First words of caution:
- There is great parking on the south side of the highway. You can run across the highway to the huge road cut on the north side. There is large bit at the west end of the turn out on the south side.
- An intact piece of serpentine is safe and fine to bring home. However, do not cut or grind this rock. Serpentine rock frequently contains naturally occurring asbestos, but this is only a hazard if you grind/cut the rock and make dust.
First a bit on rocks and their chemical content. Sialic rocks have a high silicon (Si) and aluminum (Al) content, typically found in continents with such light colored rocks as granite and granodiorite. Sialic are sometimes referred to as Felsic if they are rich in feldspar and silica rich (Rhyolite (extrusive) and Granite (intrusive)). Mafic rocks have a relatively lower silicon and aluminum content, and relatively higher magnesium and iron (Fe) content. Mafic rocks include such as basalts (extrusive) and gabbro (intrusive). Ultramafic rocks have very high iron and magnesium contents and are almost always ocean crust; typically these rocks are intrusive peridotite. Serpentine is metamorphosed peridotite. Because of the shear and deformation serpentine undergoes it has smooth surfaces, often slickensides.
What you see in the road cut is serpentine rock, so named because ostensibly the mottled pattern resembles the skin of a serpent. These rocks are heavily metamorphosed peridotite. This serpentine is part of the Feather River Belt of deep ocean rocks sandwiched between Calaveras Complex to the east and the Shoo Fly Complex to the west. The Feather River Belt is a bit of old oceanic crust that was jammed into the Sierran oceanic trench with the overlying rocks of the Shoo Fly Complex. The Feather River Peridotite Belt and the Shoo Flex Complex are roughly of Devonian (~400 million years ago) to early Pennsylvanian (~320 to 300 million years ago) age; during the Antler Orogeny (orogeny = mountain building even).
Like all EarthCaches you’ll need to answer some questions. Log you cache, but sent your answers to the CO shortly thereafter, otherwise I will be forced to delete you log. Get it wrong, no problem, all good faith efforts will be honored
(1) Name and GC code of the cache.
(2) Grab a baseball or larger chunk of the serpentine rock. Describe the coloration of the rock;
(i) mottled or uniformly colored; and
(ii) the range of colors.
(iii) Run your finger over one of the faces of your rock. Is it smooth or rough.
(3) From the description above tell me about the what the original rock was and how it was changed
References:
Alt, D & Hyndman, D (2016). Roadside Geology of Northern and Central California. Mountain Press Publishing Company.
Durrell, Cordell (1987). Geological History of the Feather River Country, California. University of California Press.
Koningsmark, Ted (2011). Geological Field Trips: Sierra Nevada. GeoPress.