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Friday, 28 October 2011

Erratics on the coast


I came across this photo showing erratic boulders sitting on a wave-cut platform on Anglesey, North Wales.  It is assumed by geomorphologists that these erratics have been left behind in this exposed location because the Devensian till in which they were contained has been washed away by coastal erosion processes.  So in a sense they are "residues" which show us that there was a glacial deposit here -- and hence that there was a glaciation........


Now think about the Giant's Quoit and the other "giant erratics" on the coasts of Devon and Cornwall.  I have already put up a number of posts about these (use the search facility to find them) -- but I am reminded that geomorphologists have got themselves into quite a lather in trying to explain their presence.  The general conclusion is that glacier ice never got this far, and that the massive boulders must therefore have been carried by icebergs or on sea ice.  One of their problems is that this is a sinking coast (see my posts on isostasy in the SW of England) -- therefore the coast must have been higher in the past than it is today.  Also, as we know, every big glacial episode has been accompanied by a substantial drop in sea level -- almost always in excess of 100m.    So how, in those circumstances, do we get floating ice transport of very large blocks into the situations where we find them today?  I am not aware of a single geomorphologist (and there are many of them who have expressed opinions) who has confronted this issue and come up with a sensible answer. 

Well, here's my answer -- these erratics were indeed transported by glacier ice into the positions where they are now found, but this was so long ago that all of the other deposits have long since been eroded away.  They are probably there somewhere, in the offshore zone of sediments......  so the parallel with Anglesey is very close indeed.

The only alternative explanation is that the erratics were transported by floating ice.  But for that to happen, the coastal zone would have to be depressed isostatically so that the "glacial shoreline" was approximately where it is now.  Now then -- 100m of isostatic depression requires 300m of ice loading.  That argues for quite a big glaciation of the Celtic Sea, Bristol Channel and the South-west Peninsula -- exactly what I have been arguing for ages, and exactly what the glaciological models show.

If anybody else has got a better theory for the emplacement of the giant erratics on these coasts,  I will be very glad to hear of it.

43 comments:

Constantinos Ragazas said...

Brian you write,

“If anybody else has got a better theory for the emplacement of the giant erratics on these coasts,  I will be very glad to hear of it. “


My understanding of your entrainment method:

Under stress the glacier bedrock breaks and freezes to the ice. As the glaciers advance the bedrock frozen to the ice will be entrained. Very sensible!

Could entrainment also occur in other ways? Consider this.

Outcrops above the ice surface break off by weathering or other geological processes (earthquakes, hot lava seepage, etc.) and fall on the ice surface. These will over time follow the surface gradient of the ice and be transported by gravity to the lowest point. In most cases this would of course be the sea coast.

Question. What is known of the provenance of the giant erratics on these coasts?

Kostas

BRIAN JOHN said...

Entrainment? Not that simple -- freezing zones and sliding zones on the glacier bed are difficult to predict. Where ice is thin under a polar thermal regime, ice will almost certainly be frozen to the bed -- as ice thickens, basal temperatures go up and sliding becomes possible. There are many factors involved -- see my posts on entrainment.

And do rock fragments fall onto the ice surface? Of course they do -- and they make up the bulk of the debris in supraglacial moraines, medial moraines and lateral moraines as seen on all of those spectacular pics I have shown. But of course you have to have rock surfaces exposed above the ice surface -- and in the case of big ice sheets you don't have any! So entrainment can only happen at depth.

Origins of the giant erratics? See my posts -- some are known, and others not. Some Scottish and Lake District sources have been suggested.

Constantinos Ragazas said...

Brian,

I don't mean to belabor the point but I have some followup questions. If you don't wish to pursue this any further I'll understand.

On the origin of giant coastal erratics you write,

“Some Scottish and Lake District sources have been suggested.”


If so, wont that mean these erratics came overland? And wont this rule out the “flotilla of icebergs” idea? Never made much sense to me! And reading your comments about isostacy and glacier ice you are also arguing against this.

I accept entrainment can be very complicated with many factors involved.

The entrainment you have described involves 'erratics from below' the glacier bed which froze to the ice and were carried away by the glacier flow.

Concerning entrainment of 'erratics from above' an exposed outcrop dropping onto the ice surface, you write

“you have to have rock surfaces exposed above the ice surface -- and in the case of big ice sheets you don't have any! So entrainment can only happen at depth.”

Could such entrainment happen at a much latter time when the “big ice sheets” were much thinner?

Kostas

BRIAN JOHN said...

Kostas -- just look at the maps of ice directions! These origins are entirely consistent with what we know about the source areas and directions of movement of the Irish Sea Glacier.

I know you want your rocks to fall onto the ice sheet surface and then get transported all the way to Stonehenge -- but as I have explained many times before, for ice to keep moving forward, and to transport surface debris, it has to maintain a minimum surface gradient. If you want ice at Stonehenge, the Preseli Hills have to be completely covered. The source areas for the Pont Saeson rhyolites, for example, are at approx the same altitude as Stonehenge -- and in any case they are in the bottom of a valley. There is no way those erratics could have dropped onto an ice surface. Forget it!!

Anonymous said...

Dear Brian
Cogent points about the Porth Leven erratic-you have made it before but now I understand the subtly of it, it is win win for you.
I have spent many days looking at it but never was able to arrange the tide to touch it.
The Axe Groups I -III are sometimes said to be from ultramafic outcrops off southern Cornwall now submerged but exposed during the Neolithic.
Is is of help.
R.A.Ixer

Constantinos Ragazas said...

I was actually thinking about the coastal erratics and how they could have gotten there. Responding to your invitation for alternative explanations! In my proposed view, the ice does not need to keep moving forward! Just the erratics on the ice surface be moved by gravity.

Of course, if we have a 'fingerprint data bank' for erratics, as we have for criminals, we could have answers to all our questions. In time we will know the truth … but in the meantime we have debates!

Kostas

BRIAN JOHN said...

Thanks Rob -- interesting points.

Kostas -- I have seen lots of stones on lots of glaciers, and they do not just slide over great distances. They tend to stay put, or in some cases they sink into the ice because of the fact that they absorb heat and melt their way down. They are also often surrounded by debris -- so the sliding surface is not smooth, but is rather like coarse sandpaper. You would need an incredibly steep ice surface to get the sliding which you dream about. I would hazard a guess at 45 degrees or more, depending on the friction involved. Please forget it!!

Ivoe T. Rowel said...

Big Stones in Glaciers,
Not that I support any theory but I understand that some of the stones at Stonehenge have a single worked, i.e. smooth, face with the remaining three faces being rough and unworked.
Could the single smooth face be the result of the stone travelling at the base of the glacier over a sandpaper-like surface?

BRIAN JOHN said...

Well, I suppose it's possible that some faces of some stones might be naturally smoothed during glacial transport, and some people think they have seen striations on them -- so nothing should be discounted. But the bluestone with the grooved edge and the one intended for use as a lintel (with mortice and tenon joints) are a bit difficult to explain by natural processes! So we have to accept at least some shaping by our jolly Neolithic stone masons......

Anonymous said...

What is the evidence the mortice and tenon joints were Neolithic rather than Roman? The Romans knew such techniques while Neolithic people show no knowledge of this elsewhere. And the Romans were at Stonehenge two thousand years ago.

Anonymous said...

mmmmmmmmmmmm
Nice point.
Jean-Robert Sabot

Geo Cur said...

It was the sarsens that tended to have one dressed face ,the one facing into the monument ,but as they were not entrained the theory couldn't apply to them .Despite being hard rock ,it is worth mentioning that "green" sarsen is said to be far easier to work . Considering this and the greater volume of debitage to be expected from dressing the sarsens ,which I believe is not not reflected in the finds at the monument , it might suggest that they were dressed ,at least roughly , closer to the quarrying site .

BRIAN JOHN said...

Anon -- far be it from me to defend the archaeological brotherhood, but they have always argued that the Neolithic builders of Stonehenge were very clever fellows who tried to apply woodworking techniques to stone. As you say, there is no evidence for this anywhere else. But neither is there any evidence (as far as I am aware) of the Romans having turned up and put mortice and tenon joints on top of all the trilithons. After all, why would they bother? I think I'm with the archaeos on this one.....

BRIAN JOHN said...

Geo -- many authors have commented on the fact that sarsens are very easy to work when they are freshly extracted from the ground. That seems reasonable -- the same is true of some well known building stones as well.

But quarrying SITE? I think I'll go for the plural on that -- and maintain the view that the sarsens were collected up from many sites within easy striking distance of Stonehenge.

Anonymous said...

The city Bath was founded by the Romans! Why wont “the Romans having turned up and put mortice and tenon joints” at Stonehenge? They used such techniques before.

Is it a stretch to argue they could have used such techniques at Stonehenge? Is this a greater stretch than arguing the archeologists “have always argued that the Neolithic builders of Stonehenge were very clever fellows who tried to apply woodworking techniques to stone “ ?

Where is the evidence that Neolithic people knew such woodworking techniques?
If we can't understand “why would they[the Romans] bother” about Stonehenge, how could we possibly claim the Neolithic people would have bothered more? Selective intentionality ?

BRIAN JOHN said...

Anon, I'm really not bothered one way or the other -- I'll let the archaeologists argue with you!

Geo Cur said...

Brian , ok I missed a possible S , fact is though we don't know how many or where .

Anonymous said...

I cannot comment on the relative amounts of bluestone to sarsen debitage as I have only looked at the bluestone 'cut' from the debitage after removal of the sarsens-although many a sarsen has been included in the bluestone cut by mistake.
Those total debitage data are with the 19th 20th and 21st cent. excavators.
It would be on some slight interest to know. But how would you distinguish true debitage from residual sarsen fragments?
R.A.Ixer

Geo Cur said...

Anon , there are obvious problems with degradation of organic materials but scroll down to p4 for an Early Neolithic example of mortice and tenon .
http://www.e-a-a.org/TEA33.pdf

Anonymous said...

Geo Cur,

The wood may be from “5102/5101 BCE” but the “keyed mortise and tenon joint” is not!

What Early Neolithic tool could have made such clean rectangular hole through the thick timber to fit the wood peg through? You need metal tools for that.

For a well preserved timber as this obviously is, the age of the wood does not tell us the age of its use.

Geo Cur said...

Rob , I was thinking along the lines that the volume of Sarsen debitage would be expected to be greater than the Bluestone debitage because of the relative size of the dressed surface areas of the two types of orthostat and their numbers ,which is not reflected in the finds .Also I would guess that the sarsens would be more likely to be the target for souvenir “chippers “ ,therefore increasing the likelihood of fragments .The number of recovered Sarsen mauls does however suggests that there was certainly dressing of Sarsen on site .

BRIAN JOHN said...

re that pegged joint, I must say I found the article and the evidence quite convincing. OK -- the wood might be older than the carpentry, but the chances are that it is not THAT much older. I wouldn't have too much of a problem in thinking that Neolithic men could have made cuts like this (they do look quite rough) with sharpened stone tools.

Geo Cur said...

Anon ,as was mentioned in the article that example is not the only one from the period .If the metals had been used it would have been apparent to the excavators and if matal had been used why would the builders want to use wood that was thousands of years old .The Chinese site of Hemudu is probably the oldest example.Click on the first link after the page opens for some Finnish tools .http://paleoplanet69529.yuku.com/topic/20238/Finnish-neolithic-woodworking-tools#.Tq29lXJS2jM

Anonymous said...

The building skills such carpentry required would make possible the building of great structures, wagons on wheels and war devices. Certainly long wooden boats and catapults too. And in combination with the stone masonry attributed to the Stonehenge builders, large palaces and temples.

Where is the evidence? Can you prove me wrong by chopping down a mature oak tree with a stone axe? Make a video of it and posted on YouTube!

Geo Cur said...

Anon ,flint axes have been shown to be very effective at chopping down trees , how do think the posts for monuments like Woodhenge or even the Mesolithic posts at the car park site at Stonehenge were felled .Like you I couldn't possibly make such an axe chop , down a oak tree and film it ,but clearly apart from the filming it was an every day event in the days before metal .The Hindwell palisaded enclosure built circa 2700 BC used 1400 timbers a metre in diameter and 8 m tall . A couople of moderns playing about but it might give you an idea http://www.youtube.com/watch?v=sT99CvsSt1Q

Anonymous said...

More examples of “pegged joints” do not make the joints any older!

Having metal does not make wood obsolete!

Why would the users use old wood? Because it was available!

Anonymous said...

Geo Cur,

Checked the Finn tool. Looks in mint condition! Has anybody tried chopping a mature oak tree with it?

Of course, anything in the internet must be true!

There must be a black market out there for Finnish Neolithic stoned tools!

Geo Cur said...

Anon ,the Finnish tool was woodworking not for chopping down trees ,axes were used for that. The other examples of mortice and tenon were all dated pre metal .

Here is the other German well p11 http://www.e-a-a.org/TEA29.pdf

The stone monuments have survived but precious little left in wood but plenty evidence of the use of massive timbers in palisaded enclosures ,timber circles , and even timber halls .How do think the timber for these was felled ?

The Stonehenge Enigma said...

Anon and Geo Cur

You don't chop down trees with stone axes on there own. Its a waste of time and effort.

If you wish to cut a tree down you burn it at the base first by building a fire and leaving it for a day or two to burn and smolder at the base if it doesn't fall down on its own accord - you then chop out the charcoal (much easier than chopping wood).

The reason archaeologist can date the post holes these trees come from is the charcoal left behind once the post has rotted (charcoal doesn't rot)as in the Stonehenge car park.

As for the joint - it is very similar to the one found at the bottom of the Solent in 2010 when they found a boat dated around 6000BC, just 900 years earlier.

What is more amazing that archaeologists even with this evidence still place Mesolithic/Neolithic men in 'Mud Huts' in Britain although the excavators noticed that these techniques are used in "constructed in log cabin style" - such finds don't match the academic model.

No doubt the some academic will come up with a idea that foreign tradesmen (like polish plumbers) were imported just to at the joints to mud hut Britain's 'Ceremonial' temple.

RJL

Anonymous said...

RJL,

I accept your statement “You don't chop down trees with stone axes on there own.”

The rest of what you say is dubious in the least. Such building techniques as “pegged joints” would have enabled the construction of many other things like wagons and weapons.

Its more than the 'dress style' that separates Neolithic from Modern! We can imagine prehistoric men using primitive tools as a modern engineer would. But it's not the tools that make things. It's the mind!

Such advanced engineering capabilities require advanced and sophisticated culture that would leave behind many artifacts and signs of civilization, including records and symbols. All lacking in this case!

You can, of course, create a 'marketplace of ideas' and supply it with 'evidence' and 'little miracles' to support the customer's beliefs. Religious cults are very good at that.

Geo Cur said...

RJL , The pre burning method is unlikely and unnecessary ,maybe ringing trees a year or more before felling was practiced but there is no evidence for either plus charcoal is also found in the bottom of post holes of minimal diameter . Reasons for finding charcoal at the bottom of post holes are , the burning of the end to be inserted in the pit to make the it less likely to rot and the burning of the timbers above ground whether purposefully or accidentally means that the charcoal eventually falls into the post pie when the timber has rotted .

Anon . I thought all that was in the past but it seems we are still sufficiently chauvinistic to believe that clerks are a sign of civilisation and sophistication .It was less than century ago that Stonehenge was considered by many to have been built by the Romans or later , since the radio carbon revolution we now realise that that you don’t have to have spoken English or played cricket to be sophisticated .

If you are looking for local artefacts and symboling why not try Newgrange and Knowth both predate Stonehenge and both have non representational rock art that has clearly been reused in their fabric .Have a look at the macehead from Knowth then look at the corbelling ,it makes a mortice and tenon childs play , even the drainage works better than a lot of modern housing ,when first excavated the passage at Newgrange was bone dry and has remained so . But if it is civilisation and sophistication as is often the case you have to look outwith Britain and Ireland ,much earlier too .Chauvet at 30,000 BP may not be symbolic enough for you but Blombos at 60 ,000 is not representational and neither is the Oldisleben at 120,000 BP .Auditorium cave in Madhya Pradesh has cup marks dating from at least 290,000 BC .Definitely not English speaking , so can’t be civilsed , but possibly tea drinkers what about Harappa http://en.wikipedia.org/wiki/Harappa contemporary with Stonehenge . Much earlier with symbols Gobekli Tepe http://en.wikipedia.org/wiki/G%C3%B6bekli_Tepe and Catal Hoyuk too . The list is endless and the dates are getting older by millennia each year .

Anonymous said...

Geo Cur,

We may use the same words but mean different things!

By 'sophisticated' I mean the intricacies of a well developed “beautiful mind”. Not etiquette in tea ceremonies!

“...cup marks dating from at least 290,000 BC “ ???

Really now! Your 'dates' are astonishing! Not even carbon dating can take us that far back! Soon you'll take us back in Eden!

Geo Cur said...

Anon , radio carbon dating is not used to date rock art ,if that was what we relied on we would have Lucy and geology dated to suit the “ well developed beautiful mind of “ Bishop Usher and have no evidence of anything “sophisticated “ prior to 4004 BC , sophistry indeed .
The cup marks from Bhimbekta were dated to the Lower Paleolithic because they were discovered below an horizon of heavily calcite cemented Middle Paleolithic sediment that excluded the possibility post depositional disturbance . No comment on the other sites from the wee list ?

The Stonehenge Enigma said...

GEo Curs

"The pre burning method is unlikely and unnecessary.... Reasons for finding charcoal at the bottom of post holes are , the burning of the end to be inserted in the pit to make the it less likely to rot"

So you days chopping it down (if not weeks and dozens of axes, which you would have to remake) only to burn it in the end once it was down?

Spoken like a true academic with no practical engineering or agricultural skills - hence my earlier comment about the need for multi-disciplined degrees.

RJL

T Hinchliffe said...

RJL Do you have any practical agricultural or engineering skills yourself? I ask in the spirit of enquiry.

Geo Cur said...

RJL ,once again you fail to support any claims with evidence whilst there are plenty of examples world wide of how to fell trees with stone axes none of which involve pre-burning ,try this ,no pictures sadly . www.bahs.org.uk/05n2a1.pdf Your guesses about my skills or lack of are equally mistaken .

Anonymous said...

Geo Cur,

Are these the same people that have dated the German well built from old wood with “pegged joints”?

But then again, what does all this have to do with Stonehenge? That you feel it does only shows how tenuous your argument is to need the 290,000 year old rock markings as evidence! Of what?

Can your understanding of Stonehenge explain how the “rhyolitic debitage ” at Stonehenge got to Stonehenge from Preseli? Analyze that!

BRIAN JOHN said...

Again, we have got somewhat off topic here, folks. This was supposed to be about erratics, and a discussion about what ancient men were capable of seems to have arisen out of a simple point re the smoothing and shaping of stones, and the capability of using "woodworking techniques" for making mortise and tenon joints on stone.

As far as I am concerned, the evidence looks perfectly convincing that in the Neolithic, people were quite capable of shaping stones and using various techniques for fitting stones together. Plenty of other examples from Europe. Not sure why others should have such a problem with that......

Tony Hinchliffe said...

I'd still like to ask RJL politely "what practical agricultural or engineering skills do you possess?" No response so far....

Geo Cur said...

The wandering from the topic was due to an inability to accept that people five thousand years ago built monuments that required abilities far more advanced than the basic skills required to produce a mortice and tenon joint . A similar misunderstanding of the much longer history of symboling only exacerbated the problem .
Anon asks “But then again, what does all this have to do with Stonehenge? That you feel it does only shows how tenuous your argument is to need the 290,000 year old rock markings as evidence! Of what? “

The evidence of Newgrange , Knowth , Harappa , Gobleki Tepe Chauvet ,etc was to show that symboling and sophistication in architecture were around a lot earlier than the mortice and tenon at Stonehenge . Contrary to your assertion that “such advanced engineering capabilities require advanced and sophisticated culture that would leave behind many artifacts and signs of civilization, including records and symbols. All lacking in this case! “

Anonymous said...

Geo Cur,

Where there is evidence we consider the evidence. But where there is no evidence, we shouldn't make it up!

And how do we make up 'evidence'? By confusing the 'date of the wood' with the 'date of its use', for example. Are you arguing 5000 years ago people dug wells for water? What passes as 'evidence' too often in the case of Stonehenge is 'interpretation' by true believers. Leaves the rest of us to sort 'facts from fiction'.

The artifacts found at Harappa are evidence for Harappa! It is clear and convincing evidence they had 'invented the wheel' and were able to build wagons driven by oxen. If these artifacts were found at Stonehenge, there would be no dispute! But to argue that the evidence found at Harappa is evidence for Stonehenge … the logical fallacy is too obvious! Or do you believe that capabilities of people at one place are true for all places? Do the aborigines today make rockets?

Don't get me wrong! This is not a 'value judgments'. I am not saying making rockets is superior to tracking wild life! In fact, I don't make value judgments of people at all! We are what we are by the same laws of nature that make each of us who we are.

In my humble opinion, civilizations advance because conditions are favorable. Like weather, food production and population size. In my view it has nothing to do with 'gene pools'. North Europe 'awakened' at a latter stage simply because the climate became more favorable. And commerce with other cultures intensified.

Constantinos Ragazas said...

Brian,

The title of the post is “Erratics on the Coast”.

“Erratics” – not local
“Coast” – between sea and land

Off topic? I don't think so!

Kostas

Geo Cur said...

Anon ,the reason Harappa and the other sites much closer to Stonhenge were mentioned was not “evidence found at Harappa is evidence for Stonehenge “ but was in relation to your earlier comment “Such advanced engineering capabilities require advanced and sophisticated culture that would leave behind many artifacts and signs of civilization, including records and symbols. All lacking in this case! “
Not only was this something simply what you would like to believe about “advanced engineering capabilities “ , we could all suggest our own equally subjective and unhelpful points , but the reasoning itself was muddled .There are plenty of examples of pre Stonehenge monuments in Britain Ireland and Brttany requiring more advanced capabilities than the use of a mortice and tenon joint .These monuments come with or without symbols and artefacts . Equally , artefacts and symboling have a much longer history but are not necessarily a sign of advanced engineering capabilities . There is evidence for all of the above but none to show that the Romans were responsible for the mortice and tenons at Stonehenge,anymore than medieval stonemasons .