USGS Bedform Movie Explanation
David M. Rubin, from the USGS Western Region Coastal and Marine Geology Program at Menlo Park, CA, published a book called Cross-Bedding, Bedforms, and Paleocurrents, which is available from SEPM and online at http://walrus.wr.usgs.gov/seds/bedforms/. He has posted some movies on the web site which demonstrate how various types of cross stratification are created. We will use these movies in lab to help interpret various styles of cross stratification.
First, here is an explanation of what you see in the movies:
The computer model that generated these movies starts with a bedform shape, which is constant throughout the movie. It is defined by the grey shaded surface. "Laminae" are deposited by moving the surface downstream and slightly up for each time step. Deposited laminae are represented by white lines. If the surface intersects the laminae, they are eroded, just like if laminae are exposed to an erosive current, they are eroded. Thus, the laminae change length and geometry as the "bedform" (surface) migrates.
All the movies associated with the publication are all listed at http://walrus.wr.usgs.gov/seds/bedforms/animation-toc.html. They are categorized by the shapes of the bedforms and photographs of real bedforms are also included. This is an EXCELLENT resource (although the movies currently use an out-of-date plugin so they can't run in a modern browser - I've reported this problem.)
Start with a movie of real current ripples in an experimental flume. This will give you a sense of how irregular ripples can be. (Drag the marker at the bottom of the movie to go back to a specific place. Click on the play button to rerun the movie.)
Linear Ripple or Dune
Here is a model-produced version of the migration of a linear ripple or dune:
Sinuous Ripple or Dune
Here is a model-produced version of the migration of a sinuous ripple or dune:
This movie ends with the top surface of the ripples/dunes getting eroded flat. The white lines represent the laminae that intersect the upper surface. Note how complexly shaped they are. Next the cross stratification is rotated to show how different surfaces show different cross stratification styles. Drag the marker at the bottom of the movie back and forth to explore these variations. Note how important it is to have a 3-D view of the cross stratification to interpret bedform geometry even when it is relatively simple!
Lots of other movies are available to explore as you wish. There are also LOTS of photos of different model-produced stratification styles and some outcrop photos associated with the online book (see http://walrus.wr.usgs.gov/seds/bedforms/animation-toc.html).