The Other California: A "River" Runs Through It...And So Do Movie Crews

The term "river" and Southern California are rarely used in the same sentence. There are some things in the south part of the state called rivers, but anyone not from SoCal could ever mistake these miserable trickles of water for a real river. And yet...there are canyons in southern California that rival the Grand Canyon in depth. There are any number of impressive gorges that cause one to wonder how they ever came to be in this dry climate.

We discovered one of these canyons during our brief exploration of the Santa Monica Mountains last summer. We first crossed the mountains the previous day over a steep, narrow winding road over a high pass.  But the next morning we crossed the mountains by simply following a canyon all the way across. How did the little stream in Malibu Canyon manage to carve its way across the mountains?

The Santa Monicas are geologically a very young mountain range, having been pushed up only in the last few million years. One would not expect that rivers would be able to effectively carve deep canyons in such a short period of time, but that doesn't take into account several other factors: the increased precipitation during the ice ages, almost yearly flash floods, and the effect of rare but astonishing "atmospheric river" storms.

Malibu Creek, which has carved the deep canyon in the photos above, was in its present path before the Santa Monica Mountains began rising. As the mountains were pushed up, erosion by the river was able to keep pace, and Malibu Canyon was the result. Such rivers are called superposed or antecedent streams. It is an impressive (if not overly busy) way to cross the mountain range. Malibu Canyon is particularly deep because it has a much larger watershed than other creeks that don't cross the range.

In the heart of the mountains we encountered Malibu Creek State Park, a strikingly beautiful area of high sandstone cliffs. As we drove through, I had a sense of déjà vu, even though I was sure I had never been here before...

We took a side road above the park to check out some exposures of the Monterey shale. The Monterey is composed of diatomite, shale and chert that was deposited in deep marine basins along the California Borderland during the Miocene epoch. It is a source of much of California's oil reserves.

Along this stretch of the Mulholland Highway, the Monterey Shale is severely folded.

Looking down into the heart of Malibu Creek, I again had that sense of déjà vu. Korea looks like this doesn't it? Or is it the foothills of the Rocky Mountains? Or was it the high prairies? Well, actually those places don't look like this at all, but we think they do because Hollywood has made use of this landscape as a geographical substitute in numerous movies and television shows. The MASH set was in the canyon below the exposures of the Monterey (below). I could almost hear the helicopters...

A short distance down the road we found the Paramount Ranch, a former studio property that is now a part of the federally administered Santa Monica Mountains National Recreation Area. Dr. Quinn, Medicine Women was filmed here, as were a great many movies including Paleface" (1948) and "Son of Paleface" (1952), "Gunfight at the OK Corral" (1957), "Fancy Pants" (1950), "The Virginian" (1946), "Whispering Smith" (1948), "The Forest Rangers" (1942), "Miracle of Morgan's Creek" (1944), "The Perils of Pauline" (1947), "Geronimo" (1939), "The Streets of Laredo" (1949), "Buck Benny Rides Again" (1040), "Ruggles of Red Gap" (1935)," "Gunsmoke" (1931), "The Plainsman" (1936), "Hopalong Cassidy Returns" (1936), "Wells Fargo" (1937), "Union Pacific" (1938), "The Adventures of Marco Polo" (1938), "The Adventures of Tom Sawyer" (1938) and "Reds" (1981).

A western-style movie set is still maintained, and the park is still used for film productions. The day we came through, someone was preparing to shoot what the ranger described as a hip-hop video as imagined in the wild west.

We were star-struck! Well, ok, maybe not, but it was fun to imagine the history that infused this place, even if the 'history' was all imagined in the first place. I always wondered why Little House on the Prairie was surrounded by hills and mountains covered in brown grass and oak trees instead of prairie flatlands (Little House was filmed outside the Simi Valley a few miles to the north).



The Other California is my continuing series of the lesser-known geological sites in our beautiful state. Thanks to Mrs. Geotripper for the use of some of the pictures above!

10 Reasons I Love Teaching Geology at a Community College

Oh, I cannot resist a meme involving top ten lists! Eric Klemetti at Eruptions started it on the topic of liking volcanoes. He was followed by Callan at Mountain Beltway on geologic structures, Siim Sepp at Sandatlas on sand,  Silver Fox at Looking for Detachment (on detachment faults, of course), and Hollis at In The Company of Plants and Rocks (on Wyoming). I teach geology at a community college, and I can't imagine a greater career. My list isn't on a specific geologic topic, it's about the joy of teaching about the Earth...

What happens when you teach geology at a community college?

We get to study anything we want
When I was in the master's program at Reno back in a different century, I had trouble settling on a geological discipline. I finally pursued a project in neotectonics, but I was constantly distracted by other stuff that was going on. Being at a community college means I have few opportunities for directed research, seeing as how I teach a large number of classes and labs. If a student asks a question on some area of the earth sciences, I get to track it down. Some years it's dinosaurs and paleontology. Other years it's seismology, or metamorphic rocks of the Sierra Mother Lode. I love learning about all of it.

We get to travel a lot to interesting places
I take my students a great many places around California, the American West, and the world. If you are are regular reader of my blog you already suspected this...

We get to meet interesting people
There seems to me to be no truer melting pot in our society than the campus of a community college. In a classroom you have people from all kinds of backgrounds thrown together: conservative and liberal (and "don't care"), rich and poor, young and old, people of faith and atheists, and people of many different cultures. The college lecture hall and especially the college laboratory is a place where people meet as true equals, and some actual communication can take place between people who would otherwise never interact. And I get to meet and learn from them all! I figure that in my 23 years of teaching, I have met something like 10,000 people. I obviously can't remember every single one of them, but it is kind of neat to be in some public place and have a stranger walk up and say "Hi, Mr. Hayes! You were my teacher years ago!"

We get to achieve goals:
Sometimes I sit in a restaurant or wander slowly through a store and I think about how the business has been there for whatever total of years, and the employees put in their shifts and look forward to their breaks. There is always a lot of hustle and bustle, there is always routine. The faces change, and sometimes the owners change, and at the end of the day, what has changed? A few more people have been fed, some more items have been sold, and for what purpose? Making money, and staying in business for another day. That is all well and good for a capitalist system, but what a mind-killing bore. There is nothing to work towards. There are no goals, at least not for the drones who do all the work (I imagine the owners have a goal of dominating their particular business; I learned that from the game of Monopoly). As a teacher, I and my students have a challenge, one that has a beginning and an ending. It may be that a student's goal is to pass the course somehow and move on, so they can start a career and...make money like a good capitalist. But I have the challenge of guiding students to know something more than they did when they started (what we call student learning outcomes). And the information is something that enriches their lives (and in the odd case, maybe even saving their lives; think earthquake preparedness).

We get to be creative
The art of teaching constantly requires thinking up new ways to enable learning on the part of the student. I have to challenge myself to break out of a mold when something isn't working. And I love drawing; by the end of the day I am usually covered with colored chalk. I've also grown to love photography, too.


Geology is fascinating
I can't speak for others, but I taught business math for a few semesters early in my career. If I had been teaching business math or accounting or economics for the last two decades, I think I would be nuts by now. I just can't imagine making certain subjects interesting. But there isn't a day that goes by when something fascinating isn't going on somewhere in the world. The stories are sometimes tragic, and sometimes awe-inspiring, but never boring. Even the most pedestrian subjects can be made interesting in some way (if you work at it)...even soils. We always have something strange and bizarre to talk about in class.

We help people discover a world outside their own
This isn't the same as the note above about traveling. Not all my students can go on field trips, but we get to open their eyes to the existence of strange and wonderful places beyond the confines of their normal everyday lives and hometowns. Every geological process involves examples from all over the planet; we use multimedia examples from our own lives or those of others to illustrate these incredible earth systems.

We get to contribute to the health of planet Earth
The decisions about how best to keep the planet liveable for 7 billion people are being made for the most part by those with a financial stake in outcomes and the politicians they lobby. Their priorities are not always correlated with everyone else's best interests. Without money, our best hope for just outcomes in environmental issues are the votes of an educated population. The community college system is on the front lines of providing that education.

We get to make a real and lasting difference
I am proud of what many of my former students have accomplished. A decent number of them went into geology, but more satisfying to me is how many of them are teachers now. There is a cascading effect of positive outcomes when students become teachers

Let's see, that's nine. What was the tenth? Oh yeah...

We're gonna get rich
Because teachers are held in the highest regard by society and especially by politicians. The politicians think that the people who are responsible for educating our children and teenagers deserve the highest possible compensation for their many years of academic preparation and daily challenges in the classroom. Now, if you actually believe in the accuracy of this last item, I recommend maybe not going into the field of education for a career. But if you want to look back on your life and say I changed things for the better, give a thought to teaching. And it's not too late to start no matter how old you are, or what career you are in now.

The Other California: The Volcanic Mountains of the City of Angels

Volcanic mountains in Los Angeles???

Well, sure there are. Of course we need to define what is meant by "volcanic mountains". If you are imagining a chain of active volcanoes spewing out lava flows down Wilshire Boulevard, well, no there aren't any of those, despite what you might have seen in a Tommy Lee Jones movie a few years back.

Extinct volcanoes, maybe? No, I'm not really aware of any of those in the L.A. region either. I've never noticed any suspiciously conical hills in the metropolitan area in my travels that way. If you want to see a city that will probably be the setting for a great disaster movie (but not a very pleasant event if it were to happen for real), you might try a place like Auckland on the North Island of New Zealand. That particular city is built on four dozen volcanoes, including some that erupted only 600 years ago (below).

Yup, if you are a Kiwi, you can latch onto a 'hot' property here.

But there are volcanic mountains adjacent to Los Angeles. They just aren't actual volcanoes, as the original vents have long since been eroded away. The city is practically surrounded by exposures of volcanic rocks. There are some on the Palos Verdes Peninsula, in the mountains above Orange County, and in the hills near Glendora and Pomona. Perhaps the best place to see volcanic rocks near LA though, is the Santa Monica Mountains and their offshore extension, the Channel Islands.

I am guilty of ignoring the Santa Monica Mountains. I grew up in Southern California and never knew of them being much more than those hills up behind Malibu, and a place where I once attended the Renaissance Faire. I spent my available weekends up in the high country of the San Gabriels, the San Bernardinos, or the Sierra Nevada. This last summer I found that I was missing out on an extremely interesting and diverse mountain range. It has a geology professor's dream of intriguing rock exposures: metamorphic, plutonic, sedimentary, and as the post title suggests, volcanic rocks.

The volcanic sequence is not of a familiar origin. The so-called Conejo Volcanics were mostly extruded on the seafloor, only occasionally rising above the waves as islands. Volcanic activity is usually associated with divergent or convergent plate boundaries. The Conejo lavas were not exactly associated with either situation.

At divergent boundaries, the oceanic crust is pulled apart, releasing pressure on the underlying asthenosphere, a hot layer that is close to the melting point of the olivine-rich rocks of the mantle. The decompression allows a small portion of the asthenosphere to melt, producing basaltic lavas. At convergent boundaries oceanic crust is driven into the mantle underneath the adjacent oceanic or continental crust. As the rocks sink into the hotter asthenosphere, the added oceanic water acts as a catalyst or flux, lowering the melting point of the surrounding rocks. Bodies of magma called plutons rise through the crust forming volcanic arcs like the Andes, the Cascades or the Aleutian Islands.

Hey, the Santa Monica Mountains don't look like this at all...

A few other volcanic systems aren't related to plate boundaries. Thermal plumes, or hot spots, are responsible for sequential series of volcanoes on the sea floor (Hawaiian Islands) or on continents (Yellowstone caldera).

For most of the last 30 million years Southern California has been dominated by the lateral motions of the San Andreas fault system, a transform plate boundary. The fault system has not been a simple line, and the position of what we call the San Andreas fault has shifted about a number of times. One of the biggest changes was a shift eastward as the Gulf of California opened up around 4 or 5 million years ago. Transform boundaries are not normally associated with volcanic activity, but there are definite exceptions, and that seems to be the case in the area that is now exposed as the Santa Monica Mountains and Channel Islands.

The Santa Monica Mountains are part of the California province called the Transverse Ranges. They are one of only two mountain ranges in North America that are oriented east-west instead of the usual northerly trend (the High Uintas of Utah are the other). The discovery and acceptance of plate tectonics provided an elegant explanation for the origin of most of the world's mountain ranges like the Alps, the Himalaya and the Andes. The acceptance of the theory seemed to deepen the mystery of the origin of the Transverse Ranges. How could a northwest trending transform boundary produce an east-west trending set of mountains? Or did these mountains have anything to do with the transform faulting at all? Some researchers suggested the structure of the Transverse Ranges predated the onset of transform faulting. The Transverse Ranges are a treasure trove of ongoing and potential future research in structural geology and landscape evolution.

Tanya Atwater and the Educational Multimedia Visualization Center at U.C. Santa Barbara have provided us a marvelous animation that offers an excellent explanation for the onset of volcanism in the ranges around 13-16 million years ago (there are many excellent animations available for free download on the site).

As you watch the video unfold, you will see that the crustal blocks that constitute the Transverse Ranges have been literally rotated 90 degrees or more, having been caught between strands of the fault system. As the blocks rotated, extensional forces caused openings in the crust, forming deep sedimentary basins that collected thousands of feet of sediment, and also allowing magma to rise from the mantle forming volcanoes on the seafloor,  These are the rocks that make up much of the Santa Monica Mountains.

This animation is courtesy of the Education Multimedia Visualization Center.

Download this and many others at this site.

These volcanic rocks formed more than ten million years ago. Tectonic conditions have shifted and much of the region is now under the influence of compressional forces that are much less likely to result in volcanic activity. But it makes for a chaotic mess of earthquake activity as events like the 1994 Northridge earthquake can attest.

The Santa Monica Mountains are a confusing patchwork of federal, state, county, and city parklands and include large areas of private property. Land use ranges from densely urbanized areas (the mountains include parts of Hollywood and Beverly Hills ) to untrammeled wilderness. A good place to start planning your exploration is the website for the Santa Monica Mountains National Recreation Area. A look at the park map reveals the logistical headache of trying to administer the parklands.

The Other California is my ongoing exploration of the less-known geological corners of our beautiful state. Tens of millions of people visit the California Coast every year, and on one Labor Day weekend something like 800,000 people may hit the beaches at Malibu. In a year less than 500,000 will visit parks like Malibu Canyon or all of the federal units combined. The interior parks are great places to escape the crowds.

In the next post, we'll take a look at an unexpected canyon in the Santa Monica Mountains, and what does Korea have to do with anything?

So Much for the Green Flash...But is the Coast Toast?

So much for the green flash...the consensus seems to be that I didn't see one, although I stubbornly hope that I saw it but just didn't happen to photograph it...anyway, courtesy of wikipedia, here is what one type of a real green flash, a mock-mirage flash, looks like.

I'm thinking that seeking a green flash and failing like I did is a bit like those first few times panning for gold. You'll see pyrite (fool's gold) a dozen times and think it might be gold, but when you see a gold flake for the first time you'll never misdiagnose pyrite again. I shall keep searching for the green...

I enjoyed the sunset that night immensely. There were all kinds of things going on, in the sky and over the water. The optical effects of a setting sun are always interesting of course. The distortion almost looked like a bomb blast. Or a light bulb...

It was interesting to see the amount of damage that has been done by wave action. This used to be part of the parking lot at Leo Carrillo beach. This is a look at the future; rising sea level and declining supplies of sand can only mean that coastal erosion will be increasing in extent and intensity.

 The seas were calm on this late summer afternoon. The effect was hypnotic and peaceful.

There were dozens of pelicans flying west along the coast line.  I love how they glide just inches above the waves; they are such graceful fliers. The Brown Pelicans almost disappeared in the 1960s as they absorbed so much DDT in their diet that their eggshells weakened and broke. With the ending of the use of DDT as an insecticide in the 1970s their population rebounded strongly.

What a shame it would have been if they had disappeared entirely. Does anyone besides me and Steven Spielberg think that they look like resurrected pterodactyls?

The sun sank below the horizon and the clouds briefly turned orange and pink. We headed back to camp up in the canyon.

I glanced at one of the rocks on the beach that was being used as rip-rap (wave barriers). The holes made me wonder about something. Fifteen years ago a movie was released to a certain amount of derision among geologists: Volcano. Tommie Lee Jones played a gruff but lovable emergency services director, and Anne Hecht played the geologist with the heart of gold. The volcano sucked (literally it sucked: Anne's friend in the movie got sucked down a hole into the lava). There were earthquakes, there were exploding buildings, there were lost children, and melting people.

Obviously, the idea of a volcano erupting in downtown Los Angeles is preposterous. There must not be a volcano within hundreds of miles of the city. I mean, there aren't any volcanic mountain ranges in southern California...right? The faults are all strike-slip and thrusts. It just doesn't seem likely. So how to find out? Maybe, a look at the interactive geologic map of California from the California Geological Survey (clue: most of the volcanic rocks on the map are pink or orange...).

Is the coast really toast, or is it already toasted?

Searching for the Green Flash...

No, not a super-hero. The green flash that sometimes occurs as the sun sinks below the horizon. It lasts for a few seconds and to be honest, I'm not sure I've ever seen it. I got to thinking about it because Andrew Alden over at About Geology mentioned it in a recent post.

I was doing my darndest to catch it last summer on what had been a kind of somber day. We had visited the site of the St. Francis Dam disaster earlier in the afternoon for the first time, and after a long drive, we arrived at our campsite at Leo Carrillo State Park along the Malibu coast (yes, there had been another lucky reservation cancellation).

The canyon where the campground was situated was already deep in the shadows, so we headed out to the beach to watch the sunset. The pelicans did a nice job of helping to frame the setting sun...

So here's the thing: just as the sun hit the horizon, my autofocus started dropping in and out of focus, so I just snapped shots hoping one of them might catch something of the flash. I think I caught it below...but what do you experts out there think??

The Other California: "Surely they didn't build it there" The 2nd biggest disaster in California history

Hindsight is harsh.

Sometimes choices and judgements are made to save time, to save money. Sometimes choices are made in unfortunate ignorance, in a time when no one could have foreseen or recognized the right choices to be made. Sometimes there is no one there to provide perspective, to provide alternatives. And then people die. Lots of people.

Ask folks what they think was the worst disaster in California history and many will get it right. Upwards of 3,000 people died in the 1906 earthquake in San Francisco, and the event has shaped the psyche and attitude of many people in the state more than a century afterward. And it was brought about by a natural event.

The second worst disaster in the history of the state is far less known. Some might guess another earthquake, like the Long Beach quake of 1933 (115 dead) or the Loma Prieta earthquake of 1989 (63 dead). Historians might point to the Port Chicago munitions explosion of 1944 (320 dead). Few people are aware that it was the collapse of a dam, and that the collapse was the result of many poor choices. Hindsight is a harsh judge, but many of the mistakes were "before their time" so to speak. The fact that it happened maybe has prevented worse disasters in the intervening years.

Time (and a great deal of government effort) has erased much of the record of our state's second worst disaster. As far as I could see there is not a single plaque or monument, either concerning the horrific event, or commemorating those who were lost. There is a small cemetery where some of the victims were buried.

Looking at the slide area on the left side of the picture above, it is hard to believe that a 200 foot high dam was anchored there, in the incompetent mica schist. It is hard to believe that the failed slopes in the picture obscure an even deeper and bigger megaslide.

It is hard to look at the flat ridge on the right side of the picture above and realize that no one ever though to check the effect of soaking the seemingly solid conglomerate in water. It is glued together primarily with gypsum, a mineral that dissolves in water. The rock falls apart when saturated.

Maybe the most stunning realization is that the schist and the conglomerate are separated by a fault zone. An inactive fault by all appearances, but a fault nonetheless. They built the dam on a megalandslide, and on a fault zone.

It is difficult to envision that on the night of March 12, 1928, the recently completed dam failed so catastrophically that the floodplain in the photos above and below was inundated with 140 feet of water flowing at a rate of 1.7 million cubic feet per second (California's biggest river, the Sacramento, averages 30,000 cfs, and the record flood on the river was 650,000 cfs).

What happened?

As Ron and Randy correctly surmised, Friday's mystery photo was about the destruction of the St. Francis Dam in 1928. I consider it one of the most important geological events ever to happen in the state, not because a great many people died, but because they died as a result of a disregard or lack of knowledge concerning human construction projects and the geological foundations on which they are built. Earthquakes and volcanic eruptions are inevitable geologic events, but the events of 1928 were completely avoidable.

In the early twentieth century, Los Angeles was at a crossroads. The city was growing fast, and the water needs of the metropolis far exceeded locally available supplies (according to city officials anyway). The story of how the city stole (legally stole, but stolen nonetheless) the water from underneath the people of the Owens Valley is a legend of California history. The fact that much of the water went to irrigation in the San Fernando Valley instead of the city just added to the scandal. Having completed the Owens Valley Aqueduct, one of the largest public waterworks ever conceived, the city needed someplace to store the water locally, especially in preparation for drought conditions. William Mulholland, the superintendent of the predecessor to the Los Angeles Department of Water and Power, oversaw the design and construction of a series of reservoirs around the Los Angeles Basin. Nine were constructed, and St. Francis Dam in San Francisquito Canyon above the Santa Clarita Valley was the largest, with a storage capacity of 38,000 acre feet. The dam itself was about 200 feet high, and just over 600 feet across. It was a concrete gravity-arch dam, one that depended on the nature of the rock in the abutments to maintain stability.

Construction was begun in 1924 and complete in 1926. During the construction Mulholland directed that the dam be made 20 feet higher than in the original plans, but he made no alterations at the base to compensate for the additional weight of the water. The filling of the dam took another two years, and was complete on March 7, 1928. On the morning of March 12, the dam keeper noted a leak of muddy water and alerted Mulholland. Small leaks of clear water from dams are usually expected; muddy leaks from a dam are very bad.  Mulholland declared that the mud was from some recent road construction and that the dam was safe. 12 hours later, the dam keeper was dead, the first victim of the collapse of the St. Francis Dam.

To his credit, Mulholland took the blame for the disaster. Although he was never convicted of any crime in the matter, his career was over. He died seven years later.

Accounts at the time suggested that failure occurred as water channeled through the conglomerate along the fault contact. A reassessment of the failure by J. David Rogers finds multiple causes for the disaster, with the reactivation of the ancient landslide being the most important factor, along with hydraulic lifting of the dam which was caused by water pressing against the topmost part of the dam (which had been made higher without compensating at the base). Rogers lists many other deficiencies, including the weakness of the rocks in the dam abutments (I refer interested readers to this very fascinating pdf by Rogers that provides a blow-by-blow analysis of failure of the dam and a great deal of background information on the disaster).

Incredibly, despite the total evisceration of the dam, the central part remained standing, a 200 foot high monument to the destruction. After a sightseer fell off the top (his "friends" had tossed a rattlesnake at him), the city quarried holes in the base, filled them with five tons of dynamite, and blew up the remaining tower. Other blocks were also destroyed, as if they were trying to erase all memory of the event. One of the blocks was the "outcrop" I used in the Friday mystery photo.

The U.S. Geological Survey has a (much appreciated) photo archive from which I have gathered these photographs of the aftermath. In the photograph below, the fault line dividing the Vasquez Conglomerate from the Pelona Schist can be clearly seen (the lighter Pelona in the foreground, the dark Vasquez on upper ridge). The fault is inactive, and no earthquakes are implicated in the failure, but had the dam not failed, rising water pressure along the fault could conceivably have eventually caused renewed quake activity. The phenomenon has been noted elsewhere.

Blocks of concrete weighing thousands of tons were carried in the floodwaters nearly a half mile downstream. The magnitude of the disaster is hard to comprehend. Normal rivers have trouble moving boulders only a foot across. Besides the sheer magnitude of the flow, debris from the landslide buoyed up the blocks.

The block below was a half mile downstream. It measured approximately 63 feet long, 30 feet high, and 54 feet wide.

It is hard to find much that is positive in this disaster, but changes were made in the aftermath. The input of qualified engineering geologists became a requirement in dam-building, and much more attention was paid to the geological setting of reservoir sites. Boulder Dam on the Colorado River, one of the largest dams in existence is not in Boulder Canyon. Following the St. Francis disaster, the site of the dam was changed to Black Canyon when it was decided that the rocks that would anchor the dam were more stable there.

It would not be at all correct to say that we learned every possible lesson in dam construction. The 1963 tragedy at Vaiont Reservoir in Italy and the 1975 collapse of the Teton Reservoir, Idaho are vivid examples of unlearned lessons.

Hindsight is harsh. But it can be a teacher, too.

The Other California: Adding Context to a Friday Mystery Photo

I am always pleasantly surprised by the insight of the commenters on this blog, especially in light of yesterday's mystery photo. I provided very little in the way of clues, but many of you saw things that I didn't when I was standing on the hill above the outcrop. Part of the difficulty was that I was on the top of the hill without binoculars, and the photo (taken by Mrs. Geotripper) was on an extreme zoom. I was also flying blind, so to speak, with no maps or field guides about the road we were on. So today I am providing lots of context to judge what's going on in the picture. This will be a two-parter...

First is the google image of the site (above), and a wider view of the outcrop (below). One thing that stands out about the San Gabriel Mountains is the extreme steepness of the terrain.  As I have noted several times (here, for instance), the only flat places are on river floodplains or the tops of landslides. That makes the green thicket of trees on the upper left of the GoogleEarth image immediately interesting. Why are the trees there, but not in other parts of the canyon floor? The second thing to note is the constriction in the canyon on the downstream side of the floodplain forest. Why is it there?

The third feature is the linear nature of the canyon itself. Streams tend to erode sinuous gorges unless the underlying geology (i.e., weak, easily eroded rock) is guiding the direction of the river carving. Why is that canyon so straight?

Here is the context shot. The dark oak tree in the upper left can be seen in the GoogleEarth image in the lower center as a dark dot where the canyon widens just downstream of the forest. The "outcrop" lies in the midst of the wider floodplain at the very bottom of the satellite image.

The linear canyon is rather adequately explained by the presence of a fault juxtaposing two kinds of rock. The rock on the west side (below) is an Oligocene terrestrial conglomerate-sandstone called the Vasquez Formation (known also as the Sespe Formation).

The gray rock on the east side (right side in the GoogleEarth image) of the fault is the Pelona Schist, a rock composed of parallel layers of muscovite mica and quartz grains. The rock is not overly tough, as the mica is soft and the layers separate easily. The Pelona is subject to landsliding...

Here is a final context shot for the mystery (below). The story is perhaps becoming clearer: the San Gabriel Mountains have been rapidly uplifted, so that even weak rocks have been deeply eroded and formed steep slopes. Fault lines cross the mountains, including this locality, so the canyon eroded in a straight line that divided the Vasquez Formation from the Pelona Schist. The Pelona is prone to slope failure, so ancient landslides blocked the stream, forming temporary lakes that filled with sediment, providing a nice environment for the growth of a riparian forest. The constriction in the canyon is the site of the old landslide.

All of which doesn't exactly provide an answer to yesterday's mystery. Many commenters (who never cease to amaze me with their insight) suggested the idea of a debris flow, and this is correct in a way. Others noted the regular pattern of holes and suggested that the outcrop has been quarried or otherwise altered by people. Also true, in a way.

Before I stumbled onto what was going on, I was thinking that this was an outcrop of the Vasquez conglomerate, but I was bothered by the bleached aspect (the Vasquez is red-brown). I thought maybe I was looking at a displaced fault sliver in which hot acidic groundwater had altered the original material. But when I saw the holes much later while processing the digital images, I realized I was looking at something much different. And something very tragic...

The next post may be titled "They didn't. Surely they didn't put it there..."