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 (lo) to be 58.0 cm, and measuring from end to end the final length (lf) to be 18.3 cm.
Here are my calculations:
e = (lf – lo) / lo * 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.