Structural geology. It’s like Cluedo with less murder.

In the summer, I went for a walk along the coast near where my folks live in South Devon. While I was there, I noticed some rather spectacular geology, so photographed a few bits of it while I was on my walk. I was initially disappointed because had neither my notebook or compass-clinometer and therefore couldn’t do any in-depth geology, except for in a ‘oohh, that looks nice’ kind of way.  As I walked along the coast, I noticed that things were starting to get complicated, so I took the photos and then went back to my walk, in the knowledge I’d think about it more later.

Now, in December, over 5 months after actually looking at these rocks, I have a bit of time to think about the history in a little more depth, giving them the true attention they deserved. I was right, it wasn’t as simple as I’d originally thought and a little geo-detective work was needed to untangle what was going on. 

I was walking along the coast on a gorgeous July day and saw this fantastic outcrop. Obviously I had to stop and snap a picture as it seemed to be showing ‘S0’ which is the original bedding surface of these rocks before they were metamorphosed by the first stage of deformation ‘D1’, which formed the second planar surface, ‘S1’. Notice that the original bedding surface and the deformed surface are slightly oblique to each other? That’s because of the stress direction of the deformation event that created the S1 surface. I’ve marked on the photo below my interpretation for this outcrop, although feel free to disagree with me, as I’m no structural geologist by any stretch of the imagination! 

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After this, I was enticed to carry on walking down the coast to see if there was anything else I could learn about what had happened to these rocks. I know I’m a nerd, but go with it. 

So a little further down the coast I came across some rather interesting structures called en enechelon tension gashes (outlined in blue). These showed me that there had to have been a second stage of deformation, ‘D2’ which was shearing the rocks in a direction that was oblique to the first stage of deformation. Wow – two deformation phases within 20 paces, I was starting to enjoy this walk more and more!

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A little further along the coast, after being licked to death by a large Golden Retriever, I saw some lovely folding so again snapped a photo, thinking that they followed the same deformation style as the previous outcrops I’d been looking at.

But the more I looked at the folds, the more I thought there was something wrong with my initial interpretations. The folds were clearly affecting the ‘S1’ surface, but the axial planes (shown by the dotted lines below) were perpendicular to the S1 surface. This means that the principle stress direction had changed, and that there was a third deformation event affecting these rocks!

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Well, that was fun. And all of that was seen in a really short distance – maybe 1km at the most. After this the outcrop became confined to large cliffs and the tide was coming in. I also needed a little bit of time to contemplate my findings with a biscuit and some tea (very essential walking provisions), and then carry on my walk along a rather nice part of the South West Coast Path. Well, it’d be rude not to on such a lovely day – and I certainly had a lot to think about!

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One Response to Structural geology. It’s like Cluedo with less murder.

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