In part one, I talked about the San Andreas Fault and how it worked to create the topography of the San Gabriel Mountains. In this part, I’m going to show you some of the signs of the fault’s activity in the Wrightwood area.

The day I arrived at the campground, I saw a bulletin that there would be a volunteer-led hike the next day exploring the San Andreas. Of course, I was thrilled!

The hike was led by a former elementary school teacher who was also a naturalist in the area for many years. She’s not a professional geologist, but a very knowledgeable and enthusiastic amateur. I wish I had brought a notebook, because I don’t remember the names of every rock she pointed out.

I do remember that she called this one actinolite. It was green, which didn’t come out so well in the picture, and obviously there’s a little quartz vein running through it. Actinolite is the product of metamorphism of basic (mafic) igneous rocks, like basalt. And it should be no surprise at all to find that in an active fault zone.

I don’t remember what she called this, if anything. It might have been something I saw on my own and said “Oooh, pretty!”. I have to admit, my geoskills are not good enough for me to have much confidence in my ventures through my field guide, so I’m going to take a pass on speculating what this one is. But isn’t it pretty?

I’m sure she identified this one, but I’m not sure how. I think she might have called this a marble. And since marble is metamorphosed limestone, and since the origin of most limestone is marine life, this rock used to be, way way way way back millions of years ago, at the bottom of the sea. And now it’s sitting on a mountain 6000+ feet above sea level. Hooray for faults!


We found a lot of rubble like this along the trail. We were walking on the Pacific plate side of the fault, not that I think it matters. But the fault has been uplifting these mountains, exposing them to erosion and frost heaving, and breaking them apart. You can see from all the orange pieces that there’s a lot of iron here. She called that “earthy hematite” although my field guide calls it “massive hematite”. In any case, it’s iron. And there’s lot of it. Which is a sure sign that there’s been a lot of igneous activity in the area at some point.

I posted that first picture yesterday. Both of them show fault flour or fault gouge. It’s everywhere in this area. The fault has been chewing up the crust here for a long time and leaving this pulverized stuff. In the second picture, I’m looking across the fault from the Pacific plate to the North American plate.




Here are a few examples of slickensides.The red striations are due to friction from the fault, which means that these rocks were actually at the interface of the fault as the plates scraped against each other. That’s so exciting! The direction of the striations indicates the direction of the fault’s movement. In the closeup of the third one, you can see that the friction caused a whole layer of the rock to be cooked.

Something that our guide mentioned during the hike was that she had been told (by whom, she didn’t say) that there were still trees living in the area that had had their tops sheared off by the action of the 1857 earthquake I mentioned in part one, but she had never seen one.

Well, I might have found one. A couple days later, I was walking around the campground and found this pine tree.

I have no trouble whatsoever believing that this tree is over 150 years old. And look at that ragged top…doesn’t it look like someone just ripped the top of the tree off? And those upper branches look a little too robust, you know what I mean? I don’t know if that’s really the explanation for the appearance of this tree, but maybe.

So how about some more fault evidence?

This is Jackson Lake. It’s a sag pond, which means that it lives in a depression caused by the action of the fault. There’s a spring here, and it’s also fed by rainfall and snowmelt, but there’s no outlet anymore because the creek that drained it was dammed in 1916.

I remember a time when I was a kid when there was a population explosion of ladybugs at this lake. They were everywhere. It was awesome.

Here’s another example of the fault’s effect on water. This is near the western end of Swarthout Valley, and you see how the vegetation in the middle is wildly different from that on the lower slope of mountain, or even the vegetation immediately in front of me? Instead of the typical “we don’t need a lot of water” chaparral-type plants, you’ve got bright green water-loving things like aspens and willows. That’s because the fault has forced the water table much higher in that area than in the surrounding areas, making water much more accessible than elsewhere.

This is the scar of a mudflow that occurred in 1941 when rapid snowmelt saturated a ridge that was very unstable due to repeated fault action. It buried several houses and occasional mudflows occur to this day.

This photo was taken from the green arrow on this map.

This location is called Inspiration Point, and I’m looking south. If the day is clear enough, it’s possible to see Santa Catalina Island, which is about 80 miles away. Today was not such a day, unfortunately. But I am looking in the direction of the course of the San Gabriel River, which eventually drains into the Pacific between Seal Beach and Long Beach. At this end, though, the course of the river has been changed by the Punchbowl Fault, one of the accessory faults to the San Andreas. Just like the interaction of a stream and a fault carved out Cajon Pass, here the fault caused the San Gabriel River to split in two, with one tributary going to my right in the photo, and carving Vincent Gulch, and the other going to my left and carving Prairie Fork Canyon. This is much easier to see in the map than in the photo, because you can’t actually see the bottom when you’re standing there!

This is Mt Baden-Powell as seen from Inspiration Point.

And this is Mt Baldy (aka Mt San Antonio) from Inspiration Point. Less biology to hide the geology. I love it!

Here is a more zoomed in photo of Mt Baldy, this time from Vincent Gap, a point where Highway 2 dips south just northeast of Mt. Baden-Powell. Here you can see something really cool. See that tilted layer of rocks? You are looking at the Vincent thrust fault. Whereas the San Andreas is a strike-slip fault, where two plates slide past each other, in a thrust fault, one plate is overriding the other. This causes older rocks to lie on top of younger rocks, the opposite of the normal situation.

The rocks above the fault are gneisses and mylonites that are between 248 and 65 million years old. Below the fault is mostly the Pelona schist, which formed around 65 million years ago. Both layers are intruded by granitic rock that’s around 30 million years old.

Here is some of that Pelona schist close up. Once you notice it, you realize it’s everywhere.

By the way, from Vincent Gap there is a trail up to an old, abandoned gold mine. My mom and I didn’t take the hike today, but I might be able to dig up some pictures of me up there when I was about 12 years old.

In part three, there will be more tectonic goodies!