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.

The Other California: A Canyon as Deep as the Grand, and a Road for no Reason

OK, not quite as deep as the Grand, but who's to quibble over a few hundred feet? That's what I love about California: it has a lot of famous places that everyone has heard about, places like Yosemite, Death Valley, and Big Sur, but it also has many corners that are known locally perhaps, but are not all that prominent in the public consciousness. They are parts of state parks or national forests, sometimes private lands, and they offer geologic treasures that would be national landmarks in any other setting. That is the theme of my sporadic posts on the Other California, a description of these unheralded places.

The beautiful skyline in the photo above comes from the high point on the Glendora Ridge Road, a paved highway with no picnic areas, no campgrounds, no signed overlooks, and no particular destination or purpose that is obvious. The road connects Baldy Village at the east end of the San Gabriel Mountains with roads rising out of the San Gabriel River and the urban sprawl of the Los Angeles Basin around Glendora and San Dimas.
For such an unpublicized road, the Glendora Ridge Road is spectacular. It winds across the top of a high ridge, a fault block caught between the now inactive San Gabriel fault and the highly active Sierra Madre-Cucamonga fault system along the steep mountain front. The rocks include beautiful exposures of Proterozoic gneiss and schist, some of the oldest rocks in California, along with Mesozoic or early Cenozoic granitic rocks (some gneiss is shown in the photo at the end of this post). To the north, the road provides a bird's-eye view of one of Southern California's more dramatic wild areas, the Sheep Mountain Wilderness. It always seemed inconceivable to me that such a place could exist just 10 miles as the crow flies from the urban sprawl of the Los Angeles basin. The wilderness encompasses nearly 8,000 feet of vertical relief from the bottom of the East Fork of the San Gabriel River to the summit of Mt. San Antonio (Mt. Baldy). The canyons are immense, with depths exceeding 4,000 feet in a few places. The rivers (another SoCal rarity) are so rugged that few trails reach them, and at least one of them is a natural trout fishery (Upper Fish Fork). The wilderness is an ideal habitat for the rare Nelson's Bighorn Sheep. I was lucky one year while hiking on the boundary of the wilderness; I chanced upon a herd of a dozen or more. Being who I am, I fumbled in my pack for a camera (one of the old-fashioned kind with this stuff inside called 'film'). They heard the noise and skittered away. I got a great picture of a dust cloud...

Mount San Antonio (10,064 feet) is the highest point in the wilderness. The summit is hidden by clouds in the pictures below, but the mountain is one of the most familiar sights on the Southern California skyline. Thousands of people hike to the summit every year along a trail that starts at the Mt. Baldy ski area. There are two other approaches to the summit, but the number of hikers every year on those routes probably numbers in the dozens. A long and rugged trail approaches from the north part of the wilderness over Dawson Peak and Pine Mountain. Perhaps one of these days my brother will guest-blog about the time we, uh, "misplaced" him on this trail during an Explorer Scout backpack trip...


The other trail to the summit starts at Baldy Village and climbs almost 6,000 feet to the summit. To call this a difficult trail is nearly an understatement, but climbing it was one of the great adventures of my youthful days. Hot and dusty in the lower reaches, and spectacular views the whole way up. It's a good place to catch sight of the bighorn sheep.

There is no official Sheep Peak, by the way. It is the informal name of the prominent peak in the center of the wilderness area, 8,007 foot Iron Mountain (the prominent peak on the right in the picture below). I've never had the privilege, but the climb to the summit is said to be one of the toughest hikes in the south state, with great views and a chance to explore some old gold mines along the way.
Glendora Ridge is paved and well-maintained. That it exists is kind of a mystery to me, and I welcome any explanations as to why it was built. According to Russ Leadabrand (a travel author and Sunset Magazine contributor in the 1960s and 70s), the road was constructed in the early 1930s. I have thought of one possible reason. After my comments a few weeks back about the hazards of living in Southern California, especially adjacent to the high mountain ridges, it occurs to me that Baldy Village would have but a single access road if Glendora Ridge Road wasn't there. A single way out in the event of a major earthquake, a major flood, a major fire, or big landslide. The same problem applies to the developments in San Gabriel Canyon off to the west, a highway that also connects with Glendora Ridge. It looks to me like Glendora Ridge Road is the evacuation route... Whether it is or is not the escape route, it is a great road to drive. Unfortunately the local kids with their newly minted driver licenses know about it too, and a Google search on the subject of the road is likely to turn up stories of tragic car crashes as much as anything else (one of my high school buddies survived a 400 foot plunge off the highway). But for us last month it was uncrowded, and with the clear December skies, a beautiful excursion looking towards the high mountains, and the crowded valleys below.

There are some great rocks exposed along the highway...