Hunting for Fossils in the Sierra Nevada...Wait a Minute, What Fossils are there in Granite?

Photo by Mrs. Geotripper

Last night I attended a great presentation on the dinosaurs and other (more interesting) Mesozoic reptiles found in California. It was given by Dick Hilton, a former prof at my school, who is currently teaching at Sierra College in Rocklin. I was invited to head up to the Sierra Nevada to look for fossils with Dick and fellow prof Noah Hughes, and I jumped at the chance.

But...fossils? In the Sierra Nevada? Isn't the Sierra Nevada composed of granitic rock? Granite and other plutonic rocks develop from cooling magma deep in the Earth's crust, an environment that is neither conducive to life, nor to the preservation of fossils. A quick look at a geologic map reveals that the Sierra is only about three-quarters exposed granitic rock. Most of the remainder is composed of metamorphic slate and metavolcanic greenstone, with a fair amount of serpentinite (California's state rock).

Metamorphic rock is the product of taking pre-existing rock and subjecting it to extreme heat and pressure. The resulting rocks, with names like slate, phyllite, schist, marble and quartzite, often bear little resemblance to their previous form, their protoliths. Any fossils that might have been part of the original rock are often destroyed in the process. There is a rich record of tectonic events leading to the formation of the Western Metamorphic Belt, a story too complex to even summarize in a short blog post. Long story short, fossils shouldn't be found in the Sierra Nevada. The rocks have been too distorted and altered. For the most part...

The Mariposa formation is a deposit that formed on the bottom of a deep sea off the coast of California in Jurassic time. The shoreline lay east of where it is today, and the Sierra Nevada was a different place: a series of active volcanoes led to a coastal forest. Dinosaurs, pterosaurs, and primitive mammals roamed the forests and floodplains. In the sea, large swimming reptiles including plesiosaurs and ichthyosaurs were to be found. The Mariposa is somewhat less altered than many other rocks of the metamorphic belt, and because of this, a few fossils have been found, fossils that enabled geologist to figure out the age of the rocks, an important step in unraveling the complex geologic history of the region.

The Mariposa formation is exposed along Highway 49, and we were searching for fossils in the vicinity of Don Pedro Reservoir. To my great delight, after a bit of sweating and slipping down wet grassy slopes we found some interesting specimens.

The most common fossils include the bivalve Buchia, an important diagnostic fossil indicating a Jurassic age for the unit.

I almost missed a small belemnite, which looks a bit like a fossilized cigar. The cylindrical fossil is the internal shell of a squid-like creature.

The prize find for me on this day was a small ammonite. Ammonites are relatives to the pearly nautilus which lives in today's seas. They can be thought of as an octopi with a shell. I have a spotty history with ammonites in the sense that for the last quarter century, I haven't been able to find any, and it hasn't been for lack of trying. I once stood in front of a productive outcrop with a noted paleontologist, and I watched him walk up and pull an ammonite out of the cliff face. I've been back to that spot many times over the last twenty years, and I have yet to find another. But today, I flipped a rock over, and there it was, a little tiny ammonite.

I didn't make that best find of the day, though. That honor belonged to Noah, my fellow prof at Modesto Junior College. The sample below shows a sprig of a species of redwood tree. This is an astounding find to me. Redwoods survive today in just three places in the world, on the northern California coast, in the Sierra Nevada, and in a small grove in China. But the trees once ranged across the northern hemisphere, and as Noah's find shows, they have been around for more than 160 million years. Think of it this way: dinosaurs once roamed through forests of redwood trees. And here in the rock was a distant ancestor to the Sequoia trees that grow just a few miles up the hill from our fossil site.

We were reminded of dinosaurs one more time today as we were driving home. We passed a large herd of modern dinosaurs who were displaying the kind of herd behavior that we think the large plant-eating dinosaurs displayed during the Mesozoic Era. Luckily they didn't attack!

Dick Hilton wrote the guide about the history of Mesozoic reptiles in the California region, and the book remains the best source of info about mosasaurs, ichthyosaurs, plesiosaurs, and the handful of dinosaur species found in the state. You can get the book at http://www.ucpress.edu/book.php?isbn=9780520233157 or any other online seller. I give it my highest recommendation!