More mud

Remember the mud I saw on Pimmit Run?

Turns out that West Virginia mud pulls many of the same tricks as Virginia mud. Here’s some mud cracks I noticed on Sunday afternoon on the shoulder of New Route 55 in eastern West Virginia:

Notice how West Virginia spices her mudcracks with chunks of Silurian quartzite and fresh crabgrass. A tasty combination!

Where on Google Earth? #199

For the first time ever, I have won a Where on Google Earth? that Ron Schott hosted, heavy on the clues.

Now that it’s my turn to host, I’d like to spice things up a bit with a dynamic view. Using Google Earth’s historical imagery feature (the little “clock” button up top), I managed to see this suite of very different views of the exact same spot over time:

Click through twice for full-sized versions. Note in particular how much this scene changes between photos #2 and #3, only three days apart! This must be a very dynamic place indeed. You have already noted of course the one constant in these images, down there in the southeastern corner.

No Schott Rule on this one… have fun! To play the game, find the location and leave its latitude and longitude in the comments of this blog post, as well as an explanation of the geological significance of this location.

Remember that this is #199, which means that the winner of this episode of Where On Google Earth? gets the honor of hosting the 200^th episode. “200,” like “30,” is a nice round number… hint, hint.

Eastern Worm Snake

While on our structural geology field trip this week, my GMU students and I encountered an eastern worm snake, Carphophis amoenus amoenus. The little charmer at first reminded me of a boa, like the ‘rubber boa’ I once found in California (a real animal, not made of rubber), and then I convinced myself it was a glass lizard… But upon the return to civilization, I was able to consult several webpages and confirm that it was in fact an eastern worm snake. Check it out:

A distinctive “thorn” at the tip of the tail:

Very squirmy and constrictional little fellow: nosing into me and poking me with his tail “stinger” repeatedly.

Overall, a cool critter!

…We were also visited by swallowtail butterflies of several flavors:

Fine fellows

What do I have in common with luminaries like Mike Fay, James Balog, and Stuart Pimm? We’re all this year’s cadre of Fine Fellows, recipients of free Gigapanning equipment and training (at a conference in Pittsburgh in November) on how to use it. Ron Schott is of course the king of geology Gigapanners, though he has some competition, and my acquisition of a Gigapan robot will up the ante a bit, hopefully to the benefit of the entire geology-interested Internet.

Here’s the full list of Fine Fellows for this year:

Mr. Richard Bahr Fire Science & Ecology Program Lead, National Park Service, Wildland Fire
Mr. James Balog Founder and Director, Extreme Ice Survey; Professional Photographer; Affiliate of the Institute of Arctic and Alpine Research, University of Colorado; Affiliate of Byrd Polar Research Center, The Ohio State University
Mr. Callan Bentley Assistant Professor of Geology, Northern Virginia Community College
Dr. Laurel Bestock Assistant Professor of Egyptology and Ancient Western Studies, Brown University
Dr. Faysal Bibi Postdoctoral Fellow, Institut International de Paléoprimatologie, Paléontologie Humaine (IPHEP), Université de Poitiers
Dr. Raymond S. Bradley University Distinguished Professor in the Department of Geosciences and Director of the Climate System Research Center, University of Massachusetts Amherst
Dr. Andrea Chavez Michaelsen Postdoctoral Research Associate in Climate Change and Environmental Public Policies, University of Florida (UF) – Universidad Nacional Amazonica de, Madre de Dios (UNAMAD)
Dr. Julia Clarke Associate Professor, Jackson School
of Geosciences, University of Texas at Austin
Dr. Nathan Craig Assistant Professor of Archaeological Anthropology, Penn State University
Mr. Brett Engstrom Consulting Botanist & Ecologist
Mr. J. Michael Fay National Geographic Explorer & Directeur Technique, Agence Nationale des Parcs Nationaux, Gabon
Dr. Joshua M. Feinberg Assistant Professor, Institute for Rock Magnetism, Department of Geology and Geophysics, University of Minnesota
Dr. Alex Fischer Program Coordinator, Center for International Earth Science Information Network, Columbia University
Dr. Michel Hermelin Professor, Environmental Geology Group, Universidad EAFIT, Medellin, Colombia
Dr. Krithi Karanth Postdoctoral Fellow, Geospatial Analysis for Sustainable Ecosystems and Development, DeFries Lab, Columbia University
Mr. Chris Linder Research Associate, Woods Hole Oceanographic Institute
Dr. Stuart Pimm Doris Duke Professor of Conservation Ecology, Duke University
Dr. Surangi Punyasena Assistant Professor, Department of Plant Biology, University of Illinois
Dr. Sara Schneiderman Research Fellow in Anthropology, St Catharine’s College University of Cambridge, and Dr. Mark Turin Director, Digital Himalaya Project & World Oral Literature Project, Museum of Archaeology and Anthropology, University of Cambridge
Dr. Janet C. Steven Assistant Professor of Biology, Sweet Briar College
Dr. Rebecca Tharme Senior Freshwater Scientist, Global Freshwater Program, The Nature Conservancy
Dr. Kim Williams-Guillén Conservation Scientist, Paso Pacifico
Dr. Qingling Zhang Research Scientist, Urbanization and Global Change Group, School of Forestry & Environmental Studies, Yale University
[Fellow TBD]
National Oceanic and Atmospheric Administration
[Fellow TBD]
Yellowstone to Yukon Conservation Initiative

I was selected for this program not because I applied, but because I blog. It’s not the first time that geoblogging has opened up new opportunities for me, but it’s probably the most significant recognition to date. I’m very grateful for it, and also gratified to see that fellow geobloggers are also being recognized for their fine work.

What about the rest of the geoblogosphere — got any “geoblogging got me a free robot camera” stories to tell?

Mud

A few more photos from Pimmit Run … of mud.

This mud has lots of interesting features, including dessication cracks showing lovely 120° triple junctions connecting up with their neighbors, raindrop impressions, and animal tracks.

Tracks of at least three species here:

(Clicking on this one will make it bigger)

Mud: not as fascinating as some things, but it can share some modest insights.

Crenulation lineation

Hiking last Sunday in Rock Creek Park, DC, I saw this boulder and my eye was immediately drawn to the linear pattern running from upper left towards lower right (Swiss Army knife at upper right for scale):

Because that photo is not especially large, let’s zoom in a bit to two sections… Here is Photo 1, annotated to show the areas we will look at next:

Here’s a cropped and higher-resolution look at the diagonal lineations that caught my eye:

There are lots of different linear elements that can show up in rock fabric (as distinguished from the many kinds of planar elements that could be found). Some lineations are primary, but the ones that interest me are secondary (i.e., tectonic in origin). Let’s rotate our perspective, moving to the left of the first photo, and turning our head ~70° to look towards the right. This closer look at the left face of the boulder reveals the origin of these particular linear elements:

…They are crenulation lineations, essentially very small folds that deform the cleavage of these highly-foliated rocks. The crenulations’ fold axes were popping out in very slight 3D relief on the face of the boulder that initially caught my eye, like tectonic “ripple marks.” On the right side of Photo 3, you can see the lineations (fold axes) stretching away into the blurry distance.

In addition, some of the convex-outward crenulations had been breached, which means that the trace of the foliation was outcropping along the same trend as the fold axis. This is a variety of intersection lineation: two planar elements intersecting in a line. In this case the planar elements are [a] the foliation and [b] the outcrop surface.

(The other, more “classic” variety of lineation is a mineral stretching lineation, like the lineated gneiss I showcased last November.)

So, how should we interpret these rocks? I’d say that an initial foliation was imparted to them due to shearing along the Rock Creek Shear Zone, a prominent north-south-trending zone of smeared rocks in northwest DC; about 1 km wide. The foliation formed perpendicular to an original σ₁ maximum principal stress direction. Later, the stress field changed, and deformed this pre-existing foliation. The new σ₁ was oriented (using Photo 1 as our reference) from the lower left towards the upper right. The new σ₂ was oriented parallel to the crenulation fold axes (upper left towards lower right). And the new σ₃ was oriented in the direction perpendicular to the main outcrop face — that’s why the folds pooched out in that direction. (It offered the least resistance to being pushed.)

Recall that we saw something similar in the snow back in February.

Anyhow, I had just gotten through discussing lineations with my GMU structure students, so I figured I should photograph this particular outcrop for their benefit…

…and, I suppose, for your benefit as well, dear blog reader.

Using bedding / cleavage to detect overturned beds

One of my students wrote to me this morning with a question about the relationship between bedding, cleavage, and folding. He asked:

I am not sure how we use the relationship between bedding and cleavage to interpret fold limbs.  It seems if bedding is near vertical and cleavage is closer to horizontal, this would be an upright fold limb.  To be overturned, wouldn’t the bedding need to be closer to horizontal?  I guess I don’t understand how does cleavage help dictate the bedding orientation.

So here’s the deal: when rock strata (layers) get compressed, they develop a couple types of structures: one is that they tend to fold, and the other is that they tend to cleave. Cleavage and folding have a distinctive relationship.  Say bedding starts off horizontally-oriented, and is subjected to a horizontal compressive stress. Cleavage will form that is vertically-oriented (perpendicular to σ₁). As deformation proceeds and the bed begins to shorten by buckling up and down, the cleavage “tips” over (rotates) as the top of the bed moves towards the fold crest. (I have previously discussed a similar aspect of vergence, using S and Z folds. The same thing that applies to the axial planes of parasitic folds also applies to cleavage.)

Assuming a simple single episode of deformation (no overprinting), the orientation of the cleavage plane will be approximately the same as the axial plane of the main fold (an imaginary geometrical plane that “divides a fold into left and right halves”).

Here’s a quick sketch I just drew of a folded bed (yellow) being cut by cleavage (parallel brown lines):

In the example on the right, the cleavage and folding agree with one another (that is, the folds’ axial planes and the cleavage planes are parallel): one episode of compression could produce these two structures in these orientations. In the example on the left, the cleavage cuts across the fold at an angle which is close to orthogonal (perpendicular) to the axial plane of the folds — this is an impossible situation to produce with a single episode of deformation.

If you were to find an outcrop of the yellow layer (circular zones of exposed rock in the diagram below) that showed the relationship between the cleavage and the bedding, you can interpret the overall structure:

Outcrop 1 shows bedding and cleavage dipping in opposite directions. Outcrop 2 shows bedding and cleavage dipping in the same direction, though bedding is dipping more steeply than cleavage.

Because of this relationship, Outcrop 2 is best interpreted as an overturned limb of a fold. But Outcrop 1 doesn’t make any sense as it is drawn above. The best way to interpret that circular exposure is shown here:

…That is: it is an upright limb of a fold, not an overturned limb.

So: if you have a steeply-dipping bed cut by more-shallowly-dipping cleavage, pay attention to the direction of the cleavage’s dip: (a) If it is dipping in the opposite direction as bedding, your fold is upright or asymmetric. (b) If your bedding and cleavage are dipping in the same direction, your fold is overturned. If the bedding and cleavage are both close to horizontal (and part of a larger fold), then you’ve likely got the limb of a recumbent fold. If bedding is vertical and cleavage is horizontal, you’re likely on the nose of a recumbent fold, where the axial plane cleavage is intersection bedding at a right angle. If bedding is horizontal but cleavage is vertical, then the deformation likely hasn’t proceeded very far. Obviously, checking for geopetal structures like cross-bedding or mudcracks can help you determine whether the beds are overturned or not from a purely sedimentological point of view.

Hopefully this post helps elucidate the structural relationship between bedding and cleavage a bit more. If not, read here about a classic example in Wisconsin (Van Hise Rock).

Photos from Eyjafjallajökull

My friend Barry R., now residing in PostDocVille, Denmark, took a trip to Iceland last week to check out the eruption of Eyjafjallajökull. Unfortunately, by the time he got to the volcano, it was no longer spouting lava, but the scene is dramatic regardless.

You can sample some of his photos below, or see the whole album on Facebook.

Waterfall:

Glacial terminus and moraine:

Ash on ice (steam rising beyond the hills):

Where the volcano has melted the local ice:

Thanks to Barry for letting me share his volcano photos here. He’s the second University of Maryland alum to do so! It seems to be a trend…

John McPhee reading

The Lilster and I caught John McPhee give a reading tonight at our local uber-cool bookstore. Check it:

First time I’ve seen McPhee live. Great stuff!

Accretionary wedge reminder: April 23

A friendly reminder that I’ve volunteered to host the next edition of The Accretionary Wedge, and I’ve chosen “heroes” as the theme.

I invite all participants (geobloggers and geoblog readers alike) to contribute stories of their heroes. It’s time to pay tribute to the extraordinary individuals who helped make your life, your science, and your planet better than they would otherwise have been.

The deadline for submission of posts will be Friday, April 23. That gives you until the end of next week.

Once you’ve published your piece, leave a link to it at the original “call for posts” post. On the weekend of April 24-25, I’ll aggregate all the submissions into a thoughtfully-composed masterpiece post and put it up for everyone to savor. Thanks in advance for your participation.

If you’re a geoblog reader, but not a geoblogger yet yourself, then I’ll be happy to publish your story here. We all have heroes worthy of sharing, right? I encourage readers to get involved; I’ll be happy to help facilitate.