Saturday, February 28, 2009

Geology Lesson #2

February has been too much fun. Let's get serious and learn something here on the old blog. (Not that learning can't be fun.) Since we have a resident geologist at Pasadena Daily Photo, let's take time out for a geology lesson. After all, there's geology all over Pasadena.

Here's our geologist, Becca, posing like a product model with a sedimentary deposit we found in the Hahamongna watershed basin. Becca adores sedimentary deposits. She explained it to me while I was taking the picture, but I didn't remember a word of it. So she sent me this email:

"In our last lesson we learned about graded bedding, in which the particle size decreases as one moves up a particular layer or bed. In contrast, this sedimentary deposit is a disorganized mix of pebbles and cobbles. A geologist would characterize this kind of rock unit - one in which a variety of particles are observed - as 'poorly sorted.' Upon closer inspection the geologist would also note that these are the same rock types that make up the San Gabriel Mountains, located several miles north of the Hahamongna watershed."

Then she throws a pop quiz into her email:

"How do you think these rocks were moved all the way from the San Gabriels to this basin?"

Becca gives three choices for an answer: wind, water or glaciers. I say it's water. Hahamongna's a watershed basin after all. I don't know when the glaciers were here last and I'm no geologist, but I'm pretty sure wind doesn't move rocks.

"Another observation about this deposit," Becca says in her email, "is that it's been heavily weathered. Weathering is the process by which rocks are broken down mechanically (size and shape changes) and chemically (the removal of minerals and addition of new minerals). Notice all the plants growing in the rock unit. This is an example of a physical weathering process called root wedging, during which roots force their way into the rock/deposit, breaking the rock into smaller pieces."
We also noticed some of the rocks had a rust-like coating. Becca says oxidation occurs when iron-rich minerals are exposed to oxygen, creating the reddish-brown stain. Aha! Like rust. Oxidation is a chemical weathering process, like she described. So this sedimentary deposit is going through both chemical and mechanical weathering.

And, Becca says, the end product of weathering is the development of soil. I love that. It's so logical.

I hope I was right about the rocks being moved by water. I was applying logic, after all. It's less logical for them to have come down from the mountains by glacier, don't you think? And by wind? There's no logic in that whatsoever. Whimsy, maybe, but really it's just plain dangerous.

14 comments:

Cafe Pasadena said...

This lazy K9 would rather have more fun - February isn't over yet, P! And, eat too. (I'd like a piece of your yesterday's photo)
Don't you have a resident husband at your PDP offices as well?

Dina said...

Root wedging! I really needed that term. Thanks for it and all the rock talk. But now you've put the picture into my head of rocks sailing through the air.
That white rock is so heart-like.

Greg said...

Nothing like a bit of geology lessons! I failed geology at school mainly because the class was friday afternoons and my girlfriend had that line off :-)

Julie said...

When my daughter was coming toward the end of Year 10 in High School and trying to decide her subjects for the final two years, I suggested Geology. After all, the school she attended managed to get 8 of the top 10 places in the state each year in Geology. She looked me straight in the eye and said: "Rocks! You want me to study rocks ... for two years!"

She chose Spanish.

Good post, Petrea ...

Bibi said...

In my next life, I am going to be a geologist. I am just fascinated by layers of sediment and rocks! I'll say it's water that brought the rocks to that place, too.

Petrea said...

I do have a resident husband, Cafe, and he's plenty of fun.

Dina, the old saying is "when pigs fly," not rocks, am I right?

Hi Greg, welcome from Hobart! I think Becca would have failed you too. She's pretty strict.

Hi Julie! Wow, what an honor, two Aussies today. I think at Becca's house it must have been the other way around. Her mother saying: "Rocks? You want to study rocks?"

Bibi, I hope Becca will let me know if I'm wrong. If so, I'll post it.

altadenahiker said...

I could read this stuff all day long. Love geology. I also love Julie's comment.

Laurie said...

I love this, Petrea. Thanks for a wonderful lesson. I have always been fascinated with geology. Austin was filled with geologists -- the University of Texas had a great department and the surrounding hills around the city were filled with fossilized limestone.

My love of rocks ended up in a love of gemstones. So this can be a dangerous hobby...

annebanan said...

Geology was one of my favorite subjects at U of Illinois back in the (our) day. I'd love to look at the rocks in this deposit, because if a glacier moved them, there might be other clues on some of the rocks, like long scratch-like marks that one rock made on another as the glacier slowly slogged them along. I don't know anything about the geology of California except that some serious plate tectonics happens/happened there, so I'll just guess it's glaciers. BTW, I love sedimentary deposits too, because in this part of the country (IL) they're often full of fossils. Fossils are very cool.

altadenahiker said...

Ask Becca what she thinks about rock dust as a soil amendment. I started using it this year for my raised beds. It comes in a pretty Dr. Earth box, but it's kind of costly, considering it's, uh, well, now I feel silly. Yes, I pay $10 a pound for granite dust. And I can't even remember who told me this was a good idea.

Kathy H said...

My daughter's in high school geology. I forwarded this post to her. :)

We took a local seismological tour with a friend who's a Caltech seismologist. Amazing stuff around here!!

Ms M said...

Fun to learn new things! The only geology I had was years ago in jr high science. I would guess either water or glacier.

Petrea said...

Karin, dog-walking with Becca is a treat. You and Albert will have to come along one day.

Ha, Laurie, I love rocks but never fell in love with gems. I still bring home rocks and sticks - they're all over the house. Does that mean I never grew up?

Anne, we await Becca's answer to the question. Glaciers or water? Some inland parts of southern California were once beneath the ocean, so that's a possibility, but it's not the kind of water flow I was thinking of.

Well, Hiker, it IS Dr. Earth. Is it working?

Kathy, a seismological tour sounds cool. Becca showed me pictures of the San Andreas fault. Amazing. Not the same as a tour, however.

I don't think I ever had a geology class, Ms M. It's a pity, because it's fascinating stuff. Good thing I've got the prof to pal around with.

Petrea said...

The answer is finally here! (Not because Becca didn't get it to me, but because I didn't get it to you):

"Yes, water is the transportation agent! In this case, water means runoff (i.e., rivers and streams). It's true that southern California was once covered by ocean, but the rocks in the photo were carried much more recently by moving water."