Skepticism

Geology Word of the Week: Dip

This past week was a little hectic (advisor #2 visiting from Wyoming + two days of mass spectrometry time), so I’m just getting around to posting the “Geology Word of the Week” now late on Sunday night. However, all is well as long as I make this post before midnight!

This week’s geology word of the week is a word I’m sure you’ve all heard before: dip.

You take a dip in a pool, you dip your tango partner, you make chip dip for a party, you feel weightless for a moment as you go over a dip in the road, you dip into your savings to buy a present for your sweetheart, you watch the sun dip below the horizon, you chew some dip tobacco (gross… I hope not!), but do you measure strike and dip with a Brunton compass? If you’re a geologist, then yes! You measure strike and dip all the time.

In geology, dip is the amount that a layer (or sequence of layers) of rock is tipped from horizontal. Dip is closely related to strike, which is the compass direction perpendicular to the dip. Together, dip and strike are important measurements for geologists to take in the field as these two measurements allow geologists to map out the 3D structure of rock layers on a 2D map.

Dip and strike are measured with a Brunton compass, a fancy compass that is a must-have for the field geologist. By the way, Brunton compasses work better when you wear your cap backwards. It’s true.

Rocks layers are almost always deposited horizontally, so why many rock layers tilted? Most commonly, tilted layers represent rock layers that have been tilted because of squishing and uplift. These rock layers were originally flat, but they’ve been uplifted and tilted because of geological forces. In some cases, the tilting may have been present when the rock formed. For instance, many sandstone rocks represent sand dunes that have solidified over the centuries from loosely-consolidated sand into solid sandstone rock. Sand dunes have characteristic shapes because of the wind. When these sand dunes are preserved, the tilting is preserved, too, and is called cross-bedding.

Here’s a great movie that show how cross-bedding in sand dunes forms and is then incorporated into solid sandstone rock:
Cool Cross-Bedding Movie

Here’s a picture of some great cross-bedding in giant sand dunes in Sedgefield, South Africa:

And for those of you who find cross-bedding a little (yawn) boring, here’s a slightly- um- different view of that same cross-bedding:

Okay, now that I’ve woken up (hopefully) those of you who may have zoned out, let’s review the new geology lingo:

Dip: the amount a rock layer is tilted
Strike: a direction perpendicular to dip
Cross-bedding: inclined beds in sediments (sand dunes, river sediments) preserved in solid rock
Brunton: a fancy dancy compass that every geologist owns. You can measure strike and dip with one of these.

Woo-hoo! It’s only 11:30 pm EST. I managed to post a Geology Word of the Week this week after all! Off to sleep now as I’m on the mass spectrometer again tomorrow.

Evelyn

Evelyn is a geologist, writer, traveler, and skeptic residing in Cape Town, South Africa with frequent trips back to the US for work. She has two adorable cats; enjoys hiking, rock climbing, and kayaking; and has a very large rock collection. You can follow her on twitter @GeoEvelyn. She also writes a geology blog called Georneys.

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7 Comments

  1. I put my hand upon your hip
    when I dip you dip we dip
    You put your hand upon my hip
    when you dip I dip we dip
    I put my hand upon your hip
    when I dip you dip we dip
    You put yours and I put mine
    and we can get down low and roll it round

  2. I have to say yay for the geology content, it’s been far far too long since I did it for A-levels 6 years ago.

    And I do remember when on field trips measuring strike and dip, so interesting to learn how the world works.

  3. For even better pictures of cross-bedding, take a look at Opportunities images of cross-bedding in the walls of Victoria crater.

    As to practical uses of dip and strike, talk to cavers – vadose follows the dip, while phreatic trends along the strike. Critical in determining passage history (not to mention awfully helpful in exploration). But we don’t use a Brunton generally, too delicate (use Suuntos or similar, often with a separate compass and clinometer units, *very* well sealed :) ).

  4. Better pictures? Really? Yhan that second one? That is some amazing cross-bedding.

    Great post, I love to pull out my trusty Brunton and impress people with my ability to find the strike and dip, weirdly people are often unimpressed.

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