In parts one and two, I talked about how the San Andreas fault formed the San Gabriel mountains. While I was spending the week sleeping in those mountains, I took field trips to a couple geologically fascinating sites in the California desert that are not directly related to the San Andreas, but are nevertheless evidence of the fascinating tectonic history of California.

The first stop was Rainbow Basin, near Barstow. This lies in an area that was once occupied by a shallow lake. The area is also the home of the Calico fault, which runs parallel to the San Andreas. On this map, the Calico fault is #21; the San Andreas is #30.

Like the San Andreas, it is a right lateral strike-slip fault. Also like the San Andreas, it has a bend in it. Which means that it creates local compressional forces while the blocks on either side try to slide past each other.

This is the beautiful result:

Darn it. I was going to build up the story into this great crescendo and pop that picture on you as a breathtaking climax, but I just couldn’t contain myself. What you are looking at is the Barstow syncline. A syncline is a fold that forms a trough, as should be obvious from the picture. (The opposite is an anticline, which forms a crest.)

Because of that bend in the fault, the sliding becomes compression. Because the Calico is much smaller than the San Andreas, the effect is not quite as dramatic as creating the Transverse Ranges, but it is no less beautiful. It’s the same physics, but in miniature.

At the top of the fold, you see an angular unconformity, basically a missing part of the sedimentary record. The folded rocks are about 15 million years old. They folded and eroded and no more layers were deposited until about a million years ago, when that top layer that has some vegetation started being deposited.

The layers here are mostly mudstone and sandstone, deposited at the bottom of the aforementioned lake. But there are other layers, too, like volcanic ash (bright white in this photo):

Those layers have been folded so much, they dip down under the road and come up on the other side.

The green layers, by the way, are metamorphosed volcanic rock. I wonder if they’re related to the Pelona schist.

Well now that I’ve ruined the story, I’m going to be quiet while I let you enjoy the rest of Rainbow Basin.

I bet you weren’t expecting another, even bigger picture of the syncline. Did you find yourself breathing heavy when you saw it again?

I grew up in Southern California and I never knew this place existed. It could be because it’s so remote.

My GPS was all “You’re on your own, sweetie.”

However, it is near what is quite possibly the most awesome intersection in the world.

It’s Fossil Bed Road because a whole slew of Miocene fossils have been found here. In fact, a whole interval of geologic time was named after this area, the Barstovian stage, which was 16 to 12 million years ago.

My second field trip was to Amboy and (sort of) Pisgah craters. These are a couple of cinder cones in the Twentynine Palms area. Cinder cones are basically baby volcanoes that spit out cinders for a while, maybe make a lava flow or two, and then give up. They are usually very symmetrical and attractive.

First I went to Amboy, and to get there requires driving through an ice age lake bed, Bristol Dry Lake.

The lake bed is now covered with salts left behind when the water evaporated, which are being mined for commercial purposes. In the distance, across what feels like the never ending salt flats, you finally get a view of the crater.

This is designated a National Natural Landmark, and there’s a parking area and picnic tables. They were completely deserted when I was there. But you get a nice, close view of the crater from a (thankfully) shaded observation deck.

See the lava flows around the crater? They are quite extensive, as you can see on the map.

There are both pahoehoe flows,

and aa flows.

These are basaltic lava flows, which you can tell by the dark color. This lava has a very low silica content, which means that gas escapes easily and you don’t get the big explosive kinds of eruptions. The lava just calmly flows out. This is the Hawaiian kind of eruption, not the Mt St Helens kind. The only difference between pahoehoe and aa is the water content. Pahoehoe has more water and flows more easily, and when the surface cools it has a ropy appearance. Aa has less water and has a more crumbly texture.

There is a trail to the summit, which is only about 220 feet up, but considering the temperature, I thought it best to defer the hike to a possible future visit.

After my brief but scorching visit to Amboy Crater, I decided not to go hunting for Pisgah Crater. I did, however, catch sight of it from Interstate 40.

The lava flows around Pisgah are quite extensive. I had driven about 10 miles past the crater and I was still driving past lava flows next to the interstate.

You can see in this map just how large the lava flows are, and that quite a bit of them are covered with sand.

Not far from Amboy, I found this, which has nothing to do with either cinder cone, but is just beautiful.

I believe these are the Marble Mountains, which are found in the Trilobite Wilderness Area. I love these names!

I hope you enjoyed my brief tectonic tour of Southern California. I’m going to leave you with a portrait of my daily visitor during my week in the mountains.