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Friday, 3 October 2014

Fracture Plane -- Abermawr


Following last winter's storms and a series of rockfalls, a beautiful long, straight, flat fracture plane has been exposed at the southern end of Abermawr Bay in North Pembrokeshire.  It compares very nicely with the one at Rhosyfelin..........

Here the rock is thin-bedded Ordovician shales and sandstones.  In addition to the plane now exposed in the cliff face there are others running at all sorts of angles --  one of them is running more or less horizontally, and as we can see in the photo it has given rise to a newly-formed bench or platform a couple of metres above present HWM, where big slabs have broken off and then been smashed up.

Archaeologists please note -- this is NOT a Neolithic quarry made by intrepid seamen who shipped all their handy slabs of rock off along the coast in curraghs or log boats.  A flat rock face with big detached slabs beneath it does not make a quarry.....

But there has been some opportunist human interference, as we can see from the graffiti!

6 comments:

Myris of Alexandria said...

Scale please.
Relative lengths are needed.
M

BRIAN JOHN said...

This one is about 25m long -- as you can see if you look carefully, the fracture coincides with the alignment of bedding planes in the strata. Some of those bedding plane junctions are coherent, and others have fractures on them. Natural enough, when you think about it.....

chris johnson said...

A couple of years ago we had a brief discussion on Rhosyfelin as a shear resulting from eg earthquake. There was some evidence I recall that the area does suffer earth movements periodically. I think this came up again recently - Myris?

What this Abermawr photo reminds us of is that sheer faces can be constructed by natural forces - pembrokeshire is well endowed with examples.

Costantinos Ragazas said...

Chris, you write ”A couple of years ago we had a brief discussion on Rhosyfelin as a shear resulting from eg earthquake.”

I remember that discussion well, since I was part of it. My argument at that time was such an ”earthquake sheer” will have TWO sides to it. The Rhosyfelin NW rock face stands alone.

With the evidence Brian presented more recently of rounded and abraded rocks, there can be no doubt ”torrents of meltwater” tore through Rhosyfelin, engulfing the Crag in water.

The only question is when. And if MPP released the Rhosyfelin rc dates we may be able to know.

Kostas

chris johnson said...

The other side - as you put it - would be below ground. And below what has been currently excavated.

I think many laymen look at Rhosyfelin and remark on the sheer face. With an idea about monoliths being quarried it is tempting to jump to conclusions.

Brian is pointing to some natural explanations, I think.

BRIAN JOHN said...

Quite correct, Chris. If there was a fault here, the downthrow side would be below ground level. Alternatively, there might have been lateral movement. But I'm not aware of any slickensides -- must look carefully for them next time I go over there.....