THE BOOK
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Wednesday, 30 January 2013

Periglacial stripes (again)

 The stripes exposed in the Avenue excavation, across the road from Stonehenge.  (Acknowledgement:  Tim Daw's web site)

I have had a few interesting exchanges with Charly French and Mike Allen on the subject of those famous periglacial stripes -- which were, according to Prof MPP, obvious enough features in the Neolithic landscape to have caused the builders of Stonehenge to build the monument here rather than somewhere else.  I have never found that explanation at all convincing -- and indeed I still consider it fanciful in the extreme. 

However, these ridges are indubitably interesting.  Charly and Mike think that they are Devensian in age and periglacial in origin.  This is Mike's latest message, which I hope he will not mind me sharing:


The rest of the slope (ie outside the alignments in the Avenue) contains a number of parallel discontinuous stripes as do several other areas in the same fields. These all run diagonally down slope and are on average 10 to 15cm across and about the same depth with irregular V shaped profiles. The are filled with typically buff to reddish brown silts to silty clays usually stone-free but sometimes containing well patinated flint.

The molluscan assemblages (when preserved) contain typical restricted open country (cold stage) assemblages - but from memory I cannot remember any of the large Pupilla muscorum sometimes found in cold-stage assemblages.  The assemblages were depauperate and species-poor so no diagnostic rarities were recorded from this area. These 'stripes' are typical or and similar to those I've seen and recorded in Sussex, Dorset, Hants etc.

With the Avenue stripes the orientation is the same as the adjacent periglacial stripes and the spacing about the same. Hence our contention that these deeper linear gullies originated as shallower periglacial stripes forming probably in the Late Devensian. The samples produced no shells ... or too few to take further to analysis but I will look through the archived material.

Obviously the origin of these is of interest if not fascination. If purely run-off and solution it does seem odd to have a series of parallel and straight linear grooves. That being said, I don't comprehend the precise formation process that created periglacial stripes as such strong parallel straight features ubiquitous across the southern chalk.





 What fascinates me about this info kindly provided by Mike is that the grooves run DIAGONALLY down slope.  If that is the case they cannot, I think, be periglacial in origin, since all of the periglacial stripes I have ever seen run directly down a slope on the maximum gradient.  And we are just as much in the dark with respect to the precise processes involved.

This all confirms me in the belief that these are solutional rills, possibly of very great age (ie not Devensian but maybe much older, forming and deepening over several glacial / interglacial cycles) and in some way STRUCTURALLY CONTROLLED.  I have no idea what the bedding of the chalk is in the vicinity of Stonehenge, but I think I might speculate that the chalk beds are dipping and that the solutional rills are developed on the strike of these beds as they outcrop at the surface.  A glance at the detailed BGS geological map might give us guidance on the matter.......

All further comments (from Mike or Charly or anybody else) will be welcome!

The Rhyolitic "debitage" around Stonehenge

 A thin section numbered SH80 -- acknowledgements to the authors of the paper cited below, and to Wilts Arch and Nat Hist magazine.  This rhyolite is very different from the Rhosyfelin ones -- so where does it come from?


There is a new paper by Ixer and Bevins, with a comment by Mike Pitts, in the latest edition of the Wilts Arch and Nat Hist magazine.   Thanks to Rob for passing it on.   It's very difficult to follow, since the authors spend most of the time  discussing very subtle differences between the 14 samples which they looked at, and deciding which category or sub-category to place them in.  The essential conclusion seems to be that the rhyolite fragments examined (which have still not been tied to any standing stones at Stonehenge) are mostly from the Rhosyfelin area -- the similarities with the samples taken from the crag are close enough for "micro-provenancing" in just one case, but in the others the authors have to admit to provenancing from the general area.  They do not speculate as to how wide this area might be -- so it could be covered by a grid 500m x 500m in extent, or it could be even larger.

One general problem that I have with the paper is that it tends to talk of the "Stonehenge rhyolitic debitage" as if it is homogenous or as if they have examined enough of it to be sure that any conclusions drawn are incontrovertible.  This is the problem that all scientists face -- when we have a sample of limited size, how wide are the conclusions we can draw?  It's not a bad idea to bear in mind that the parts of the Stonehenge "stone floor" or regolith or debris layer examined and sampled thus far is very small -- and as I have said before, there may be several "debitage" groups with quite different petrographic characteristics on other parts of the Stonehenge site or in other parts of the Stonehenge landscape. 

I wasn't entirely convinced by the attempt to push samples from SH 80 into the Rhosyfelin assemblage, and would like to know what the characteristics of rhyolites from other outcrops in the region might look like, and how they might compare with the samples examined.

Mike Pitts says:  "It is notable that all the samples matched in this study to Craig Rhos-y-felin come from debitage and not from megaliths (although Ixer and Bevins (201111a and b) have suggested that buried megalith SH32e may also come from Craig Rhos-y-felin). One of the distinctive features of the rhyolitic rocks is that they are flinty – they have a good conchoidal fracture. That makes them relatively easy to break up, if they are standing as monoliths at Stonehenge. But it also makes them suitable for making portable artefacts. There are flaked bluestone ‘tools’ from Stonehenge (including some from the stone floor). Which of these are made from debris created when stones were dressed on site? Which are made from broken up megaliths? And which were made in Wales and brought to Stonehenge by people visiting, perhaps on a pilgrimage of some kind? Clearly the distinction has important implications for how we understand Stonehenge."  Mike is seeking to open up the debate here and to avoid sticking to the standard story, but he is still seeing the world through the same tunnel as all the other archaeologists.  He fails to recognize that there is a further possibility, and he should also have asked this question:  "Which flaked bluestone tools -- and indeed which bits of debris -- might have come from smaller rhyolite stones found in the neighbourhood and which might be assumed to be small glacial erratics?"  I am referring here to stones which might have been too small to use as orthostats, or which might just have been the wrong shape, or which might have been too badly damaged during glacial transport and subsequent frost shattering.



Wilts Arch & Nat Hist Mag vol 106 (2013) pp 1-15

"A re-examination of rhyolitic bluestone ‘debitage’ from the Heelstone and other areas within the Stonehenge Landscape"
by Rob A. Ixer and Richard E. Bevins, with a contribution from Mike Pitts

ABSTRACT
Recently it has been proposed that the Stonehenge rhyolitic debitage can be distributed into five petrographical groups (A-E) (and that at least three of them (A-C) are from rocks cropping out at Craig Rhos-y-felin). This supersedes an earlier classification scheme of this important category of Stonehenge material. The earlier 1980s scheme, based on lithics found close to the Heelstone, divided the rhyolites into two groups (A and B) and sub-divided the larger into two further sub-groups (Bi and Bii). Re-examination of this earlier material together with other Stonehenge rhyolites has allowed the two schemes to be compared and integrated.
The original 1980s Group A lithics are identical to the present Group B, (both are small groups). This group is described in detail so completing the petrographical descriptions of the Stonehenge rhyolitic debitage. Despite bearing feldspar megacrysts this group shares sufficient petrographical characteristics with rocks from Craig Rhos-y-felin to support the view that that location is the geographical origin of the group.
Lithics belonging to the 1980’s groups Bi and Bii, however, are randomly distributed amongst the present A and C groups and there are no strict correspondences. The designation Bi and Bii should therefore be abandoned.
Using the new scheme it should now be possible to map more precisely the distribution of the rhyolitic debitage in the Stonehenge landscape to inform such questions as to the number of rhyolite orthostats originally present and their fate.

Monday, 28 January 2013

More South Pembrokeshire Erratics


A splendid photo by Adrian of the Flimston Churchyard and some of the seven recorded erratics.

My thanks to Adrian James for a batch of splendid photos of other South Pembrokeshire erratics to go with the one at Loveston.   I won't reproduce all of them here -- but encourage you to take a look at Adrian's site:

http://pdboyinsuffolk.blogspot.com/2013/01/other-erratics-around-castlemartin.html

This is an interesting comment from Adrian about the Flimston Collection:

Flimston Chapel churchyard (SR92399558). There is a substantial collection of erratics in this churchyard. Some have been used as headstones for the graves of members of the Lambton Family who died in military service. Others have been left sitting in one corner of the enclosure. All of these stones arrived in the churchyard when the chapel was renovated and re-consecrated in 1903. There are 7 of them. A pamphlet, which describes the features and memorials in the yard, printed at the time of the opening of the chapel in about 1914, gives us these vague details:
No. 1 Boulder, at the head of Lady Victoria Lambton's grave was taken from just opposite Flimston Cottage. A 'brecciated spherultio, albite, trachyte or rhyolite.' Many occur in Pembrokeshire. This one 'seems to fit best with those of Romans'Castle in the character of its spherulites and groundmass.' Most of these appear to have travelled over 30 miles from the N. West separated from their parent rocks by St Brides Bay and Milford Haven, and by a considerable mass of high ground.........
[Flimston Cottage stood at SR927955, about 0.3 km ESE of the chapel and just north of the old clay pits.]
......No. 2 Boulder, from Pwlslaughter, which stands in the opposite North corner. [Bullslaughter, SR942944 - approximately 2.25 km SE]
No. 3 Boulder from Bulliber Farm [About 2.25 km WNW, at SR905968]
No. 3 Boulder from Merrion pond. [ About 2 km NE].
No. 5 Boulder from Lyserry Farm
No. 6 Boulder from Lyserry Farm.
No.7 Boulder from Lyserry Farm.
[Lyserry is about 3.4 km ENE of Flimston chapel, at SR9556967]
 
---------------------------------
 

Another erratic -- also from Loveston Farm?  The nice thing about the erratics of this area is that they are pretty obvious -- the only local rock type is Carboniferous Limestone -- so anything that isn't limestone is an erratic......

The Devensian Cilgwyn Moraine

This is a false colour image of the Cilgwyn area -- courtesy Henry Patton -- which shows the small irregularities in the land surface because "false lighting" has been introduced from the NE.  On the top image you can see the mounds and terraces of fluvioglacial and glacial material; on the bottom image I have added a line to highlight where I think the edges of a glacier lobe might have been, at the peak of the Devensian Glaciation about 20,000 years ago.

The high ground on the right is Carningli -- a rocky summit which has signs of till and glaciated rock slabs on its northern and north-eastern flanks.  I'm still open to the idea that ice may have flowed right over Carningli during the Devensian -- but dating is very difficult.

At bottom left we see the inlet of the Gwaun Channel meltwater drainage system -- assumed by the authors of the big glacial lake paper about Glacial Lake Teifi to have been a spillway for Glacial Lake Brynberian.  See the previous posts for details.

I live slap in the middle of this area, and I have to say that I have not yet seen any trace of glacial lake deposits -- and am rather unconvinced about the spillway too.  But there are certainly pretty impressive terraces of sand and gravel that look to me like kame terraces -- along the right edge of the photo.

At the southernmost extent of the lobe, as shown on the lower photo, the morainic clutter is very impressive too, with a number of distinct mounds, vary large erratics scattered all over the place, and generally a very rough land surface which makes farming difficult -- very different from the areas where fluvioglacial materials have been dumped.

A lobe here would accord well with the idea of a highly irregular ice edge, with many lobes and embayments coinciding with areas of lower and more hilly land.

I'm still working on this, and will keep you informed...






Wednesday, 23 January 2013

Ice marginal lakes come and go....


A photomosaic of ice fronts, marginal lakes and outwash plain on the south coast of Iceland.  The sea is right at the bottom edge of the photo.  This is a very high definition image -- click to enlarge as much as possible, and you will pick out great detail.  Note that some meltwater streams follow the ice edge before decanting into marginal lakes, which in turn decant into the sea.  The meltwater routes are well marked -- here and there we see distinct channels being formed, but because almost all of the features beyond the ice edge are made of unconsolidated sediments in this case, the channels are ephemeral.


South Greenland ice sheet edge.  I suspect that this photo was taken in the autumn, after the fall of the first snows of winter.  the lakes are still ice-free.  there are a lot of them -- and if you zoom in you can see some spillways snaking away from the ice front, along depressions or natural cols in the undulating bedrock terrain.


Another South Greenland ice sheet edge, with abundant meltwater filling low-lying parts of the landscape but not actually impounded by the ice edge.  The overall slope of the land surface is away from the higher foreground towards the top right corner of the photo.  If you look carefully you can see that most streams are flowing away from the camera position.  there are some rock-cut spillways or channels.  The lakes in the foreground are full, whereas those in the distance have had higher water levels than those of today -- you can see the light-coloured strips along the waters edge -- these are strandlines, maybe just a few metres above present lake levels.  This is a very dynamic environment.  Each lake has a different history and a different duration.

Glaciated terrain in Svalbard

Two nice satellite images from Svalbard.  The top one is from the NW of the island, and the lower one is from the N part.  The top image shows typical jagged upland terrain, with many knife-edged ridges and steep peaks -- the shadows of these peaks on the snow is what gives the terrain a ragged or jagged appearance.  Don't know what the geology is.......

The lower photo shows an area with more extensive snowfields and plateaux, and fewer jagged peaks.  More interesting for glaciologists, maybe, but not for mountaineers.

Another glacial lake


One of my favourite photos of a glacial lake margin.  This is from South Georgia -- photo by Chalmers Clapperton.  Note that this lake has a wildly oscillating surface -- it's quite probable that these shorelines are not in a tidy time sequence at all.  Also, note that the strandlines are well marked where the rocky slopes are relatively gentle, but less well marked at the far end of the lake where there are quite active scree slopes.

Monday, 21 January 2013

Hammerstone or erratic cobble?


I found this picture on a web site belonging to somebody who helped with the 2011 dig at Rhosyfelin.  She refers to "quarried rubble, flint flakes and hammerstones" which perhaps somebody less enthusiastic might have referred to as "frost-shattered scree, rhyolite fragments and erratic cobbles and boulders."  But there you go.  There is no stopping these enthusiastic archaeologists once they get going........ obviously as soon as they start digging they are left in no doubt as to what they are looking at.

Another thing I learned from this blog is that somebody called Alasdair Pike is trying to work out the source of the gold found at the site.  Yes, I did say GOLD.  So is this site a treasure trove as well as everything else?  There is no end to the wonders of Rhosyfelin ---  I can't wait for the TV spectacular........

On Spillways

These two photos are from the Ouimet Canyon spillway in Canada -- 2.5 km long and over 100m deep.  It has carried a phenomenal amount of meltwater from the glacial lakes that developed at the end of the last glaciation as the Laurentide ice sheet was retreating back to its core area.  But this feature is not the result of a single catastrophic event -- the channel has a very complex history, and is now accepted as a "composite" channel which has probably been used many times, over millions of years.....

This is the Red Rock Pass, which carried vast quantities of water from the overflowing of Lake Bonneville in the USA.  It's a  very impressive feature, and is justly famous -- but again it seems to have been used more than once.


Click on this one and give it careful scrutiny.  Beyond the ice edge, this is complex terrain, with many areas of glacial and fluvioglacial deposits among the bedrock outcrops.  The terrain is hilly, and there are many meltwater lakes.  But note that the water NEVER flows back into or under the ice.  On the other hand there are long stretches of ice front where the meltwater flows in well defined streams along the ice edge, going from one lake to another.

Sunday, 20 January 2013

Ice Dammed Lakes

The shorelines of Glacial Lake Missoula in Montana, USA.  These are nicely picked out by winter snowbanks.  Shorelines or terraces like these are excellent indicators of ephemeral lakes impounded by ice margins.  They may or may not have formed in a sequence, as lake levels fell from the highest to the lowest marked position.

 Laminated or banded sediments left high and dry when Lake Missoula was finally drained.  These are relatively course, made of "rock flour" -- a sign of turbulent water and very heavy sediment loads coming into it from the melting ice margin.  Note also the dropstones which have dropped into the sediments on the lake bed as icebergs and ice floes melted on the surface of the lake.

Shorelines of Lake Bonneville -- another vast lake formed during the wastage of the ice sheets in the United States.  The history of these lakes, and of the landforms associated with them, is now quite well known.

A detailed map from a paper by Krister Jansson of the glacial lake sequence in a part of Labrador-Ungava.  Here the cold-based ice sheet was retreating broadly northwards,  down the surface slope.  So the situation would have been broadly similar to that of Glacial Lake Brynberian.  Ponded water was effectively held up by the retreating ice margin.  Lake deposits, washed surfaces, spillways and shorelines all go to build up a picture of how the ice edge retreated.

In seeking to discover whether Glacial Lake Brynberian really did exist, we really need to know whether there are glacial lake sediments within its proposed edges, whether there are shoreline traces (even very faint ones) like those in the illustrations above, and whether there are Spillways worthy of the name, by which the lake might have drained.  At the moment I am not entirely convinced on any of those fronts.  Work in progress......

Saturday, 19 January 2013

My blog site photo album

Not everybody knows this, but if you want to review all of the photos used on this blog since the day it started (there are now almost 1000 of them) you can find them in my Picasa album, here:

https://picasaweb.google.com/lh/reorder?uname=112312986974547323674&aid=5688145723498324801

or here:

https://picasaweb.google.com/lh/reorder?uname=112312986974547323674&aid=5339881857493156577

Depending on which browser you are using, you should be able to see them in various formats, in various sizes, and arranged in various ways.  What you can't do, straight from the album, is go to the post in which a photo first appeared.  Leave that with me -- I'm working on it....

Change of Title

 A fantastic photo of Unterer Grindelwaldgletscher, from the Glaciers Online web site. 
(Nothing to do with Stonehenge)

Dear All

Please note that I have changed the title of the blog, to more accurately reflect what it is all about.  I have been thinking for some time that "Stonehenge Thoughts" was not really a very good title for the blog, given that some of the comments published on the site are not very thoughtful at all (!!), and that every blog in the world involves somebody or other thinking and then blabbing on about something or other which might be of marginal interest to the great mass of humanity.

I've also noticed that it is the Ice Age dimension that makes this blog quite attractive to many students of Stonehenge, and which makes it stand out from many other web sites and blogs about that old ruin.  So that might as well be reflected in the title.  The URL for the site will of course remain unchanged.

Finally, when I analyse Google searches, I see that many of those who come to the site are coming to look at things which appear on Google images -- relating to glaciers, moraines, surges, ice sheets and ice caps, Pleistocene chronology, Greenland and Antarctica.  In other words, they are looking for information about glaciers and the Ice Age, and not specifically about Stonehenge.  So there must be a lot of Earth Science students out there, who are using my site as an information resource.  That's great -- and again it confirms that a name change is a good idea.

So there we are then.. hope you all approve...

Brian

The site of Lake Brynberian


Here is a new map (based on a computer-generated terrain map from Henry Patton) showing the "landscape context" for Rhosyfelin.  The "lighting" for this model is in the south, so the north-facing slopes show up clearly.  Click to enlarge.

Rhosyfelin is shown with the orange dot.  The "basin" which is supposed to have held the waters of Glacial Lake Brynberian is clearly shown, defined in part by the northern slopes of Mynydd Preseli.

Tuesday, 15 January 2013

Those old valleys

I found this map in an old paper by Eyles and McCabe (1989) and have tidied it up a bit for clarity.  But it's rather interesting, showing the "tunnel valleys" in and around the Irish Sea, Cardigan Bay and St George's Channel.  These are the old bedrock channels, often deeply cut to well beneath sea level  -- Milford Haven, Wexford Harbour and Waterford Harbour could also have been shown with the aid of arrows going "with the trend".

What we see here is a pattern of drainage which is essentially dendritic, and which has its origins far back into the Tertiary and maybe even earlier.  The key components of the uplands and lowlands have thus been in place for many millions of years, so that when the Pleistocene glaciations occurred the "grain" of the country, and the alignment of these old drainage routes, have strong influences on the patterns of both glacial action and fluvio-glacial channel cutting.  Much meltwater was directed into these old valleys -- as we can see in the case of the Gwaun-Jordanston valley system, where subglacial channels have been deeply cut into the floors of older shallower valleys.  The remnants of these old valleys can still be seen as "shoulders" high above valley floors.  In some cases the positions of interfluves have been moved, and in other cases (as with Cwnm Gwaun) they have simply been lowered, so that we now have humped long profiles on channel floors. (This is a good diagnostic feature for a sub-glacial meltwater channel.....)

As I argued many years ago, and as now seems to be acceptd by the new generation of Geomorphologists, these channels are MUCH older than the Devensian.  they have been used over and again, someimes by meltwater flowing subglacially and sometimes by water flowing subaerially -- in spillways from the glacial lakes of the area.

Is all of this relevant for our understanding of Stonehenge?  Not really, but there are implications here for our understanding of landscape evolution at Rhosyfelin.

Some of the subglacial meltwater channels near Fishguard -- the big one near the base of the photo is the Nant-y-Bugail Channel, so deep that it was used in WW2 for the creation of long tunnels cut into the valley sides for the storage of mines and other ordnance.  These valleys have been superimposed onto a landscape of much older valleys, orientated in more or less the same direction.


Monday, 14 January 2013

North Pembrokeshire glacial map

 Click to enlarge.....

In my attempts to understand the Pleistocene history of the Rhosyfelin site, I came across this old map made many years ago by Prof David Bowen.  It incorporates info from many different sources.  The red dot (added by me) shows the location of the Rhosyfelin dig.  There are lots of glacial features (including many that are quite fresh) to the north of it, so even without knowing whether Lake Brynberian actually existed or not, and without having any shorelines to work with, we can say with some certainty that the dig site lies somewhere quite close to the Devensian ice limit in North Pembrokeshire.  More to follow.......

Friday, 11 January 2013

The Rhosyfelin sediment sequence


Following the contribution  from Barry, let's have a look at the central hypothesis here.  The maps above come from Etienne et al, and are based on detailed work in the lower Teifi Valley and more scattered bits of work elsewhere.  Their main contribution to glacial geomorphology was to show that the the lake deposits (rhythmites or varves of clay and silt) up-valley from Cardigan were laid down in glacial lakes at a time of ice advance.  That's a very different scenario from that of Charlesworth, who argued in 1929 that there were great lakes in this area AFTER the peak of the last glaciation, at a time of ice wastage.  That's the conventional or accepted scenario, and it's what happens most often in ice marginal situations as well.

So what the new work means is that if there are tills and fluvioglacial deposits in the area, dating from the Devensian glacial episode, they should be ON TOP OF the lacustrine deposits.  That indeed seems to be the situation in the Teifi Valley.  But what about the rest of the area shown on the maps above?  There have been hardly any systematic borehole investigations, and there are very few natural exposures we can look at -- so what we have in map (f) above -- the one enlarged in the lower illustration -- has to be looked on as a working hypothesis, to be tested against field evidence. 

For a start, I have already mentioned on this blog that I'm not that happy with either the ice directions portrayed on the map, or with the straight smooth nature of the ice edge.  Ice edges in this sort of undulating terrain look much more like this:


So unless we can find evidence of lake deposits in the areas shown as being occupied by water in the lower map, we should keep an open mind.  Maybe the area of Lake Brynberian was actually occupied by a lobe of ice rather than by a lake? 

There are two critical altitudes here, if we follow the hypothesis of the researchers.  One is the 220m water level, shown on the map, with a spillway at 220m via the Rhosddu Channel near Crymych.  The other -- at an earlier stage -- is at 115m, with a spillway into the Gwaun Channel at Cilgwyn.  (Other lower spillways are also postulated for stages during which the ice front was out in Cardigan Bay, ponding meltwater against the coastline.)

So we are talking about two hypothetical lake shorelines, one along the north face of Preseli, below Carn Goedog and Carn Alw; and another much lower down, in the currently farmed area, coincidentally running quite close to Rhosyfelin, running through Tycanol Wood, and then running through to Cilgwyn.  Unless these lake levels were VERY short-lived, we might expect to see some evidence of them in the landscape.  We need to look more carefully.  But at the moment I have reservations, since I do not see a clear spillway at Cilgwyn (which is where I live) and instead I see a large moraine, with banks of sand and gravel which seem to me to be kame terraces rather than lake deposits.

With respect to the sequence at Rhosyfelin, I think that the digging team in 2012 has got down to the top of a layer which seems to me to be till.  They refer to it as a "surface" or floor, and clearly think that this was the floor of the quarry on which all those heroic quarrymen worked back in the Neolithic.  I think it is a sedimentological rather than anthropological feature -- although I would have no problem with the lower part of it, near the tip of the rocky outcrop, being used in Iron Age times (or whatever) as a camp site with a hearth. 

So how do we fit the Rhosyfelin sequence together?  There may be lake deposits under the till.  Only time will tell.  The fine-grained material which seems to be above the till might represent a short-lived lacustrine situation, or it might be a "redistribution" layer, made of older lake deposits from upslope which have carried down towards the valley floor.  Then we have that thick sequence of slope deposits, partly periglacial and partly seeming to indicate warmer climatic conditions. 

Where does all the shattered rock debris come in?  I think it might rest on the till -- that would make most sense to me, given that we have had 10,000 years of oscillating cold climate following ice retreat and culminating in the "cold snap" of the Younger Dryas.

We eagerly await the results of the pollen analysis and radiocarbon dates from the excavated sides of the dig, to tell us just how large the time-span is between the current ground surface and the "floor" of the 2012 excavation.


The 2012 Rhosyfelin dig site.  The possible Iron Age surface (with camp site and hearth?) is to the left of the furthest plank.  The pit which is supposed to have held a standing stone is clearly visible.  I suspect that the "surface" seen in the dig is the top of the till layer.  Subsequent slope deposits are seen on the right-hand edge of the dig.

Wednesday, 9 January 2013

Comment from Barry about the Lake

 This is a very interesting note from Barry -- precisely in tune with some thinking I have been doing lately.  I'll do a more detailed post on this soon.  Lake Brynberian is worth thinking about........

I also note Barry's comment about BGS taking a preferred "human transport" position in the bluestone debate, maybe based on advice from Richard Bevins.  I have always found it surprising that Richard and other eminent geologists should accept the non-evidence for human transport and ignore the considerable body of evidence pointing towards glacial transport -- but maybe these big organizations like the National Museum don't like having internal dissent, and maybe all employees have to sign up for the same position, regardless of the merits of the case.  Politics trumps science, yet again......



------------------------------------

As a keen follower of S.T. I noted with interest your 3 Jan 2013 item on the stratigraphy of the Rhosyfelin dig site.  Proposing that the semi-stratified materials found there may coincide with levels 6 & 7 in the stratigraphic columns depicted, implies the presence of a till deposit lying below them, which I recall is something you have raised in earlier issues of the blog.  It would be even more pertinent if such a till was found to overlie a glaciolacustrine deposit, which is suggested by Etienne et al's schema of pro-glacial lakes, including what they term Llyn Nevern. (In line with a more consistent nomenclature this should be Llyn Nyfer).  On seeing the cross-section adjoining the notorious reclining rhyolite 'orthostat' in September, I wondered whether the lowest silty or clayey layer exposed in the trench might represent such material, actual or re-worked, which would tend to upset your view of the stratigraphy, but acknowledge that the upper surface of this layer is perhaps more undisturbed and planar than might expected if lake deposits were superceded by till or fluvioglacial material.

All I know is that I have always thought that the Brynberian depression looks like a lake basin, so have been happy to go along with Etienne's theory, which has more recently been endorsed by the BGS in their mapping of the Fishguard district.  However, the drift identified as flooring the Brynberian basin is shown by them as Irish Sea till - as a pretty extensive sheet - on what observational basis I have no idea.  Its absence at Rhosyfelin is unfortunate, and I'm not sure one can really surmise that it lies even deeper below all that rocky fill; seems unlikely to me.  Another puzzle is that the only glaciolacustrine deposits: "massive and laminated clay" which I can spot on the BGS map appear as small fragmentary occurrences around what would have been the northern upper rim of the lake (as well as a bigger area on Tregroes Moor south of Manorwen).

Another thought.  If one imagines Llyn Nevern being filled from the Llantood intake point, what would have happened at Cilgwyn?  Would it have acted like a bath being filled without a plug to stop outflow; or could there have been a plug of pre-existing glacial fill at the head of the Gwaun Valley, which for a period prevented discharge?  From poking about in your area, I could convince myself of the plug otion.  What do you think about this, I wonder?

For information I am attaching a letter commenting on the survey sent to BGS last October.  On the vexed question of their allying themselves to the bluestone human transport cause, their belated reply to me indicates that "we took advice from a number of sources, including Dr Richard Bevins of the Museum of Wales ..".  So there you have it!
 

Friday, 4 January 2013

Rhosyfelin Dig Site


As a service to mankind in general, and Kostas in particular, I have marked the approximate dig site at Rhosyfelin with a greenish blob on this satellite image.  The river flows along the shady area in the wood from the south (bottom of photo) towards the north, curving gently along the bend in the valley.

The topography of the valley is actually quite complicated here, with many relict features which I attribute to meltwater erosion on a substantial scale, maybe partly during the Devensian glacial episode but probably mostly in earlier glaciations including the Anglian.  The meltwater erosion might have been subglacial.  The zig-zag in the road has messed things up a great deal, but if we eliminate the activities of the road-builders we can see that there is a strange double channel cut on the north side of the infamous Rhosyfelin rocky ridge -- water must have been carried north-eastwards into the main channel.

As far as the slope deposits are concerned, as exposed in the dig, they have come from several different directions, moving NE (up against the rock face in the southernmost part of the dig site) -- then E, and then SE along much of the edge of the dig as shown in the photos. 


In this photo the direction of debris movement has been more or less from behind my highly reputable friends towards the bottom of the photo (NW towards SE).

Thursday, 3 January 2013

Ten thousand missing years

 Assorted climatic graphs showing climate oscillations in the period 20,000 BP to the present -- and a lot else besides.  Note that the time sequences do not all run in the same direction!

I have been intrigued for a long time by the fact that we know very little indeed about the period 20,000 BP to 10,000 BP in the UK.  That's not quite true -- we do know a fair bit now about the period 13,000 to 10,000 BP, since there were quite violent climatic oscillations at that time, marking the REAL end of the Devensian glacial stage.  I'm convinced that the maximum extent of the Devensian ice in South and West Wales was around 20,000 years ago, and that the ice then melted back very quickly and maybe catastrophically so that it had disappeared completely by about 18,000 years ago.  During that 2,000 years or so, there would have been significant accumulations of fluvioglacial sands and gravels in many places, including -- in Pembrokeshire -- the Monington area, the Cardigan area, the Western Cleddau valley around Mathry Road, and the Mullock Bridge area near Dale.  During this period of ice wastage the climate was severe enough to maintain permafrost close to the ground surface, as evidenced by fossil frost wedges and involutions in some deposits.

There are three oscillations which geomorphologists have argued about for a long time, referred to variously as Zone 1 (Older Dryas), Zone 2 (the Allerod Interstadial) and Zone 3 (Younger Dryas).  Zones 1 and 3 were cold episodes, and Zone 2 was a warm interlude.  You can pick up most of these violent oscillations in the graphs above, based on global oxygen isotope sequences including data from Greenland and Antarctica.

The biggest debate in the UK concerns Zone 1 -- did it really affect the environment in the UK, or was its impact damped down somehow during the long period of gradual climatic warming following the retreat of the Devensian ice?  But the evidence does seem consistent in showing that Zone 2 was really rather warm, and that Zone 3 was seriously cold -- so much so that in many parts of the UK the glaciers in the uplands were regenerated for a few centuries and even created fresh moraines and other features.

If we are looking at the sedimentary / stratigraphic record in Pembrokeshire the interesting thing is that there is so little of it, given that we are looking at ten thousand years of severe periglacial conditions, but with a very gradual and erratic global warming going on.  There are few traces of these oscillations either in peat bogs, boreholes or coastal sections.  Why? We can assume that the absence of a peat bog record indicates simply that there were no peat bogs at the time -- and indeed that there were few circumstances in which organic material could rot down and accumulate.  So the landscape must have been bleak and barren, with extensive snow-patches and a very sparse tundra vegetation across which Pleistocene animals roamed -- including woolly mammoths, cave lion, deer, musk oxen, brown bears and wild boar.  There were certainly human beings living in caves as well -- as we know from Paviland, Caldey Island and elsewhere.

 Stratigraphic columns for the superficial deposits on the Pembrokeshire coast.  Key:  1: bedrock platform or raised beach platform. 2: erratic boulders. 3: raised beach gravels. 4: thick lower head (periglacial slope deposits). 5.  Irish Sea till and equivalent local till. 6: fluvio-glacial sands and gravels.  7:  thin upper head and rubble-drift.  8:  loess and modern soil horizon.

But why are there so few periglacial slope deposits in locations where we might expect considerable thicknesses?  There are a number of possible explanations.  It may be that it was very cold and arid for much of the time, with permafrost actually inhibiting slope processes and not permitting thick accumulations beneath cliff faces and on valley sides.  It may also be that (as in the case of Abermawr, above) there was no handy slope to provide frost-shattered debris capable of moving off downslope, since the old cliff-lines had already been buried during 70,000 years or so of periglacial conditions during the buildup to the glacial maximum.  At Abermawr and Poppit the Lower Head, developed during the Early and Middle Devensian, is more than 5 m thick in places, overlain by Irish Sea till from the glacial episode, and then by a mixed material (which I have called "rubble drift") made of fluvioglacial materials mixed up with frost-shattered slope deposits.  This material is rarely more than 2m thick, passing upwards into loess (in some places) and the modern soil horizon formed over the last few thousand years.

Statigraphically, my instinct is that the superficial layers on top of the rock debris at Rhosyfelin are the equivalent of layers 6 and 7 in the diagram above.  Do these layers represent a full 20,000 years of development?  My suspicion is that they do -- but we really need some radiocarbon dates and pollen analyses to point us towards the truth -- if there was ever enough organic material to be subjected to analysis.

The big stone at Rhosyfelin, resting on its bed of broken bedrock fragments and enveloped by pseudo-stratified slope deposits.  The situation at the end of the 2011 dig.

The situation at the end of the 2012 dig.  Note that the slope deposits on the right are over 2m thick in places.  There is no obvious disruption or unconformity here -- again leading to the suspicion that this is a single suite of deposits which has accumulated continuously over a long period of time.

MPP's Stupendous Erection and the Cacophonous Waters


 One of the slides studied by Rob Ixer and Richard Bevins, showing the Jovian fabric of the Rhosyfelin rhyolites.  It's amazing what you can build out of a few chips of rock, if you REALLY try.....

Continuing my look at Ch 17 -- the one about the bluestones -- in MPP's latest book.  Here we come to the Erection of a Stupendous Hypothesis, which in effect rewrites the history of the British Neolithic.

From p 287 onwards, we are seriously into the territory of the hypothetical, if not the fantastical.  On the basis of his conviction that Rhosyfelin is the site of a bluestone quarry, and that Carn Goedog is another, MPP then takes the suggestions by the geologists that Carn Breseb and Carn Ddafad-las might be the sources of some bluestones to build a theory that the preferred bluestone quarrying sites were on the northern flank of Preseli at rock outcrops in the upper part of the Nevern Valley.  (That pretty well coincides with what I have been saying about the most likely locations for glacial entrainment by a glacier flowing over the Preseli ridge, but let that pass......)  Not content with that, the Professor says that he and Colin went fighting through the jungle a bit lower down the valley (probably near Felin y Gigfran) where they identified "two likely quarry sites beside the cacophonous waters".  Indiana Jones, eat your heart out.....  before we know it, we will have a veritable multitude of quarries.

So - forget about the south side of Preseli and Done Bushell's "prehistoric Holy Land", and concentrate on the north.  Castell Mawr is the site which attracts MPP's attention, and before he has even started serious excavations there, he is flagging it up as "the largest henge in Wales, a fitting social and political centre for the people of the bluestones."  He wants a big stone circle there, or maybe two, supplied from all these local quarries  -- reinforcing the idea that this was a local "ritual centre".  He even decides that the stone-movers would have carried their stones along the flattish valley floor until they reached the "gentle incline that leads out of the Nevern valley towards Castell Mawr a mile away."  Next, we move to the idea of a "powerful polity" in and around the valley, a people whose earliest ancestors had brought a megalithic tradition with them into west Wales when they came in from the sea -- and who then celebrated their power and their ancestry by building stone circles supplied from nearby quarries.  Then, later on, for some reason or other, the momentous decision was made to dismantle these circles and to transport them overland over a distance of 180 miles to Stonehenge...........

Wonderful, stirring stuff, based on no evidence whatsoever.

Next, Newport comes into the frame as a possible embarkation or dispatch point for the bluestone cargoes.  But no -- Colin Richards has apparently convinced MPP that sea transport was not challenging enough for our Neolithic ancestors, and that it would have been far more thrilling for thousands of people to have worked together in "great social spectacles" by laboriously hauling the stones overland.  Time would not have mattered to the stone pulling crews -- the important thing was status or renown!  And there would have been good food and few excellent raves along the way.  (And a Mother of all Raves waiting for you at Durrington Walls when you reached your destination....)  So vast numbers of eager volunteers would have pulled the stones along "an established network of routeways leading eastwards from Preseli."  These routeways would have followed the flat-bottomed glacial valleys so as to avoid the thickly-wooded valley sides.  (MPP does not explain why these valley floors are supposed to have been less thickly wooded than the valley slopes, but let that pass.)  So the preferred route seems to be the Nevern Valley travelling eastwards, over the watershed to the Taf valley, then on to Carmarthen, up the Towy Valley into the Brecon Beacons (collecting the Altar Stone on the way), over into the Usk Valley, and onwards towards a Severn crossing point and into England.  It is no coincidence, says MPP, that much of this route is today followed by the A40 road, since that too was based upon the principle of minimising transport costs.

The final bits of the epic saga relate to the nature of the societies which developed in Britain during the neolithic.  MPP thinks that migrating Continental farmers may have come in from the sea into West Wales and North Wales around 6,000 BP, creating two very wealthy and powerful societies based upon farming and animal husbandry.  He claims that the archaeological evidence of tomb types and enclosures supports this idea.  Then for some reason there was a mass migration from Pembrokeshire to Salisbury Plain around 5,500 - 5,000 years BP.  Did they ALL set off on this epic exodus?  MPP thinks that they might well have done.  Later on, after 30 generations or so, Preseli might have been remembered as a place of "ancestral significance" -- so in a mighty gesture of corporate solidarity and sacred significance, the whole of the Neolithic society of the day was moved to fetch all those stones from the Nevern Valley and to carry them to Stonehenge, thereby linking the ancient sacred centre with the new one at Stonehenge.  So Stonehenge became "a monument of unification, bringing together groups with different ancestries in a coalition that encompassed the entirety of Southern Britain, if not the entire island."  Hmmmmm....

MPP thinks that it might not have been all sweetness and light as far as the gigantic corporate stone-shifting enterprise was concerned, since there is evidence of some conflict at the time in the Welsh Borders -- but he won't let that spoil a good story.  Stonehenge remains in his mind as the great centre of political unification, mobilizing an entire society in its creation at the navel of the cosmos, bringing together the people of the sarsens and the people of the bluestones, the people who were pastoralists and the people who were farmers.

So this complex and stirring tale is the gigantic edifice which MPP and his colleagues have created, based largely (but not entirely) upon the supposition that Rhosyfelin is a Neolithic bluestone quarry from which stones were taken to Stonehenge.  Without Rhosyfelin, the whole story falls to pieces.  We might refer to the whole elaborate saga as MPP's Stupendous Erection, supported thus far by a strange consortium of sychophantic archaeologists, well-meaning but ill-informed volunteer field assistants, gullible media people, enthusiastic university press officers and TV producers thinking about the next archaeology block-buster.  Are none of these people capable of asking serious questions and insisting on rational replies?

I wonder if Rob Ixer and Richard Bevins imagined what the consequences might be when they did that rather neat bit of geological provenancing a few years ago? That was, after all, just a refinement or a continuation of a lot of work done by many other geologists over many years.  On very small foundations, mighty edifices are constructed.

More to the point, I wonder how long it will be before MPP's Stupendous Erection begins to collapse under its own weight?  I had better stop this post here, before I get carried away by my own metaphor........

Wednesday, 2 January 2013

Sad tale from Rhosyfelin



A bit of gossip.  I have heard, quite independently, from several of those who were involved in the 2012 digs at Rhosyfelin and Castell Mawr about the prevailing ethos on site.  They had assumed, I suppose, that the digs were designed to discover whether or not these sites have any archaeological significance or not -- with respect to local prehistory at first and then maybe with the wider world.  But no -- the ethos was as follows:  "It is already established that Rhosyfelin is a quarry site, from which stones were taken to Stonehenge during the Neolithic.  Don't bother to question that assumption; any other explanation is a waste of our time.  What we are doing here is assembling evidence which supports the hypothesis.  So go forth, dig deep, and find it!"

So much for science and the scientific method.  I had hoped that with so many expert people involved, and with the use of so much scientific gadgetry, there would at least be some objectivity on the site, and some explorations of alternative hypotheses.......... for the sake of the many volunteers who were giving their time for free.   But it appears that mentions of glacial geomorphology, natural processes and erratics were given short shrift in discussions and explanations.  Two of my informants have said they were quite shocked to find "closed minds" where they had expected academic rigour.

Really, I find all that to be rather sad........

Tuesday, 1 January 2013

New year Quiz


Over the Christmas hols, my wife crept up on me and took this photo of a book I was obviously enjoying.  A gigantic prize awaits the lucky reader who correctly guesses the title of the Book!!  And no -- it is NOT "Alice in Wonderland" or the Holy Bible.

MPP on the matter of quarries

 The big stone at Rhosyfelin at the end of the 2011 dig.  On the left edge of the stone MPP apparently sees stone rails where others see nothing...

Having dealt -- to his own satisfaction -  with glacial matters, in chapter 17 of his book, MPP moves on to consider the distribution of Preselite and other stone tools before moving to take a look at Boles Barrow and its problematical bluestone.  The discussion doesn't take us anywhere very much -- he prefers the view that the bluestone found at Heytesbury House was not the one taken from the barrow, whereas I prefer to take the view that it was.  We won't get much further with that argument........

On p 274 of his book MPP refers to Stanton Drew and Glastonbury, presumably seeking to discredit the idea that the bluestones are glacial erratics.  His arguments are rather convoluted, but he clearly misunderstands what I mean by bluestones, and he has not done his research properly on either ice movement directions or on glacial transport.  The Stanton Drew stones might well be erratics -- they do not seem to be from the immediate neighbourhood -- and I have never claimed that they "should" have come from West Wales.  All I have suggested is that the Stanton Drew stones might have come from the W or NW rather than from any other compass direction.  When the full geology of Stanton Drew is published, we will see if that suggestion is correct. With regard to Somerset and Glastonbury, MPP asks why the builders of Stonehenge would have bothered to collect stones from so far away while they ignored stones much closer to hand, on Salisbury Plain.  I do not know what he is on about here -- I have consistently argued that both the bluestones and the sarsens were probably collected up from around Stonehenge, and with regard to the sarsens at least, David Field and other archaeologists now seem to agree with me.  But then, it seems to be one of MPP's ruling hypotheses that the sarsens came from the "sarsen fields of Marlborough Downs" and that the bluestones came from the bluestone quarries of West Wales....... of which more in a moment.

Chapter 17 continues with a very polite but nonetheless dismissive analysis of the "Tim and Geoff" theory about the desirability of bluestones and their "healing properties" assumed to be associated with the locations of sacred or healing springs in the eastern parts of Preseli.  MPP gives his learned colleagues pretty short shrift -- and it's a bit unclear whether he is rejecting the whole idea of spotted dolerite and dolerite quarries or just the idea of the magical and healing rocks as desirable objects.

 Carn Goedog, beneath whose slopes MPP sees a possible Neolithic quarrymen's settlement.

We then get to the recent work of the MPP tribe in the Preseli foothills.  Here the prevailing hypothesis is that there was "a northern emphasis for the bluestone sources" and a transport route that did not involve the use of the Eastern Cleddau River, Milford Haven or the Bristol Channel.  First, MPP looks as Carn Goedog and Waun Mawn, hypothesising that there was a quarrying community living at and taking stones from the former, and a large stone circle (now dismantled) at the latter.  It appears that the "geophysical survey" failed to find any evidence for this stone circle, so that idea is now abandoned in favour of a hypothetical stone circle at Castell Mawr.  (The team was looking for stone holes at Castell Mawr in 2012, but did not -- as far as I can make out -- find any.....)  MPP refers to "heaps of broken stones" as providing evidence of recent quarrying at Carn Goedog, and also mentions a gully or depression close to the edge of a rock outcrop which contains three long stones "far away from the rock face to be something more than chance rock-falls."  In pondering whether these might be abandoned monoliths in a quarrying context, he also refers to an "excellent natural ramp" running from here down the side of the hill towards the Brynberian Valley.    Our dear professor is very good at seeing what he wants to see, where others like myself see an entirely natural and somewhat degraded rock outcrop of dolerite on the northern flank of Mynydd Preseli........

Of more interest is the idea that there are traces of nine rectangular buildings on the slopes beneath Carn Goedog, opening the possibility of a small Neolithic settlement here, maybe followed by further settlement in the Bronze Age.  I suspect that MPP will seek permission for a dig at this site in future years -- he will want to show that this is the site of a cluster of Neolithic quarrymen's cottages.

 MPP, Richard Bevins and others examining the rock face on which a precise match has been made between a sample from here with some of the rhyolite "debitage" at Stonehenge.

And so to the great adventure at Craig Rhosyfelin.  MPP says that he and his colleagues had marked it out as a probable bluestone quarry before Rob Ixer and Richard Bevins matched up some of the Stonehenge debitage with the "Jovian" fabric and other characteristics of the rhyolites at this rocky outcrop in the Brynberian Valley.  So in 2011 the team did a geophysical survey on the flank of the rock and then started to dig.  In describing the results, MPP waxes lyrical, referring to "an ancient ground surface" protected under "layers of soil washed down from higher up the valley."  Then he says:  "When we started finding hammerstones on that ground surface, we realized that we had not just a prehistoric quarry but a perfectly preserved one -- the Pompeii of prehistoric stone quarries."  Then they found the big stone in the pit, and in describing this discovery MPP becomes positively euphoric:  "Someone had left behind a monolith when the quarrying had ended.  We could hardly believe our luck. This was a smoking gun; the game was up for anyone still trying to argue that the bluestones were not quarried in Preseli during the Neolithic, and then taken to Wiltshire.

Then MPP talks of the "stone rails" and the position of the monolith "just in front of a drop in ground level" -- and uses these "facts" to suggest that the monolith (if it had not been abandoned) was about to be lifted onto a sledge or placed onto rollers........... and then he even talks of "the prehistoric track leading out of the quarry" ---- again demonstrating his extraordinary capacity for seeing what he wants to see and portraying it as established fact.   

Hmmm.... not so fast, my dear fellow.  Have you ever stopped to consider that your smoking gun might just have been used to shoot yourself in the foot?  Do you feel no pain?

To be continued....

Happy New Year!