I’m about to share a huge part of my childhood with you. When I was a kid, my family spent most weekends every summer camping in the Angeles National Forest near Wrightwood, CA. We also went up there in the winter to play in the snow. That’s the nice thing about living the Los Angeles area: you can drive a couple hours, play in the snow, and drive home where there is no snow.

We went on a lot of ranger-led hikes (something they don’t seem to do anymore, which I think is a huge loss) so I always knew that whole area was resting right on top of the San Andreas fault. I was familiar with the terms “fault flour” and “slickensides”, even if I couldn’t tell you precisely what a fault was. I just knew it was going to be the source of The Big One, something that lives uneasily in the backs of the minds of everyone who lives in California.

(Fault flour)


With a new appreciation for what faults are and all the effects they have (and ironically, writing this the day after the East Coast Earthquake), I spent last week reliving that bit of my childhood.

For orientation, my campsite was approximately at the green arrow. The fault runs on a diagonal, roughly parallel to Highway 2 east of the Big Pines Junction and Highway N4 west of it.

If you were to follow Highway 2 (and consequently, the San Andreas) east a bit further, you would see that it cuts right through the town of Wrightwood. In fact, the 1812 earthquake that destroyed the Great Stone Church (and killed 42 people) at Mission San Juan Capistrano probably had its epicenter in or near Wrightwood.

Another major earthquake occurred along the San Andreas in 1857. This one was called the Fort Tejon Earthquake. This was a huge quake, estimated to be a 7.9, (over 100 times stronger than yesterday’s East Coast quake) which left a surface rupture scar over 350 km long. Because the Los Angeles area was so sparsely populated, only two people died. I may (may!) have found some evidence of this quake in my campground (stay tuned). This was the last major quake to occur along the southern segment of the San Andreas.

Paleoseismology studies have estimated that major quakes occur along this part of the San Andreas every 140-160 years. It’s now been 154 years since that quake.

For some more orientation, take a look at this map of the San Andreas fault. Notice how it runs along a roughly northwest-to-southeast path. There is a portion that runs a little more east-west than the rest of it, and that portion is just above Los Angeles. That part is called “The Big Bend” of the San Andreas, and it creates the beautiful mountains Angelenos get to enjoy in their backyard.

The San Andreas is a right lateral strike-slip fault, which means that the two plates that meet at the fault, the Pacific plate to the west and the North American plate to the east, are sliding past each other. That’s all well and good when your fault is a straight line, but what happens when you get a bend like that? It’s hard to move in a straight line when your path bends sharply. The result is a local area that acts more like a convergent plate boundary than a transform boundary, and the rocks have nowhere to go but up. The result: mountains!

Those mountains are, specifically, the Transverse Ranges, so called because they have a more east-west orientation than the typical north-south orientation of California’s mountain ranges (the Sierra Nevadas, the Cascades, the Coast Ranges). The Angeles National Forest is in the part of the Transverse Ranges known as the San Gabriel Mountains.

To the east of the San Gabriel Mountains is the San Bernardino Mountains. Between them lies Cajon Pass, which anyone who has driven between Los Angeles and Las Vegas on Interstate 15 is familiar with. The formation of the pass itself is because of the San Andreas.

As the mountain streams, specifically Cajon Creek, that drained toward the southeast from the San Gabriel Mountains intersected the San Andreas, their courses changed, eroding a broad amphitheater out of the alluvial deposits that had formed on the northern side of the mountains and shifting the drainage divide by five miles. This not only created Cajon Pass, but caused a valley to lie between the mountains and their alluvial deposits. You can see that quite readily in this map:

One of these streams, Lone Pine Creek, a tributary of Cajon Creek, runs right alongside the San Andreas. The fault and the stream worked together to carve out Lone Pine Canyon, the more scenic of the two routes from I-15 to Wrightwood.

(Lone Pine Canyon)

(Lone Pine Creek)

So given all that history, there should be some really interesting rocks in the area, right? You’d have to be blind to miss them. And apparently I was blind for the several years of my life, because I sure never noticed these.

These first ones are from a small trail near the visitor center at Big Pine Junction. Incidentally, this spot lies at the highest point on the San Andreas, 6,861 feet, as you can see from this USGS marker.

Here are just a few of the rocks I found on this trail.

Those first two were within a stone wall, and the last one (of two rocks) were next to a stone wall. I believe the first one to be a basalt or maybe dolerite, and the second to be a granite. I’d like to call the two rocks in the third picture chalk, but I’m probably wrong about that. In these pictures, I’m standing a very, very short horizontal distance from the fault on the North American plate side, but I have no idea where these rocks were quarried from.

I haven’t the foggiest idea what to call this, but it reminds me of Neapolitan ice cream.

I really, really want this to be marble. The blue didn’t come out so great in this picture, but it reminded me of the big hunk of blue marble at the Smithsonian that was quarried from Riverside, which isn’t too far from here.

This one absolutely fascinates me. It sure looks like sandstone on the right (with a quartz vein running through). I want to call that metaquartzite on the left, but that’s probably not right. Whatever it is, it sure is stuck on there, with a little transition zone between them. I’m sure that’s got to be some kind of contact metamorphism.

How about some biology?

This is a Jeffrey pine. I love these trees because they have this jigsaw puzzle looking bark:

that has this delightful sweet, vanilla smell. I wish I could capture the aroma for you. I’ve been sniffing the bark of every pine tree I come across.

Growing nearby, as they usually do, was a manzanita:

They have this deep red bark. Manzanita means “little apple” in Spanish, and sure enough:

Not ripe yet, obviously, but these plants were so important to the natives that they held big celebrations when they did ripen. In fact, owning tracts of manzanitas was one of the few instances of land ownership among the Native Californians.

This is the very rare High Voltage Pine…

And here’s a woodpecker. Because they amuse me.

In Part Two, more evidence of the San Andreas in action.