One of California's most distinctive and mysterious bodies
of rock is well exposed at Shell Beach, north of Bodega Bay
in Sonoma County. All photos by Andrew Alden.

A big swatch of the Coast Range is a set of rocks that once baffled generations of California geologists. It's a dog's breakfast of different things, most of them familiar in the region, mixed together with no pattern that anyone could make sense of. The geologists who explored California were no slouches, but all they could do was to map these suites of rocks in a catch-all category called Franciscan melange.

Around 1970 the new theory of plate tectonics found just the place for Franciscan melange, and Shell Beach is just the place to ponder and admire it. I've made several visits there and don't recall any shells—maybe a better name for it is Melange Beach. And right nearby is another mystery from the ice ages. For anyone into geology, Shell Beach is a great workout.

Melange, we now know, is what happens to rocks in subduction zones, which is where one tectonic plate plunges beneath another. Before the San Andreas fault began carrying coastal California sideways to the north, the plate west of us was being subducted directly eastward against North America. (A remnant, called the Juan de Fuca plate, is still doing that off the Pacific Northwest.) Rocks and sediments caught between the plates were mixed and tumbled like snow in front of a snowplow. And that's what melange represents, and that's how the Franciscan got so scrambled. Shell Beach shows us the whole range of the Franciscan in one compact site.

First let's get oriented on the geologic map (from U.S. Geological Survey map MF-2402).

At the top, the Russian River enters the sea at Jenner. Shell Beach is due south of the "Qt" symbol, part of Sonoma Coast State Beach. You can also see it from offshore on the California Coastal Records Project site. "Qt" stands for Quaternary terraces, which I told you about down at Pebble Beach. Up here there are only two terraces mapped, but subtle signs indicate more of them higher up. "KJfs" stands for Cretaceous-Jurassic Franciscan sandstone, but it includes a large share of melange. The tiny orange dot represents Mammoth Rock, which we'll talk about later. Here's the view from the terrace looking south. That's Bodega Head in the farthest distance.

The cliffs are all melange. Most of it is a shale and sandstone matrix that has been thoroughly disrupted by tectonic mixing. The sea stacks out in the water are chunks of hard rock within the melange that have resisted erosion. Where these crop out of our rounded oak-dotted hillsides, the local geologists call them knockers. But resistant blocks occur in all sizes, both larger and smaller.

The stairs down to Shell Beach pass by a big greenish body of serpentinite, our state rock, in the gully. It's worth a detour to inspect them. This soft rock type doesn't form knockers. (I should remind you that all collecting or defacing of rocks is prohibited in this state park.) The beach is small, with dark sand and not much of it, and the coast is cool and breezy—not a place for surfing, picnics or volleyball. What's special about it is the range of rock colors in one place. I'll give you a small sample.

The palette does cluster around green and blue. Greenstone, shown below, is ancient seafloor lava that has been changed by time and pressure, but not enough to hide its original pillow shapes.

Chert is a flinty rock that acquires subtle translucent colors, setting off its waxy luster, during subduction.

What excites geologists, and may catch your eye, is that Shell Beach also exposes the soft matrix rocks that held and polished these boulders during subduction. Matrix is seldom seen elsewhere because it quickly turns to soil or washes away. In addition to all these is eye candy, things you just want to turn into background images or jigsaw puzzles.

If you have time, take the trail north from the beach toward these two ancient sea stacks. The first, Mammoth Rock, is in the center and the second behind it to the right is Sunset Rocks. Some 125,000 years ago, these stood among the waves and endured until the land rose and the sea fell away.

The stacks are a mixture of rock types that is largely blueschist, a tough stone formed by high-pressure metamorphism. At Shell Beach, chunks of it extend the palette all the way to indigo. The second stack has a real treat—polished spots that have been interpreted as marks left by ice age mammoths that used the rock as rubbing posts, just as cattle do today. KQED showed you these in its Ice Age Bay Area series in 2008.

I have presented more detail about the polish here and more about the rocks here.

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^{Pebble Beach is on the coast just north of Bean Hollow State Beach. Come for the pebbles, stay for the rocks.}

It was a weekday the last time I visited Pebble Beach, on the San Mateo coast just south of Pescadero, and several busloads of children came and went. They flocked to the pebbles, flung them high, and were off in their buses 15 minutes later leaving the beach to me. I did the same here when I was a kid, and you can too—the pebbles are wonderful—but I can show you a better time today.

Pebble Beach (also called Bean Hollow Beach North) is a great place to look at the deep sea floor without getting your feet wet. The rocks exposed there are scrubbed clean by surf and nicely tipped up on edge for easy viewing, and they present many instructive mysteries that students actually pay professors and buy textbooks to learn.

But first, about the pebbles. You get to the beach on Route 1 going south from San Gregorio Beach, which skirts the coast on flat land that is largely planted to artichokes and ranchettes. This tableland is a marine terrace or wave-cut platform. See the setting in the photo below, from the California Coastal Records Project.

The San Mateo coast is famous for these terraces, especially down around Santa Cruz. They are land that was cut flat by the sea during the Pleistocene epoch, then lifted by tectonic activity. The terraces consist of beach sand and offshore sediment from tens of thousands of years ago, and most of the pebbles of Pebble Beach are recycled from that Pleistocene gravel.

Now let's look at the geologic map of the area (from USGS Open-File Report 98-137):

Pebble Beach is right where the symbol that says "63" is. The areas marked "Qmt" are the marine terraces. If you examine the red contours, which show elevation at 50-meter intervals, you'll see that the terraces occupy at least three different levels.

The rocks on the beach, revealed as the terrace is eroded away, are mapped in green and belong to the Pigeon Point Formation, a thick pile of former deep-sea sediments dating from the Late Cretaceous epoch, some time between 100 and 65 million years ago. That "63" symbol means that the beds of rock, originally horizontal, are tilted to an angle of 63 degrees. And here they are, exposed again just as the sea left them in the Pleistocene.

The surface between the steeply dipping rocks and the horizontally bedded terrace deposits is called an angular unconformity. In this case it represents a gap in time of at least 65 million years.

The next five photos are cross-section views, shot looking directly down that 63-degree angle at the rocks.

Here's where fine-grained siltstone, light-colored sandstone and cobblestone conglomerate are stacked. There's another angular unconformity between the siltstone and sandstone. And look at the wavy top surface of the sandstone—the conglomerate clearly arrived in a rush, pushing into the sandstone while it was still loose sand. This was a very active environment, an offshore basin where large amounts of coarse sediment washed down periodically in underwater landslides. In fact, it was rather like offshore California today. The many small stones embedded in these rocks were eroded free to become the gravel in the marine terrace. So the pebbles of Pebble Beach have been recycled two times, and maybe more.

Here are some more signs of vigorous activity, including load marks from heavy gravel landing on soft silt:

Rip-up clasts are chunks of fine-grained seafloor that were swept up by strong sand flows:

Drag-folded laminations show how overriding sand flows deformed the soft silt beneath them:

And here are some tiny thin veins of bitumen, ancient organic matter that may once have been a mat of vegetation:

The final attraction of Pebble Beach is the honeycomb weathering, often called tafoni, that forms when salt air attacks clean sandstone:

Too bad the schoolkids can't stay all afternoon! But you can.

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There is something about these rocks that draws you in. Maybe it's the way they jut out of the ground? Or perhaps it's the "Stonehenge" way they form an enclosed circle? Or maybe it's just a nice place to get out of the wind? Whatever it is, they seem to pull you towards them. And once you are there, they almost call out to be touched. The rocks, long known as the "Sunset Boulders," have attracted rock climbers for years. I've climbed these rocks before. But like so many other people, I had no idea I was touching history.

During the Pleistocene, 10 to 20,000 years ago, this place was very different than it is today, inhabited by massive mega-fauna; bigger elephants, lions, bears and wolves, than we see today. While those big animals went extinct thousands of years ago, they left their mark on this place.

Looking around these rocks it is easy for me to imagine the herds of Columbian Mammoths lumbering from the nearby wallow to rub against the boulders. I can picture huge herds of camel and horse grazing nearby. Yes, those animals evolved here in North America and then crossed into Asia where they thrived and survived. Weaving my way between the boulders, I can imagine how the predators could have used these rocks as an ambush site. I envision a huge saber-tooth cat slinking between the craggy rocks, looking to pounce on an unwary bison. I can see the prides of American Lion, similar but much larger than African Lions, basking on the tabletop boulders after a big kill. I can also picture the ultimate predator making their campsite here when that first hunting party foraged deeper inland. Yes, humans were here too. And I'm sure the same pull these rocks have today existed back then.

This seems like a sacred place to me. Sacred to history. So when you visit these rocks think about those who came before you. Think about the mammoth and the bison and the camel and the horse. Think about the lions, tigers, bears and wolves. And think about those first people. Tread lightly and respect this wonderful place. With care, these rocks will be here long after we all become part of history.

Special thanks to the San Diego Natural History Museum for contributing artwork and HD video to our story. Also, to see more artistic representations of Pleistocene mega fauna, by the artists who contributed to our segment, see:

Laura Cunningham's artwork

Artwork of Joseph Venus

William Stout's wonderful murals

Watch the Ice Age Bay Area television story online.