Thurs-Demo: The one with Warts

Although I mainly teach introductory geology, I have taught other upper-level geology courses. One of my favorite topics is bedding structures (ripples graded beds, mudcracks, etc). The differences in those bedding structures formed by moving water compared to those formed on land by moving air are particularly helpful when interpreting the environment of formation of a particular unit of sedimentary rock. Combine a stack of these units into a stratigraphic column and you have a record of environmental change over time. Compare one stratigraphic unit to an adjacent unit of the same age and you have environmental change over spatial scales. Combine the vertical analysis of time with the spatial analysis and you have a four-dimensional study of an area. Time and space. All from a set of lovely sediments all piled up together (more or less depending on your field area).




Here's a lovely example of some bedding structures from the Chinle Formation (Triassic) of Utah. There are some interesting drag marks and some other traces that my ichnologists might be able to tell more about. But the lumpy ridges are what struck me. They aren't normal ripples where you might see some distinct cross laminations to give you a sense of fluid flow direction. They're also kind of "smooshed" at their crest, suggestive of some kind of soft-sediment deformation after they formed.

My current thought is that these are adhesion structures. These are ripples, ridges and bumps (wonderfully called "warts") that form when wind blows loose sand across a wet surface. If you live near the beach, or - for those of us in colder climes - walk across a parking lot on a windy winter day, you may see little bumps as the snow bounces across and then sticks to the wet pavement.


Here are some "nivean adhesion warts" that formed as snow blew loose snow across a wet sidewalk last winter. The orange BB is 6mm in diameter.

You can make your own adhesion warts - all you need is a wet paper towel and some fine sand. Here's a quick video:



Oh, and since my department has a fancy new camera, it would be cruel of me not to show you some of these sand grains bouncing across a wet surface in slow motion, here you go (I suggest going full-screen for this one)


Adhesion warts are probably my favorite bedform. In part it's got a great name. But also, they are clear indicators of terrestrial sedimentation. You really can't form these things underwater because there's no surface tension to allow the sediment grains to "stick" to each other underneath the water.

Dune Week

Looks like folks are talking about sand dunes and eolian transport on the geoblogs this week.

Here's some video I shot of blowing sand - you can see at least two distinct modes of movement - large grains are either tumbling individually, or avalanching down the slip face. Smaller grains, however, are picked up by the wind and bouncing (saltating) as they are transported.