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Sunday, 27 April 2014

Rhosyfelin Foliation Plane


Thanks to Rob and Richard (whose papers I have been re-reading), and thanks to our mysterious friend Myris, I have been trying to get my head around the geology of Rhosyfelin.  I think I am getting there.

As I now understand it, the flattish rock face at Rhosyfelin, which runs away into the distance, coincides with the main "foliation plane" along which the foliated rhyolites with Jovian fabric are exposed.  Sampled point number 8 is just on the left edge of the photo, in the foreground -- but rocks with very similar fabric are exposed all the way along this face.  If there are thin sections of samples from other points along the face, we look forward to seeing them in due course, and to seeing how much lateral variation there may be in the fabric.  If the fabrics are very similar all the way along the 60m or more of the rock face, I am a bit mystified as to why the geologists said that they were now able to fix the provenance of some of the Stonehenge rhyolitic debitage to within a few metres.  Maybe there is more to come on this, in future articles.

The joint planes and fractures (on many different scales) that run into the rock face are at about 90 degrees to the face itself, and as suggested in a previous post:

http://brian-mountainman.blogspot.co.uk/2013/09/the-rhosyfelin-fracture-pattern.html 

As mentioned in that post there are other big fractures that can be followed along the rock face, running more or less horizontally if you look at the strike but with a dip down towards the SE -- ie approx 90 degrees from the sloping rock face.  If you scrabble about on the ridge crest, as I have done, you can pick up some of these sloping surfaces.

Because the foliation plane has given rise to a major structural weakness in the rocks at this site, debris of all shapes and sizes has peeled away from it and has gathered in the litter / scree which is now being investigated with such enthusiasm by the archaeologists --  who are of course desperate to see everything as "quarry" debris.........

The interesting thing for me, arising out of all of this, is that the foliation plane in question runs deep into the earth beneath the area where the archaeologists have been digging and once ran high up into the air above the remnants of the present day ridge.  What this means, in turn, is that the rhyolite fragments at Stonehenge might well have come from a part of the ridge which was once many metres above the present-day ridge crest.  As I mentioned in my SCRIBD article some time ago, the "proto-Rhosyfelin" might well have looked very different from the ridge of today, having been whittled away by glacial, fluvio-glacial and periglacial processes over something like half a million years.

http://www.scribd.com/doc/150104599/A-Long-History-of-Rhosyfelin

(Note -- if you look at the SCRIBD article, you may well get pretty angry with masses of adverts plastered all over it.  That's something SCRIBD has started doing -- but there should now be a mechanism on your computer which enables you to go to "Ad Choices" and turn all these nuisance adverts off.)










1 comment:

Dave Maynard said...

I use something called AdBlock and find it very good. There is a little number showing the number of ads blocked, it went from 4 to 37 when I checked your SCRIBD document.

Dave