Local Geology: Wallace Creek

Hey there every peoples!

As you may have noticed, I was on quite a hiatus during this holiday season. Way too much going on. Hopefully when I start school next week I’ll settle back into my old routine. But now that I am back to writing blog posts, what shall I write about to not only break the silence but to also kick off the new year? The answer came to me while I was at Carrizo Plain National Monument this weekend: Wallace Creek.

Wallace Creek may seem like nothing special. Hell most of the year it’s just a creek bed. It doesn’t have any of that lush, picturesque vegetation that you find around creeks and streams. And yet Wallace Creek often shows up in discussions on geology and has even been featured on the History Channel program “How the Earth was made”. Here’s a clue to why Wallace Creek is of such renown: look at the photo below and see if you notice anything odd about it:

What is so odd about Wallace Creek?

If you look carefully, you can see that the creek bed starts off straight, but then veers off away from where you are and then veers off back towards the plain. If you still can’t see it (I was holding my camera as high as I could), then this aerial photo ought to help:

Aerial view of Wallace Creek

There you can not only see the funky bend but also another bend to the left. That is an ancient bend that the creek once flowed through but has since run dry because it was cut off from the main channel. This is all because Wallace Creek runs across the San Andreas Fault. For those of you who don’t know, the San Andreas Fault is an 800 mile fault line that runs north to south in California and is one of the longest faults in North America. San Andreas is what is known as a right lateral fault. Lateral movement occurs when rocks on one side of the fault slide past rocks on the other side in a horizontal fashion with little (if any) vertical movement. Right lateral means the direction its going: if you and a buddy stood on opposite sides of the fault and faced each other, you would be moving to your buddy’s right and vice versa.

But how did the creek get its shape? While the fault moves at about 1.2 inches a year Wallace Creek did not take its odd shape (called an offset channel) gradually over time as might be suggested by this fact. Rather, its shape came about very suddenly. Instead of moving gradually over hundreds or even thousands of years, Wallace Creek has been twisted by jumps made during earthquakes. Look back at the aerial photograph. See the channel on the left? It’s what’s called a beheaded channel. It was once the main creek bed. It flowed straight across the fault around 10,000 years ago but a series of earthquakes every few hundred years offset the channel. Eventually the creek cut through the first bend and separated the offshoot and began carving the modern channel about 3800 years ago. More earthquakes offset the new creek creating the offset channel we see today. The ancestral channel is but a ghost; a stream without a source. Wallace Creek may be the largest and most dramatic example, but right next to it are several smaller streams that have also become offshoot and beheaded channels.

So if Wallace Creek moves in leaps and bounds during earthquakes, when was the last time it moved? History can help us with this one. Farmers and ranchers have been working on Carrizo Plain since the time of the Spanish. But they had no idea that they were sitting on a time bomb. On January 9, 1857, the land seemed to come alive as it shook violently. San Andreas had unleashed an earthquake just north of Carrizo Plain that caused Wallace Creek as well as the rest of the valley to offset by 30 feet. The earthquake was one of the most powerful in American history: an estimated 8.0. The quake was felt from Marysville south to San Diego and east to Las Vegas, Nevada. The nearby Kern River had its current turned upstream and its waters ran four feet deep over its banks. A cattleman on the Plain reported that his circular sheep pen was converted into a crude s-shape. The surface of the earth along the fault was ruptured for almost 220 miles.

Because of the arid climate of Carrizo Plain most of the evidence of the restless San Andreas Fault has been preserved. Not only can you see the many stream channels that so dramatically demonstrate the movement of the earth, but also the fault itself. Since there has been little rain to cause erosion, you can still see the rift that San Andreas left like a great scar on the land:

View of Carrizo Plain from atop Caliente Ridge. The San Andreas Fault is the short ridge just below the mountains

Wallace Creek is a fascinating look into the workings of the earth. Had it not been for the San Andreas Fault, it would be nothing but a barren, unexciting gash in the land. But Wallace Creek is just one of the reasons I have become so fond of Carrizo Plain. I hope to one day show you what an incredible place it is. But for now, I’d like you to take a moment to ponder on the marvels of the Sand Andreas Fault at a little place called Wallace Creek.

Till next time!

Cajon Pass

Hey there every peoples!

Alright, I have a break, so I am going to write a post I have been meaning to write for a very long time but just never got around to it. I have tried to remember as much as I could.

Way back in the early days of april me and my dad attended a second field trip with the San Bernardino County Museum. This trip took us to Cajon Pass, which sits east of Redlands in the “Inland Empire”.  Cajon Pass is a geological and paleontological laboratory, hosting a rich array of fossils and geological features. We spent all day driving around the pass visiting fossils sites and taking in some spectacular views.

The most prominent feature of the pass is the San Andreas Fault. San Andreas is the biggest fault in California and is responsible for many of our worst earthquakes. The falult runs directly across the pass, with the Pacific Plate to the west and the North American Plate to the east.

A view of the San Andreas Fault running across Cajon Pass

The presence of Pelona schist and a sag lake testify to the fault’s presence. Just so you know, a sag lake is a lake that is formed right on a fault when ground water seeps up through cracks created by movement of the fault. In the case of Cajon Pass, this type of lake is represented by Lost Lake. I guess it isn’t really lost if the curators can take people to it.

Another neat geological feature is the Mormon Rocks (or as the curators called them: “Rocks of Jesus Christ of Later Day Saints”). This is where we had lunch and a few people had a good time climbing around and exploring the rocks. Me, on the hand, stayed behind with Eric and aggravated Kathleen with our chatter about Diablo II. The Mormon Rocks were once though to be part of the Punchbowl Formation but were later found to be older than said formation. The formation was named after Devil’s Punchbowl, a geologic feature further to the east (we didn’t get to see it).

Mormon Rocks

Cajon Pass has also yielded many fossils. There are many fossil bearing layers through out the pass and because of the constant motion of the earth there, they all jumble together. Take this site for example:

A great jumble of prehistoric rocks

I cn’t remember the name of the formation on the right, but I remember it’s middle miocene in age, abround 16 to 12 million years old. And interesting thing to note is that the formation is a terrestrial deposit but at one point yielded a whale vertebra. How is that possible? Remember, this area has undergone massive remodeling thanks to tectonic boundries. What the curators think happened is that the whale was buried in an older layer at a time when the pass was underwater. The whale died and it’s vertabra was buried. Later, during the middle Miocene, the bone eroded out of it’s origonal geologic unit and was redeposited when the middle Miocene unit was being formed. So instead of a whale finding it’s way inland (like that one whale, Humphry), the vertebra was instead reworked from an older layer. Isn’t geology fascinating!

Now look at the slanted layers to the left. They are cretaceous and paleocene rocks. I don’t remember much but I remember Eric talking about a plesiosaur vertebra being found in those layers, possibly in the paleocene layers. I just can’t remember. Sorry.

Anyway, they even took us to a fossil site with fossils still in the ground! They said that those fossils have been there for 22 years. That’s because they have been left there so that they can take people to the site to see fossils in their original state. The matirx consists of sand mixed with pebbles, which seems like an unlikely place to find fossils. Indeed, the fossils were fragmentary, consisting moslty of teeth:

A fossil horse tooth in situ

Fragment of rhino tooth, possibly Aphelops

And of course, we can’t forget the curators, who braved hell and high water (and me) to take us on another fantastic trip! Can we hear a big round of applause for:

Chris Sagebiel, purveyor of geological wisdom

Eric Scot, Master of Fossil Horses and Pringle Enthusiast

Kathleen Springer, Senior Curator of Geology, Queen of Rocks, Bane of All that is Doug...

Nah, I’m just kidding. She a wonderful person to be around. But all in all, these people really make the trip. Instead of just some tour guide, we get the people who actually work out here. Thanks again for the wonderful trip guys.

Till next time!

Fossils from the Punchbowl Formation, including a horse jaw and a camel skull

Local Geology: Estero Bluff

Hey there every peoples!

Sorry for being a little off topic lately (and will be again very very soon) but I promise I will keep things relevant. So here’s some geology. Last week my Geology class went on its first field trip. Mr. Grover took us to a piece of land up in north county, north of the town of Cayucos. It’s a place called Estero Bluff, and while it has a very nice beach, it sports some interesting geology (I may be a little spotty; I tried to remember as much as I can).

Mr. Grover ain’t lying when he says we have such wonderful geology in this area. He demonstrated this by bringing us to Estero Bluff. He brought us here because Estero Bluff demonstrates many geological processes. For example, notice the terrain:

Estero Bluff

It’s a flat open terrace. But sitting on the terrace is this hill:

A hill sticking out like a sore thumb on the Continetal Terrace

So how did this happen? It’s a combination of things. The first is a change of sea level. Sea levels have changed so much throughout the earth’s history. It rises and drops with the growth and waning of glaciers and other factors. But a drop in sea level doesn’t explain the terrace alone (as we’ll later see, uplift also had a hand in it). Erosion is always at work, whether we see it or not. And what went on here at the bluff was a process called differential erosion. Everything erodes at different rates for various reasons and in the case of Estero Bluff, it’s the hardness of the rock. The hill here is made up of layers of chert:

Layers of chert that make up the hill

When you have a blob of chert in a site that’s mainly made of soft soil, the soil will erode away while the chert stays relatively intact since it’s much harder. And that’s how that’s how this hill came to stick out of a flat terrace.

Now, the other reason for the terrace is uplift. Did you notice the chert layers were at a slant? That is due to the process of geological uplift. This is where the earth pushes up (that’s often how mountains are formed). And Estero Bluff is unique because it sits on the edge of two continental plates. Currently the Pacific Plate is being subducted (pushed under) the North American Plate. And this subduction has caused the layers of the earth here to scrunch up and tilt on their sides. It’s wholly possible that the chert layers could be vertical in a few million years.

This uplift is the reason why Estero Bluff is unique. It represents an Ophiolite, which Wikipedia describes as “a section of the Earth’s oceanic crust and the underlying upper mantle that has been uplifted or emplaced to be exposed within continental crustal rock.” The chert represents old ocean sediments. It rests near the top of the Ophiolite because it was among the top layers when the crust began to scrunch up. Usually below sea floor layers we get basalt lava with what are known as “pillow” structures:

Pillow lava basalts (not the best example though)

Pillow lava is formed under water, but they will get their own post eventually. Estero Bluff illustrates the top of the Ophiolite in this pocket beach, where we can see the layers of basalt, chert, and younger conglomerates:

Mr. Grover expalining the different layers

Finally, Estero Bluff has one last goody for us:

A bluse schist enjoying the California sun

It’s called schist. It’s a form of metamorphic rock formed from basalt. It is formed at high pressure but low temperatures. Generally it cooks at temperatures of 200-500 degrees Celsius at depths of 15 to 30 kilometers. So how did it get here? Let’s review: Estero Bluff exhibits layers of rock uplifted from the crust and upper mantle and schist is formed from basalt. I think what happened is some of those pillow lavas got sucked down into the subduction zone and formed schist. Then through uplifting they were brought to the surface, where they came to rest on the beach and get polished by the ocean.

Whoa man, that was a mouthful! How’d I do? Because I’m better with paleontology than geology. Anyway, I might feel another rant coming on, but after that, back to the good stuff.

Till next time!