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Sunday, 28 November 2010

Main spotted dolerite source area





Been up to the top of Carningli in the snow today -- quite delightful.  The visibility was excellent -- and I got this pic of the eastern end of Preseli.  Click to enlarge the photo.  The rocky outcrops on the mountain ridge include Carn Meini (immediately to the left of the TV mast), Carn Breseb, Carn Gyfrwy, and Carn Ddafad-las.  These are all possible source areas for the spotted dolerites at Stonehenge -- but the consensus at the moment is that the majority probably came from Carn Goedog, which is the massive tor on the north-facing hillslope, immediately below the TV mast in the photo.

There are some rhyolites in this area too, but the rhyolites at Stonehenge seem to be very variable -- and some fragments seem to have come from the lower land in the foreground of this photo.

15 comments:

Robert Langdon said...

Brian well done braving the cold weather. The map of the Mesolithic Period show that this area would have shorelines on both sides of the peninsula to support your theory, although from the map carnalw would be closest to the shoreline and most convenient - look out for deep earthworks were boats would be moored to load the stones as we find at Stonehenge as confirmation.

http://www.abc-publishing-group.co.uk/Images/Preseli%20Mountains.jpg


Regards
Robert Langdon

BRIAN JOHN said...

Sorry Robert -- I don't believe a word of it. I spent most of my professional life as a geomorphologist looking at Pleistocene shorelines, in the UK, and the Arctic and Antarctic. I think I know what to look for. I have seen no trace of a shoreline anywhere in the areas where you want your shorelines and harbours to be. According to your map, the Mesolithic sites at Hoyles Mouth, Wogan Cavern, Nab Head, Nevern Estuary etc would all have been deeply submerged if there was any truth at all in your "submergence" maps. Yours is a jolly fantasy, but it is not supported by the facts.

And you cannot simply dismiss the huge weight of evidence from all around the UK coast that sea-level in the Mesolithic was LOWER than it is today. There are hundreds of peer-reviewed papers in the literature, all showing the same thing. One of the best reviews in the literature is the Geol Cons Review for SW England, Campbell et al, 1998, around p 38. I suggest you read it.

Kosta Dean said...

Very interesting photo, Brian. Can you tell me the elevation of Carn Meini and Carn Goedog? And are there other such hills at higher elevation to these if one was to take a straight line path to Stonehenge? I would appreciate greatly that consideration.

Constantinos Ragazas

BRIAN JOHN said...

Hi Kostas

Carn Meini c 360m and Carngoedog c 250m above sea-level. If one draws a straight line from Preseli to Stonehenge I don't think there is any higher land -- except maybe in the area around Swansea and Neath -- the outer edges of the uplands of the South Wales Coalfield.

Kosta Dean said...

Thanks Brian,
Needless to say, all this fits well with my theory!

Kosta Dean said...

Brian,

So that I have a fix on the isostatic rebound of the UK after the last glaciation period, is it correct that the elevation of the coastal area of southwestern UK was some 200 m. higher than it is today? Would that also be so for Carn Meini at the same time frame -- to say an elevation of 560 m from the 360 m it is today? Please correct me if this is wrong.

Constantinos Ragazas

BRIAN JOHN said...

Have had a look at the relief map, Kostas, and on that straight line you are concerned about, there IS higher land in the Gwendraeth - Neath area, on the moors where they approach the coast. This is all immaterial anyway, since ice does not flow in straight lines. We know that during all glaciations there was a lot of ice pushing out from the uplands of S Wales, and any ice that crossed Pembs from NW towards SE would have been pushed offshore by this pressure of Welsh ice. Lots of my earlier posts deal with this.

BRIAN JOHN said...

With respect to isostatic depression and rebound, I have also dealt with this before. A general rule is that a load of 300m of ice (for example) would lead to a land surface depression of 100m. So 300m of ice over Preseli would have depressed the Carn Meini area (now at 360m) down to c 230m. That would have been pretty meaningless in landscape evolution terms, because at the same time se-level will have been much lower too, due to eustatic effects.

At the end of the glaciation, the land would have recovered quickly at first, with the adjustment gradually slowing down to zero.

Beyond the ice edge, wherever that was, there might have been a small bulge effect, with a land surface a little higher than it is now and also settling back to equilibrium during deglaciation. That doesn't seem to have been very much in SW England, since during the Holocene the Mesolithic shoreline -- for example -- seems to have been in pretty much the same relative position (ie a bit lower than today) right across W Wales and S Wales and down the coasts of the Severn Estuary and the Bristol Channel.

That's where the submerged forest evidence comes into the frame, as I have explained in my posts re Robert's theory....

You have got your isostasy upside down!

Kosta Dean said...

Brian,

I recognize the complex dynamics of isostatic/eustatic effects. I was hopping for a simple answer! No matter. Nothing of essence changes. It's not so much the absolute but the relative altimetry that matters anyway. And that we don't have. Or do we!

I think of the UK as a row boat floating in water. You push down on one side and the opposite side rises. And if you push down too much, you get water in the boat. Or if you load it up with a heavy weight the whole boat sinks down or unload it and the boat rises. Is there a geological axis for such isostatic / eustatic rocking of the UK? Perhaps I am asking more than geological science can answer at this time.

On a different matter. What is the evidence that the lintel sarsens were carved and shaped by the same people and at the same time as these were erected?

Constantinos Ragazas

BRIAN JOHN said...

The only places where you can pick up these "relative altimetry" effects are in those parts of the UK where there are raised Holocene shorelines -- eg in parts of western Scotland and in Donegal. Elsewhere, isostatic recovery has been less than the eustatic rise -- so the sea has been transgressive -- rising faster than the land. See my post in early June in which i try to explain all of this!

The trouble with your boat analogy is that the UK doers not act like a single rocking block -- the crust is much more flexible than that, so there are "bends" and flexes which are still imperfectly understood....

re the lintels, I'm not aware of any evidence that shows that the lintels were shaped at the same time as the big sarsen uprights were erected. Most of the archaeologists simply ASSUME that.

Kosta Dean said...

Thanks Brian, I appreciate your comments. Archeologists do assume too much!

Robert Langdon said...

I think costa may have a point! There is a complete chapter in my book on Geological so called 'proofs' that has been proven wrong in the last 50 years (too many to list on a blog site)- Doggerland is a clear case of Geological back pedaling.

You may be interested in a Times Feature today http://bit.ly/fXFuV1
that finds the same navigational aids I have found 'inland' that have now been found on the coast line - which clearly shows boats and water in the neolithic period.

Isostatic rebound goes down then up then down again - the problem is that you have no idea on what cycle it's currently undertaking - it's quite possible it will act like a waterbed and this up and down movement will continue (going again up - but not as much then down) like a diminishing sine wave - until a force is applied to stop the movement, i.e. another ice age - Geologists and Archaeologists need to be honest and admit when they are just guessing!!

Robert Langdon said...

Sorry Guys the Times link wont work without an account - money spinner!!

I've copied it to my FREE site:

http://www.abc-publishing-group.co.uk/Megalithic.htm

RJL

BRIAN JOHN said...

Thanks Robert -- the Times piece is interesting -- somebody rang me up to tell me about it, as well. Wouldn't have a problem with some of what Dividson says -- of course standing stones would have been used as waymarks both inland and on the coast. Where people travel, they need aids and warning re dangerous areas, passages into port etc.

I hope your book is not going to do the old trick -- used by pop-scientists for years -- of looking for wacky theories held by just a few geologists (or whatever), pretending that those ideas are universally accepted, and then knocking them down and coming out with statements such as "that all shows how cockeyed mainstream science is. Don't believe a word they say. Believe me instead!!"

Sorry -- you are talking nonsense about isostasy. We do have a pretty good idea what is going on, and how isostasy works. The thing that complicates the picture sometimes is tectonic movement associated with mountain building, continental drift and other large-scale crustal deformations -- but even that is now pretty well known.

The important thing is that whatever your hypothesis may be, you must have internally consistent evidence which is verifiable and which stands up when data from a wide range of different disciplines comes into the frame.

Kosta Dean said...

Robert, thanks for that link!

I did read it. I was impressed by the size and weight of that Grand Menhir Brisé, but I am not sold on the 'theory' put forth.

From the fragmentation and position on the ground I assume this menhir was once standing erect. Otherwise it wont serve much as a 'navigational marker'!

Brian, what do you think. That megalith was transported and erected by Neolithic men?

We 'see' what we think!

Constantinos Ragazas