The next time you go to the beach, whether at the ocean or beside a river, take a closer look at the sand and think about the stories those grains can tell!

Sand . . . we play in it, we stroll on it, we make castles out of it, but what do we really know about it?

Most people use the term “sand” to refer to loose material on a beach, but sand is actually a grain size measurement used by geologists to describe sediments varying in size from about 1/16 mm to 2 mm in diameter. Sand can be found not only along ocean beaches, but along flowing rivers and beside land-locked lakes.

Mineral and volcanic sands are terrigenous, meaning that they come from the land. Mineral sands start as mountains and boulders and are gradually broken into smaller and smaller particles by weathering and erosion. Being alternately heated and cooled through the seasons, rolling and tumbling around in mountain streams and rivers, rocks are broken apart by the water’s movement. The black sand beaches of some islands (e.g. Hawai’i) are volcanic in origin. Lava rocks, chunks of cooled molten lava, are broken down into sand over time by physical and chemical weathering.

Some sand is biogenous, meaning that it comes from living things, such as mollusk shells and corals. The energy from water breaks shells down into tiny pieces that eventually are washed onto beaches. Parrotfish in the tropics use their teeth to scrape off and grind down bits of coral reefs; the undigested particles pass through the fishes’ digestive systems and end up as sand on tropical beaches!

If you examine the beauty of beach sand with a magnifying glass, the shape, size, and color of the sand grains can tell many stories.

Shape indicates the age of sand grains. Different materials break down at different rates. For example, quartz is very sturdy and takes longer to become sand than the calcium carbonate material of mollusk shells. Younger sand has sharp and angled edges; it needs time and wave action to become rounded. Sand particles that are smooth and well-rounded are evidence that they have been rolled around for awhile.

Wave action is the story told with grain size. The story is simple: small waves move small sand grains around; large waves move large grains. In addition, Aeolian transport, or sand movement driven by winds, shifts the sand around on the beach, shaping and reshaping, covering and uncovering.

Color offers evidence for source materials – sand particles that are clear, tan, gray, or brown might be quartz or feldspar, black or gray particles could be ilmenite (titanium oxide) or magnetite (iron oxide); garnets are red, mica is silvery black or gray, and shells are purple, white, black, or brown.
The chorus from the song “Mountain in my Hand” (on the Only One Ocean CD by the Banana Slug String Band) explains it best:

“I’ve got a mountain in my hand from the rain-washed land; down by the sea now is where I stand, with this mountain in my hand trying to understand; Oh, oh, wonderful sand.”

Additional Links

• Sea Grant North Carolina

Detail of the rock garden in Ruth Asawa's 'Garden of Remembrance' at UCSF. All photos by Andrew Alden.

Stone is more than the peculiar specialty of geologists. All of us have dealt with stone since our deepest ancestors' days in the African savanna, and I welcome the variety of viewpoints that center around it. The child, the builder, the fabricator, the gravestone cutter and the artist see stone in special ways. Lately I've noticed the affinity for stone that Ruth Asawa displays in her art.

Ruth Asawa has created public art in the Bay Area and elsewhere for more than 40 years. Unlike her signature woven wire pieces, Asawa's public commissions are in bronze or stone. The bronze is carefully sculpted and cast, but the stone is as nearly natural as can be. Let's look at two interesting examples in San Francisco.

The Buchanan Mall (Nihonmachi) Fountain is an installation meant for use and enjoyment as an integral part of the Japantown center. It immediately calls to mind the traditional Japanese rock garden in the streamlines of its cobblestone pavers and island stones.

Bronze sculptures, one of them a fountain, sit in two stone circles like islands or ponds. Both were inspired by origami.

Both circles include island stones and are harmoniously faced with Californian river rocks. It interests me that Japan and California, facing each other across the Pacific, both owe their geology to subduction, the plate tectonic process of clashing plates that pushes their rocks together into mountains.

The Garden of Remembrance is in a courtyard at San Francisco State University, built to commemorate the imprisonment of Japanese-Americans during World War II—a troubling act of U.S. public policy and a formative event in Asawa's life. It has two parts, a grassy square and a waterfall rock garden.

Apparently Asawa's intent for the square was to bring together rocks from each of the ten War Relocation Centers. But the first stone I approached told me immediately that geology had thwarted this purpose.

This classic peridotite, streaked blue with serpentine minerals, could not have come from California's Tule Lake or Manzanar. The first would have been black Cascades lava, the second Sierran granite or gneiss. This boulder comes from the Klamath Range. Clearly these stones were symbolic, not literal representatives of the camps. It was a relief at that point to put down my geologist's mindset, because an artwork like this should not distract anyone from its true purpose.

Later I learned that the Rohwer, Arkansas, camp where Asawa spent the war years (along with George Takei, Janice Mirikitani and more than 8,000 other American citizens) was located in a marshy lowland with no rocks at all. The same was true of the camp at Jerome, Arkansas.

While there is some color among the ten boulders, most are structureless mudstones, dark and mute. They populate their lawn in a scatter that schematically matches their pattern on the American map. (In that context, the blue peridotite represents Tule Lake and its exceptional role in the camp system. Maybe my geologist's mind has something to work on after all.) Asawa collaborated with stonesetters Isao Ogura and Shigeru Namba in selecting and placing the stones here, and in the uplifting rock garden next to the lawn.

Even at shady times of day or in the typical fog, this garden must be a bright place thanks to the light-colored Sierran boulders. On a sunny day, the combined direct and reflected light makes it almost dazzling. The top photo of this post shows a closeup. The stones themselves are luscious, coaxing even the geologist to simply stand and gaze.

The gist of the 2008 TV story was that geologists are now able to use special paleoseismic techniques to analyze earthquake faults and determine their seismic history over several thousand years. By noticing patterns in earthquake activity over long periods of time, they can also make predictions about when major events are likely to happen in the future. They determined that a major event of 6.8 or higher happens every 140 years or so on the Hayward Fault. It's been 143 since the last one.

Engineer Khalid Mosalam

A 2003 report by the USGS found that there is a 62% probability of at least one magnitude 6.7 or greater earthquake in the 3-decade interval 2003-2032 within the San Francisco Bay region. With odds like this, I'm grateful that there are people like Khalid Mosalam and his colleagues at the Pacific Earthquake Engineering Research Center's Shaking Table Laboratory who dedicate their careers to learning how to make the built environment that we live in, work in and travel on more safe in an earthquake.

I'd included about a minute of video from the Shaking Table Lab in the 2008 piece but I always regretted that I wasn't able to show more of this facility. So when we started putting together an entire episode focused around the theme of earthquakes, I thought a short segment about the Shaking Table would be perfect for this show.

Substation equipment getting shaken up on the table

When we were there shooting in 2008, they were testing some electrical substation switches which were interesting but definitely not as dramatic as some of the other structures they build and shake in three axes, often until collapse. The generous engineers at PEER were able to provide us some videos of other structures they tested including a two story house, a masonry wall and a bridge pier support. The 20' x 20' table is one of the largest in the world to be able to move in three directions (translation and rotation) so, according to Mosalam, it's an extremely important piece of equipment at UC Berkeley and has contributed to important research that will result in people being safer when the next 'big one' hits.

^{The carbonate crust called tufa coats a high boulder over the Salton Sea, testifying that the Colorado River fed mighty Lake Cahuilla there in the geologically recent past. Photo by Andrew Alden.}

Geologic research is a remote and dusty undertaking that can look tedious to most people. But if it bears on subjects we fear, like earthquakes, even painstaking background projects can be blown out of proportion. A recent example from Southern California went from an intriguing journal article to "we're all gonna die" TV stories in the space of a day. Maybe it will help you spot the next time this happens in the Bay Area.

The article, published in the prestigious Nature Geoscience on June 26, presented the fruits of years of heat and tedium on the foul waters of the Salton Sea, in southernmost California. The Salton Sea occupies a sinking basin, the Salton Trough, that would rather be part of the Sea of Cortez except that the Colorado River has built a delta that dams it dry. As the river wanders over its delta, draining sometimes south and sometimes north, it periodically creates a geologically temporary lake in the Salton basin that has filled and dried out many times in the last few million years. (A short video by the University of Redlands illustrates the process.)

The repeating lake is known as Lake Cahuilla, and the Salton Sea is today's version of it. The Salton Sea is actually a terrible blunder, formed in 1905 when an irrigation canal tapping the Colorado River burst its banks and flooded a huge area of farmland in the dry bed of Lake Cahuilla. The breach was eventually fixed before the reborn lake could drown everything from Mexicali to Indio, but floods continued to be a problem until Hoover Dam was built upstream 30 years later.

The Salton Sea covers up the southern end of the San Andreas fault, and that's where our science story begins. A research team from Scripps Institution of Oceanography crisscrossed the drowned land in a small boat with a state-of-the-art CHIRP sonar system. They mapped a swarm of short faults running from the end of the San Andreas toward the end of the Imperial fault lying to the south—a tectonic configuration called a stepover. Here's the figure showing how it all fits together.

_{Tectonic setting of the Salton Trough and Salton Sea. SJF is San Jacinto fault, SSAF is southern San Andreas fault and IF is Imperial fault. Green grid is ship tracks in the dashed box; awz (acoustic wipeout zone) marks disturbed sediments. (Scripps)}

From their sonar database the researchers extracted a decent record of recent ruptures on some of these stepover faults. My figure below shows, schematically, how the sediment record is built as downdrops on the fault combine with sediment being deposited on top.

The researchers ran this process backward to reconstruct the history of faulting for a few thousand years into the past.

What made things interesting was how this record of stepover quakes meshed with records of lake floods and San Andreas mega-quakes. The correlation is imperfect, but intriguing: every time the lake was reborn in a flood, the stepover faults gave way, and half of the recent mega-quakes coincided with stepover quakes. To connect the dots further, the researchers said that "at least one of the last three dry-basin floods coincides with an earthquake on the SSAF (southern San Andreas fault)." In their cautious words, "We propose that loading by (Lake Cahuilla) may have induced failure on faults beneath the Salton Sea that, in turn, has the potential to trigger an earthquake rupture on the SSAF."

That's all ingenious science (seismic oceanography, in the desert!), but not news. To get news takes a press officer with a nose for a hook, and the hook is that the SSAF hasn't broken in some 300 years when we might (given the recent record) expect it to happen more often. Has our failure to let Lake Cahuilla be reborn held off triggering the next Big One? Well, has it??

That question is something for geologists to wave their arms about at meetings and parties, not hold out to the world as a sure thing. But between the paper, whose abstract is world-readable, and the press release from Scripps we can see the story begin to morph. It's a scattershot press release, meant to serve any news angle, that immediately confuses today's managed Salton Sea with wild prehistoric Lake Cahuilla. It includes a great hook, a quote about the "overdue" SSAF likening it to "a woman who is 15 months pregnant." The quote is only part of the background to the science, just a tangential part of the paper itself, but irresistible to the troutlike mind of the news reporter.

First out the chute on June 26 was the San Diego Union-Tribune's website, where Gary Robbins immediately removed all scientific uncertainty in his first sentence: "The Salton Sea east of San Diego is a deceptively dangerous backwater, hiding faults that repeatedly produce powerful earthquakes that jolt all of Southern California, says a new study by the Scripps Institution of Oceanography."

Later that day a Los Angeles TV station rewrote Robbins's story under the headline "Mega Quake Around the Corner?" with the lead sentence, "Man's interference with Colorado River floods that used to regularly flow to the Salton Sea may have 'stopped the clock' on a regular series of big earthquakes, setting the stage for a mega quake that could wreck Southern California, scientists said Sunday." The accidental flood of 1905 has become "interference" with regular flows to the Salton Sea, and the reasoning has turned upside-down: If we had let the Imperial Valley drown, Southern California's "Big One" might already have happened and been a few tenths of a magnitude unit smaller. How anyone wins under that scenario is not explained.

After a few days, more and better journalists weighed in. Some of them had actually studied the Nature paper (as I have). Many, including Discovery News's Tim Wall, apparently did not. USA Today's Dan Vergano did well. So did Chris Clarke on KCET's website. Charles Q. Choi did fresh reporting, as usual, for OurAmazingPlanet.com. Science News's Devin Powell wins my prize for his succinct, factual and well-written account. As you peruse the items collected by Google News under a search on "salton earthquake," look for those items that get the details right—specifically the fact that the Salton Sea arose by mismanagement, not on purpose.

The Bay Area has excellent media when it comes to our own earthquakes, with seasoned staff and good support from academic and government experts. But still—if scary news comes up, look around the science mediasphere and wait a few days before deciding what you think. Even better, track down the news at its source whenever you can. Learn who to trust.

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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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^{The tsunami of March 11 broke docks and damaged boats in Santa Cruz Harbor. Most earthquake-generated tsunamis in this part of California will cause similar levels of damage. Photo courtesy Sequoia Hughes of Flickr under Creative Commons license.}

Last week the Bay Area got a tiny taste of Japan's seismic disaster when tsunami waves stirred our waters, a giant agitating the San Francisco Bay and coast with a flick of its pinky. The waves we saw overwhelming the east coast of Honshu were attenuated to small surges here at the opposite side of the Pacific Ocean.

In central California, we will always have good warning of these big seismic tsunamis because they are not created nearby. Our tectonic setting is not conducive to Japan or Sumatra-style tsunamis. But I said seismic tsunamis, the ones that earthquakes cause. There are two other kinds that mean you should always practice tsunami awareness when you're at the beach. And National Tsunami Awareness Week, scheduled by purest coincidence for next week, is a good occasion to train yourself and your family.

Standard tsunami awareness is pretty simple, simple enough to put on a sign that says, "In case of earthquake, go to high ground or inland."

_{Photo courtesy Bruce Manchon, all rights reserved}

That refers to an earthquake that you feel, not one you hear about on the radio. I can be a little more specific. Don't worry about small earthquakes, namely the short, sharp shocks we feel often around here. Worry about a long-lasting earthquake, one with slow rhythms. If one of those happens while you're at the beach, look—you want to leave anyway, because a large earthquake like that may mean trouble at home. If the sea starts acting strange, do what the sign says, period. Otherwise, follow your usual earthquake protocol: Get away without dawdling, drive warily with your radio on, remember your family plan, use your phone no more than absolutely necessary.

The tsunamis that arrive from distant quakes, or teletsunamis, come with several hours of warning. The nearest earthquake faults that could send a damaging tsunami our way—subduction zones—are off northernmost California, part of the Cascadia seismic zone that stretches up the Oregon and Washington coast into Canada. A tsunami arising from a magnitude-8 or larger event up there would arrive here at least a couple hours later. Tsunamis from major earthquakes in Alaska, far eastern Russia, Japan and the Philippines will give us much longer warning times. There are enough people on a typical beach, with phones and text devices and radios, that you should be able to count on sufficient warning even for a Cascadia event. In addition, local emergency responders will be out in person to warn beachgoers. (If you're on the beach alone, be more alert.)

If you hear about an approaching tsunami, I must advise you: don't be irresponsible and rush to the beach. We're all intrigued by geological phenomena, and every red-blooded geologist has "witness a tsunami" on his or her geological bucket list. But remember the person taking pictures at Crescent City (a town also ravaged by a tsunami from the 1964 Alaska earthquake) who was washed out to sea. Think about the surfers who wandered around Santa Cruz Harbor, risking themselves and worrying others, as the waters rushed in and out.

However, if you choose to ignore my advice, then you should do as I wish I could have done, and proceed in a responsible manner to a safe place high above the water, obeying authorities, not congesting emergency escape routes, prepared for the worst. UC Santa Cruz geologist Christie Rowe did that and recorded the tsunami's arrival. She adds, "I would advise people not to panic, to check the West Coast Tsunami Warning Center website and select a vantage point well above the predicted wave height."

But not every tsunami is a seismic tsunami. Two other kinds of tsunamis, not monitored by dedicated networks, have a chance of happening somewhere in the world during the average lifetime: landslide and impact tsunamis. A landslide tsunami, caused by large mass movements into or beneath the sea, is quite plausible along our steep coasts and rugged offshore seafloors. Be wary of one even after a relatively small local quake. An impact tsunami, caused by an object from space crashing into the ocean, has no upper size limit and no preferred location. The odds are small but every beach in the world, ours included, faces the risk. So be like a sailor and always keep your weather eye out.

Learn more:
California tsunami information
National Tsunami Awareness Week
tsunami.gov
West Coast/Alaska Tsunami Warning Center

And check out QUEST's story "Scary Tsunamis":

QUEST on KQED Public Media.

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_{Get out of town and learn about Round Top, Oakland's own volcano, with an EarthCache. All photos by Andrew Alden.}

By now you must have heard of geocaching, the self-guided sport that takes players into the great outdoors by combining GPS wayfinding technology with geographic clues on the web. In the basic game, you get the location of a hidden box on a geocaching website—just a latitude and longitude. Using your GPS unit, you make your way to that spot, find the container and score a point by recording the achievement.

This is a good way to have fun in the outdoors with a minimum of rules and structure. Millions of people have played, and more than a million geocaches are registered around the world.

The first time I was exposed to geocaching was in 2004: I was out to explore the southern Diablo Range, and my friend came along to find some caches there.

_{A geocache in an ammo box lurks in a boulder pile; a logbook and assorted trinkets are inside.}

He found his first cache of the day in a boulder pile, a place with a superb view of the inner Coast Range. He opened the box, traded one of his trinkets for another, and left his name in the logbook. Then we sat and talked about what we were seeing in front of us.

_{Concretions and tafoni in the southern Diablo Range.}

I showed him concretions sticking out of sandstone beds; we poked our heads through the large erosional holes called tafoni. We photographed old smelting works, collected a few stones, visited a mineral dealer at home, lunched on the porch of an abandoned house.

That's the way to learn about geology. A ramble along a path or roadside in the company of a teacher is the best way to learn, but such a thing can be hard to arrange. I've done that kind of teaching to groups—it's fun and rewarding and a lot of work. An EarthCache is the next best thing: instead of finding a hidden box, you just show up. And instead of trading trinkets, you read a lesson about what's in front of you.

There are more than 10,000 EarthCaches listed on the EarthCache website. Each one has a geology lesson associated with it approved by the Geological Society of America, plus a task you must perform to prove that you visited. You email your proof to the EarthCache site, and that's what it's all about.

A few EarthCaches in the Bay area:

Round Top, Oakland (shown here)
South of Fort Funston, San Francisco
View from Windy Hill, San Mateo Peninsula
Volcanic rocks of the Pinnacles
Marine terraces in Santa Cruz
Point Reyes earthquake trail

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_{The Hayward fault crosses Oakland's Temescal Regional Park. Photo courtesy Andrew Alden}

Earthquakes will never be as predictable as the weather, I think it's safe to say, but if you get to know your nearest faults then earthquakes will be less of a surprise. When our local faults rupture, you will be less likely to panic and more likely to get through the event unscathed. So when I urge you to friend a fault, it's not to infect you with a scientific hobby but to bring you a lasting practical benefit.

A fault is a crack that has had movement along it. Geologists find them everywhere, and geologic maps are festooned with them. Almost all of them are inactive, though, and they're generally obscure even to practiced eyes. You can see what I mean on the U.S. Geological Survey's zoomable geologic map of the Bay Area.

_{US Geological Survey image}

Here's a part of it showing the area around Fremont. We can't be expected to worry about every heavy line, can we? Thankfully, no; and not the colors and symbols either. That's geology stuff: bedrock and contacts between stratigraphic units. But the two heaviest lines are important. On the left is the Hayward Fault and on the right is the Calaveras fault, both capable of major shocks.

_{US Geological Survey image}

Another USGS map called the Quaternary map focuses on just the active faults—well, the sort-of active ones that have moved during the Quaternary Period. You need to know only two things about the Quaternary: it's pronounced "qua-TERN-ary" and for our purposes it includes the last 1.8 million years of geologic time. Now a fault that hasn't moved in a million years, like a volcano that hasn't erupted in a million years, is not much of a threat. The color codes on the faults match the time of the latest fault movement, angry red being historic time (namely, 1868).

There are two good ways to visit our local faults. One is visiting them in parks (see the list below), and the other is the freelance approach of tracking them through the neighborhood. For that, the best tool is the State of California's Alquist-Priolo Earthquake Zone Maps, which have just been placed online by the California Geological Survey. Mandated by the Alquist-Priolo Act of 1972, these maps display the locations of faults that have ruptured during Holocene time, which is geologese for the last 11,000 years. Unlike Quaternary faults, activity on these Holocene faults is a pretty sure thing. And the maps display the detailed fault traces as mapped by geologists, superimposed on a high-quality topographic map.

With these, you can drive or stroll an area and assess the land for yourself. In places like downtown Hayward, the signs are plain and plentiful. In many others, you'll wonder what the heck those geologists were seeing. (The answer is that they were looking at historical aerial photos and data, finding subtle clues on the ground, and doing a lot of connecting dots.) You'll have a head-scratching good time, and you won't see your landscape the same way again.

Visit the San Andreas fault:
Fort Ross—Take Fort Ross Road east about 0.5 mile and spot the painted line across the road; an interpretive trail is nearby.
Olema—Take the Earthquake Trail near the Bear Valley Visitor Center in Point Reyes National Seashore.
Los Trancos Ridge—This ridgetop park above Palo Alto has an earthquake trail along the fault.
Sanborn County Park—South of Cupertino in the Santa Cruz Mountains is this park with the 2.5-mile San Andreas trail along the fault trace.
San Juan Bautista—The fault runs just yards east of the mission here.

Visit the Hayward fault:
Berkeley—Tour the fault in "Bear territory" including the infamous football stadium built across the fault in 1923.
Oakland—Lake Temescal park displays the fault in several places.
Hayward—The historic downtown and old City Hall straddle the fault, and signs of steady (aseismic) creep are abundant here.

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Kirkwood Mountain Resort.

It's that time of year when people head up to the Sierra Nevada Mountains for some great skiing and snowboarding. This season is looking to be a good one with a handful of big storms dumping huge amounts of snow on resorts and in back-country terrain. Although deep snow cover obscures the view of some of the geology, the views from chair lifts and other vistas while skiing is a fantastic opportunity to think about the geologic evolution of these mountains.

My favorite ski area in the Sierra is Kirkwood Mountain Resort, which is along State Route 88 approximately three to three and a half hour drive from San Francisco and 20-30 minutes from the town of South Lake Tahoe. In addition to great terrain, Kirkwood has some fantastic geology, some of which you can ski right up to and check out in detail.

The rocks exposed at the surface on the mountains of Kirkwood are various volcanic rocks ranging from about 6 million to 15 million years old (depending on exactly where you are). The volcanic deposits at Kirkwood are nothing like the smooth lava flows you might see on the Big Island of Hawai'i. They are more similar to the recent volcanic deposits seen on the flanks of and in areas adjacent to the Cascades volcanoes in northern California, Oregon, and Washington.

Many of the cliffs exposed at Kirkwood during the winter are beautiful volcanic debris flow deposits that have up to boulder-sized chunks of igneous rock within a fine-grained rock. These rocks are interpreted to be the deposits of mixtures of mud, sand, and volcanic rock debris that flowed down the flanks of the now-extinct volcanoes. I forgot the name of the specific trail — please comment if you know — but there is a great run where you can take a short break to catch your breath and walk up to some outcrops of these debris flow deposits.

So, next time you're sitting on the chair lift waiting to take that next run, look around at these beautiful mountains and picture the ancient volcanic landscape that created the terrain you're skiing.

Images: (1) Kirkwood from Mal Parkington / Flickr; (2) Glove Rock from Dean_In_SF / Flickr

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Click here for a larger version of the Nile Delta.

The Sacramento-San Joaquin Delta has this classic, triangular shape but with a major caveat — it’s inverted. That is, instead of the delta splitting into numerous channels in a downstream direction, it is characterized by numerous channels coming together in a downstream direction. The geologic history of the greater Bay Area helps explain this rather unique delta geometry. Unlike the Nile, Amazon, Mississippi, and other major river systems, the location where the Sacramento-San Joaquin rivers meet sea level is: (1) well inland of the coast and (2) strongly controlled by the topography of the region.

The Sacramento-San Joaquin Delta is known as a bay-head delta, which is when a delta forms at the head of a large estuary like the San Francisco Bay. When sea level was much lower during the last ice age the river met the sea at the position of the Farallon Islands. As sea level rose and the valleys that are now the Bay flooded, the river mouth moved inland to its current position. The complex topography of the Bay Area — a result of active faulting associated with the San Andreas, Hayward, and other faults — has forced the channels in the delta to come together at Carquinez Strait.

Future sea-level rise will affect the delta region, especially Suisun and Grizzly Bays, significantly. Even a relatively small rise will change the character of these wetland areas. Further east, near Antioch and Lodi, the delta is actively subsiding (sinking), which could exacerbate the negative effects of a rising sea level even more.

Images: (1) Nile River Delta; credit: Wikipedia, (2) Basemap from FlashEarth, annotation by me.

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