What's Burning Up Tonight: The Blue Cut Fire in Southern California

Source: http://www.fire.ca.gov/general/firemaps

California is burning. Sometimes, the fires are burning in unfamiliar places, places we've never visited. Logically, we know they are tragedies, that people are losing their homes, that beautiful forests are disappearing, but there's nothing quite like having a familiar place go up in flames.

Fires are part of the natural landscape, an outgrowth of a Mediterranean climate where no rain falls for six months out of the year. Plants and animals are adapted to fires. But things have changed, and some of the fires have become terrifying disasters. First of all, the growing population is expanding into ever more dangerous terrain, places that burn regularly, whether developed or not. But second, the effects of global warming are intensifying droughts that have left most of California dry and vulnerable.

The Blue Cut Fire is burning in an area I know well. I grew up at the foot of Lytle Creek and Cajon Pass, and have traveled over the pass perhaps hundreds of times over the years. The fire exploded in one day to more than 25,000 acres, and conditions offer little hope of any type of containment very soon. An unknown number of structures have already been destroyed, and an unprecedented 35,000 houses are threatened in places like Wrightwood, Phelan, and Lytle Creek. Something like 80,000 people have been evacuated. I hope and pray that it will be contained and controlled soon with a minimum of damage.

I wrote about the region a couple of years ago. Lone Pine Canyon is in the middle of the area that is burning, so the places seen in these pictures will be looking very different now. What follows is my post of March 31, 2013:

The Other California: Getting a Good Look at California's Faults

The San Andreas fault in Lone Pine Canyon near Cajon Pass in Southern California. Photo by Mrs. Geotripper

If there is anything that people know about California, it's the San Andreas fault. In a vague sense anyway...California has a lot of earthquakes that many people assume happen along the San Andreas (most don't), that there are occasional BIG ONES that happen on the fault (some of them, but not all), and if popular culture as expressed in movies like the original 1978 version of Superman reflects common knowledge, no one knows where it is or what it looks like (oh, and California is going to fall into the sea).

Still, the San Andreas is a fundamental part of the structure of California, and it is indeed capable of causing a great deal of seismic havoc, with some parts of the state considered at a high risk of moderate to large earthquakes. If one is curious about seeing the state's most famous fault line, there are a great many places to explore it, some in surprisingly visible locations. We are exploring one of them today in a segment of my "Other California" blog series.

Thousands upon thousands of cars follow Interstate 15 daily across Cajon Pass on their way to Las Vegas or their jobs in the L.A. Basin. I imagine few of the drivers know they are crossing California's most famous fault just a few miles north of the Interstate 15/Interstate 215 above San Bernardino. For a few moments, travelers are treated with a spectacular view up the linear track of Lone Pine Valley (the picture at the top of today's post shows the view, taken at 65 mph). I explored the lower end of Lone Pine Canyon and the "mysterious" Lost Lake (really not mysterious) in a post several years ago (click here to see it), but a weekend or two ago I had a chance to explore the upper end for the first time in a few decades.

The Mormon Rocks (or Rock Candy Mountains) expose the 18-20 million year old Cajon Formation. Interstate 15 climbs towards Cajon Pass on the skyline.

The road through Lone Pine Canyon takes off from Highway 138 about a mile west of the junction of 138 and Interstate 15. The road climbs through some beautiful exposures of the Cajon Formation  (the tilted exposures of the arkosic sandstones and conglomerates are called the Mormon Rocks or the Rock Candy Mountains; I've written about them before). The rocks have been tilted and folded due to their proximity to the San Andreas fault. The road surmounts a low pass and drops into Lone Pine Canyon.

The San Andreas fault is not a single break, but is instead a half-mile wide system of different slices of crust. Constant shearing over the last few million years by fault motions has left the rock fractured and easily eroded, forming the distinct linear valley of Lone Pine Canyon.

The series of fault blocks have formed a series of benches and scarps on the southwest side of the canyon, and a few offset channels are easily seen in the chaparral-covered hillsides (above).

Approaching the upper end of Lone Pine Canyon, we can see an odd hill in the middle of the valley. Looking at the steep slope to the right, it is apparent that we are looking at a rather large debris avalanche, caused no doubt by an earthquake along this stretch of the fault (above). The road skirts the lower end of the slide, confirming the nature of the hill with the broken and brecciated rock (below). There are plenty of candidates for the causative event; major earthquakes happened along this stretch of the fault in 1857(info here), 1812 (info here), and as many as a dozen other times in the last 1,500 years. There is a distinct warning in the earthquake history for this stretch of the San Andreas: the recurrence interval of large quakes is just over a century but it has been more than 150 years since the last major event.

The road reaches the top of the canyon, and a short walk along the ridge at the summit provides a spectacular view back down Lone Pine Canyon towards Interstate 15, and the two largest mountains in Southern California, San Gorgonio Peak and San Jacinto Peak. The fault passes between them on the way to Palm Springs and the Salton Sea.

 There is a large outcrop at the head of the canyon that exposes highly sheared rocks from the fault zone. The original rocks may have included the Pelona schist and granitic rocks but they are almost unrecognizable in hand samples.

But one thing is kind of cool: how often can you hold a major fault zone in your hand?

The road through Lone Pine Canyon ends in the village of Wrightwood, the subject of our previous post about a recurring mudflow originating in the Pelona Schist on the high ridges above. As if they didn't have enough to worry about...

The Other California is my continuing blog series on the geologically fascinating places in our fair state that don't always make it onto the tourist postcards.

The Airliner Chronicles: the San Andreas Fault in Southern California

The San Andreas fault at Cajon Wash and Lone Pine Canyon at the eastern end of the San Gabriel Mountains

Oh how I love flying. I don't get all that many chances to do it, and flying on days with clear weather happens even less often, but sometimes it works out. One of my first blog series was called the Airliner Chronicles, and in it I featured geology from an aerial perspective. I haven't flown in a number of years, but I had a brief trip to Southern California last month, I scored a window seat, and managed to chose the correct side of the plane, so I got to see some marvelous perspectives of the San Andreas Fault.

Mount San Antonio is the highest peak in the San Gabriel Mountains at 10,069 feet (3,069 meters). The highway in the foreground is Interstate 15 heading towards Cajon Pass.

What made the views so memorable was that they were in the San Gabriel Mountains, which just a few weeks ago were made into a National Monument. These were the mountains of my youth, the peaks and valley where I learned to camp, hike, and climb. I've seen these mountains from down in the canyons, and from the tops of the peaks, but never so well from high above in a plane. The flight was too short!

The San Bernardino Mountains are higher than the San Gabriel Mountains on average, but not generally as rugged. I was on the wrong side of the plane for good pictures, though.

The San Andreas fault slices through 600 miles of California, but the most rugged stretch of its path is along the northern edge of the San Gabriel Mountains. The fault actually crosses the Transverse Ranges, offsetting the San Gabriels from the San Bernardino Mountains at Cajon Pass. The presently active stretch of the fault has shifted at least 130 miles in a right lateral sense (when standing on one side of the fault, the other side shifts to the right; a left lateral moves the opposite direction). The grinding of the rocks along the fault produces a powdery material called gouge that is easily eroded. The fault reveals itself as a series of linear valleys that cut across the mountain ridges. I've provided labeled versions of most of the pictures showing the trace of the fault.

The San Gabriel Mountains are moving northwest along the fault.

I passed the small mountain village of Wrightwood, a town with a lot of geological problems. Not only does it sit directly on the San Andreas fault, it also lies at the base of a steep mountain ridge composed of deeply sheared Pelona schist. When wet, the schist is prone to failure, and there have been a number of serious mudflows over the years including an event in 1941 that buried 190 acres. The scars of the landslides were easily visible from above.

The village of Wrightwood on the San Andreas fault. The Heath Canyon Mudflow started at the scars on the left side of the photo.

The San Andreas has produced a number of very large earthquakes in Southern California, most notably in 1857, an event with an estimated magnitude of 7.9. The quake fractured the ground for a distance of 225 miles (350 kilometers) from Cajon Pass to Parkfield in the Central California Coast Ranges. The ground shifted as much as 30 feet (9 meters), with an average of 15 feet (4.5 meters). Trees that were damaged in the quake can still be seen in Wrightwood. The average recurrence interval over that last few thousand years has been about century, but it varies widely from a few decades to more than 300 years. It is considered one of the more likely faults to produce large earthquakes in coming decades.

Wrightwood on the San Andreas fault in the San Gabriel Mountains

A bit further I could pick out the Mountain High Ski Resort and the deep gorge of the East Fork of the San Gabriel River. The heart of the newly established San Gabriel Mountains National Monument, the East Fork is around 4,000 feet deep, almost as deep as the Grand Canyon.

Mountain High Ski Resort and the East Fork of the San Gabriel Canyon

The canyon is unique. It was the focus of a huge political battle in the 1970s over a wilderness designation that brought much of the canyon under protection from mining and development. There are herds of Nelson Bighorn Sheep and natural trout fisheries in the depths of the canyon tributaries. Development was always a dicey proposition. There is little in the way of flat terrain, and terrible floods have scoured the canyon bottom on numerous occasions. There is a paved "bridge to nowhere" in the lower canyon. The road it was once connected to was washed away long ago.

Further west there is a fine example of a shutter ridge. These are mountain ridges that have been moved laterally by the fault in such a way as to block of the river drainages from the other side of the fault, deflecting the streams sideways.

A shutter ridge along the San Andreas fault in the San Gabriel Mountains

The winding road in the picture is the Angeles Crest Highway, which was opened for traffic in 1956. It is an anachronism, built in a time when development was encouraged in the national forests. It goes for 66 miles from La Canada in the L.A. Basin to Wrightwood. In such steep terrain, keeping the road open is a challenge. Landslides and slope failures are commonplace along the highway.

Shutter ridge on the San Andreas fault

 A special geological treat from the air is Devils Punchbowl, a Los Angeles County Park. The park lies on an ancient strand of the San Andreas fault. The sandstone and conglomerate of the Punchbowl Formation was twisted into a plunging syncline, a downward pointing fold with an axis that slopes to the west (to the right in the picture below).

The Devils Punchbowl, a west plunging syncline in resistant layers of conglomerate and sandstone.

Devils Punchbowl is a marvelous little park with memorable hiking trails that explore the steep fins of sandstone. You can see some pictures from ground level of the region in this post from last year.

Offset stream near Palmdale. The dark line is the California Water Project.

As we neared the town of Palmdale, I could see examples of offset streams. These are stream channels which have been shifted and separated by the lateral motion along the fault.

My flight also included a stretch of the fault system a little farther north. I'll try to include it in the next post.

Finding Faults and Bottomless Lakes in Southern California

My previous post on the geologic hazards of living at the eastern edge of the San Gabriel Mountains resulted in a fair number of comments, essentially all correct, pointing out that the local inhabitants are threatened by earthquakes, landslides, mudflows, floods, fires, and...pet-munching critters. Some also pointed out that every locality faces natural hazards, which is very true, but not all places are equal! One of the serious hazards of living in California is the ever-present threat of major earthquakes, due to the proximity of numerous active fault systems. But ask a Californian (or anyone else for that matter) the name of a fault in our state, and the answer is invariably "the San Andreas". Ask for the name of another, though, and most people blank. The next question one might ask is "how close do you live to the San Andreas, or any other fault?" Unfortunately, that question may result in a blank stare, too.

So, today's post is about finding fault in California. If one thinks he or she has never seen the San Andreas or any other fault, they might find that they actually have, but just didn't know it. The San Andreas in particular is spectacularly exposed along several major freeways leading out of the Los Angeles Basin, along Interstate 5 at Gorman, Highway 14 at Palmdale, and on Interstate 15 over Cajon Pass. That's where we are exploring in today's adventure. A few miles north of the junction of I-15 and I-215, the Interstate climbs a hill, and an exceedingly straight valley appears to the northwest (the picture above). It is called Lone Pine Valley, and it is one of the better exposures of the San Andreas. One can leave the highway at Kenwood Avenue or Cleghorn Road and follow the original Route 66 to see some interesting fault features, including an exposure of the rocks in the fault zone at the Blue Cut.

But first we have to find the Bottomless Lake! It is not clear to me how the whole thing started, but a short Google search reveals discussions of the mysterious bottomless lake in Cajon Pass, including this delightful little excerpt:

"I was there swimming not to long ago and saw a man catch a fish that looked like a bass but the eyes were huge and almost popping out! He was gonna take it home to find out what it was but released it fearing it might die and it turn out to be a pre historic fish. I just believe that its a bass that was way deep down under and its eyes popped out as it was surfaced. But what has my attention is the rumors of lost lake having no bottom! Any info?

Anonymous (not verified) on Mon, 09/06/2010 - 9:32pm.
THATZ TRUE LOST LAKE HAVE NO BOTTOM THEY CHECK ALREADY"

I always find myself wondering who the mysterious "they" are...

Anyhow, Lost Lake is a sag pond along the fault trace, which is fed by springs (groundwater is often forced to the surface along fault lines). I am guessing that the "bottomless" rumor came along when the association of the lake with the fault became locally known... a line of thinking that might go... "faults go to the center of the earth, or something like that, so a lake on the fault won't have a bottom either". Of course one can see the bottom of the lake in the GoogleEarth image, and the crime report I read noted that divers looking for the body mentioned that the the lake is barely 20 feet deep.

The lake has been a trash heap and hangout for undesirable people over the years, but it looked on my visit like the Forest Service was trying to improve the situation. There was not as much trash as I remembered from the past, and signs noted that a recreation fee was required for parking. The cattails were healthy too.
A short walk up the hill provides a view of the fault scarp and the Blue Cut, an exposure of the mangled and broken rock within the fault zone. In the picture below, the San Andreas cuts through the left flank of the hill in the distance and crosses the freeway in the left margin of the picture. Lost Lake is the small pond in the center of the picture.
The GoogleEarth image below shows the fault relations at Lost Lake, including a very nice example of an offset stream. The San Andreas is a right lateral fault, meaning that an object across the fault from the observer is moving to the right (note the arrows in the picture at the bottom). Terraces, scarps and channels at Lost Lake have been extensively studied to construct a chronology of past earthquakes at this site, and there have been some big ones (an older reference is included below). Two historical events involved nearby portions of the fault, the 1857 Ft. Tejon earthquake (with a magnitude in the vicinity of 8), and possibly in 1812, an earthquake that caused serious damage as far away as Mission San Juan Capistrano. The average recurrence interval may be as little as 100 years.

And all of this can be seen from a quick detour off the freeway! It's a great place to learn about faults.

Weldon, II, R.J., and Sieh, K.E., 1985, Holocene rate of slip and tentative recurrence interval for largfe earthquakes on the San Andreas fault, Cajon Pass, southern California: Geological Society of America Bulletin, v. 96, pp 793-812

The Other California: Cajon Pass and No Strange Sci-Fi Creatures

One evening as the sun went down
And the jungle fires were burning,
Down the track came a hobo hiking,
And he said, "Boys, I'm not turning
I'm headed for a land that's far away
Besides the crystal fountains
So come with me, we'll go and see
The Big Rock Candy Mountains

In the Big Rock Candy Mountains,
There's a land that's fair and bright,
Where the handouts grow on bushes
And you sleep out every night.
Where the boxcars all are empty
And the sun shines every day
And the birds and the bees
And the cigarette trees
The lemonade springs
Where the bluebird sings
In the Big Rock Candy Mountains.

The Big Rock Candy Mountains, by Harry McClintock

Wow! Thanks for all the guesses and commentary on yesterday's mystery photo. Your answers all made sense, but the winner is the Rock Candy Mountains (also known as the Mormon Rocks).

I grew up in Southern California, and was quite familiar with the Rock Candy Mountains in Cajon Pass, and I thought the rocks were related to the song above, which you might remember from the movie "Oh Brother, Where Art Thou?". I can't find any evidence that they were the mountains of the song, but I vividly remember doing field work along the train tracks, and seeing a dozen men or more riding on each train rumbling down the pass.
Millions of people see the rocks every year as they speed up Interstate 15 on their way to Vegas or Mammoth, and I doubt that one in a thousand ever stops to have a look. It is worth your time! Cajon Pass is a geological puzzle, complicated by the fact that the San Andreas cuts right through it, dividing the San Gabriel Mountains (below) from the San Bernardino Mountains.

The rocks are Miocene-aged (18-20 million years) gravels and arkosic sandstones of the Cajon Formation, and were deposited in alluvial fans that flanked hills and mountains in the Mojave Block off to the northeast. The rocks have been stressed and twisted into a series of folds in the area adjacent to the San Andreas. During our mapping exercises on the site, I got one of my geological lessons in life: asked to identify the rock in an outcrop, we had a long discussion of whether it was quartz sandstone, arkose, or fine-grained conglomerate. The answer? Deeply weathered granite bedrock, not a sedimentary rock at all (the picture below is definitely conglomerate though; that's a quarter for scale on the bottom left)!

As C.B. and others noted, the holes are tafoni, formed as the cement holding the rock together is dissolved, and wind removes the particles. It feels like the rocks are watching you with hollow eyes sometimes. The small caves and openings are favorite nesting spots for raptors and other large birds.
As to the sci-fi reference, I always thought that the Star Trek episode "Arena" with the Gorn had been shot near these rocks, but I thought I'd better check and make sure before I posted the picture yesterday. Sure enough, I was wrong: the episode (and numerous other movie scenes) were filmed at the Vasquez Rocks on Highway 14 between Santa Clarita and Palmdale.

One can certainly argue that the Gorn was not the most ridiculous sci-fi creature ever filmed, but I always thought it looked like a Mardi Gras celebrant who got lost on the way to New Orleans. And then the poor guy got caught in a death match with Captain Kirk...I can hear him now "Hey, I can't get my mask off! Hey, what are you doing with that bamboo and gunpowder?"

The Other California is an ongoing exploration of some of the lesser known geological regions of our beautiful state.