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Sunday, 24 May 2015

Craig Rhosyfelin -- a seriously cracked-up crag

 Part of the rock face exposed during the archaeological dig.  If you get close enough to it and look along it, it certainly looks very flat, and one might think of it as "unnatural" in some way, and therefore a quarry face.  However...... things are not so simple.

I was intrigued the other day with Myris's comment that the "quarry face" at Rhosyfelin is  "a large planar surface that appears not to be structural in nature (absence of slickensides) and is difficult to explain if totally natural.  Not a fault plane nor master joint plane.........."

I commented on it at the time,  and have been back to take another look.  I am now more than ever impressed that the whole crag is so seriously riddled with multiple fractures running in all directions that it is a miracle that any of has survived the ravages of time and the Ice Age.  There are many posts touching on this subject, for example:

I published this rough illustration of the fractures in two dimensions.  The third dimension is the fracture plane(s) running along the face itself.

Things are actually a great deal more complicated still.  Let's concentrate for a moment on the "large planar surface" which might also be called a "sub-planar foliation surface" since it coincides with the foliations in the rock which have been given so much attention by the geologists.  It is not actually a single sub-planar surface at all, but a rock face made up of fragments of multiple surfaces.  Some parts of the rock face project at least 1.5 m out beyond other parts -- so we have recesses and projections when we examine the site carefully.  When we look at the tip of the spur, we see some of these surfaces end-on:

 Close-up of the tip of the spur.  Here we can see the steep dip of the "sub-planar foliation fracture surface" on the right, and then behind it, across the photo from right to left, a further 6 (at least) other fractures, more or less parallel.

 A close-up of another part of the spur tip.  There are multiple fracture surfaces coinciding with the foliation planes, some of them 10 cms apart and others less than 1 cm apart.

Looking in the other direction, down the slope from near the right edge of the photo with the blue and orange lines on, we get another cross-section of what is going on in the interior of the crag.  Again, we see multiple fractures related to the foliation planes in the rock.  There are at least ten pronounced fractures, and many more hairline cracks.

Now here's a question for the geologists, which was raised in a letter to me from Barry.  Why do these foliation plane fractures occur, in some places very closely spaced, and in others widely spaced?  Is it something to do with crystal alignment or arrangement, or something to do with cooling surfaces?  Rob and Richard, advice please?

So much for the fractures related to the foliation planes.  Then we get to all the other fractures.  I started to record them by compass directions, but gave up because there are so many of them, some traceable over 50m or more, undulating up and down along the rock face, and others restricted to just one semi-detached slab.  The message is that hundreds of these fractures are discontinuous and localised, developed in response to quite localised stresses.  So I would agree with Myris that there is no master fault plane or joint plane here, and certainly no bedding plane since we do not have any identifiable "beds" of rock as we do on sedimentary rock exposures. These are, after all, metamorphosed volcanic rocks, and the main processes of deformation shortly after their emplacement would have been related to cooling and contraction and maybe loading beneath later igneous and sedimentary accumulations.

So here are two more pics just to confuse things even further.

 Another exposure on the rock face, showing one long and undulating fracture running from left to right, and a series of steeply plunging fractures which are not parallel.  Just to the left of centre, there is a V-shaped arrangement of fractures.

This is the most complex arrangement of fractures on the whole rock face.  In a word, wholesale chaos in three dimensions......

Back to the original point.  This is certainly a highly complex rock face -- worth a structural geology field trip on its own account -- but I do not agree that it is difficult to explain if totally natural.  I see nothing whatsoever on the rock face to suggest human intervention, apart from the cosmetic work recently undertaken by the archaeologists. 

What intrigues me rather more is the role of geomorphology in all of this.  I mentioned the possible role of cooling, contraction and loading beneath later Ordovician and later rocks, but the more I think about it the more convinced I become that the rock wall is actually a meltwater channel wall, swept clean by turbulent meltwater flowing over the col at the base of the spur and then downslope in a subglacial channel.  I'm also quite attracted by the idea that there may have been actual fracturing of the crag as a result of ice loading during the Anglian Glaciation and -- to a lesser extent -- during the Devensian or Last Glaciation.  This process of compression / tension / pressure release might well have been a powerful factor in explaining the entrainment of blocks into the base of the over-riding glacier.  Another possibility is that we are looking at the results of unloading or pressure release linked to ice wastage.  We see evidence of this happening in many glacial environments today -- I have discussed this before in the blog.

Quarry face?  I suggest that we just forget the idea.

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