Structure

Complex cobble, Lincoln Park

Geology was my first major at the University of Washington, and although I eventually got bogged down in the chemistry and calculus, I loved the field trips. We rafted down NW rivers, hiked in the mountains and even chartered a plane for an educational - and spectacular - flight around Mt. Rainier.

My favorite course was one called Structural Geology, in which we used the patterns of rock to understand their history and formation. From a hand sample, we learned to use the patterns of intrusions, folds and colors to trace its story.  It was challenging, but fun.

Since then, I have been fascinated by rocks with complex stories to tell. It may be hard to see in this picture, but the middle of this rock (found this morning in Seattle) is a contact between two different kinds of rock, both with strong, but perpendicular linear elements. One half  is dark with layering along the rock's long axis - the other half is much lighter in color, and with layers at a 90 degree angle. Weird, huh?

Weird...and beautiful.  I don't know what to call it, or how it formed; I'll know more after I've polished it and can study it a bit more.

Using the "Search Image"

Petrified Wood, Alki Point

When I patrol the beaches near my home, looking for colorful or unusual rocks, I try to keep a mental image of what I am looking for, what scientists call a "search image." This refers to having a visual sense of what you're after when you set out, which can give you an advantage in spotting one specific kind of rock among thousands. Normally, I look for bold patterns, or bright colors - the easiest things to see. But today I made an effort to look at black rocks. My goal? To find specimens of petrified wood.

As I have mentioned in earlier posts, there is a form of black petrified wood that I have found many times on local beaches, though normally as small cobbles that reveal their patterns only after polished. I have no idea where the rock originates, but I am guessing it is somewhere relatively nearby, simply judging by the relative abundance of the stuff.

So I set out today with the idea of concentrating on finding these pieces of dark wood. This may be why I found this piece within 15 minutes of searching the beach, by far the largest piece of petrified wood I have ever found along the shore.  Did my "search image" make the difference, making this rather drab piece of rock stand out?  I think it did.

Though this piece looks drab now in its unpolished state, I will try and post a picture of it after polishing - look for it a few weeks from now. Until then, I will probably be back looking for other pieces of streaky black rock...