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Mar 27 2012

The Mystery of Strike and Dip

There are things that remain somewhat mysterious to me. You can read about things like strike and dip for months, years even, but they’re words. You know dip refers to something tilting down. It’s right there in the word. Strike is more vague. I usually think of it as “striking out,” perhaps for parts unknown.

And if you’d asked me to demonstrate strike and dip in the field, I would’ve laughed in your face. The whole concept has a sort of je ne sais quois to it. I can recognize the words. I know it’s measured with a compass. I had absolutely no bloody idea how to see it.

Until now. Observe. Lockwood will show you in four photos of Sunset Bay, OR he posted on Twitter and gave me his kind permission to use, with the captions taken from his tweets.

#1: Oblique view of a single bed- rocks in sand lower middle right show up in 3 & 4 as well...

Now, experienced geologists and geology students will have no problem seeing what Lockwood saw. To folks like me, this is just some bent-looking rocks. And you might, like me, be too busy wrapping your head around the idea that apparently bent rocks are actually straight to even consider strike and dip.

That’s okay. Learning this shit takes time. And that’s why inveterate teachers of geology mess about drawing things on photos for you: to help you see.

#2: Annotations on #1...

POP goes the bedding plane. Okay. So we’ve got one bed in the strata to work with. Awesome! We can begin to see WTF’s going on. Remember, we can be the plywood, and see how that bedding plane has been scooped out by erosion – it’s like we’re standing inside an up-ended layer cake with a great big slice taken out of it.

As for strike and dip, we’re still at sea. But notice that the layer cake is, while raised from the horizontal, not standing straight on end. It’s leaning.

#3 Looking directly into strike; rocks in sand down the lower middle same as in #1...

Okay, so we’re looking into the strike. What strike? Harf?

#4 Annotations on #3. One can easily forget how abstract concept of strike and dip can be to beginners.

And I still can’t claim to understand it, but at least it’s beginning to pop out now. And the dip emerges with gorgeous clarity. This abstract concept is becoming something concrete. Someday, perhaps not too far in the future, I’ll be saying, “Oh, right. That is totally abstract when you’re first starting out. Forgot about that. Whoops!”

That day still isn’t here. But I found a nifty video that helped bring some of this stuff out of Abstractland into the town of Concrete (not the one in Washington), and a series of photos that showed how to measure it, and someday, when I at last have succumbed to the temptations of a smartphone, I’ll be able to somewhat intelligently, and perhaps even nonchalantly, walk up to an outcrop and begin using its nifty geologic compass app to measure strike and dip all by me lonesome. That’s one of the things I love about geology. Ordinary folk without advanced degrees can do it.

Especially when they have friends like Lockwood helping to make the abstract concrete.

7 comments

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  1. 1
    Nance Confer

    That was fun! Thanks.

  2. 2
    Southern Geologist

    This and the earlier post about tracking the strata were both quite enjoyable.

  3. 3
    Sam Coleman

    I agree that sometimes we geologists forget how confusing the concepts of strike and dip are to a beginner. Thanks for the reminder – thinking back, I was at least as confused as you the first time someone handed me a compass and told me to find the strike and dip of an outcrop.

    1. 3.1
      geocatherder

      So was I, and it took several accumulated hours of looking at faults, thinking through apparent strike and apparent dip, and getting the hang of the whole thing. The first time I did a field exercise, I could tell by the look in the professor’s eyes that he didn’t think I’d ever graduate. But I did finally get it in the end.

      Dana, next time you talk Lockwood into a field trip, insist he bring his Brunton and show you how to use it. There’s nothing like measuring these things to get your mind around them.

      1. Lyle

        I agree when I took a geology field camp in 1972 it was learning how to measure dips and strikes with a Brunton that beat the concept into my head. Yes I had heard about it in Structural Geology class, but having to do it in the field made the difference. The area was from the Black Hills thru Yellowstone to East of Butte,Mt. The trip which ran from 7 to 7 was learning geology from a fire hose. (Of course being from MI, our geology was more subdued and hidden)

      2. Lockwood

        Unfortunately, when my apt. flooded nearly a decade ago, the Brunton was lost, along with most everything else. Dana’s camera geotags photos, though I’m not sure what all is stored in the metadata (direction of view?). However, I think I could get the idea across with just a simple toy compass and a protractor. Likewise, dip can be acquired with a string taped to a protractor on one end, and a weight attached to the other end. A Brunton is more accurate & precise, as well as more durable and flexible, but getting at the ideas can be accomplished without that much investment.

  4. 4
    David Utidjian

    Dana,

    Thanks for all that. I have just learned in 20 minutes of reading and lecture something I never knew. I don’t know how long it would take for me to learn such a thing normally (lectures and books and field trips) but my guess is a lot longer than 20 minutes. I have a nice Brunton kicking around somewhere like the one in the pictures so now I just gotta find some outcrops.

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