THE BOOK
Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Thursday, 25 April 2013

The Middle Neolithic Population Upheaval



 Artist's impression of an 8,000 year-old Neolithic settlement.  Nice little huts -- and note the stepped pyramid in the distance!!

 Some interesting info on the BBC web site recently -- apparently there is a move towards the idea that there was a major population surge across western Europe in the Middle Neolithic.  Obviously this has a bearing on our debates about the size of the Mesolithic population in Britain, and the extent to which there was continuity or transition from this largely hunting and gathering society into one that was much more numerous, dynamic and innovative -- leading of course into the period of the Beaker culture.  If these researchers are right, this great wave of settlement came from the east within a relatively short time span -- this might well have a bearing on the MPP theory of people coming from the WEST and carting lots of petrified ancestors with them on their journeys to the site of Stonehenge.

Quote:  "The team found that the genetic signatures of people from the Early Neolithic period were either rare or absent from modern populations. And only about 19% of the Early Neolithic remains from Central Europe belonged to the H haplogroup.  But, from the Middle Neolithic onwards, DNA patterns more closely resembled those of people living in the area today, pointing to a major - and previously unrecognised - population upheaval around 4,000 BC."

23 April 2013 BBC web site:

Making of Europe unlocked by DNA

http://www.bbc.co.uk/news/science-environment-22252099

http://www.bbc.co.uk/news/science-environment-17857641

Farming 'spread by migrant wave'


Stonehenge and the Aurochs Migration Route



Forget about ancient ancestors, healing stones, solar solstice alignments and periglacial stripes.  Stonehenge is where it is because it's on an aurochs migration route.  The OU team that was involved in the latest investigations of the Mesolithic traces near Amesbury has been putting this press release out and about, and now it's all over the internet......  some of the headlines are quite splendid.  Aurochs-burgers, anybody?

 -----------------------------

 This is typical......

New Stonehenge Theory Suggests Ancient Hunting Site

http://www.inquisitr.com/634075/new-stonehenge-theory-suggests-ancient-hunting-site/

Posted: April 24, 2013

A new Stonehenge theory suggests that the ancient ruins may have been a hunting site. The new theory comes after archaeologists discovered a site with evidence dating roughly 5,000 years before the structure was built.

The site, which was occupied continuously for more than 3,000 years, also had evidence of burning, thousands of flint tool fragments, and the bones of wild aurochs.

Together, the data suggests that the area around Stonehenge could have been an auroch migration route. The site could have been for feasting in ancient times, with the migration route drawing different cultures in the region together for the purpose of hunting the extinct giant cows.

Lead researcher David Jacques of the Open University in the United Kingdom, stated, “We may have found the cradle of Stonehenge, the reason why it is here.”

The mystery surrounding the ancient stone structure has existed for decades and new Stonehenge theories are ruthlessly debated among scholars. The stone structure was erected about 5,000 years ago in the plains of Wiltshire, England. But no one can definitively say why the giant stone structure was built.

Some theories for its existence include a burial ground, a place of worship, a sun calendar, a symbol of unity, or even that Stonehenge was created because of a sound illusion. The structure consists of large megaliths, also called sarsens. They are up to 30 feet tall and weigh upwards of 25 tons.

It also includes smaller bluesones, which weigh up to four tons. Researchers believe the massive boulders came from a quarry near Marlborough Downs, which is 20 miles from the mysterious site. The bluestones likely came from Preseli Hills in Wales. The hills are almost 156 miles from it.

The new Stonehenge theory could also help identify the people who first built the massive structure. Aside from a few giant pine posts that could be totem poles, which were raised between 8,500 and 10,000 years ago, there has been little evidence of occupation predating Stonehenge. The new research also suggests that the ancient boulders could have been raised in honor of the sacred hunting ground.

Wednesday, 24 April 2013

The Stonehenge rock types -- time for an update?


These two diagrams, courtesy Rob Ixer, show the composition of rock types from part of the Stonehenge Layer.  The diagrams probably need to be updated.   Note -- they do not represent the WHOLE of the Stonehenge Layer, most of which has never been properly examined.  Who knows what treasures there are, still to be uncovered?

Last year, in October, I published the following information, some of which probably needs to be updated.  There has been such a flood if geological information in recent years that it's difficult to keep up -- the geologists are rolling together some of the rock types which were previously differentaited, and finding petrographic reasons for differentiating others that were previously assumed to be the same,  such is the way with science.  All updates gratefully received -- I will amend the list as necessary and then publish it again.

So here goes:

1.  There are 31 dolerite orthostats, of which 14 have been sampled in 1991 and 2008.  Some are standing stones and some are stumps.  Some are spotted and some are unspotted. (I am a bit mystified as to why the unspotted dolerites do not appear on the diagrams above -- stones 45 and 62 are made of unspotted dolerite.)  Rob has made the point that the differences between the spotted and unspotted dolerites are "a chimera" -- presumably on the basis that there are wide variations within the Preseli tors and other outcrops.  But the latest thinking is that Carn Meini is probably NOT a source....

2.  There are five crystal vitric ash flow tuffs represented in the orthostat collection.  (Stones 40, 48, 46, 38, 52c.  (Four distinct types?)  There is not much debris to match these in the Stonehenge debitage, but similar fragments are found in the great cursus field.  Research is ongoing, but they may come from the Preseli area. In the latest paper by B+I, Carn Alw is ruled out as a source location, but it is suggested that there are four other locations for similar broadly similar rock types -- as yet unidentified.

3.  There are four volcanic ashes -- stumps 32c, 33e, 33f, 41d. (These are not sampled, and so all we can do is speculate...)

4.  There is one calcareous volcanic ash stump -- number 40c (Again not sampled?)

5.  There are 2 micaceous sandstone stumps -- numbered 40g and 42c.  (More info is eagerly awaited on these......)  There are also lumps of Lower Palaeozoic sandstone scatterd about in the debitage -- the largest lump weighing c 8.5 kgs.  From SW Wales? Work is apparently in progress in an attempt to find the source areas for these samples.

6.  There is another calcareous sandstone -- the Altar Stone (stone 80).  sampled more than a hundred years ago, but not since.  Probably from the Senni Beds of Carmarthenshire or Powys? (Not from Milford Haven)  Interestingly, no debitage has been recognized in recent digs from this stone or from anything like it.

7.  In the debitage there are lots of fragments of volcanics with sub-planar cleavage -- matching the Rhosyfelin rocks?  The "rhyolite with fabric" is not all the same -- but most appears to be from the Pont Saeson area.  There are NO matching orthostats.

8.  There are also some basic tuffs in the collection of fragments from the debitage -- two lithologically different types.  From the Fishguard Volcanics?

9.  Other lithics in the stone collections from the debitage -- some stones are adventitious / introduced / modern, but some (eg haematite, greensand, slate, Mesozoic sandstones and gabbros) appear genuine, and need further research.

10.  In the course of the recent geological research, 6,368 rock samples have been examined and classified -- and organized by archaeological context.  the total weight of samples thus far is in excess of 70 kg.  Most fragments are very small, weighing on average about 11 grams.

11.  Almost half of the material in the debitage is sarsen -- I suppose we should not be surprised by that, but it would be good to know how many types of sarsen there are, and where they came from.....

12.  This recent research matches pretty well with what I said in my post dated 3 December 2011:
http://brian-mountainman.blogspot.co.uk/2011/12/bluestone-rock-types.html

I reckoned then that there are about 30 different rock types represented in the "bluestone assemblage" -- and unlike Rob, I give significance to the small bits as well as the orthostats, since I am interested in glacial and other processes and want to know where they came from and how they got here.

======================

Stonehenge Bluestone Types

1.  Unspotted dolerite ---- monoliths  45 and 62.  Carn Ddafad-las?

2,  Spotted dolerite -- densely spotted.  Monolith 42  -- Carnbreseb? 43?

3.  Boles Barrow dolerite -- spotted?  But similar to stones 44 and 45? From Carnmeini / Carngyfrwy area?

4.  Rhyolite  -- stones 38, 40, ignimbrite character.  Ash-flow tuffs (dacitic). Not Carnalw ? May be from different sources?

5.  Rhyolite --  stones 46 and 48, rhyolitic ash-flow tuffs.  Carnalw area?  Same source?

6.  Rhyolite fragment from a different source from the above types

7.  Laminated calcareous ash -- stumps 40c, 33f,  41d

8.  Altered volcanic ash -- stump 32c, 33e?

9.  Rhyolite -- another type -- stump 32e.  Related to Pont Saeson / Rhosyfelin samples?

10.  Micaceous sandstone -- stumps 42c, 40g (Palaeozoic -- South Wales origin?)

11.  Rhyolite -- lava -- stone 46

12.  Rhyolite -- flinty blue -- different lava?  stone 48

13.  Spotted dolerite with whitish spots --stones 33, 65, 68, stump 70a?, stump 71?, 72

14.  Spotted dolerite with few spots -- stone 31, 66?

15.  Spotted dolerite with pinkish spots -- stones 150, 32, 34, 35A, 35B (one stone), 39 (?), 47, 49, 64, 67, 69, 70

16. Spotted dolerite -- moderate spots -- stone 37, 61, 61a?

17.  Unspotted dolerite -- stone 44 -- different from stones 45 and 62

18.  Very fine-grained unspotted dolerite -- stone 62

19.  Silurian sandstone -- Cursus -- fragments

20.  Devonian sandstone -- Altar Stone -- Devonian Senni Beds -- Carmarthenshire or Powys

21.  Sarsen sandstones -- various types -- packing stones and mauls

22.  Jurassic oolitic ragstone -- Chilmark?

23.  Jurassic glauconitic sandstone -- Upper Greensand?

24.  Gritstone unspecified fragments (Maskelyne, Judd)

25.  Quartzite unspecified fragments (Maskelyne, Judd)

26.  Greywacke unspecified fragments (Maskelyne, Judd)

27.  Granidiorite -- Amesbury long barrow 39

28.  Quartz diorite -- ditto

29.  Hornblende diorite -- ditto

30  Flinty rhyolite -- fragments from Pont Saeson (see 9, 11 and 12 above.  same source?)

31.  Rhyolite fragments -- with titanite-albite intergrowths (source unknown) 

Monday, 22 April 2013

Latest statistics

I have just seen the latest info from Blogger on page hits -- the site has now had more than 300,000 page views.  Thanks for all your support, everybody!  The map also shows hits from all over the world, so many more people are using the site than those who contribute to our discussions.  I hope they are picking up useful and entertaining material.  Long may it continue.....  I'll try to keep abreast of developments, but I do have other things going on in life.....

Saturday, 20 April 2013

Flights of Fantasy......



After a pleasant day mending the roof, I got round to watching that programme.  It's called "Stonehenge -- the Missing Link" and it's part of the Flying Archaeologist series featuring Ben Robinson.  It's on BBC iPlayer here:

http://www.bbc.co.uk/iplayer/episode/b01s1ll4/The_Flying_Archaeologist_Stonehenge_The_Missing_Link/

I must say that I was distinctly underwhelmed.  There was some interesting information in the programme, but I found the central thesis -- that valleys and ridges had different ritual significance, and that there was a profound symbolism in Mesolithic and Neolithic times attached to springs and rivers -- singularly unconvincing.  Of course different components of the landscape have different significance economically, but why the need for RITUAL significance too?

What I heard was a good deal of fantastical enthusiasm but no actual evidence. People put settlements near water because they (and the animals which they hunt) need water, not because rivers and springs are sacred or magical.  At least that point did come over in the last part of the programme, relating to Vespasian's camp, where it was explained that people congregated there and used the location for over 2,000 years, probably because that was where animals came down to drink and where hunting was easier and probably more successful.  In exactly the same way, tribal people in Australia and Africa know that animals MUST come to their watering places regularly, making hunting a great deal easier.

I still find it intriguing that there seems to be a whole generation of archaeologists out there who insist on seeing sacredness, ritual significance and  MEANING in the simplest of things where all I see is evidence of people scraping together a living, trying to survive, and seeking to minimise effort.  And, of course they occasionally had a bit of a party, and had fun.


Slate microlith at Vespasian's camp

This is the published info about the slate microlith from Vespasian's camp.  Slate is notoriously soft -- even the good quality slates used for roofing purposes in western Britain.  So I'm not sure that this little flake would have been of any use for anything very much.  I remain to be convinced that there was any human agency in the fashioning of it.

If it is really slate, it could have come from many different places -- including North Pembrokeshire.

It is not beyond the bounds of possibility that it has indeed come from a glacial erratic.

Vespasian’s Camp: Cradle of Stonehenge

April 19, 2013 By
Current Archaeology
http://www.archaeology.co.uk/articles/features/vespasians-camp-cradle-of-stonehenge.htm  

Extract:  "Most intriguingly, we also recovered a small worked slate point shaped like a microlith: the only prehistoric slate tool we know of ever discovered in the UK. By itself this would be interesting, but the fact it was found in a Mesolithic context just over a mile from Stonehenge makes it very significant. Tim Darvill says the slate is a kind of metamudstone, which would make the tool more durable. We are still trying to determine the provenance of the stone. There is no slate in the Stonehenge area – indeed, the nearest source we know of is in north Wales. It could have been fashioned from glacial erratic, though we are not aware of any such slate erratics in the vicinity. The alternative is that this points to a significant movement of people and ideas, pre-dating what went on at Stonehenge by thousands of years. All signs point to this site being regarded as a special place to gather."

Mesolithic settlement at Stonehenge



Haven't seen that TV programme yet, but this is a report reproduced on Stonehenge News about the thesis that the area has been continuously settled since the Mesolithic.  How "special" this is cannot be ascertained -- since an area like Stonehenge is of course intensively investigated.  So sampling bias comes into play.  However, judgment must be reserved until we have watched the programme.....

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

Stonehenge occupied 5,000 years earlier than previously thought

by stonehengenews

Stonehenge may have been occupied five thousand years earlier than previously thought, archaeologists claim.

Excavation of a site just a mile from the stone structure provided what researchers claim is the first firm evidence of continuous occupation from as early as 7,500BC.

Earlier evidence had suggested that humans were present at the site, known as Vespasian's Camp, around 7,500BC but there were no signs anyone had lived there until as late as 2,500BC.

By carbon-dating materials found at the site, the archaeologists identified a semi-permanent settlement which was occupied from 7,500 to 4,700BC, with evidence that people were present during every millennium in between.

The people occupying the site would likely have been responsible for erecting the first monument at Stonehenge, the Mesolithic posts, between the 9th and 7th millennia BC.

Instead of being seen as a site which was abandoned by Mesolithic humans and occupied by Neolithic men thousands of years later, Stonehenge should be recognised as a place where one culture merged with the other, researchers said.

Dr David Jacques of the Open University, who led the study, said he identified the settlement after deciding to search for evidence around a spring on the site, which he reasoned could have attracted animals.

"My thinking was where you find wild animals, you tend to find people," he said. "What we found was the nearest secure watering hole for animals and people, a type of all year round fresh water source. It’s the nearest one to this place [Stonehenge]. I think it’s pivotal.”

Dr Josh Pollard of the Stonehenge Riverside Project added: “The team have found the community who put the first monument up at Stonehenge.

“The significance of David’s work lies in finding substantial evidence of Mesolithic settlement in the Stonehenge landscape [which was] previously largely lacking, apart from the enigmatic posts, and being able to demonstrate that there were repeated visits to this area from the 9th to the 5th millennia BC."

Source: Nick Collins, Science Correspondent - http://www.telegraph.co.uk/

The findings will be broadcast in an episode of The Flying Archaeologist on BBC One on Friday evening.

Tuesday, 16 April 2013

Another piece of meteorite?



In case anybody missed this, here is Pete's photo of a small fragment which he found -- derived from a barrow near the Avebury Sanctuary.  Could this be a fragment of meteorite?  Or is it a bit of clinker from the smelting process?  If this is Bronze Age I suppose the latter is possible......

From Pete:

https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-ash3/559489_10151514988553022_419258203_n.jpg
 

I found this near the Sanctuary at Avebury.
An animal had excavated out a lot of material from the center of a barrow including two types of pottery, a polished bone pin and several strange types of stone. I left them there.
PeteG

Sunday, 14 April 2013

Relief Map of the main "bluestone source area"


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

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

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

Scientific note on the Lake House Meteorite


 A thin section (greatly enlarged) of a sample from the Lake House meteorite. Image:  Salisbury and South Wilts Museum, Salisbury

This is a useful note:
http://www.lpi.usra.edu/meetings/metsoc2011/pdf/5326.pdf
 It appears that the C14 age noted (around 10,000 yrs BP) is not an actual C14 age determination on organic material, but a weathering age determined by one of the cosmogenic dating techniques and calibrated to the radiocarbon age scale.

Where does the date of >30,000 yrs BP come from regarding the date of arrival on Planet Earth?  There has clearly been some oxygen isotope work, as suggested here:  "In fact, oxygen isotope evidence demonstrates that the meteorite had been weathered in a cold climate. It was also covered in fragments of the local chalk country rock. There now seems little doubt that the specimen is a genuine British “find” and may have been transported to the Salisbury area by glaciers, in a similar way to that proposed for the rocks used to construct nearby Stonehenge."

This too:  "Starting in the asteroid belt, it reached Earth at least 10,000 years ago, landing in a frozen, unknown northern wasteland. It was then transported south by a glacier, deposited on Salisbury Plain and probably used to build a tomb by Iron Age man. It was then dug up by a gentleman archaeologist and, for perhaps a century or so, rested near the doorstep of the Lake House mansion."

http://www.open.ac.uk/blogs/meteorite/?p=1540

I suppose this dating study is published somewhere -- still searching.......

======================

THE METEORITE FROM LAKE HOUSE

C.T. Pillinger, J.M. Pillinger, R.C. Greenwood, D. Johnson, A.G. Tindle, A.J.T. Jull and M. Ashcroft.

Contact:   c.t.pillinger@open.ac.uk

Introduction:
In 1991 a very big meteorite (>60kg, dimensions: diameter 50cm x height 40cm) was brought to the Natural History Museum by the then occupier of Lake House, a country mansion in Wiltshire, UK, associated with a large estate of the same name. The circumstances concerning the likely origins of this ‘find’ are dealt with in a companion abstract [1]. Our attention was attracted to it because of its proximity to Danebury Hill (20 km east), where the only British meteorite ‘find’, collected
under controlled circumstances, was located in 1974 [2]. If the large meteorite from Lake House turned out to be paired with the much smaller Danebury find, then the mystery surrounding its origin would be instantly solved.

The sample from Lake House was confirmed as a meteorite by Robert Hutchison [3] and subsequently returned to its owners. Our electron microprobe data from a PTS made from a chip taken at the OU suggest it should be classified as type H5 similar to our assignment for Danebury.

Petrography:
The sample is a heavily weathered (W5) [4], moderately shocked (S4) [5], equilibrated ordinary chondrite (H5). Distinct chondrules are present, but these tend to have poorly defined boundaries. Porphyritic types predominate, but barred olivine and radial pyroxene textured chondrules are also common. Chondrule mesostasis is recrystallised, with grain sizes generally below 50μm.  The sample is cut by a network of veins, up to 2 mm thick, filled with secondary weathering products.

Discussion
In respect of weathering characteristics, the two specimens are entirely different – areas in our Danebury PTS are grade W1 and the whole specimen is no worse than W2, whereas the meteorite from Lake House is W5. The thin section of the latter was made from a near surface chip of the highly corroded sample; the Danebury analyses were performed using a fresh interior portion. Not wishing to have what appeared to be a very delicate specimen disintegrate, we attempted to extract an interior core from the larger meteorite using an experimental drill being developed for robotic space missions. Much to our surprise prolonged drilling was unable to penetrate more than a millimeter into a location where the crust appeared to be absent. In earlier performance tests the drill had no difficulty in obtaining 1.5 cm long cores from basalt and concrete. We conclude that the inte-
rior of the meteorite from Lake House may not be as weathered as it appears from the outside.
Because we had obtained a very precise terrestrial weathering age for Danebury, samples of
the meteorite from Lake House were removed for Accelerator Mass Spectrometry terrestrial residence dating. The first sample studied has a 14C weathering age of 10,600 +/- 1800 years BP. A second run with a better specimen gave 9500 +/- 560 years. The large meteorite is clearly not
related to the small one authenticated from the archeological site a few kilometers away. Given that it is a single stone, the meteorite represents an unusually large ordinary chondrite find.

References:
[1] C.T.Pillinger and J.M.
Pillinger, this vol. [2] C.T.Pillinger et al., this vol. [3] pers. comm. [4] Wlotzka F. 1993
Meteoritics 28:460. [5] Stöffler D. et al. 1991. GCA 55:3845-3867.

Note:  W5 on the weathering scale is defined as: beginning alteration of mafic silicates, mainly along cracks.

Saturday, 13 April 2013

Colin Pillinger's RS lecture





Thanks to Anon for drawing attention to this.  It is very entertaining -- thoroughly recommended.   The whole lecture is there, together with all of the Powerpoint slides.  Enjoy!!

http://royalsociety.tv/rsPlayer.aspx?presentationid=1035

I'll report on the dating techniques and the meaning of the dates when I have more info to hand.


Another MPP book?

Another tome about to appear?  looks like it.....

From Publishers Weekly:
http://www.publishersweekly.com/pw/by-topic/authors/interviews/article/56800-we-will-rock-you-pw-talks-with-mike-parker-pearson.html

We Will Rock You: PW Talks with Mike Parker Pearson

In Stonehenge: A New Understanding, Mike Parker Pearson, the leader of a groundbreaking archaeological study of the Stonehenge World Heritage Site, proposes an exciting new theory of the wonder of the ancient world and its lesser-known neighboring Neolithic sites.

Friday, 12 April 2013

New evidence of glaciation on Salisbury Plain?

Prof Colin Pillinger and the Lake House Meteorite

New evidence of glaciation on Salisbury Plain? Er, no, actually.  But the latest info on the BBC's "The Sky at Night" (and on the BBC web site) refers to the glaciation of Salisbury Plain as if such a thing is perfectly well accepted, which of course it isn't.  Well, not at the time Colin Pillinger wants it, anyway......

The meteorite is apparently 4.5 billion years old, and Prof Pillinger thinks it has been on planet Earth for about 32,000 years.  Presumably on Salisbury Plain, where it was later incorporated into a Bronze Age barrow at some unknown location, possibly near Barbury Castle in Wiltshire.  The reason for that piece of reasoning?  Well, the traces of chalk on the meteorite surface, suggesting that it was embedded in chalk or covered with chalk at the time when it was found and taken away by Rev Edward Duke prior to ending up on the doorstep of lake House.  There is a lot of doubt about the provenance and the story..... but that is for others to discuss.

So where did the glaciation theory come from?  I have written to Colin about this, and await a reply.  But I assume that the thesis is that the meteorite is pretty well preserved, and that it therefore landed on something soft like a snowdrift or a glacier surface (the alternative is presumably that it landed on the ground surface, which would have been hard enough to ensure the shattering of the whole meteorite and the wide dispersal of all the bits.)  The other line of evidence appears to be that there is remarkably little surface weathering of the meteorite -- and this also leads Colin to the assumption that it was covered, buried or protected by some medium or other -- and he has suggested ice.

Much as I would like to go along with this thesis, I'm afraid it doesn't seem to fit with what I know about the Devensian.  Around 30,000 years ago we were in an interstadial -- warmer -- phase prior to the rapid cooling that led to the Devensian Glaciation of around 25,000 - 20,000 BP.  I'm not even sure that there would have been extensive snowfields and permafrost at the time.  Maybe there were cold winters and seasonal snowbanks, but they would certainly have melted away in the summers.

I'm not even sure that a glaciated landscape would have ensured a "soft" landing for the meteorite.  Glacier ice is pretty hard -- often harder than a normal ground surface with a soil layer and bedrock beneath.  Even under a periglacial regime, permafrost is pretty hard too if it is at the ground surface (during the winter) or maybe a metre or two down (during the summer.)

Before going any further with this, I would like to know what the basis for the date of 32,000 BP is, and where the "radiocarbon" date of 10,000 BP came from.  I'm not aware of any publication in which all is explained.......

Wednesday, 10 April 2013

Carn Alw and spherulites


One of the Stonehenge samples examined by Thomas -- note the spherulitic structure on the bottom part of the slide and the transition to a more flaky structure at the top.  An indication  of a sedimentary origin?

 A new sample from Stonehenge, from the 2008 Darvill / Wainwright dig.  Again there is a transition from a spherulitic structure to a flaky structure.  Does this transition absolutely rule out Carn Alw as a source locality?  Not on the evidence thus far presented......

I have been looking again at the new paper called "Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical reappraisal of the petrographical account of H.H. Thomas" by Richard Bevins and Rob Ixer ----- and I have to say I am a bit less convinced than I was the other day.....

The authors tell us that the 4 rhyolite orthostats at Stonehenge (numbered 38, 40, 46 and 48) are ash-flow tuffs  with pumice, lithic and crystal fragments and in one case well-preserved glass shards. In contrast, the Carn Alw and Stonehenge samples described by HH Thomas and now re-investigated are "recrystallized, typically spherulitic rhyolitic lavas which might have originally been part of a thick lava flow or dome."  If the orthostats are classified correctly, they do not match any of the five slides examined in this new paper -- or indeed the slides originally examined by Thomas in his 1923 paper.  We have to take that on trust, because the authors do not reproduce photomicrographs from samples from any of the 4 Stonehenge orthostats -- so we do not know how different they actually are. 

So what we are talking about here is this terrible stuff called the "debitage".  Was Thomas at fault in saying that the two thin sections of rocks found at Stonehenge actually came from Carn Alw?  Richard Bevins and Rob Ixer say he was at fault, and that these two samples (Thomas's Figs 1 and 3) are different from those which they have examined from Carn Alw.  When we look closely, they do not seem to me to be all that different, but the reader does have a few problems in sorting this out, because both HHT and the new authors reproduce bits and pieces of their thin section slides, sometime this way up and other times that way up, sometimes under crossed polars and other times under plane-polarised light, and at different scales.  So real comparisons are notoriously difficult for Joe Public to make.  

We have to take it on trust that the Carn Alw rhyolites are actually very different from anything found at Stonehenge, but on looking through my references and my own field notes, I see that some of the Carn Alw rhyolites are spherulitic and others are not, and that there is great variation  in rock textures from one part of this large craggy outcrop to another.  We see from the paper that 6 samples from Carn Alw have been analysed, but we do not see the photomicrographs reproduced in the paper.   So here is a request to the authors.  Please can we see them? I promise to publish them on this site as an aid to discussion.

A lot of the discussion in the paper relates to the supposed origin of different spherulite structures.  The authors suggest that on Carn Alw the rhyolites are genuine volcanic rocks which have not been redeposited -- whereas other rhyolites may have been deposited in water, as shown by the transitions between spherulite structures and argillacious transitions to flaky or platy structures -- typical of fine clay particles settling out in waterlain sediments.

Quote:  "We consider that the spherules in sample E1993 (Figure 3 of Thomas and Figs 8a and 8b of this paper) and in sample SH08-390, collected during the 2008 Stonehenge excavations (see Fig 9) might be accretionary lapilli rather than spherules generated as a result of recrystallization of rhyolitic volcanic (sic).  Accretionary lapilli develop in volcanic ash clouds as a result of collision of liquid-coated particles with binding of the ash particles as a result of surface tension. The variable supply of ash particles of different sizes results in a concentric growth pattern in the lapilli. Accretionary lapilli falling into the sea will accumulate in the sedimentary sequence, forming an horizon related to the supply of the lapilli. The end of the supply of the lapilli will see a switch back to the accumulation of normal sediment which in the case of an environment offshore would typically be mud-dominated (ie argillitic). However, accretionary lapilli typically range in diameter between 3-4 mm but reaching up to 10 mm, considerably larger than the spherules examined here, although interestingly Lowman and Bloxam (1981) described air fall tuffs containing accretionary lapilli from the Fishguard Volcanic Group west of Ty Canol Wood, some 3 km WNW of Craig Rhos-y-felin." 

So the question is this:  have all rock samples showing "accretionary lapilli" come from unknown areas (like the area to the west of Tycanol Wood) or could some of them have come from Carn Alw?

Another question is this:  how unique are the banded rhyolites and rhyolites with spherules described by Thomas and reexamined in this paper?  The authors assign one of Thomas's slides to Rhosyfelin on the basis of similarities, but how secure is that assignation?  In other words, are there any other outcrops within or outside the mapped outcrops of the Fishguard Volcanic Group where similar banded rhyolites occur?  Or are they using Rhosyfelin as a probable source simply because they know it well -- in other words, could the assignation be down to sampling bias?

And again:  how certain are the authors that they know about all of the variations that occur within the crags of Carn Alw?

And again:  how certain are the authors that there are not other areas where rhyolites with spherules can be found?  Indeed, to their credit they suggest that there might be, and suggest that at least three source areas for the rhyolitic debitage at Stonehenge are still to be found.  That's in addition to the source areas that still have to be found for the four Stonehenge rhyolite orthostats.

There will be many more twists in this story before it is done....... and thank goodness that we have two very experienced geologists on the case.


Sunday, 7 April 2013

New Bevins / Ixer paper on certain rhyolites


Carn Alw, the crag on the north side of Preseli which was thought by Thomas to have been the source for the Stonehenge rhyolites

A new paper by Richard Bevins and Rob Ixer has just been published online -- thanks to Rob for drawing my attention to it.  This one is not about the Stonehenge rhyolite debitage and the link with Craig Rhosyfelin -- but about HH Thomas and his conviction that the source of the Stonehenge rhyolites (or at least some of them) was the crag called Carn Alw, on the north side of the Preseli ridge.  The details of the paper, and the Abstract, are here:

"Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical reappraisal of the petrographical account of H.H. Thomas"
Richard E. Bevins, Rob A. Ixer
Journal of Archaeological Science, In Press, Accepted Manuscript, Apr 2013
doi:10.1016/j.jas.2013.03.017
http://www.sciencedirect.com/science/article/pii/S0305440313001076

Abstract
The source of the Stonehenge bluestones was first determined in the early 1920’s by
H.H. Thomas who was an officer with the Geological Survey of England and Wales. He
determined that the so-called ‘spotted dolerites’ could be petrographically matched to a
small number of outcrops in the Mynydd Preseli district in south-west Wales. The
bluestones, however, comprise a number of additional lithologies, including rhyolite and
‘calcareous ash’, as well as various sandstones. Thomas was convinced that the volcanic
lithologies in the bluestone assemblage were all sourced from a small area at the
eastern end of the Mynydd Preseli, with the rhyolites originating from the prominent
outcrop known as Carn Alw. Recently, the provenancing of these rhyolites to Carn Alw
has been questioned on the evidence of whole-rock geochemistry. This raised concerns
over the original petrographical attribution. Accordingly a re-investigation was
undertaken of the rhyolite petrography by re-examining the original specimens used by
Thomas. Three of the original four thin sections studied by Thomas were re-examined,
along with a newly made thin section from the fourth of Thomas’ samples as the original
thin section could not be located. The new petrographical evidence demonstrates
convincingly that the two pairs of samples from the Preseli and Stonehenge as examined
by Thomas do not match despite his contention and confirms that Carn Alw is not the
source of the Stonehenge rhyolites which Thomas described. This reinforces the
geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is the principal source of rhyolitic debris in the Stonehenge Landscape.



So HH Thomas is under scrutiny here -- which is fine by me, since geologists have, by and large, been very reluctant to put his work under close scrutiny, even though I have done so in THE BLUESTONE ENIGMA and on this blog:

http://brian-mountainman.blogspot.co.uk/2013/03/is-bluestone-myth-based-on-scientific.html
http://brian-mountainman.blogspot.co.uk/2010/08/hh-thomas-geologist-seduced-by-dreams.html
http://brian-mountainman.blogspot.co.uk/2011/03/more-on-hh-thomas.html
http://brian-mountainman.blogspot.co.uk/2010/02/did-hh-thomas-cook-books.html
http://brian-mountainman.blogspot.co.uk/2010/03/two-great-hoaxes-piltdown-skull-and.html

On the contents of the new paper, the authors home in on the four rhyolite samples studied by Thomas and used as the basis for the "Carn Alw connection."  They point out that the matches between the Stonehenge samples and the Carn Alw samples are not at all as convincing as Thomas pretended, and that the problem of accurately matching thin section slides one against another is exacerbated in this case because Thomas did not make it clear whether he was looking at his slides through plain polarised light or through crossed polarised light.  This is a somewhat esoteric matter, and the illustrations used in the paper are very revealing in this respect!

At any rate, this is an interesting study, and we must be grateful to the authors for yet another fascinating piece of detective work.  They are perfectly justified in giving close scrutiny to Thomas's confident assertion that  "…conclusive evidence is furnished by the fact that the remaining four igneous masses of Stonehenge, the rhyolites, are identical in colour, mode of weathering, and all structural and mineralogical details with the rhyolites that occur at Carn Alw."  The authors point out that the four rhyolitic (and dacitic) Stonehenge orthostats are ash-flow tuffs, with pumice, lithic
and crystal fragments and in one case well-preserved glass shards. In contrast, they say that the Carn
Alw samples described by Thomas (and now reexamined) are recrystallized, typically spherulitic rhyolitic lavas which might have originally been part of a thick lava flow or dome.  One of Thomas's thin sections seems to match the foliated rhyolite from Craig Rhosyfelin, but the others are from unknown sources.  However, there is a suggestion that a possible source area might be to the west of Tycanol Wood, where air-fall tuffs were examined by Lowman and Bloxham in 1981.  (If that source were to be confirmed, that would be interesting indeed, since I have speculated many times on Tycanol and Carnedd Meibion Owen as being a possible area for the glacial entrainment of erratics...........)

In conclusion, Bevins and Ixer say:  "not a single rhyolite fragment from the Stonehenge orthostats or debitage from anywhere in the Stonehenge Landscape can be attributed to Carn Alw."  That is a pretty definitive statement, which may well be correct.  However, I would have liked a little more hard information on the actual samples examined from Carn Alw -- it is a big rock outcrop with considerable geological variation within it.  Where were the samples taken?  Have the authors eliminated the possibility that Thomas's samples were not taken from a different part of the outcrop from that sampled recently?

Finally, I am rather concerned by the manner in which Rhosyfelin has been pulled into this paper.  Quote:  "Bevins et al. (2012) presented geochemical evidence to suggest that the majority of the Stonehenge rhyolitic rocks were derived not from Carn Alw but from Craig Rhos-y-felin....."  As mentioned before, I am not at all convinced by that statement -- it is far too sweeping, and the authors should have said, much more cautiously,  "... the majority of the Stonhenge rhyolitic debitage from sampled locations" came from Rhosyfelin.

Quote:  The present study ".......serves to test the proposal by Bevins et al. (2012) and Ixer and Bevins (2012) that Craig Rhos-y-felin rather than Carn Alw is the main source of rhyolitic debris in the Stonehenge Landscape."  Again, that is far too grand an objective, given the very small number of samples looked at here and the very limited knowledge which we still have of "the Stonehenge landscape" -- most of which has of course not been investigated by anybody at any time.

Quote from the abstract and the conclusion:  "This reinforces the geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is the principal source of rhyolitic debris in the Stonehenge Landscape."

With due respect to the authors, what their new paper does is demonstrate that Carn Alw was not the source for the rhyolite orthostats or for the fragments collected by Gowland. As they say, they still have no idea where the source area for these fragments (or for the four rhyolite orthostats at Stonehenge) might be.  The new work does nothing to reinforce the Rhosyfelin connection.  If anything, it weakens it, since the authors are now confirming that other sources exist for at least some of the material at Stonehenge.........

Finally, what are we to make of the work of HH Thomas all those years ago?  Was he a charlatan who was actually involved in scientific fraud in pursuit of his great goal of demonstrating that the bluestones were carried from Preseli to Stonehenge by human agency?  I have long suspected that when he made his famous lecture and wrote his famous paper, the evidence he presented was partial and carefully selected, with "inconvenient" information simply left out.  He was a darling of the archaeological establishment, and he knew perfectly well what an impact his thesis would have.  In  the event his evidence and his conclusions were subjected to hardly any scrutiny -- a point which has not been made often enough.  So was he involved in a scientific fraud on a par with that of the Piltdown Skull?  It's possible.  Bevins and Ixer are far too polite to suggest such a thing in their paper -- but they do suggest that his work was less competent than it might have been, even given that he was working on his samples almost a century ago.  Was he careless?  Disorganized?  Or maybe just so obsessed with his thesis that he was blind to the subtleties of interpretation which Bevins and Ixer have now applied, when looking at exactly the same thin sections?









Stones with Character?


Pentre Ifan cromlech in Pembrokeshire.  The stones were all collected from the immediate vicinity -- there are many others still lying around.  Were the stones just the right shape and size, or did they have "meaning"??

Thanks to Chris for drawing attention to this.  I have to admit to not having read the article, but I am tempted to say -- from a reading of the abstract -- that this looks rather like mystical hogwash in that even unworked stones are given "meaning" in that they  "would have helped preserved the visual resemblance of the stones to the outcrops or boulder fields from which they were derived."  Hmmm.  I can understand that when we are looking at rock art we can bring in some ideas about "meaning" and "significance" since somebody has obviously spent a lot of time making patterns with tools, and one is justified in asking "why?" ..... but to suggest that the recycling of old stones is also significant in that this shows an acceptance of the "materiality of the slabs" and maybe some embodiment of "anthropomorphic symbolism" seems to me to be going too far, by quite some distance.  People re-use old stones because, well, why wouldn't they?

At least Prof Scarre seems to accept that stones were usually locally sourced -- which shows that he accepts a degree of utilitarianism and practicality on the part of the megalith builders.  But then to say "the careful selection of megalithic blocks suggests that they incorporated and materialised memories, powers and associations of place" leaves me wanting some evidence.  Prof Scarre is not alone here -- as we have debated at length on this blog, all three of the learned professors (GW, DT and MPP) assume that the bluestones -- in particular -- were invested with magical or sacred properties, either because they were supposed to have healing properties, or because they incorporated the spirits of the ancestors.  (Of course if you are arguing that they were collected from a very distant place and transported over a vast distance you have a find some justification for that mighty enterprise.)  The utilitarian view -- that stones in megalithic monuments were used simply because they were in the right place at the right time, and because  they were a convenient size and shape -- does not get much space in the learned texts, probably because it is too simple.

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

Stones with character : animism, agency and megalithic monuments.


Scarre, Chris (2009) 'Stones with character : animism, agency and megalithic monuments.', in Materialitas: working stone, carving identity. Oxford: Oxbow Books, pp. 9-18.
Abstract
Recent studies of megalithic monuments have shown how they incorporate blocks, sometimes taken from different locations, which link the monuments to features of their local landscapes. The slabs were often left unworked, or only minimally shaped, which would have helped preserved the visual resemblance of the stones to the outcrops or boulder fields from which they were derived. The careful selection of megalithic blocks suggests that they incorporated and materialised memories, powers and associations of place. The recycling of carved and shaped standing stones in the passage graves of Brittany illustrates another approach to the materiality of the slabs, one which draws upon anthropomorphic symbolism. Some later monuments too have carved motifs, and those motifs may imply they were thought to embody ‘human’ qualities. An ‘animistic’ or ‘anthropomorphic’ reading of these blocks may provide additional insights into the social practices and beliefs which lay behind the construction of megalithic monuments.
Also:
 Scarre, Chris (2009). Megalithic Quarrying: Sourcing, extracting and manipulating the stones. Oxford: Archaeopress.
  • Publication type: Books: edited
  • ISSN/ISBN: 978 1 4073 0405 2

Saturday, 6 April 2013

The Stones of Trefelin: a parable



The stones were always there -- or, to be more precise, since the Devensian glacial episode, about 20,000 years ago.  Our house is built in the middle of an extensive undulating zone of morainic hummocks which I refer to as the Cilgwyn moraine, close to the eastern end of the Gwaun Valley.  The moraine is full of boulders and smaller rocks of all shapes and sizes (mostly derived from within a mile or two), but there are some outcrops of local rhyolite as well, particularly along the banks of the river.

The first house, which I call Medieval Trefelin, was probably built in the 1400's, since it is first recorded in the ancient documents in 1532.  It was very small -- probably more like a hovel, with walls a metre thick made with locally collected boulders and rock rubble.  Those walls are embedded within the southern part of the modern house.   Like many of the labourer's cottages of this time, there were probably no internal walls, and there was probably only one storey.  Why was it built here, on the south-facing valley side close to a ford across the river?  Shelter would have been a prime locational factor, but maybe a few stony fields would have provided a living for a smallholder's family.  Maybe the property was owned by the Barony of Cemais and occupied by a tenant who was a labourer on the estate.  We don't know the date at which the property passed from being leasehold to freehold.

Victorian Trefelin -- the facade built by S Howells in 1879.  He was clearly a man with pretensions and cash available -- shaping dolerite blocks is incredibly difficult and time-consuming, since it is much harder than granite.

The next house we know about is Victorian Trefelin, dated to 1879, when somebody called S Howells built the main house as we see it today.  He was not a member of the gentry, but he must have been a yeoman farmer or a merchant with pretensions, because the new house had 6 or 7 rooms on two storeys.  Also, Howells excavated into the hillside and built one gable end right against the slope.  But the most impressive thing is the use of stone.  It's all local dolerite, probably collected from the morainic debris in the adjacent fields -- but it's all shaped and dressed very beautifully, showing that this was a man who had resources and pretensions of grandeur.


Random stone facing on the wall of Trefelin Fawr,  dated 1979. Dolerite, quartz blocks, and volcanic ash blocks are incorporated, but the foxy brown stone is rhyolite from Sychpant -- attractive in colour, and often with nice rectangular shapes and tidy corners.

The next house is Trefelin Fawr, which is where we come in.  We bought the property in 1976, and immediately started to convert it from a small rectangular house to an L-shaped larger dwelling.  The extension which we built was faced with whatever local stone we could pick up in the garden and fields, but mostly with rhyolite from the Sychpant farm quarry, about 2 miles away.  Why from there?  Because our builder had a nephew who lived there, and because that rhyolite has a wonderful colour when fresh and another wonderful colour when weathered.  Also, it breaks naturally into rectangular blocks.  Also, it cost us nothing -- apart from the heavy labour involved in sorting out the rubble in the quarry, selecting the stones we wanted, and carting them back to Trefelin.  And why did we want to put natural stone facing onto the extension?  Partly for aesthetic reasons.  Partly because we knew this would enhance the value of the property in the longer term.  And partly because, in its infinite wisdom, the Pembs Coast National Park has decided that stone facing is "vernacular" and has always tended to give planning consents for stone-faced extensions and refusals for everything else.  Economics, sociology, culture and politics all come into play.......

Trefelin Fach, built in 1982 and faced with really random stone mostly collected from the garden.  Most of these stones would have been rejected out of hand by any self-respecting builder, but because I did the work myself there was a certain amount of bloody-mindedness involved!  And time was not an issue.  Cost WAS an issue....

The final episode involves Trefelin Fach, the smaller building across the yard, which we built for business purposes as a replacement for a very unattractive concrete block, flat-roofed double garage which was there when we arrived in 1976.  Again, we obtained planning consent for this building on the basis that it was attractively designed and also that it would be stone faced.  But by the time we built this one, we had run out of cash, and I had to do a lot of the work myself, including the slating and the stone facing.  To minimise cost and effort, I decided to use stones collected from the garden and fields almost exclusively, since that would help to clear the land and also give a "folksy" look to the stone walls of the building.  This is truly random stone walling -- very few of the stones have flat faces and rectangular shapes, and the building process was very slow because I had to use a lot of concrete and cement padding and filling, and go upwards at a rate which avoided the collapse of new sections under their own weight.  Visitors to my wife's candle workshop often comment on the building's "hobbit-like" appearance!  So what were the factors here?  Politics and planning issues, yes.  Aesthetics, yes.  Economics and financial constraints, yes.  And a certain amount of bloody-mindedness and determination to "do it my way" -- even if the stone walls in this building ended up looking rather different from the walls of Victorian Trefelin and Trefelin Fawr.

End of parable.  Read into it what you will.  But for me, the history of this house shows that the resources (in this case, stones suitable for building purposes) have always been there -- at least during the period of settlement over the past 500 years or so -- and that the manner in which those resources are used depends upon a highly complex mix of economic, political, social, cultural and even psychological factors, with some resources (eg rhyolite blocks) being used at certain times and other materials (eg dolerite boulders) being used at others.  That was always the way, and always will be......

As far as I know, there have never been any religious or spiritual reasons for any of the decisions made with regard to the building of Trefelin.  But maybe there is something spiritual involved in turning a house into a home?

Thursday, 4 April 2013

Stripes at Airman's Corner



Thanks to Tony for drawing attention to this "evaluation report" undertaken by Wessex Archaeology.  Full of interesting info!  There are "periglacial stripes" here too -- but they do not show up in all of the photos.  In some of them the chalk surface under the soil appears rather flat, or irregular and lumpy but without clear lineations.  Three of the trenches (6,10 and 11) show things which the authors refer to as "post-glacial stripes" or as "periglacial striping" -- although there is no rationale given for the use of these terms -- and no discussion of what the responsible processes might have been......

 For what it's worth, Airman's Corner sits in a slight dip, and it appears that the stripes seen in the photos run more or less directly down the gentle gradients involved -- ie perpendicular to the contours.


LAND AT AIRMAN’S CORNER
Archaeological Evaluation Report
Prepared for
Chris Blandford Associates
on behalf of
English Heritage

By
Wessex Archaeology

http://southplanning.wiltshire.gov.uk/public-planning-application-documents/00339500/00339088_Environmental_Statement.pdf

The fieldwork was undertaken by Steve Thompson assisted by Bob Davis, Dave Reay,
Catrin Matthews, Dave Murdie, Anne Connors, Simon Flaherty, Jonathan Kaines,
Christo Nicolle, Tomasz Wisniewski, Blanka Zohorjanova, Chris Johnson and Ken
Lymer.
This report was compiled by Steve Thompson with specialists report by Lorraine
Mepham (Finds), Pippa Bradley (Flint), Sarah F. Wyles, (Environmental). The
environmental samples were processed by Nicki Mulhall and Marta Perez-Fernandez.
The report illustrations were produced by Kenneth Lymer.
Discussions with Dave Norcott (Wessex Archaeology Geoarchaeologist) and Ben
Urmston (Wessex Archaeology Terrestrial Geophysicist) are incorporated into the
report text. 

Wednesday, 3 April 2013

Williams on the rills and grooves

Above I have reproduced some of the images relevant to the "periglacial stripes" debate. (By the way, these features were not "discovered" by MPP and colleagues -- Martin Trott described them in 1988, and apparently Atkinson knew about them as well. Trott saw no great significance in them, and referred to them and to solution hollows in the chalk surface.  Here is a cutting reproduced from Tim Daw's "Sarsen" blog:

 http://www.sarsen.org/2012/11/stonehenge-periglacial-stripes.html


I think the rills are anastomosing and irregular, with individual channels or gullies extending for maybe just a few metres before joining other rills or splitting -- with the sort of pattern we see on braided rivers on glacial outwash plains.  As I have said before, I do not see any evidence of any rills extending for hundreds, or even tens of metres.  If we look carefully at the second image above, we can see that there is one slight trace of a feature extending for maybe a hundred metres to the left of the Avenue embankments.  To me, that looks man-made.

On the bottom image (which Neil has used on another forum discussion) we can pick up all sorts of details.  Some of the "texture" might be down to old plough marks -- but there are many very subtle features which one can only pick up on if you enlarge the photo.  (Click to enlarge.)  There are even some slightly winding or meandering features, and some things that look like solution hollows.  I would really like to knoe what the features of the solid geology are in this area......

Finally, I came across this chapter by RBG Williams, who has written very extensively about the periglacial landscapes of Britain:

RBG Williams 1986 "Periglacial phenomena in the South Downs."  Ch 17 (pp 161 - 168) in Sieveking and Hart "The Scientific Study of Flint and Chert."

This book is long since out of print, but if you google the chapter, you will find it on Amazon, and it so happens that Ch 17 is on of the chapters you are allowed to read online..........

Here are a couple of short extracts:



Williams talks about frost shattering of chalk surfaces during the Devensian and earlier glacial episodes, sheetwash, solution processes, and the formation of rilles under a number of different scenarios.  

This all accords very nicely with what I have been saying.  As Neil has confirmed, these features are everywhere on the chalk downs.  Maybe the features owe part of their origins to surface and subsurface runoff and solution during the Devensian -- maybe when there was an intermittent permafrost table which provided an "impermeable" layer a metre or two under the ground surface.

It may be that the features are still forming today, with water concentrated in the subsurface rills when there are periods of heavy and continuous rainfall -- so that the infiltration capacity of the chalk is exceeded.













The chalk surface: grooves everywhere

The chalk cliffs at Freshwater Bay, isle of Wight (Ian West).  Although this is a distant shot, you can see how undulating the surface of the "solid" chalk is -- marked by the red line.  The chalk which looks solid is actually pretty smashed up.  The undulations are explained by reference to bedding structures, shear planes and fault lines, and even bands of flint nodules which can allow water and root penetration.  Above the red line is the regolith -- a mixture of coombe rock, clay with flints and soil with some brickearth -- up to 4m thick.


The upper surface of the chalk at Harry Rocks, Dorset (Ian West).  here the junction between bedrock and regolith is even more irregular -- click to enlarge and see the details.  The "low points" are solution pipes or rilles where water flow -- and solution -- are concentrated.


A close-up of coombe soils and the underlying chalk on Salisbury Plain.  Again the junction is highly irregular.  Note the band of flint nodules lower down.

Because it's not easy to see sections through the Upper Chalk and the soil layer on Salisbury Plain, it's best to look for parallels where the cliffs on the coastline do the job for you.  There are hundreds of photos of the chalk cliffs of Southern England available -- and some of the best are in the series of wonderful guides for geology students orepared by Dr Ian West -- for example here:


There is no reason to think that there should be a flat or regular chalk surface on Salisbury Plain but an irregular one close to the coast!

My conclusion is that there are "periglacial stripes", grooves, ridges and rilles everywhere on the Upper Chalk, and that there is nothing remotely unusual about what has been found in the Avenue close to Stonehenge.  Sometimes the grooves will be aligned along bedding planes, faults, shear planes or even bands of flint nodues where water flow and solution are concentrated.  it is also perfectly natural that there should be "regional" or "local" preferred alignments for these features, depending on the nature of the structures and weaknesses revealed in the underlying chalk.  There is nothing in the literature, to the best of my knowledge, to counter this argument.

More to follow.......

Monday, 1 April 2013

The Blog Image Library

 According to Blogger I have used 574 images on this blog since it was started.  If you want to check them out, or hunt for a particular image, you can find them all here:

https://picasaweb.google.com/112312986974547323674/StonehengeAndTheIceAge?authkey=Gv1sRgCOn01qvAu-jVDw

Well, I can get at them, and I suppose others can too.......

Glacial Lake Brynberian


Further to our discussions about the likelihood of there having been a Glacial Lake Brynberian during the Devensian Glaciation, the light was perfect the other day when I passed by.  So here is a pic of that extensive flattish area where the lake might have been located.

By the way, Rhosyfelin is about half a mile off the picture, to the left.  Whatever might have happened during the Devensian, my conclusion from looking at the glaciology of the Anglian Glaciation is that c 450,000 years ago, the ice passed right across this landscape, moving approx from bottom left to top right, covering the mountain with at least 500m of ice.  As the ice moved upslope, conditions were at one stage perfect for the entrainment of blocks and debris from the ground surface and from rocky protruberances (as at Rhosyfelin) and for movement of this accumulated debris along shear planes up into the body of the Irish Sea Glacier.