Formation of Cross Beds
When a depositional environment has sand in it and water or air moves the sand grains around, those grains can build up into piles of sediment. When the sediment piles reach a height where they are unstable - called the angle of repose - the grains will avalanche down the side of the pile and make a thin depositional layer of the grains that moved. Over time, multiple avalanching episodes will result in many thin parallel layers next to one another. These are called cross bedded laminae, because they form at an angle to the horizontal nature of the main bed. See the graphic below for an idea of how these cross-beds form over time.
Which way is up?
When cross beds form, the grains avalanche down the face of the form as previously mentioned. As with any item that falls downhill, there is a zone of "run out" at the bottom - the grains do not just roll to the base of the bed and stop immediately. The grains will fall down the side and roll a little distance along the surface until they lose momentum. This makes a sort of "tail" to the cross bedded laminae that lays on top of the bottom bedding surface which we can use to determine stratigraphic up. On the other hand, the top of cross beds are not typically preserved because subsequent movement of grains erodes this part. The result is a rock that has a base portion which shows the "run out" of the avalanching grains and a top that is truncated by the next bedding plane.
The photo to the left shows a well-formed set of cross beds. The same photo is shown next to it, this time with outlines showing the bottom and top portions of a single cross bed along with inner lines that follow a couple of cross bedded laminae. Note how the tops of the cross bed lines are cut off by the top of the bed and the lower parts of the cross bed lines comes down to and then follow along the bottom of the bed. This shows us that this bed is in a stratigraphic up position.
It's a topsy turvy world
The two photos below both show cross bedding, although the one on the left is overturned and the one on the right is not. Can you pick out the top and bottom of the beds in each as well as some internal cross bedded laminae to prove this to yourself?
en Espanol Spanish version - cross bedding (Microsoft Word 36kB Feb2 09)