Fred Allebach is a member of the City of Sonoma’s Community Services and Environmental Commission, and an Advisory Committee member of the Sonoma Valley Groundwater Sustainability Agency. Fred is a member of Sonoma Overlook Trail Stewards, as well as Sonoma Valley Housing Group and Transition Sonoma Valley.
Rodeo Beach geology field trip
The other day I had a chance to go to Rodeo Beach on the Marin Headlands, on a little household geology field trip. I was the driver and companion, and my partner was the geology student sent on assignment to identify chert, greywacke, and pillow basalt.
The above-noted rocks are part of the Franciscan Complex, which is an accreted Mesozoic terrane of former ocean bottom that was scraped off the Farallon Plate as it subducted under the North American Plate. The Marin Headlands and the California Coast Range are made up of this accreted Franciscan Complex rock, with the constituent rocks at different stages in the rock cycle. Rodeo Beach rocks represent all three rock cycle stages: igneous, metamorphic and sedimentary.
The subduction of the Farallon Plate, over the last 200 million years, is associated with various mountain building events. The creation of the Sierra Nevada granitic batholith being one prime example. The Farallon Plate has a remaining fragment that is still subducting off the California Coast. This is the Gorda Plate, on Cape Mendocino near Petrolia. Here, at the Mendocino Triple Junction, the San Andreas Fault, North American Plate and the Gorda Plate meet. The Gorda Plate is subducting to the southeast, driving volcanic activity farther inland at Mounts Lassen and Shasta. Sonoma’s thermal waters: Fetter’s Hot Springs, and the Geysers, represent the trailing edge of this ancient subductive volcanic phenomena.
Back on the beach, we found the assigned rocks, and began to notice that the beach itself was an aggregation of cool-looking sand and gravel made from the local rock formations. Apparently, this type of beach is unusual in California. Most beaches are made from sediments washed down by rivers.
On the north side of the beach were many surfers, and they were fun to watch. Since no materials are allowed to be taken from a National Recreation Area, we took pictures while strolling, observing and noticing features about the assigned rocks.
Greywacke is an unremarkable looking, texturally immature, lightly metamorphosed sandstone. What would you expect from a prematurely accreted ocean bottom? Chert is a fine-grained sedimentary rock, frequently with micro fossils. Chert was a favorite material to make stone tools. Why? Because of the concoidal fracturing properties made possible by chert’s microcrystalline silica content. Chert does not limit the tool maker by having to deal with pre-existing crystal cleavage planes, and this allowed for intentional shaping. Chert on the Marin Headlands can be seen in dramatic outcropppings of folded, twisted shapes known as clines and synclines. Pillow basalt is an igneous rock that gains its peculiar shape through lava being erupted under water, or better said, by subaqueous extrusion. This underwater lava extrusion can occur on tectonic spreading centers or near underwater volcanoes.
With all these cool geologic processes happening at Rodeo Beach, one might start wondering, how did this all get here and why? Of course, this was part of the assignments purpose, to pique thought and wonder, and as my scientific associate and life partner’s professor said, it’s so involved, “it makes your head hurt.”
Well, let’s not be shy to attempt an explanation. Heat energy is what makes earth a live planet. The earth’s molten core is what’s driving all this continental and oceanic tectonic crustal movement, evidence of which is seen at Rodeo Beach. The heat energy from the core drives convection currents through the mantle, that move the crustal plates on the earth’s surface around. The plates smash, grind, slide and subduct, and in the process, the crust deforms, making mountains and volcanos. That’s plate tectonic theory in a nutshell.
Out on Rodeo beach, the waves lap up like any other day. The Farallon Plate is gone, subducted, transmogrified into Sierra granites. Now at the continental margin is the San Andreas Fault, a strike-slip, or transform fault. Along the San Andreas Fault the Pacific Plate runs to the northwest, sliding along the North American Plate, building up pressure every 100 or so years that is released as earthquakes. That’s why we need to wire book cases with our geology books to the wall.
Who would have thought a geology field trip to Rodeo beach could have such far flung implications!?
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I was living in northern California in 1980 when Philip, a college friend from Australia, and his wife Dianne came to visit me. I was looking for something for us to do together for the afternoon so I suggested that we go to Rodeo Beach, just north of the Golden Gate for the afternoon. I knew it was just a small beach with brown sand, and it was nothing like the wide white beaches near Phil and Dianne’s home in Capel, just south of Perth. But I thought the drive would be nice and it would show them a little of what it was like to live in California.
The beach was small and nearly deserted when we arrived, so we, as many people do, took our shoes off and wandered slowly along letting the small waves wash over our feet as they swept in every couple of minutes.
We made our meandering way along the shoreline, avoiding the most intrusive waves and I mindlessly picked up a small flat stone and tossed it out onto the water to see if it would skip over the surface. But stones don’t skip well over lumpy oceans like the Pacific; not nearly as smoothly as they skip over millponds. I picked up another and tossed it out over the water again but this time I stooped down as I tossed it so that it didn’t hit the water so hard, but that didn’t work either. I tried another, and another, but still had no luck.
I picked up another flat rock and glanced down at it sitting in my hand before I threw it. It was round and oval-shaped, and a little smaller than the palm of my hand, but it was flat, with a gentle bulge on its top and bottom. It was also very smooth, as smooth as fine china. It almost glistened in the sunlight. It was a pale grey and there was a small white patch, the size of a small postage stamp, on its upper side. It was worthy of a better examination and I immediately discovered another postage-stamp-sized white patch on its underside too.
As I looked more closely I noticed that while it was mostly oval shaped, part of its edge was straight, not curved. I looked at this straight edge more closely and saw that the it was indented and finely sculpted into elongated grooves. Suddenly and immediately I recognised the grooves as being the hinge side of a shell; a type known as a pelecypod. I had seen drawings of these grooves in anatomical drawing of shells but I had never seen a real one. Then I knew that what I had in my hand was not a flat stone at all but a perfect slicified internal cast of the whole animal shell. The small white patches on the upper and lower sides were all that remained of the animal’s original shell, and they were firmly stuck to the cast.
As soon as I realised what I had in my hand I looked at it much more excitedly. The straight edge which I recognised as the hinge was intricately grooved and a perfect cast of the internal edge of the shell where the two sides joined together. They were as smooth as glass.
The cast had faint pale concentric ovals on it showing the internal pattern of the shell, and I guessed that the two small patches of shell, still firmly attached to the top and bottom sides were the places where the muscles that closed the shell were originally attached. The cast itself was now tough hard silica and it had survived being broken out of the rock that had been its home for many thousands, or probably millions of years, and being washed around in the waves until I had picked it up.
It was no longer a flat rock in my hand, but a beautiful fossil of an ancient animal.
I took it home with me, and then back to Australia when I returned there in 1983 and it has occupied an important place in my collection of various rocks and minerals ever since.
Occasionally I show it to people might be interested in it, and those who can appreciate fossil shells often say that it is the finest specimen they have ever seen. And I remember how close I was to skipping it back out into that lumpy ocean that day in 1980.
40 years later when I had moved back to California I returned to that beach with my wife to see if I could find more fossils like it, but I could not find any. I found many grey rocks that I recognised by their general shape as being internal casts of shells, but they were far too beaten and worn by the waves to be of any great value as fossils. I showed some to my wife but she thought that they were just unremarkable rounded rocks. However when I placed four of them of different sizes in a row and showed her that all of them were the same color, and that they all had two straight sides, and the internal angle of the two straight sides on all of them was about 140 degrees; and that the remaining curvature was the same on all of them, she began to see them for what they really were.
But they could not stand on their own and tell their story; they were too badly damaged and needed an interpreter.