Catoctin Fold Calculations

Last weekend Alan and I had the opportunity to lead a field excursion with some of our fellow GMU/NOVA geology students/professor to some of the outcrops we have been scoping out lately. This particular day found us at Thoroughfare Gap along I-66 in Fauquier County Virginia, where some large outcrops of the transgressive Chilhowee Group are on display. We like these outcrops because they illustrate part of the rifting of Pangaea’s super-continent predecessor, Rodinia. As you move “up sequence”, or eastward in this case, the rock types move from a course grained arenite (Weverton), to a muddy shale (Harpers), to the Antietam sandstone which is not actually preserved at this location, but what this implies is a transgressive sequence. The depositional environments for each became deeper as the Iapetus Ocean crept up the eastern seaboard.

Just below the Chilhowee Group in the stratigraphic column, and a little further west from the outcrops we were looking at, lays the Catoctin Formation, a flood basalt from the same Rodinian rifting event and a favorite among Virginian geologists. So far today Callan Bentley has already posted his “Friday Fold” exhibiting a nicely chevroned kink in the Catoctin, and riding on his coattails I couldn’t resist sharing my own Catoctin fold (even though I gave up the rights to it).

In this rock, the more competent, stiff layer has been distorted and mashed inside the softer surrounding matrix. This ptygmatic fold (intestine-like) includes some elliptical and boxy isoclinal folds that are just screaming for some numerical analysis. So I complied.

If we assume that the stiff, lighter colored layer was originally straight and measure the length of that original position we can calculate the new folds elongation (e) to determine a percentage of how much the layer has shortened. Using a string to run along the many folds I found the original length (l_o) to be 58.0 cm, and measuring from end to end the final length (l_f) to be 18.3 cm.

Here are my calculations:

e = (l_f – l_o) / l_o * 100

e = (18.3 cm – 58.0 cm) / 58.0 cm *100

e = -68.4

Therefore the fold is now 68.4% shorter than it originally was. If that happened to me there would be a two-foot tall redhead walking around, and I would have to trade in my guitar for a ukulele.

Last Sunday was the first time I have ever found folding in the Catoctin, but maybe now I will keep a sharper eye about.

 

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The Walls of Versailles

I spent part of my morning cruising around the Palace of Versailles through the use of www.googleartproject.com. If you are not familiar with this webpage, Google has taken the “Street View” technology and used it to document numerous art museums across the world. Not to get too cliché or cheesy, but to me this is one of the true great applications of the internet and maximizes the potential the internet has had as a tool. I have been fortunate to travel to places like the Louvre in Paris and the MoMA in New York City, and also to have such a comprehensive collection of museums like the Smithsonian just down the street from where I grew up. Now the people that do not have the opportunity can at least get a sense of what it is like to stroll the corridors of King Louis’ great palace.

How can I relate this to geology? While the paintings and architecture were enough to keep the aesthetic eye pleased, the building itself is an amalgamation of incredible rocks with jointing, bedding and foliation to excite even the most novice geologist enthusiast. Take a look at the example below, and get an idea of what I am talking about:

The picture below is one area that particularly stood out to me:

Here is a highlighted version to show what really caught my eye:

To me these slabs of rock look like a fault breccia, and could be referred to as either a psuedotachylite or a suevite depending on the originating environment. If formed in a fault setting it is psuedotachylite, and if it is from a meteorite impact it is suevite. In both rocks, the more competent rock is broken into angular chunks while the less competent rock is pulverized by fault friction into a glass. In the pictures above the darker colored areas are the glass that contain the large angular rocks.

Here is a zoomed in picture with an unfortunate amount of blurriness:

Maybe in combination with “Where on Google Earth?” we can start a contest of  ”Where on Google Art Project?”.

All images copyright Google. Thanks.

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