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Sunday, 14 April 2013

Relief Map of the main "bluestone source area"


This is a high-definition image from the Fishguard geological map -- with additional notations.  It shows the (inaccurate) Devensian ice limit, which should be shown in some places a few miles further south.

I have also added some of the other key locations discussed recently on this blog -- including the Cilgwyn and Gernos Fawr moraine sites,  Carn Alw and Carn Goedog (recently discussed in the context of the rhyolites and dolerites that have found their way to Stonehenge), Maiden Castle (a very fragile tor which must have lain outside the Devensian glacial limit), and Rhosyfelin ---  where all the recent fun and games have been happening.......

I hope this is helpful for those who are unfamiliar with the local geography.  Click to enlarge.

14 comments:

Dave Maynard said...

Brian,

What is the evidence for the Devensian spread south of Crymych? Is it morraine material or something similar?

Just interested as I live in Hebron and not noticed anything like this.

Also, if the 'bluestones' were entrained in the Cilgwyn/north of Preseli area, would the shape of the range tend to set the glacier moving in a south east direction towards you know where?

Behind this is I guess, the question of the impact of topographic features 100's of metres below the ice surface on the direction of flow of ice. I would have thought it would have been quite small, with other factors, many of which we cannot recreate being more influential. But I might be wrong.

Dave

BRIAN JOHN said...

Dave -- I think that map evidence is based largely on the presence of fresh till -- it's shown on the geological map.

Remember that we are talking about the Irish Sea Glacier here -- not a tinpot little glacier which had its origins in the Preseli Hills. The ice may well have been 2,000 feet thick over the top of Preseli -- we just don't know yet, but we can go on what we know about glacier and ice sheet long profiles. Ice movement direction is determined above all else by THE DIRECTION OF THE ICE SURFACE SLOPE -- the shape of the land beneath does not have much effect, except when the ice is really quite thin.

chris johnson said...

Very interesting map!

Recently we speculated on "Lake Brynberian" but there seems to be nothing to the North that might have restrained the waters.

The striking feature of the land S/W of Rhosyfelin below the hill tops is that big bowl which makes one think of a lake, or after recent days a meteor crater.

Perhaps the Lake House meteor fell in Prescelli?

Constantinos Ragazas said...

Chris,

"Lake Brynberian" – if it existed as Brian and I argue it did – was a glacial lake. Glacial lakes often form when glacier ice dams the meltwater collecting in a region. For example, The Irish Sea Glacier I argue formed such an ice dam along the mouth of Bristol Channel to form a glacier lake over Salisbury Plain.

When the glaciers retreat or melt, the ice dam becomes the egress draining the lake. Eventually, that develops into a river. The river at Craig Rhosyfelin I argue is such a river. It should not surprise you, therefore, that in the present landscape “[there is]nothing to the North [of Rhosyfelin] that might have restrained the waters.”

Hope this clears the matter for you.

Kostas

BRIAN JOHN said...

Kostas -- I think Glacial Lake Brynberian MIGHT have existed -- the terrain makes it possible. The idea of a glacial lake over Salisbury Plain is nonsense -- you just have to look at a topographic map to see that there is no possibility of containment of the type that we see in the basin of the Nevern River and its tributaries.

chris johnson said...

Brian,
The river Nevern does not look, to my untutored eye, like a meltwater valley. By contrast the Gwaun and the Cych do.

I think Kostas has a "marginal lake" in mind. So partly restrained by mountain on the southern shores, and restrained by a glacier to the North. If so then a meltwater valley would have been scoured out when the glacial wall failed. The Nevern and tributaries look to have formed more gently.

I hope you can explain more.

I am still curious about that huge bowl below the mountain ridge above Rhosyfelin. Could a meteor strike have had a hand in its formation, or is it purely the result of glacial forces do you think?

BRIAN JOHN said...

Chris -- parts of the Brynberian and Nevern Valleys do have many of the characterustics of deeply-cut meltwater channels similar to Cwm Gwaun and Nant-y-Bugail but smaller. Thje gorge downstream of Rhosyfelin is quite spectacular, as is the Nevern Valley just below the confluence with Afon Brynberian. Zoom in on Google Earth to have a closer look!

The "bowl" to which you refer is very wide and broad -- it seems very different from the meteorite craters that we know about. I think it is a natural feature related to long-continued fluvial erosion under a temperate climatic regime -- nothing to do with either glaciers or meteors.

Constantinos Ragazas said...

Brian,

Glacier Lake Salisbury is speculation that explains many of the 'facts on the ground'. At least to me. This hypothetical glacier lake could have been part of or twin lake to Glacial Lake Harrison that others have speculated and written about.

It would be very revealing if we had a computer simulation of what happens when the coast along Bristol Channel (or even the mouth of Bristol Channel) is topologically blocked by a glacier (say 1000 ft high) and meltwater fills in the territory behind it.

It shouldn't be too hard. Possibly Robert has done this already to get his glossy inundation photos in his book.

Kostas

Robert John Langdon said...

Loads Kostas

But you need to put your hand in your pocket and buy a copy ;-)

In the meantime you and Brian might like to see Antarctic Stonehenge -

http://www.newscientist.com/article/dn23339-antarctic-ice-grows-as-climate-warms.html

RJL

Constantinos Ragazas said...

Robert,

You are now being a 'modern academic' asking fee for facts! So out of character.

If you have such computer stimulations showing what happens to the landscape when the mouth of the Bristol Channel gets chocked up by the Irish Glacier (to say 1000 ft high), while meltwater collects behind it, why don't you email it to Brian to post. We can thus have your 'inundation' and my 'ice sheet' too!

We can then have a vigorous debate. And you can defend your modified version of 'inundation' by a vast glacial lake. And away from your original 'rising ground water' or sea levels that Brian ridiculed in the past.

What exactly are we seeing in the Antarctic Stonehenge you linked us? Can't identify the blue columns in the glacier or how they formed.

Kostas

Robert John Langdon said...

Kostas

Your supposed to be the mathematician - do the maths and prove Brian and the others wrong (if you can!).

It's a simple calculation - you estimate the cubic cupacity of the ice on Britain at the end of the last ice age - Brian's provided lots of maps to that end.

Then look at the displacement of water in both the surrounding seas (again Brian has already provided those) the shortfall (and there is a shortfall if you look at Jelgersma 1979 - which dates back to the period your interested in 18,000 BCE) and you are left with what must had still on the landscape.

Adjust (add) it against the current groundwater table (the Avon runs at about 225 feet at the end of the Avenue) if your calculation can get it above 350+ feet for this area, you will have a scientific argument that the plain was flooded - which Brian will probably completely ignore.

Good Luck!

RJL

Constantinos Ragazas said...

Robert,

You can dump gallons of water on a pavement but you wont get a pool of water if the pavement is flat.

What I had in mind were the topography of the region and if such could have contained a glacial Salisbury Lake when the mouth (or coast) of Bristol Channel is blocked.

Any explanation of the pattern of 'blue columns' in the Antarctic Stonehenge you linked? I find that intriguing! Is this pattern typical of how glaciers in contact with water melt? Brian I am sure knows. But he's not saying!

Kostas

BRIAN JOHN said...

Yes -- that's quite a spectacular picture. As icebergs adjust themselves in the water they may flip or tilt in many different directions. What you then have are interactions between the ice layering (originally the snow accumulation bands in a glacier) and enhanced melting on the waterline and more enhanced melting from rills running down the sides of the iceberg from melting in the sunlight. Result -- things of great complexity and beauty.

Constantinos Ragazas said...

Brian,

So the 'blue columns' in the photo are formed 'from rills running down the sides of the iceberg'.

This is very interesting. Since these columns of rills are so uniform in size and regularly spaced. If the edge of this ice sheet was circular and not straight, I can see how such meltwater rills running down the edge could carve the Aubrey Holes in the chalk bedrock!

Kostas