Another stone lifting theory

Without charge, and in a spirit of festive goodwill (because Wales did so well in the Rugby World Cup) I offer this to the archaeologists.

In looking at something else, suggested by Geo Cur, I came across this nice little picture, indicating a possible mechanism by which Neolithic boatmen might have picked up heavy elongated stones from the sea floor in areas where there are high tides (for example, on the Pembrokeshire coast).

 (After Geraldine and Matthew Stout, 2008, with respect to Newgrange in Ireland)

The idea is that you drag your stone down across the mudflats at low tide and mark its position.  Then you wait for the tide to rise, position your boat over the stone, and let the falling tide drop your boat down until it is positioned on top of the stone.  Then you lash the stone and the boat up together.  Then you wait for the tide to rise and lift the boat and the stone -- and with the stone now clear of the bottom, off you paddle in your boat all the way to Somerset, where you deliver your under-slung cargo to your waiting friends who have their rollers and sledges at the ready for the last part of the journey.  If you are really smart, you might be able to drop the stone, on a falling tide,  directly onto a sledge which has been pre-positioned on the bed of the estuary.

The article in which this idea is proposed has a rather wacky -- and I think unconvincing -- theory that the Breton word "bronbag" (meaning "boat's breast") used for describing megalithic standing stones indicates that big stones were moved in this way by water.  I think it much more likely that this word is simply  a descriptive one, applied to standing stones which looked, from a distance,  like boats positioned on end and bedded into the ground.

All good fun.......

 Source:
http://www.e-a-a.org/TEA33.pdf
Words as Archaeological Finds
A Further Example of the Ethno-Philological Contribution to the Study of European Megalithism

by Francesco Benozzo, University of Bologna, Dipartimento di Lingue e Letterature Straniere, Via Cartoleria 5, I-40124 Bologna, Italy.
francesco.benozzo@unibo.it

See also:
Stout, G. and M. Stout 2008. Newgrange. Cork University Press, Cork.

Iceberg transport

Our discussion about the dumping of erratics on distant beaches prompted me to show these two photos:

The top photo shows a big tabular berg typical of those that calve from the biggest and most active glaciers in Greenland.  Normally, when seen from above, those bergs look clean, but if by some freak a big berg like this flips over, the bottom photo shows what it can look like.  There are huge quantities of debris here, embedded into the ice.  In some cases this may be a layer carried within the glacier on a shear-plane -- in other cases this may be the actual glacier bed that we are looking at.

 Zoom in close on the bottom photo if you can -- and you will see many very large boulders melting out of the till and debris-rich ice.

Where floating glacier snouts are crumbling or collapsing into the sea (as in the "Frozen Planet" sequence from Jakobshavns Isbrae) most of the debris from this smashed-up ice is dumped on the sea bed very close to the glacier snout, or maybe within a few kilometres as the floating brash ice is moved away by tides and currents.

But where big slabs of ice are floated away and caught up by ocean currents, we clearly have the potential for tremendous amounts of sediment to be widely dispersed onto the sea bed, hundreds or even thousands of miles from the calving glacier front.  And if an iceberg like this (or even a small remaining fragment of it) washes up onto some distant shore, we have a simple mechanism for the dumping of boulders like the giant erratics of Devon and Cornwall.

Here are a couple of iceberg track maps, to show just how far these icebergs can travel.

Bulldozing glaciers

The spectacular time-lapse footage in the first "Frozen Planet" programme should leave one in no doubt that glaciers are capable of tremendous things in the process of landscape change -- moving very fast on occasion (for example, when a surge is under way) and shifting large quantities of debris.  However, one might be left with the impression that glaciers act as bulldozers, pushing debris ahead of themselves as they advance.  That is actually quite a rare occurrence.  One place where this does occur is at the snout of the Thompson Glacier on Axel Heiberg Island.  Have a look at these:

Click to enlarge

The top photo shows the push moraine beyond the glacier snout, and the lower one was taken out near the outer edge of the push moraine, looking back at the snout.  That gives a nice impression of the sheer scale of the phenomenon.

There are some rather unique factors here, in that the glacier is (or was, in the 1960-70 period) advancing strongly along a valley thickly infilled with fluvio-glacial sediments frozen by permafrost.  Great slabs of these frozen gravels have in effect been bulldozed, because the base of the glacier is not sitting on top of the gravels but is at a lower level -- so the forward movement of the glacier is strong enough to displace the gravels rather than riding over them.  Wonderfully impressive, but not all that common.

Most glaciers, as they advance, slide on the bed if basal ice temperatures are around the pressure melting point, or -- in truly polar situations where the glacier bed is frozen onto the bedrock interface -- by various processes of internal deformation.  One of these processes is shearing or thrusting -- and there are spectacular examples of thrust planes on the snout of Thompson Glacier as well.

Here is another photo from Thompson Glacier:

The main feature of this photo is the meltwater stream gushing out from a tunnel high on the glacier snout, but the whole of this ice face is controlled by shearing or thrusting, with ice riding up over slower-moving debris-laden ice at the base of the snout.  These thrusts are responsible for the delivery of the masses of debris from within the glacier, or from its bed, up to the ice surface.

For more photos of this remarkable glacier (which I visited in 1963) see the Glaciers Online web site, and go to Axel Heiberg Island and then to Thompson Glacier.

http://www.swisseduc.ch/glaciers/axel_heiberg/thompson_push_moraine/index-en.html

Super erratics and erratic clusters

 I came across this on the fantastic "Glaciers Online" site, organized by Swiss Education.  This is the famous "super erratic" at Bremgarten in Switzerland.  Notice that it's in four large pieces -- so as in the case of the Big Rock at Okotoks in Canada, we have in effect a "giant erratic cluster."

http://www.swisseduc.ch/glaciers/index-en.html

If you look further on the Glaciers Online site there are many other spectacular photos of erratic clusters, like this one on Svalbard:

These are granite erratics that appear to have been transported as a group from a long way off -- reminiscent of the Darwin Boulders in Patagonia.

Here's an even more quirky occurrence from Broggerbreen in Svalbard -- a rectangulat piece of granite that looks for all the world as if a stonemason has been hard at work on it!

How are erratics like this entrained?  Well, there appear to be two (at least!) prerequisites:  high stresses on the glacier bed, creating fractures which weaken the coherence of the rock and permit quarrying to take place; and then freezing-on at the bed, with large stones picked up and carried off downglacier.  Here is a rock surface (again in Svalbard) that has been broken up by subglacial processes and "prepared" for exploitation:

Some blocks have already been dragged away.

I remain convinced that in the right circumstances extremely large masses of bedrock can be dragged away by overriding ice and transported for hundreds of kilometres within a glacier.....

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.

Enjoy!

I was inspired by those wonderful images last night on "Frozen Planet" to take a trawl around the northern lands on Google Earth.  Above are two images from Ellesmere Island, northern Canada.

The top one is particularly impressive -- we might call it "The Big Bang" except that there probably isn't much of a bang.  But no less than seven glaciers colliding -- the forces are unimaginable.

Glaciers can do almost anything, folks!  Carrying a few pathetically small stones from West Wales to Stonehenge?  A doddle.....

Click to enlarge.

The Devensian ice limit in North Pembrokeshire

On 17 April I published this map -- indicating my latest thinking on the extent of the Irish Sea Glacier in North Pembrokeshire.

I have now found four places where there are hummocky deposits which I interpret as morainic and fluvioglacial accumulations of a terminal nature -- in other words they show end moraine or glacier terminus positions.  One is the Pont Ceunant moraine, which I have previously described -- search for the blog entries via the search facility on the blog page.  There are other accumulations at Cilgwyn, around the eastern end of the Gwaun Valley and near the cattle grid on Waun Mawn, near the standing stones described also on this site.  Around Llanychaer in the Gwaun Valley there are other deposits of till and fluvioglacial mounds on the valley floor  suggesting that the valley is old and that the deposits are young.  I have suggested before, in various articles, that the complex of valleys here is indeed very complex, containing the best examples in the British Isles of sub-glacial meltwater channels and also features of many generations -- modified during a number of glacial episodes.  Some sections of the channels are genuinely sub-glacial (dating maybe from the Anglian and Wolstonian glaciations) and other sections may be marginal and sub-marginal, maybe old but freshened up by re-use during the Devensian.

The Gwaun-Jordanston system of meltwater channels in North Pembrokeshire -- a map which I made in 1965 as part of my doctorate research.  Click to enlarge.

It now seems to me that during the Devensian the Irish Sea Glacier pressed inland from Fishguard as far as Llanychaer, effectively blocking the meltwater outlet to the north-west and diverting large quantities of meltwater through the Nantybugail Channel southwards and south-westwards towards Trefgarn Gorge.  After that, as the ice edge retreated, the other channels to the south of Fishguard were also used, one after another.

 Google Earth image of the terrain on the northern flank of Preseli, east of Carn Goedog.  Part of the craggy tor is on the left edge of the photo.  Note the channel-like features on the mountainside -- natural, or made by animal hooves?

Other feature which have a bearing on this new interpretation are the channel-like features which occur on the northern face of Mynydd Preseli to the east of the main road where it goes up towards the summit col, and between Carn Goedog and Carn Alw above the route of the old drovers' road.  For years I have thought of these as made by the hooves of thousands of animals driven eastwards during the days of the cattle drovers, but now I am not so sure.  In some places there are several of these "steps" or channels roughly parallel with one another on the hillside, and I now think they may well be marginal or sub-marginal drainage channels cut along of close to an ice edge.  I hope to do some more work on these channels.

It is also increasingly apparent to me that on the Carningli upland, on the ridge around Carnedd Meibion Owen, and on the main Preseli upland ridge, there are abundant signs of ice-smoothed slabs, erratics, perched blocks and sheets of till which suggest that glacier ice DID actually cover these uplands during the Devensian -- maybe for just a few centuries.  This is where Henry Patton's models of local ice caps around the peripheries of the Welsh ice cap come in very handy -- and I suggest that there were indeed a number of small thin local ice caps over these uplands, maybe sometimes isolated from the larger ice masses to the north and west, and sometimes incorporated into them.  So there would have been an oscillating junction between active ice belonging to the Irish Sea Glacier and sluggish or stagnant ice on these thin local ice caps.  Details still to be worked out.  Watch this space......

Here is a photo of the edge of the ice sheet in North Greenland.  There are four types of "terrain" here.   First, on the left we see the ice sheet edge -- remarkably clean here, because this is polar ice, probably frozen to its bed.  Second, we have small ice caps on the higher land.  They are permanent, in the sense that they are always there, summer and winter, over decades and probably centuries.  Third, there are the areas where there are perennial snow patches interspersed with terrain where the snow melts each summer.  It will not take much of a climatic deterioration, or an increase in snowfall, to incorporate these areas into the local ice caps.  And fourth, there are the snow-free valleys and broader lowlands, where summer melting is sufficient to remove each winter snowfall.  If we were to imagine a steady deterioration of the local climate here these areas would coalesce one by one, until the whole landscape would appear white from above.  It would then be very difficult to fix the actual edge of the ice sheet with any precision.  I imagine something like this scenario for the periphery of the ice sheet in North Pembrokeshire during the Devensian -- and by extension, for the Somerset - Wiltshire area during the Anglian Glaciation around 450,000 years ago.

The high point of my academic career......

As my son said yesterday, being insulted by that smart-ass Graham Norton on the BBC One Show HAS to be the high point of my academic career!  It was on last night -- here are a couple of stills:

 The animated map is quite wonderful, in a grotesque sort of way -- based (sort of) on raw materials which I provided them with.  I asked them to let me check it before it went out -- but they only had 3 days from filming to transmission, so there it is......

Two days of my time, and 3 hours of filming last week -- and they cut down all my careful explanations of the theories and their pros and cons into a sound-bite of a few seconds.  This is after all a family show -- mustn't be too taxing.  What they wanted was some wacky stuff from two eccentric engineers (Garry and Bruce) and a party pooper (me) to entertain the viewers while they tucked in to their suppers......

What the hell -- it's all quite entertaining -- and at least some people will now know that maybe glaciers have something to do with Stonehenge!

Here is the link:
http://www.bbc.co.uk/iplayer/episode/b016k0vn/The_One_Show_25_10_2011/

The Stonehenge piece lasts about 5 mins, and starts a couple of minutes into the show.  Enjoy!!

The Stonehenge Layer

We seem to be getting deeply into this topic on some of the other threads, so let's go with the flow, even though Rob might not have anything more to say.

I think this is an important topic -- and Kostas is quite justified in asking the question "Is the Stonehenge Layer confined to Stonehenge, or is it found across a wider area of Salisbury Plain?"  If it is found just in tight proximity to the stones, then I suppose we have a justification of the view that the layer is strictly man-made, being associated in some way with stone working or stone destruction.  If it is found (with bluestone or foreign fragments in it) miles or tens of miles from Stonehenge, then we are in interesting territory!

The above pics show the pitted and broken surface of the chalk (showing the multiplicity of stone sockets), packing stones and the brownish layer of superficial materials nicely labelled up during the 2008 Darvill and Wainwright dig.  This is what the two professors say about the layer:

The ‘Stonehenge Layer’

Our excavations within Stonehenge in 2008 (see CA 219) confirmed what earlier excavations had hinted at: namely that the Bluestones started to be broken up and chipped away more or less from the time they were set up in each successive arrangement. The great spread of flakes and debris usually referred to in the archaeological literature as the ‘Stonehenge Layer’ is not, as once thought, the debris from a one-off act of dressing the stones prior to their erection. Instead, these flakes have accumulated over millennia and include evidence for the use of Bluestone to fashion axes. Furthermore, detailed analysis of the finds from our excavations, now well under way, has highlighted two other important points. First, that some kinds of stone that we found as flakes and blocks in the excavated sample are not represented amongst the existing range of pillars standing at the site. Detailed petrological work by Rob Ixer and Richard Bevins has racked the source of one such rock type – a kind of rhyolite – to probable source outcrops near Pont Saeson, on the north side of the Preseli Hills (despite a speculative note in British Archaeology Nov/Dec 2009, page 7, suggesting a source elsewhere in Wales). Second, that interest in the Stonehenge Bluestones did not cease in prehistoric times. In the 4th century AD a shaft was dug adjacent to Bluestone Stone 35a that was refilled with rich dark soil. It was then ritually sealed by the placeme up, would have marked the position of the shaft. Coins, pottery, brooches, surgical instruments, and possibly also a curse-tablet from earlier excavations show that Stonehenge was just as much a sacred spot in Roman times as it had been earlier.
From
T Darvill and G Wainwright
The Stones of Stonehenge
http://www.archaeology.co.uk/news-features/the-stones-of-stonehenge.htm
Issue 252 of Current Archaeology

---------------------------
We have seen references to bits of bluestone at the Cursus and at Woodhenge, and then we also have other bluestone flakes and chips turning up elsewhere -- not to mention the MPP theory of bluestones being used at "Bluestonehenge" -- and perhaps I could ask a question of you guys who know some of the other excavations:  is the soil sequence the same as that shown in the top picture above, away from Stonehenge?  And what's the "dark layer" which is sometimes mentioned?  Is that in a consistent stratigraphic position?

Ancient wisdom from Monty Python

http://youtu.be/zWANZjTwAaQ

The stuff of nightmares

I wonder if the archaeologists ever wake up in the middle of the night and ask themselves the following questions? 

1.  If Stonehenge was built by people who literally travelled hundreds of miles to collect stones from far distant places, how come nobody else did the same thing?  Were these fellows just that much stupider, or smarter, than everybody else?

2.  How come all the sites they chose for their "bluestone quarries" were far away, in the west?  How come they were not at all interested in other perfectly nice stones, closer to hand, to the south, east and north?

3.  Since we know that there are about 30 different rock types at Stonehenge (in the standing stones, stumps and debris) does that mean that there are about 30 sacred sites, and 30 quarries?

4.  If there were 30 sacred sites out there somewhere, how come nobody else thought they were sacred too?

5.  If these splendid fellows were so keen on spotted dolerite, how come they carried so many other "rubbish stones" to Stonehenge as well, only to smash them up when they got there?

Just asking, not for the first time, and still waiting for the answers.......

Anybody else having problems?

Tony says that some of the pics are chopped off on his computer screen .... anybody else having this problem?

Rhosyfelin geology

The BGS has made available for public use, via its fantastic web site, a full geological map of the UK at a 1:50,000 scale.  It's superimposed on the Google imagery, so you can get to wherever you want and find out what the geology is......  you can even click on a site and get a description of the rock type.

http://maps.bgs.ac.uk/geologyviewer_google/googleviewer.html

 

Above is an extract from the map showing the Rhosyfelin - Pont Saeson area.  The site of the famous "bluestone quarry" is slap in the centre of the photo, where the minor road has a distinct Z-bend.  The pinky-purple colour shows the extent of Fishguard Volcanics, which also sweep from here towards the NW, with a number of outcrops on the northern flanks of Carningli and Dinas Mountain.

The green colours represent sedimentary rocks -- for the most part Ordovician shales in this area.

The Fishguard Volcanics in this area are rhyolite lavas and ashes with considerable internal variation, which leads Rob Ixer and Richard Bevins to think they can tie down some of the Stonehenge fragments to specific locations at Craig Rhosyfelin with great accuracy.  We have already had some discussions as to whether you can tie down particular fragments to a location with an accuracy of +/- 2m!! 

We await with interest their paper on this matter....