So, we decided to leave Luffenholtz County Beach and head one mile south to Moonstone Beach. It's also known for its tide pools and rocky cliff face, but in addition it has sea caves and great surfing. When we arrived, we saw what else it has that Luffenholtz doesn't, and that is the mouth of the Little River. I'm pretty sure it doesn't get more beautiful than this.
There weren't green rocks strewn about at the base of this cliff, but there was an obvious trail up that we promised ourselves we would return to hike one day soon.
Capitola Beach. We looked and didn't notice anything, and just dismissed this as a possibility for fossil hunting. It was just another rocky cliff face like the hundreds of miles of cliff faces along the California coast.
When we got home from this walk, I tried to identify the green rocks from Luffenholtz Beach. I wasn't completely successful. A lot of shiny polished green stones show up on google searches. Then I emailed geologist Andrew Alden whose website and name came up when I googled this search term "Serpentine Beach Rock." He wrote me back, sent me links and thoroughly enlightened us about what we had been looking at. We had not heard of the Franciscan Assemblage (or Franciscan Complex) as this particular section of northern California's coastline is called. The geological history of it is so incredibly rich and interesting it has its own Wikipedia page. Here's an excerpt:
These rocks - which are also known as the Franciscan Complex, Formation, Series, or Group - include mafic volcanic rocks (basalt), many of which are altered to greenstone, radiolarian cherts, greywacke sandstones, limestones, serpentinites, shales, and high-pressure metamorphic rocks, such as blueschist. Although most of the Franciscan is Late Jurassic through Cretaceous in age (150-66 Ma), some Franciscan rocks as young as Miocene (15 Ma), and as old as early Jurassic (180-190 Ma) age are known. Following deposition, these rocks were then faulted, folded and mixed in a seemingly chaotic manner. Due to the lack of continuous exposures and the complex folding and faulting, it is impossible to use conventional methods to estimate the thickness of the assemblage. However, various arguments can be made that at least 50,000 feet (15,240 m) of sediment are present.Does it blow your mind to know that this is a formation that has been around for 150 million years? There we were walking around, staring up at it, wondering about its significance, its colors, its lack of obvious fossils, and not thinking at all about unimaginable amounts of time and the fiery crazy mechanisms that thrust this piece of earth up like this. And, just because it really is a spectacular complex, it does have a sparse but diverse assemblage of fossils. We didn't see them because they are microfossils.
Franciscan rocks are thought to have formed prior to creation of the San Andreas Fault when an ancient deep-sea trench existed along the California continental margin. This trench, most of which is no longer evident, resulted from subduction of oceanic crust of the Farallon tectonic plate beneath continental crust of the North American Plate. As oceanic crust descended beneath the continent, volcanic rocks, mainly basalt, making up the lower plate, and marine sediments deposited on top of it were scraped off and accreted (i.e., added) to the leading edge of the overriding plate. This resulted in widespread deformation with development of thrust faults and folding. Ophiolite (which weathers to serpentine), and rocks altered by high-pressure metamorphism (such as blueschist) were emplaced during this episode. Deformation and emplacement continued during subsequent creation of the San Andreas fault to result in a complex chaotic assemblage of diverse rock types that some refer to as a mélange.
Just knowing how old these mountains here are, I thought I should take a look at the age of the Appalachian Mountains. Okay, our mountains here are mere babies compared to the 500 million years of the Appalachians. Roger asks, "How old is the Canadian Shield?" The Canadian what, says I? So, I take a look. Wikipedia says it's 2.45-1.24 g.a. So, what is g.a. I wonder? I suggest it probably means a gazillion years. We laugh. It's geologic time. We're talking billions of years. BILLIONS! California is so young. But that's probably because we're still forming. Stay tuned!
The best part of the internet is finding answers to our questions, and the best part of the planet is its endless awesome mysteries.