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Sunday, 17 April 2011

Geological Survey maps ice limit

 Click to enlarge the map.

Been looking at the Geological Survey map of the Fishguard area (Sheet 210) published last year.  Not only does it give fantastic detail for the Fishguard Volcanics and the other rocks currently attracting attention from the geologists -- but it is also a drift map, showing the superficial deposits of Devensian and more recent age in the area.  The red line shows the Geol Survey assessment of where the devensian drift deposits (till and sands and gravels) end.  North of the line, deposits are widespread.  South of the line, Devensian glacial deposits are not supposed to exist.

The map is quite similar to others recently published on this blog.  However, it is inaccurate in many respects, and the field mappers have only included THICK deposits.  For example, on Dinas Mountain and on Carningli there is a thin spread of till south of the line -- difficult to map, and not always visible when the gorse and heather are thick.  Luckily, recent burning has stripped back the vegetation, allowing these deposits to be seen quite clearly.

I persist with my view that most of Carningli WAS covered by the ice of the Irish Sea Glacier during the Devensian.  What happened further east, on the north flank of the Preseli ridge, is a matter for further research.

28 comments:

Constantinos Ragazas said...

Brian,

Interesting post. This map with the glacier ice limit in bold red should raise some questions, as you have done so effectively. Hope more research reveals more of what really happened here geologically.

What puzzles me most about this map is how the ice limit seems to engulf the area between Wolfscastle, Carningli and Preseli. This area being so contained by the glacier limit should show some periglacial/permafrost signs. Does it?

Another curiosity. Does ice formed by a body of frozen water show some physical characteristics different from glacier ice formed by piles of show fall? It seems to me that ice formed by frozen water will be more compacted and uniform than ice formed by snow. The characteristics of melting will also be different I think. What do you think?

Kostas

BRIAN JOHN said...

Yes, the extra-glacial area (outside the ice limit) does show signs of periglacial activity, and things like the teetering tors that I mentioned in an earlier post. There are also erratics and old glacial and fluvioglacial deposits with subdued surface forms -- so they have been largely destroyed by solifluxion and other denudational processes.

There are many types of ice, including glacier ice, sea ice, rime ice and superimposed ice. true glacier ice has crystals so large that they are fused together, trapping the original air content of the snow pack into small bubbles. Frozen bodies of water do not grow upwards. They stay where they are, and freeze down to the bottom as conditions deteriorate. you see this in ponds and in Arctic lakes every winter -- and in many parts of the Baltic Sea.

Constantinos Ragazas said...

Brian, you write

“The red line shows the Geol Survey assessment of where the devensian drift deposits (till and sands and gravels) end.  North of the line, deposits are widespread.  South of the line, Devensian glacial deposits are not supposed to exist.”

So Brian, how did the bluestones get from Wales to Stonehenge? Had there been glacier flow from Preseli to Salisbury, wont the geological evidence indicate such glacier presence? It seems the scientific evidence is contrary to your glacier transport theory.

The evidence may be indicating the agency of some other geological mechanism besides typical glacier flow. Any ideas? Or are we only left with the human transport alternative?

As you know, I don't believe in the human transport theory. But I do believe in an alternative ice trasport of the Stonehenge stones!

Kostas

BRIAN JOHN said...

No no -- you have got it all wrong, Kostas. This line shows the DEVENSIAN limit -- relating to the most recent glaciation. There have been several glaciations, as explained on previous blog posts. The one that was probably responsible for the transport of the bluestones was the ANGLIAN glaciation, date uncertain but possibly around 450,000 years ago.

Constantinos Ragazas said...

So Brian, the Stonehenge stones have been lying around Salisbury Plain for 450,000 years. Do we have any comparisons of how stones would look like that have been lying around for 450,000 years? And wont the land itself have matured more over such a long period of time than the land at Salisbury Plain actually has? The soil of Salisbury Plain is said to be very young with relatively undeveloped thin top soil.

I happen to come across on the web this article on solifluxion and periglacial conditions in SW New Zealand. The article claims these conditions existed there 6,000 years ago, same as in the North Hemisphere they claim. Can we draw any comparisons between New Zealand and the UK? You may be able to make more out of the article than I did. The link to it is:http://rsnz.natlib.govt.nz/volume/rsnz_82/rsnz_82_05_011130.html

Kostas

BRIAN JOHN said...

Yes, my assumption is that the bluestones and sarsens have been lying around on Salisbury Plain -- or maybe not very far away -- for a very long time. One of the cosmogenic dating techniques will probably sort this out for us, one of these days. Mind you, the results of such dating on old rock surfaces thus far have been very questionable. For example, Prof David Bowen's chlorine-36 dating of rock samples from Stonehenge and Carn Meini have been heavily criticised:

http://findarticles.com/p/articles/mi_hb3284/is_n266_v69/ai_n28663881/

and it looks as if the more recent dating exercise by Danny McCaroll and colleagues for North Pembrokeshire has given equally dodgy results. The problem is that it's very hard to know the length of time over which a rock surface has genuinely been exposed to radiation and weathering, given changes in vegetation cover, snowcover and even soil cover that may have been eroded away. Things are even more difficult with erratics or monoliths, which may be buried or partly buried, and even moved about by natural processes over many different phases over hundreds of thousands of years. But I live in faith that one day this will be sorted.

Who said that the soil on Salisbury Plain is very young? That's not what I read when I look at the literature.

BRIAN JOHN said...

The article by Cotton and Te Punga is very old -- there are literally thousands of articles on periglacial slope deposits and processes written since the 1950's. It's difficult to extrapolate from one place to another -- while there are glacial conditions in one place there will be periglacial conditions in another, and temperate conditions in another. So yes, we can draw some comparisons -- but I would hesitate to say that we can correlate stratigraphic horizons over such a distance ..... it's difficult enough just within the UK!

The Stonehenge Enigma said...

Sorry Brian I have to agree with Kosta.

If your theory is correct and these stones were moved 450,000 years ago they would NOT be laying around awaiting collection - they would be buried deep into the ground covered by soil (not forgetting Britain was a forest from 9000BC to 3000BC and so on top of the soil was 6,000 years of forest debris)

So, if they are covered by soil and mulch, how do you know that they exist?

Back to the drawing board my old mate!

RJL
www.the-stonehenge-enigma.info

BRIAN JOHN said...

Robert, the sarsens littering Salisbury Plain have been there for far longer than 450,000 years. Some are buried, some are partly buried, and others are apparently just lying there on the surface. The reason is that soils do not just accumulate ad infinitum -- if they did, there would be soils more than 20m thick across the whole of Southern England. Soils move downslope, and they are eroded away by rainwash, river action and even by deflation by the wind. In other words, they are redistributed and eroded. There is no problem whatsoever in having large stones emplaced on the ground surface half a million years ago, and still being visible at the surface. I don't mean all of them will be visible and accessible -- many more could well be buried, in places where solifluxion has led to thick accumulations of debris, for example in chalk combes and beneath scarp faces.

Constantinos Ragazas said...

Brian,

Love it when you make a date commitment! When I do, it always gets me into trouble!

As to my claim that the soil of Salisbury Plain is relatively young ...

Salisbury Plain is generally “rendzina-dominated landscape”. From wikipedia, “Rendzina ... is one of the soils most closely associated with the bedrock type and an example of initial stages of soil development.“

You are 'burying the truth' with you 'bury the stones'. No need to! There are better explanations to Stonehenge!

Kostas

BRIAN JOHN said...

Hi Kostas

Far be it from me to pontificate about the dates of the various glacial phases! If you look up "Anglian" on my blog you'll see that I'm pretty nuanced in what I say about the glaciations and their dates -- there is a lot of uncertainty in this area, in spite of the great advances in dating techniques. some apparent "dates" are artifices, created by the characteristics of a technology. But we will get there in the end....

My debate with Robert is not dependent on precise dates -- just refer to the glacial episodes as "old" or "young" if you like...

The Stonehenge Enigma said...

You said "Soils move downslope, and they are eroded away by rainwash, river action and even by deflation by the wind."

And rightly too! BUT NOT in a forest unfortunately. In a forest they would be completely buried and take maybe a thousand if not two thousand years for the debris to erode leaving tops of the stones showing!

But the forests (according to present academic theories using tree pollen and snail evidence) did not go for a thousand years AFTER the stones were at Stonehenge!!

You can happily get out of this hole if you agree with 'part' of my hypothesis that the Bluestones were laid in 8500BC(according to the car park's carbon dating of tree remains) - which would have been just after the Ice and Tundra had gone and before the forest grew too dense to see your stones.

RJL
www.the-stonehenge-enigma.info

BRIAN JOHN said...

Sorry Robert -- I am not in a hole and therefore have no need to climb out of it. Soils do not systematically just accumulate in forested areas -- I have wandered about in pristine forests which have not been cleared since the end of the last glaciation -- and there are many areas among the trees where soils are almost absent, and where boulders lie about among the trees.

Not sure what you are on about when you say that the C14 dating of tree remains at Stonehenge dates the placing of the bluestones at 8500 BC or 10,500 BP. How on earth do you draw that conclusion?

Constantinos Ragazas said...

Brian,

From the article I referenced above there is evidence of solifluxion deposits widely spread in SW Wellington in New Zealand. This the authors feels dates to about 6000 BC. If we go by the natural symmetry of Nature, North and South, could we reasonably conclude that some similar large scale temperature conditions also existed in the UK? After all, southern UK is farther North by 10 degrees than New Zealand is South. And both the UK and New Zealand are large island masses in vast oceans.

This, of course, fits well with my 'ice transport' theory. That it does nothing for your glacier transport theory probably means that you will reject this evidence summarily.

Kostas

BRIAN JOHN said...

I don't understand your point, Kostas. If there are periglacial deposits in New Zealand dating from 6,000 BC, what does that prove? That it was cold in that part of New Zealand at the time. There are periglacial deposits still accumulating in parts of Scotland, North Wales, the Alps, Norway and many other places today. That doesn't show that we are in the middle of a glacial episode -- it just shows us that it is colder at higher altitudes.

Constantinos Ragazas said...

Brian, addressing your puzzlement in your last post to me above:

Key elements of my 'ice transport theory' are ice, meltwater and containment basins. Evidence of solifluxion and periglacial permafrost support such conditions. Evidence of these existing at a latitude 41 degrees South suggests similar overall conditions existing 51 degrees North at Stonehenge. That these should have occurred 6000 BC fits well with my theory that at the time when Stonehenge was built an ice-sheet covered the land and people were around to built Stonehenge.

It's quite possible that the ice sheet that covered Salisbury Plain is the frozen waterways Robert claims inundated Salisbury Plain. Such body of water would have frozen solid during a period of 1000 years of 'deep freeze' around 8000 BC. It all fits perfectly together! Where Robert is wrong is in assuming that the water was navigable during this period of time.

Kostas

BRIAN JOHN said...

Kostas, you appear not to have understood my point about altitude. You CANNOT say that because there was solifluxion going on at 41 deg S, there must have been solifluxion also going on at 51 deg N. There is solufluxion going on today on high mountains very close to the Equator. That does not mean that there are periglacial conditions everywhere else on Planet Earth.

We know what went on in the UK during the Youger Dryas. There was no vast expanse of frozen water covering the southern part of the UK.

Constantinos Ragazas said...

Brian,

My plausible argument was not meant to be a 'proof' that ice covered Salisbury Plain 6000BC. Rather, the evidence does not 'disprove' this. As is also true for all other evidence that I am aware of. Do you know of evidence that 'proves' ice DID NOT cover Stonehenge 6000BC?

On a different topic: What information do you have as to the origination of human skeletal remains found at Stonehenge? I recall reading that, through dental mineral tracing, some of these trace to Alpine regions while others trace to Preseli. Still others are said to trace to the Mediterranean. Please clarify if you can.

Kostas

BRIAN JOHN said...

Kostas, on the Amesbury Archer, I suggest you use the search facility on my blog or do a Google search -- there is plenty in the literature (much of it plain crazy)......

And as for a great sheet of ice covering SW Britain and reaching Salisbury Plain around 6,000 BC, we have covered all this before, ad infinitum. There was mixed deciduous woodland in Southern England at the time, and reconstructed temperature curves and sea level curves show that the climate had warmed a great deal following the end of the Younger Dryas.

Tony Hinchliffe said...

The Mediterranean origin for one skeleton found near Stonehenge has since been overruled. Mike Parker Pearson told a group at a Salisbury Museum centenary lecture that English Heritage had changed its interpretation of the evidence. As far as I know, English Heritage has not publicly corrected its original statement, made with much fanfare. I commented on this on Dennis Price's "Eternal Idol" blogsite some time ago, much to Dennis's disappointment.

Constantinos Ragazas said...

Sorry Brian about the 'ad infinitum'. I will change my approach to getting at the evidence and the truth. This really is all I am seeking from you.

My understanding is that the evidence of a great expanse of forests at Salisbury Plain comes primarily from pollen counts. And of course on the face of it this could discount both Robert's and my theories.

A couple of questions to clarify this:
1)Are the pollen different or the same as pollen found at higher elevations where forests existed North and West of Salisbury Plain?
2)Can such pollen found in the soil of Salisbury Plain have come from elsewhere, brought there probably by meltwater streams collecting at Salisbury Lake(?), by ice, snow or even by air?

Brian, our experience with interpretations of the evidence re:Stonehenge is that too often these have to be revised. Consider Tony's comment above, for example, about some claims that were recently withdrawn! So we must question everything and be open to all possibilities, if we will ever arrive at the Truth of Stonehenge.

Kostas

The Stonehenge Enigma said...

"My understanding is that the evidence of a great expanse of forests at Salisbury Plain comes primarily from pollen counts. And of course on the face of it this could discount both Robert's and my theories."

Sorry Kosta have I missed something?

The forest would discount both your and Brian's theory as your ice would have melted some many years prior to its growth and Brian's lost rocks from he's previous ice age half a million years, buried under 6000 years of debris.

But this does not exclude my theory of increased water/river levels (Brian's wording of inundation is an incorrect analogy) as rivers run through forests without hindrance as in the amazon - which is the closest representation you will find to how the Stonehenge area would have looked after the last ice age.

Hence no sliding on ice or dragging over ground - impossible hypothesis!!

As for the Amesbury Archer - he had and archaeologists have found many 'jade' stone tools - this jade comes from the Italian Alps and can only effectively come to Stonehenge by boat (as roads did not exist, especially in forests and its a long way to walk!) which also confirms how the stones were transported to Stonehenge.

RJL
www.the-stonehenge-enigma.info

BRIAN JOHN said...

Robert -- please pay attention. I have already made the point that stones lying around on the ground surface will NOT necessarily get buried by "6000 years of debris".

So now we have the "flooded forest" hypothesis...... this becomes even more bizarre!

BRIAN JOHN said...

Kostas -- pollen tends to be best preserved -- and most abundant -- in peat deposits. Pollen grains can be found in river deposits, but river flow and abrasion tends to knock the pollen grains about a bit, so they may be impossible or very difficult to identify. And yes, pollen will almost certainly have come from "somewhere else" via wind transport or transport in debris in slow=moving streams etc. But palynologists, who are clever people, generally know what environmental interpretations to put on pollen assemblages. And they can also recognize "derived" or out-of-place pollen.

Constantinos Ragazas said...

Very interesting discussion Brian and Robert!

In a nutshell, this is what I understand from your previous posts:

1)“The borehole data is consistent all over Stonehenge bottom” showing 40 cm of dark brown gravel and silt (Robert)
2)“ … there are gravels and silts all over the place, and in many different stratigraphic positions...” (Brian)
3)“There was mixed deciduous woodland in Southern England at the time” (6000BC) (Brian).
4)“...rivers run through forests without hindrance as in the amazon” (Robert). Mesolithic inundation is therefore not inconsistent with the dense forestation of Salisbury Plain at the same time 6000BC
5)“And yes, pollen will almost certainly have come from "somewhere else" via wind transport or transport in debris in slow-moving streams etc” (Brian)

Putting all this together, this is what I think:

1)The 'borehole evidence' as well as other such soil evidence from many other locations and at different stratigraphic levels -- along with the absence of a wide uniform such layer covering the entire area – are consistent with my 'ice cover theory' and inconsistent with Robert's water inundation theory.

Those parts of the land covered by ice for longer periods of time will show little or no such soil deposits. While places where ice meltwater gathered in stagnant containment pools (like Stonehenge!) will show much deposit of this kind of soil. Places where there was meltwater stream flow, (like the Avenue!) will also show deposits of gravel and silt.

These locations can be at different stratigraphic levels and locations, including at the side of a hill running down the slope of a hill in a straight track etched in ice!

2)The pollen evidence for a dense forestation of Salisbury Plain around 6000BC is at best weak and inconclusive. My thinking is that this pollen came from elsewhere. From higher elevations North and West of Salisbury Plain, where such forests may have existed.

These pollen were deposited at Salisbury Plain by any number of ways, and most likely by melting ice water collecting and depositing pollen in bulk at Salisbury Plain. That a thick presence of pollen is found at Salisbury Plain to me is more evidence that this region was at one time covered by water which later froze into an ice sheet.

Pollen is more likely trapped and preserved in snow and silt than in surface soil where it could simply be blown away.

3)An amazon river running through densely wooded Mesolithic Salisbury Plain is a nice thought, but leads to more problems than it solves.

For one thing, it restricts navigation and the size of boats that could be used. The Stonehenge stones could not have been carried there in canoes! And if the entire area was submerged in water, even the biggest sarsen at Stonehenge would have to be carried there by boat. Hard to imagine how this was possible 6000BC.

Also Stonehenge is at a higher elevation and water, therefore, had to reach that high. Thus you are forced to either have very very tall trees growing in water, or regular trees growing under water!

4)The 13 mounts in a straight line that Robert claims are evidence of man-made navigation posts are also very problematic. It is quite a task to just build such mounts in dry land, but impossible to imagine doing this when the land is covered deeply in water.

These mounts simply are not man made and they were not used for navigation purposes. These mount, however, are consistent with my 'ice cover theory'-- being formed by soil deposits in a retainer basin made of ice!

Kostas

BRIAN JOHN said...

We are getting a bit off-topic here, guys. This post is about the accuracy (or otherwise) of the Devensian limit in West Wales......

BRIAN JOHN said...

Kostas -- take a look at this -- it outlines some of the potential and the problems of palynology on Salisbury plain:

http://www.scribd.com/doc/12594106/Pollen-Archaeology-on-the-A303-Stonehenge-Improvement

BRIAN JOHN said...

A list of all Appendices, and links, here:
http://www.wessexarch.co.uk/projects/wiltshire/A303