Posted by: Alan Pitts | May 12, 2010

Structural Geology Trip Part 3: Limberlost Trail

After a light breakfast on Saturday morning we headed off towards the beautifully and  scenic Skyline Drive. We made several short stops to discuss the regional geological content and then made our way to the next field area.

The Limberlost Trail is somewhat famous, and if you think I’m making this up just check out the National Parks edition of Monopoly and you’ll see the Limberlost trail as one of the “railroads”. I  certainly applaud Milton Bradley for selecting a trail from our near and dear Shenandoah  National Park, but at just 1.3 miles in length and 100 feet of elevation gain, I think I would have picked  something a little more epic. Just off the top of my head, I would maybe go for the White Rim Trail in Canyonlands National Park…. But that’s just me.

After visiting the Limberlost trail for myself, I’m guessing that Mr Monopoly  picked the Limberlost not for of its gentle grade, crushed greenstone walkways and wheel-chair accommodations , but instead for the incredible columnar jointing ( !)

At our first stop along the trail we saw these columnar joints:

by steph schilingoThese columnar joints are in the Catoctin formation, which is a vendian age ( 650-540 million years ago) flood basalt which was later metamorphosed to greenstone.

Here is an annotated version of the picture to note several features:

Here you can see the polygonal shape of these columns. Most of these have 6 sides but a few have seven and we found some with as many as 9 sides in the field. The other point worth mentioning is the angular relationship between the joint surfaces. The columns intersect with each other at approximately 120 degrees.  What is really fantastic about this outcrop is the fact that the columns were left intact and maintained their original orientation. They experienced metamorphism but not much deformation.

A little further up the trial we saw some more columns who were not so lucky, next two photos by Nik D:

You can see how the column boundarys are no longer so crisp and regular at 120 degrees. These rocks experience metamorphism and deformation.

Here is a side view of the same outcrop :

This is very similar to the Giants Causeway in Ireland, but sheared and  some would say…..better ?

And another sketch of mine illustrated the sense of strain on this outcrop (apologies for the weird pink reflections, the waterproof paper of my field notebook is pretty glossy and is hard to photograph):

This shows a sense an East over West sense of shear.

The Catoctin tells a similar story as the Swift Run formation as they are both associated with the a rifting stage. The Catoctin flood basalts formed due to thinning of the crust, resulting in volcanism, much like what is currently happening in the East Africa Rift Valley. However these rocks, like many others we saw on the trip, tell a two part tale. One describing the conditions of its formation and the other describing future deformation.

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