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.