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Tuesday, 22 March 2011

Perched blocks and glaciation

Carn Edward, near Carningli

Carnedd Meibion Owen, near Tycanol Wood

Garn Fawr, Dinas Mountain

Carn Arthur, near Carn Meini

Carn Enoch,  Dinas Mountain

These are all perched blocks found in NE Pembrokeshire.  They are not  ERRATICS since they are all of rather local origin, transported by ice for metres rather than kilometres.  Almost certainly the agency is glacier ice, although in some circumstances blocks can be dislodged from a cliff face or from a teetering tor and roll or slide down a snow slope, to be later "dropped down" onto a precarious situation as the snow melts away.  But I don't think this can have happened in any of these cases -- there are no convenient "source locations".  So glacier ice has actually moved these very heavy blocks, several tonnes in weight, and has left them behind when it melted.

Age?  Four of the photos come from the zone of "rearranged tors" affected by thin glacier ice at the peak of the Devensian glaciation, within a kilometre or two of the ice edge.  But the Carn Arthur perched block was reputedly hurled there from the other side of the Preseli Ridge by a giant or great hero called Arthur, in the good old days. (Hero names like Arthur or Samson are often used to explain strange natural phenomena.....)  If you don't go for that explanation, and prefer the glacial one, the interesting thing is that Carn Arthur is on the SOUTH side of the Preseli ridge, well to the south of the Devensian ice edge as we currently imagine it.  That means that the block was put into position during some earlier glaciation like the Anglian, maybe 400,000 years ago......

13 comments:

Anonymous said...

That final photo looks like a Neolithic U.F.O. Is it possible....? We may have ANOTHER theory - volunteers, please step forward.

Sem said...

Nice pics of perched boulders, probably totally glacial deposits.
But.. and I am pushing here, Carn Arthur lies directly North of Gors Fawr circle, and the boulder is on the skyline, and accentuates the outcrop pointing to Carn Menyn. Did the builders of Gors Fawr use Carn Arthur and it's perched boulder as a "marker" or did they place the marker themselves?
Cheers
Sem

PS for your info only, have you seen the standing stones at SN1388 3249. There are 3 in the "stone river" virtually invisible until close up. I don't believe in coincidence and suspect both archaeologists and geologists have missed something in this "sacred landscape".

BRIAN JOHN said...

Anon -- sorry, that's now photo number 3. When I was editing them they hopped about a bit and I had to re-import.... Yes, that's a really strange one, perched on three small stones! But goodness knows what it weighs -- 3 or 4 tonnes?

Constantinos Ragazas said...

Great photos Brian! Clear evidence of glaciation! There are couple such boulders at Central Park in NYC and clearly identified as glacier. The one with the big boulder supported by three very small stones seems to suggest a more recent event! What is the explanation for these photos from the other side?

Kostas

BRIAN JOHN said...

Sorry I was unable to paste in my explanation of the blocks yesterday -- ran out of time, with friends coming to supper!

Yes, the one perched on three little stones is VERY unusual.

Sem -- the stone river. I'll do a post on that before long.

BRIAN JOHN said...

Sem -- on the matter of the "sacred landscape" that supposedly existed at the eastern end of Preseli, I remain unconvinced. See this post:
http://brian-mountainman.blogspot.com/2010/12/preseli-stone-age-holy-land.html

I don't see any evidence of sacred or ritual features being more common here than anywhere else in Pembrokeshire, and I don't see any evidence that "sacred landscapes" were demarcated in any way or separated from "mundane landscapes." I think the whole thing is something dreamed up by archaeologists like Profs D and W, as I have said many times on this blog.

Barrie Foster said...

Just a thought about galciation in the South West. Cornwall has a number of 'Logan' or 'Rocking Stones'. When I lived in West Penwith it seemed to be the received wisdom that these were left by glaciation.

BRIAN JOHN said...

Just came in from the garden -- it's almost too hot for gardening today!!

Of course there are rocking stones on some of the Dartmoor tors -- some of those tors are very delicate and teetering. But I imagine you would be referring to some of the giant erratics on the coast?

Constantinos Ragazas said...

Brian,

I am very intrigued by that photo with a big boulder resting on three very small stones. What possible glacier deposit can account for such configuration? The big boulder looks like it was gently lowered on top of the three small stones. Any other way would have certainly crushed the smaller stones holding up the big boulder. And it appears that such incident was later and after the small stone deposits. But what process would 'gently lower' a huge boulder on top of three small stones?

Are the stones of the same type and origin do you know?

Kostas

BRIAN JOHN said...

Kostas, don 't get too intrigued by the perched block supported by three small stones. It's not unique in this area -- there are other slabs and boulders supported on much smaller stones, but this is the most photogenic of them! The big block and the little stones here are dolerite, but there is a thin spread of till in the neighbourhood, particularly to the south of the tors, on the down-glacier side. When glacier ice envelops and overrides a tor like this, it's perfectly feasible for a boulder to be let down quite gently. The process may take many decades, and when the boulder finally comes into contact with what is underneath the ice, there will probably be a lot of water washing away the finer materials.

Barrie Foster said...

Brian,

Yes, I was thinking about the coastal logan stones. The perched blocks on the Dartmoor and Bodmin tors seem more likely to have been the result of erosion or weathering. I lived not far away from the logan stone above Pendour Cove near Zennor; but the best known Cornish Logan Rock is that close to Treen and Porthcurno, on the headland opposite the Minack Theatre. This was famously dislodged in 1824 by a naval party under the command of Oliver Goldsmith's nephew, who was obliged to restore the stone at his own expense.

But there are many perched blocks on the sea cliffs of the Lands End peninsula - only the true 'logans' or rocking stones receive particular mention.

Constantinos Ragazas said...

Thanks for the explanation, Brian!

Certainly, a big boulder held up by ice slowly melting away will gently lower the boulder on top of whatever lies on the ground! But a retreating/advancing glacier with a well defined edge and flow direction will dump such boulders more violently. Could such perched boulders be used as evidence to distinguish one type of glacier condition from some other?

Kostas

BRIAN JOHN said...

I see where you are heading, Kostas! I don't think so -- you find perched blocks in all sorts of glacier wastage environments. Yes, blocks and flowtill do slide down steep ice faces sometimes -- and end up in an appalling jumble down below, or else fall into meltwater streams. Some perched blocks, of course, do not appear to be perched initially, but can later become isolated when all the finer debris around them is washed away.