group of geologists came to visit and survey Big Creek Reserve.  These graduate students were from the University of Texas, and were doing field work down the coast.  They were looking for the Franciscan Complex, which is a major component of the Pacific Coast Ranges of California, and part of the San Andreas Fault system.  One of these geologists, Elaine Bohls, spent a week mapping the various rock formations at the Reserve.  She will tell you she has been hooked on rocks since she was a kid.  Elaine says that you have to really examine rocks to get to know them.  She even licks them to see what they taste like.  Each type has its own unique taste.

 

Not too surprising, Elaine found three different major rock formations at the Reserve:  the Franciscan Complex, the Salinian Block, and the Big Creek Conglomerate.  All of these formations are faulted, folded and juxtaposed against each other because of all the faults in this area.  The Franciscan Complex is well known, and fairly new, formed only 28 million years ago, when the Pacific Plate started sliding under the North American Plate.  This complex formed from blocks of oceanic crust (basalt and greenstone) and overlying sediments (chert, limestone and shale) mixed with submarine landslide deposits (graywacke sandstone and shale) shed from the North American continent and with pieces of metamorphic and mantle rock (usually altered to serpentine), which were forced to the Earth’s surface.  Evidence of the Franciscan Complex can be seen in the cliffs of Willow Creek and Jade Cove.  You know the Salinian rocks.  They are white marble and granite, and are found in the peaks of the Ventana Cones and Pico Blanco. 

As Elaine mapped the rocks of the reserve, a pattern started to develop.  The rocks determine the soil-type, and each type of bedrock contains a different combination of minerals and elements.  And guess what?!  There are different plants that like to live in the different mix of minerals.  The redwoods seem to like the Franciscan; grasses and scrub like the Coast Ridge Belt; and the oaks like the Big Creek Conglomerate.  Each type of bedrock can supply the nutrients needed for that particular type of vegetation. Sometimes, there is a fault that can cause different rock-types to come together or to allow for more water.  The vegetation can change pretty abruptly at these fault contacts.  When we are in the field, or looking at aerial photos and satellite data, we look for these changes in vegetation to help us find these possible changes in rock type or for faults. On the Reserve, there is a distinct, fairly straight line of trees, with redwoods and forest to one side and grasses to the other, along the north wall of Big Creek Canyon and Burnette Creek.  There are other areas along Devil’s Creek.  The Highlands Road and Circle M Road are two great drives that show this change in vegetation as the rocks change.

Elaine also developed a geology teaching kit for the Reserve.  She gathered several samples of each type of rock and put them in different plastic bins.  She then developed written flash cards for each type of rock.  When students come to Big Creek Reserve to study the geology, they can see all the types of rocks that are represented here, as well as a wonderful, color-coded GIS map of where to find them.  We really appreciate Elaine’s time and effort in developing this wonderful tool.

Finally, Elaine found two rock samples that she is having tested and age dated.  One is a beautiful green chert that she is having tested for radiolarians.  It is different from the other chert deposits found on the Highland Peak-Serpentine trail, in that it is found with the greenstone/metavolcanics.  In addition, one of the marble outcrops possibly has identifiable crinoid pieces.   If this is so, then we might be able to identify which types of crinoids and figure out a ballpark age of when the marble (which was originally limestone) may have been deposited.  Also, the geology of the Reserve presents a small enigma in that there is a “sliver” of Salinian rocks with the Franciscan rocks on both sides of it.  The big question that needs solving is:  “How did it get there?”  We hope Elaine will come back in the near future and help us solve this mystery.

In the meantime, it’s a good bet that those rocks on the road are from the Franciscan Complex!

By Terry Hallock and Elaine Bohls