Cross Bedding and Finding Up

Sometimes, I have a terrible time explaining something to my students in class.

Sometimes, I can redeem myself by writing a blog post clearly explaining what I couldn’t get through in class.

One of those topics is cross-bedding in rocks. Now, if you’ve ever driven anywhere in the southwestern United States, you’ve seen lots of cross-bedding.

Crossbedding of sandstone near Mt. Carmel road, Zion Canyon, indicating wind action and sand dune formation prior to formation of rock. Credit: National Park Service photo by George A. Grant, 1929

This can happen at nearly any scale, from tens of feet in thickness, to inches in thickness. So, then, how do cross beds form?

They form from the motion of sand or other sediments due to the flow of wind or water.

The motion of sand as it is blown up and over a dune.

The motion of sand as it is blown up and over a dune.

The dune (or ripple as it’s called if it’s very small) moves in this manner, with sand being eroded off the back and deposited at the front. The cross-beds form as sand cascades and is deposited along the front of the dune. If more sand is being added (as if carried in by a river) the dune will also build up taller and taller.

The migration of a dune, showing the formation of cross-bedding along the front of the dune (where deposition is occurring)

The migration of a dune, showing the formation of cross-bedding along the front of the dune (where deposition is occurring)

Later, another dune may come along and over-ride the first, cutting off the top part of all the cross-beds. The bottom part of the dune is still preserved.

How cross-beds look in a rock. The top is truncated, but the bottom is completely preserved.

How cross-beds look in a rock. The top is truncated, but the bottom is completely preserved.

And here’s what a complete stack of cross-beds might look like. In this case the flow of wind or water changed directions back and forth.

A stack of cross-beds. Multiple dunes blew through the area where this rock now sits. Compare this with the photo at the top of the post.

A stack of cross-beds. Multiple dunes blew through the area where this rock now sits. Compare this with the photo at the top of the post.

This is actually very convenient for geologists that the top is truncated and the bottom remains preserved, because this provides a means by which geologists can tell which way was ‘up’ when they’re looking a a rock with cross-bedding. Sometimes we need to know this because we might just have an isolated boulder, or we are looking at an outcrop of rocks that are steeply tilted and we just don’t know which side was originally the top.

A cross-bedded sandstone. Can you tell which way was up?

A cross-bedded sandstone. Can you tell which way was up?

Take this rock (above) for example, that was just sitting in the laboratory. Can you figure out which way was up?

Here’s a tracing of some of the cross-beds. Can you see what I was tracing? Can you tell which way is up now?

Tracings of some of the more obvious cross-beds in the above rock.

Tracings of some of the more obvious cross-beds in the above rock.

Here’s another example of cross-bedding, courtesy of Lockwood DeWitt at his blog Outside the Interzone.

Is this block of sandstone right-side-up or upside down? Click the image for full resolution. Click here to visit

Is this block of sandstone right-side-up or upside down? Click the image for full resolution. Click here to visit Outside the Interzone for an explanation of what you’re seeing. Credit: Lockwood DeWitt.

Good luck!

This entry was posted in Geology, Teaching, UREES101 by Penny. Bookmark the permalink.

About Penny

Scientist (Paleontology, Geochemistry, Geology); Writer (Speculative and Science Fiction, plus technical and non-technical Science); Mom to great boy on the Autism spectrum; possessor of too many hobbies.

5 thoughts on “Cross Bedding and Finding Up

  1. I am not one of your students, just a reader. I found this really interesting. Although, My kids would say the sand is more fun when it’s still dunes 🙂 In your photo, the left side is up (I say with little confidence) ??

  2. Hi Rhonda! I’m glad that it’s not just students reading and learning from my blog. This makes me very happy.

    Your response showed me that I might not yet quite have the explanation correct, so I added another figure showing what it looks like when cross-beds stack up.

    It turns out that in the photograph, the rock is oriented correctly. Depositional up is up in the picture. Can you see it?

  3. Here’s another example- try to figure out which way stratigraphic “up” is before reading the blurb, because I explain it pretty thoroughly (and, I hope, clearly). http://outsidetheinterzone.blogspot.com/2013/02/geo-365-feb-16-day-47-cut-and-fill.html Penny, you’re welcome to use this photo if you like, and if you send me an email address, I can send you the original. Blogger reduces them somewhat when I load them. To see a more detailed version, right click the photo, and choose “open in new tab.”

  4. Wow! Yours is a great example. I’m sending you an e-mail to the address you provided when you replied. I’d love to include your image and a link to your page. Thanks!

  5. The simplest answer – choose those lines of maximum curvature.
    These lines lie concave up toward the vertical direction.

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