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!

The Other California: Spring Arrives in the Prairie Lands

California has a rather sullied reputation these days; a lingering depression (at least in our Central Valley), high unemployment, urban nightmares, Hollywood, and a legislature that lacks the will to support the education of its children and young adults. And yet, 38 million people choose to live here. They have a lot of reasons, and some think they want to leave, but frankly California is a nice place to live. Especially if you have any interest whatsoever in geology. There is hardly any place in the world that packs so much geographic variety within its borders: coastlines, mountains ranges, deserts, and valleys. My Other California series is an on-again, off-again exploration of the geologically interesting areas that don't normally show up on our tourist postcards. I started this series with an exploration of the prairie lands of California that surround the state's vast Central Valley. Because I live on the fringe of these prairies, I venture out some afternoons to see what's happening. Friday was an exploration of Merced Falls Road in the Sierra Foothills, a short drive that included a surprise vista of Yosemite's Half Dome and Clouds Rest.

Merced Falls Road connects Highway 59 at Merced Falls to Highway 132 near Don Pedro Reservoir. It provides access to a number of ranches and the recreational areas along McClure Lake. It also includes numerous exposures of the metamorphosed sediments and volcanic rocks of a Jurassic island arc system that was slammed (in a geologic sense) into the west coast of the North American continent in late Mesozoic time. The rocks have been upended and deeply eroded, producing a landscape that looks like a neglected cemetery (the outcrops look like tombstones). The rocks include the Mariposa formation and the Gopher Ridge volcanics.

Friday was a fine day for seeing the early blooms of what promises to be a fine wildflower season. More on that in the next post. When we reached the little village of La Grange downstream on the Tuolumne River, we could see the preparations being made for an exceptional spring runoff. The snowpack in Yosemite is somewhere around 150-160% of normal after three years of drought, and the water authorities are emptying out Don Pedro Reservoir to make room for the much needed onslaught of snowmelt that begins soon. Below is the artificially induced mild flood being conducted to avoid an out-of-control flood in a few weeks.

What else did I discover? I found out my hay fever is alive and well...sniff...

A Present-Day Gondwanaland Forest

I am using today's post as a bridge back to my continuing story of the real Jurassic Parks (and Cretaceous Parks) on the Colorado Plateau in the southwestern United States. But those parks are in mostly desert environments today, and it can sometimes take a lot of imagination to see the tropical forests that existed in the region during Jurassic or Cretaceous time (of course, imagination is not a bad thing). On the other hand, there is a place in the world that has been detached from any other continent for more than 80 million years, thus preserving some elements of the Mesozoic ecosystems that existed at the time. There was a lot more to see just an achingly short time in the past, only a few hundred years ago.

I am speaking of New Zealand, land of kiwis and hobbits (thank you Peter Jackson, I enjoyed the movies a lot)

New Zealand consists mainly of two major islands, divided from each other across the narrow Cook Strait. The North Island includes Auckland, a city built on dozens of volcanoes, Rotorua, a geyser wonderland sitting in the midst of a huge caldera complex, and the big active volcanoes including Ruapahu, Ngauruhoe and Tongariro. The South Island includes the city of Christchurch, and a series of spectacular mountain ranges including the Southern Alps (remember the spectacular opening sequence of "The Two Towers"?), and the Kaikoura Range that drops 9,000 feet right into the Pacific Ocean (sort of like Big Sur, but three times higher).

Because the island mass separated from other continents prior to the mass extinctions of the late Cretaceous, mammals were present, but never evolved to dominate the ecosystems as they did elsewhere in the world, and eventually went extinct, except for some bat species. With the loss of the dinosaurs, the only vertebrates in position to take over the terrestrial habitat were the birds. With no reptilian or mammalian predators, the birds were able to evolve a flightless existence (flight is an expensive energy proposition if it is not necessary for survival). Eventually a number of very large birds evolved, including ten species of moas. The largest stood over 9 feet tall. They were mainly plant-eaters, but an extremely large eagle lived on the island as the top predator.

Unfortunately, the arrival of humans on the islands around 700 years ago spelled doom for the moas, and when they were gone the large raptors lost a main food source and went extinct as well. Only the kiwi, the small forest dwelling bird, survives today.

The trees on the island provide the other link to the past. I have included some of the huge tree ferns that evoke Jurassic forests in the pictures above (the moas aren't real. Really). The ferns in the third photo line the opening of a 75-foot deep explosion pit at Orakei Korako Geothermal Area on the North Island. I really would not have been surprised if we had been attacked by a few velociraptors in the forest there...

The ancient tree that captured my imagination the most was the Kauri tree (second photo), one of the bulkiest trees on the planet, rivaling even the Sequoia trees in my own backyard of the Sierra Nevada. The most mature trees are not quite 200 feet high, but their trunks remain thick practically to the crown. Their history extends back into Jurassic time. They once covered much of the islands, but unfortunately their wood is strong, mostly knot-free, and attractive. Something like 95% of the original forests have been cut down, and old-growth forests are exceedingly rare and precious. The trees are now protected by law, but ironically, dead trees in swamps are not, and the wood is durable enough that some of the trees from the swamps are still utilized legally, despite having been dead for hudreds or thousands of years.

The Real Jurassic Parks: Navajo!

If the question arises as to what formation is responsible for the most dramatic scenery in all the Colorado Plateau, the Navajo Sandstone would have to be near the top of everyone's list (anyone out there have opposing arguments?). It is hard to imagine a rock unit that can erode into more beautiful colors, forms and structures (and I am being totally scientifically objective here....).



200 million years ago, a vast desert had spread across what is now Colorado, Arizona, Utah and Nevada and north into Wyoming. The idea that sand dunes represent a typical desert is a widespread misconception (typical American deserts are no more than 5% sand dunes), but in this case it was true: huge dunes crept across the landscape, in places burying the underlying rocks to a depth of 2,000 feet or more. The source of the sand? Early assumptions said the last remnants of the Ancestral Rockies, but some recent work suggests that the sand was derived from vast river systems with headwaters in the Appalachian-Caledonian mountain system.

Map from Ron Blakey

The uniqueness of the Navajo lies in the structure of the rock itself, the crossbedding, and the somewhat unique range of color: in different parks the rock is white, orange, red, yellow and vermilion. The variations are apparently due to a bleaching effect of once-present hydrocarbons that dissolved, and then reprecipitated, iron oxides in structures like domes and thrust faults. The iron oxide minerals hematite, goethite and limonite provide the intense color, and in places have formed the unique "moqui marbles", spherical concretions of hematite that are popular with rockhounds.

I want to explore a couple of the Navajo Sandstone parks, and I start today with a lesser-known park, by name and visitation, but very familiar because pictures of this canyon are a staple in art galleries. I am referring to Antelope Canyon, outside of Page, Arizona. Antelope Canyon is on the Navajo Reservation, and has been established as a Tribal Park (like Monument Valley). The canyon is a small tributary to the Colorado River, and from above, you would barely know it is there: you can literally jump across the opening. Underneath is a slot canyon, more than 100 feet deep...

Over hundreds of thousands of years rushing waters have ripped away at the sandstone, but because the well-cemented nature of the rock, mass wasting is ineffective at widening the gorge. Beneath the surface is a fascinating labrinth of twisting passages and curving walls. The canyon is both a challenge and a Mecca for photographers. The play of reflected light ranges across the brightness spectrum, and at the right times the rock seems to glow internally.

Slot canyons carry their own dangers, and flash floods are a real concern here. In 1997, eleven tourists were killed when a distant storm sent waters slashing through the canyon. The Navajo Nation now carefully monitors visitation when flooding is a possibility. Still, despite entry fees and charges, the canyon is a delight to visit: there are two sections, an upper canyon with a level floor, and a lower canyon that requires some climbing and squeezing, although ladders and steps are now in place to allow fast evacuation if floods come. It is a truly beautiful place. I visited in 2007 during a day I had to drive 750 miles, but I spent a long time in the canyon, snapping more than a hundred pictures. It was magical....

Time Beyond Imagining - A Brief History of the Colorado Plateau: The Real Jurassic Parks

As we move forward in time, we reach the geologic period called the Jurassic, one of the few units of the geologic time scale that is familiar to many laypeople. We have Steven Speilberg and Michael Crichton to thank for that, and yes, dinosaurs rose to prominence in the terrestrial landscape, but lots more was going on as well, including a lot of interesting non-dinosaurs, including our own small furry predecessors creeping about in the underbrush (the mammals first appeared in the late Triassic, about the same time as the first dinosaurs).

So what are the real Jurassic Parks? In the movies, the park was an island populated by reconstituted dinosaurs that ate people and each other. The Jurassic Parks of the Colorado Plateau are the ones in which the most dramatic scenery is provided by Jurassic-aged rocks, which include the Wingate, Kayenta, Navajo, Carmel, Entrada, Summerville, and Morrison formations. The parks include the familiar Zion, Arches, Capitol Reef, and Canyonlands national parks, and the less familiar Grand Staircase-Escalante, Navajo, Rainbow Bridge, Vermilion Cliffs, and Colorado national monuments. Notable state parks include Kodachrome, Goblin Valley and Dead Horse Point, all in Utah. Many of the dramatic cliffs included within the Glen Canyon National Recreation Area are also made of Jurassic rocks. And let us not forget the actual Dinosaur National Monument, the one that actually has dinosaurs (although at last report, they did not eat park visitors).

Today's photos include a parting shot of the Chinle Formation at Petrified Forest National Park, simply because I came across it in a lost file and liked it, and a picture from near the head of the Schafer Trail in Canyonlands National Park, which is cutting across ledges of the Kayenta Formation as it prepares to plunge down the Wingate cliffs to the White Rim, discussed in a previous post.

The Schafer Trail leads to the White Rim Trail, which winds for a hundred miles or so through the heart of Canyonlands National Park. It is one of the life-list goals that I haven't accomplished yet. It pretty much requires at least two days to travel, and three or four days by bike. The Schafer Trail was originally carved by cattlemen and improved by uranium miners (they worked pretty hard...).

So, why the dramatic colorful cliffs? Something important had changed, causing the Triassic floodplains to disappear. The silts and clays and river channels were replaced by desert dune sands, which quite often form prominent cliffs when hardened into solid rock. The deserts formed because Pangea had begun to break apart, and North America was now moving northwest, into the subtropics where the prevailing air circulation results in dry conditions. We see a similar situation in the Middle East and Sahara Deserts today.