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.

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 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.

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.

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.

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.

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?

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

Good luck!