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Saturday, 21 January 2012

When did Craig Rhosyfelin come into existence?


This is an interesting question:  when the large chunk (or chunks) of rock was/were taken from Craig Rhosyfelin by one process or another, and transported towards Stonehenge, did the landscape look anything like the landscape of today?  The provenancing of some of the Stonehenge rhyolite debitage to "within a few metres" at Rhosyfelin seems to be based on the assumption that the landscape is unchanged, and that the pronounced spur sticking out into the valley of the Afon Brynberian was there at the time of rock extraction.  I have questioned that assumption, and have wondered how accurate any provenancing can be, given sampling bias and other factors -- but putting that question to one side, I'm increasingly convinced that the landscape in and around this valley has changed quite substantially during the past 500,000 years.

On the top photo above, the MPP dig site is shown with the circle, on the flank of the rocky spur of Craig Rhosyfelin.  On the lower photo, that site is in the bottom R corner, and we see the deep valleys of the Brynberian and Nevern rivers.  Both are flowing partly in deep gorges, and I think most geomorphologists would agree with me that these are Pleistocene features, created at a time of deglaciation, with great volumes of meltwater flowing either under the ice or beyond the wasting ice edge, and trying to find its way towards lower land.  This is at the same time as the creation of the huge meltwater channels of the Gwaun - Jordanston system mentioned in earlier posts.

So if these are meltwater channels created for the most part at the end of the Anglian Glaciation, it follows that they were not there (and neither was Craig Rhosyfelin) at the time when thick ice was flowing across the landscape and entraining large blocks of foliated rhyolite.  So what did the landscape look like prior to the arrival of the ice?  Probably there would have been a shallower valley here, and probably there was a substantial crag of rock -- rather like the tors of Carnedd Meibion Owen -- standing proud of an undulating land surface.  Subsequently, after the erosion of this high crag, meltwater got to work, creating the main valley and the subsidiary valley on the northern flank of the rock.

5 comments:

Constantinos Ragazas said...

Brian you write,

“I'm increasingly convinced that the landscape in and around this valley has changed quite substantially during the past 500,000 years.”


Also,

“So what did the landscape look like prior to the arrival of the ice? Probably there would have been a shallower valley here, and probably there was a substantial crag of rock -- rather like the tors of Carnedd Meibion Owen -- standing proud of an undulating land surface. Subsequently, after the erosion of this high crag, meltwater got to work, creating the main valley and the subsidiary valley on the northern flank of the rock. “

Brian, the more you initially oppose my ideas the more you finally embrace them! What you are describing now is a landscape different from what you said before. With a high ridge/crag I suggested was there before there was the Rhosyfelin outcrop.

Kostas

BRIAN JOHN said...

Kostas -- you have got it all wrong, yet again. By a "substantial crag" I mean something maybe 10m - 20m higher than the surface today. That crag summit will certainly will not have been higher than 100m ASL, and it is on the wrong side of the mountain for your wonderful sloping sheet of ice. I wish you would THINK a bit, Kostas, and simply look at some maps,before banging off these triumphalist messages......

Confused said...

Dear Brian,
Back last year (Friday 4/11/11), under the heading "Beware of the Quarry Hunter" you posted a photo of a large rock uncovered during the archaeological dig at Craig Rhos-y-felin.

Now that sourcing of stone, particularly in this area, can be achieved to a fine tolerance, it follows that the petrography of this large stone could indicate whether it has been levered from the tip of the Craig Rhos-y-felin spur, or transported by ice/humans from farther afield.

Your views on the following would be welcome:
a). If the big rock was transported from elsewhere, then provenancing the stone to Craig Rhos-y-felin would induce a problem; namely, if, as the ice-flow maps indicate, the glacier travelled from north to south then the big stone should be living on the south side of the spur but it is on the wrong (north) side.

b). From what I have gleaned from your posts on glaciers it seems that it is the base of the glacier that uproots stone from the bedrock.
If the "substantial crag" was 10-20 metres higher than what it is today, then a glacier travelling south would, logically, have carried this large stone some distance south before dumping it.
However, the position that the stone now rests in, i.e. a few feet from the spur, would, it seems, correspond to humans simply levering it from the outcrop and letting it fall.


Do you know if there were any geological samples of this large rock taken while it was exposed, please?

BRIAN JOHN said...

Confused -- as far as I can see this large stone, like lots of smaller stones around it, is just the product of normal bedrock shattering (possibly under a periglacial regime, in the Older or Younger Dryas) and rockfalls. It is exactly where you would expect it to be if it has just fallen off the crag -- no human agency needed, and no glacier needed. I try to explain this in some of the other Craig Rhosyfelin posts.

I am sure that MPP and his colleagues have taken samples from the stone, and in my view it is 100% certain that it will match the samples taken from the adjacent bedrock. The results will no doubt be published in due course, and will be flagged up as having great archaeological significance........ believe that if you want to!

Not Quiet So Confused said...

Brian,
Many thanks.
NQSC