More on quarrying

....... by which I mean glacial quarrying, the process by which chunks of bedrock are removed and entrained within overriding ice.

As a reminder, here is Prof Dave Evans's definition:

Quarrying (Prof David Evans)
Progress in Phys Geog 2004
 
Quarrying
Quarrying involves two separate processes: (1) the fracturing or crushing of bedrock
beneath the glacier;  and (2) the entrainment of this fractured or crushed rock.  Fracturing
of bedrock may take place where a glacier flowing over bedrock creates pressure
differences in the underlying rock, causing stress fields that may be sufficient to induce
rock fracture (Morland and Boulton, 1975; Morland and Morris, 1977). Fluctuations in
basal water pressure may also help to propagate bedrock fractures beneath a glacier
(Röthlisberger and Iken, 1981; Walder and Hallet, 1985; Iverson,1991a). Brepson(1979)
has successfully simulated the sliding of temperate ice over an obstacle in the
laboratory, and noted that large cavities form in the lee of obstacles, aiding quarrying.
Evacuation of rock fragments along joints in the bed is possible where localized basal
freezing occurs, for example as the result of the heat-pump effect proposed by Robin
(1976). Although Holmes (1944) originally argued that quarrying could occur beneath
both thick and thin ice, and outlined a theory based on pressure-controlled freezing of
meltwater in joints in bedrock, there is now general agreement that quarrying is
favoured beneath thin, fast-flowing ice (Hallet, 1996). Modelling studies indicate that
low effective basal pressures (0.1–1MPa) and high sliding velocities are the dominant
glaciological conditions required for quarrying because these conditions favour
extensive ice/bed separation (subglacial cavity formation)and also concentrate stresses
at points, such as the corners of bedrock ledges, where ice is in contact with the bed
(Iverson, 1991a; Hallet, 1996).

According to this theory, it may be that many of the features I have recently (again!) been looking at in the Stockholm Archipelago are very recent indeed -- formed just before deglaciation, which occurred here around 11,100 years ago.  Dave is picking up on a suggestion from many writers that when ice is very thick, movement on the bed might be negligible, with conditions effectively protecting rather than eroding the landscape beneath.  So for each glacial episode there might be two "erosional episodes" -- one at the outset of the glaciation, and the other at the end.  On the other hand there are other observations that suggest that thin ice may have a polar thermal regime on the bed, which means freezing-on and bedrock protection -- with erosion increased when the ice thickens because there may be a temperate thermal regime on the bed -- meaning high sliding velocities and considerable meltwater lubrication.  The truth is always more complicated than one would like it to be.........

Anyway, one thing that is blindingly obvious in the archipelago is that on the stoss (upglacier side) of all bedrock knolls and other roche moutonnees, abrasion and polishing features predominate, whereas on the lee side of all features plucking leaves rough and even jagged surfaces which show us where bedrock chunks have been dragged away.

Here are some photos of stoss-side slopes on RÖDLÖGA STORSKÄR:

Now here are some from the lee-sides or down-glacier sides of the same bedrock hillocks -- note how jagged surfaces, sharp edges and broken rock predominate.

 The only rounded or sub-rounded lumps of rock in environments like these are the glacial erratics dumped more or less at random during ice wastage.   It would be good to show some of these sites to certain British archaeologists, who would presumably (unless instructed otherwise) assume that all of them are Neolithic quarries littered with monoliths that were left behind by the builders of Stonehenge......

Roches moutonnees in the Stockholm Archipelago

 Two images of moulded and streamlined topography on Pre-Cambrian Basement rocks in the Stockholm Archipelago. Location -- west side of Rödlöga Storskär.  In both photos the ice has moved from right to left.  Note the streamlined ridges and gullies.  In the lower photo we see the stoss side of a large roche moutonnee -- apart from a few plucked faces on miniature roches moutonnees, all the surfaces are smoothed and rounded off by powerful abrasion.  The ice here has been under compression, probably melting on its bed.

I have done many posts on this topic before -- and will probably do more in the future, if fate decrees that I have more visits to the extraordinary and beautiful islands of the outer archipelago.  I have just spent three days out there. and these notes and pictures are the result.

Wherever you look on the washed rock surfaces there are asymmetrical forms.  We would call many of them roches moutonnees, and they occur on many different scales.  Some of them are substantial hill masses, over 20 m high, and others are measurable in centimetres.  They should not be confused with whaleback forms or rock drumlins, which tend to have abrasion / polishing features on all flanks;  in contrast,  roches moutonnees always have gentle up-glacier (stoss side)  slopes that are polished and steep down-glacier (lee side) slopes that are fractured and steep.  Different processes are at play -- and the key to the form is the ability of ice to exert immense pressure and to "quarry" vast chunks of bedrock which are then entrained and carried away.  This is how erratic slabs, boulders and pillars are formed........

In the archipelago the ice moved consistently from north to south, with minor variations dictated by local topography.  There was erosion on a considerable scale during the Devensian glaciation, with the ice of the Scandinavian ice sheet again grinding down old surfaces affected by ice many times before. Parts of the eroded surface are covered by till and fluvioglacial materials, but because the surface was covered by water after being covered by ice, it has been well washed, with fines carried away into deeper water -- leaving behind a classic erosional landscape that can be examined in minute detail.

Herewith some more photos:

 A mini-roche moutonnee, just a few cm high.  The watch shows the direction of ice movement.  Smoothed and abraded face on the right, and plucked (fractured) face on the left, shown up by the lighter colour on the rock.

 This roche moutonnee is a bit bigger.  Here the direction of ice movement is again obvious.  The stoss side is heavily striated, and the fractures on the lee side are quite complex. There have probably been several phases of block removal.

 

 Another small roche moutonnee.  Here it looks as if there has been just one phase of block removal -- the fractured face is much simpler.

 There is a more complex fracture pattern on this lee side, and the shape of the roche moutonnee is quite irregular.  Note the big crescentic gouge on the abraded surface in the background

Striations in the Stockholm Archipelago

Not all rock surfaces are susceptible to the process of scratching or striation.  In this part of the archipelago this grey fine-grained gneiss (?) reveals crossing fractures and a smoothed striated surface.  The pink granite (?) in the background contains large masses of quartz crystals, and reveals no striations at all, breaking up into a broken and rough surface.  In the foreground is a scar left by the removal of a large bedrock block on the down-glacier or lee side of this slight ridge.

Another heavily striated surface on fine-grained gneiss.  The crossing fractures are lines of weakness which are exploited by the quarrying process by which one block after anther is dragged away by the overriding ice.

 Heavily smoothed and striated bedrock, with a marked fracture scar in the bottom left quadrant of the photo.  This scar shows no traces at all of ice action, suggesting that it is very fresh.

Striations in  the Stockholm Archipelago.  Here, in an area of Pre-Cambrian basement rock with relatively low relief, there are abundant traces of glacial erosion on rock surfaces just above present sea level. The ice of the Scandinavian ice sheet retreated from this area around 11,200 years ago, at a time when the land surface was greatly depressed.  The retreating ice edge was probably floating, and the whole landscape was then deeply inundated by the sea —  to emerge bit by bit as isostatic recovery ran its course.  Isostatic recovery is still going on today, at a rate of c 1 cm per year. 

The last erosive ice to affect this landscape was flowing almost exactly north— south, and this is the direction followed by most striations.  However, there are deviations of as much as 30 degrees, since the ice adapted its flow patterns to the details of bedrock morphology.  We can even say that it adapted to micromorphology, with measurable deviations across an area of a few square metres where there are ridges, gullies and knolls.

How old are the striations?  There are occasional crossing striations, but not many.  I suspect that they are quite fresh, dating from the Weichselian / Devensian glacial episode.  I also suspect that during that glaciation, over a timespan of maybe 20,000 years, there were not many deviations in ice movement direction.  The ice always flowed from the north towards the south.  Given the micro-adaptations we can see on the striated rock surfaces, the ice was remarkably fluid, suggesting considerable meltwater lubrication — but this was not a “wasting ice” situation, since the effects of gouging, rock fracturing and block removal are in many cases quite spectacular, suggestive of situations in which the ice was “frozen on” to the bedrock surface. Striations are in themselves indicators of hard basal ice capable of pressing rocks and rock fragments into the bedrock surface over which it was flowing. I suspect rapid alternations between freezing and basal thawing and sliding — a classic situation for optimal glacial erosion and areal scouring. 

Most of the striations in the photos are less than 5 mm deep, but some are up to 2 cm deep.

West Greenland tsunami

From the BBC web site -- one of the houses swept away by the wave

Some images are appearing from West Greenland following the tsunami that struck the coast yesterday.  It looks as if one small village bore the brunt of the wave -- around 30 homes were destroyed, and four people are missing.  Damage in other communities was on a smaller scale.

Tsunamis are extremely rare in Greenland, although plenty of dangerous waves are produced in front of calving glaciers and when large bergs roll over.  This one is thought to have been triggered by an underwater earthquake -- and the water displacement (a prerequisite of all tsunamis) may have happened as a result of an underwater landslide or from a large landslide on a steep fjord side.  In the latter case, there will be an obvious landslide scar.  More info will no doubt be forthcoming -- but this is a very remote area. 

The Lower Palaeozoic sandstones at Stonehenge

 Lump of Lower Palaeozoic sandstone found during the Darvill / Wainwright excavations at Stonehenge in 2008.  This is the biggest sandstone fragment found thus far -- 

it weighs more than 8500 g.

This paper has been floating around for a long time, and is finally published.  Details:

 ========================

"The petrography, geological age and distribution of the Lower Palaeozoic Sandstone debitage from
the Stonehenge Landscape."
by Rob Ixer, Peter Turner, Stewart Molyneux, and Richard Bevins.
Wiltshire Archaeological & Natural History Magazine, vol. 110 (2017), pp. 1–16
=======================

Abstract

The three major groups of debitage found in the Stonehenge Landscape are dolerites, rhyolitic tuffs (almost exclusively from Craig Rhosyfelin, now designated as Rhyolite Group A–C) and ‘volcanics with sub-planar texture’ now designated as Volcanic Group A and Volcanic Group B. The only other significant debitage group, but only accounting for about 5% by number, is an indurated sandstone now called the Lower Palaeozoic Sandstone.
The Lower Palaeozoic Sandstone is a coherent lithological group with a slight metamorphic fabric and is a fine-grained feldspathic sandstone with characteristic dark, mudstone intraclasts. Palynological (acritarch) dating of the sandstone suggests that it is Late Ordovician or younger whilst the petrography suggests that it is older and more deformed than the Devonian (ORS) sandstones exposed in South Wales.
Spatially, as with all the major debitage groups, the Lower Palaeozoic Sandstone is widely and randomly distributed throughout the Stonehenge Landscape; temporally, almost none of the debitage has a secure Neolithic context but some may have later Roman associations. The debitage cannot be matched to any above-ground Stonehenge orthostat but may be from one or two buried and, as yet, unsampled stumps. (These are stumps 40g and 42c)
The lithology is believed to be from an unrecognised Ordovician (or less likely Silurian) source to the north or northeast of the Preseli Hills.
Although there has been confusion within the archaeological literature between the ‘Devonian’ Altar Stone, Lower Old Red Sandstone (Devonian) Cosheston Group sandstone and the Lower Palaeozoic Sandstone, all three are very different lithologies with separate geographical origins.


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

 This is a detailed and useful paper which carries forward the work on the Altar Stone (by Ixer and Turner in 2006) and examines it in the context of other sandstone fragments in the Stonehenge collections.  It is still the case that the Altar Stone is inadequately sampled and hence inadequately understood;  but the balance of probabilities at the moment is that it came from the Senni Beds near Laugharne and Llansteffan rather than from the Cosheston Sandstone of Mill Bay on Milford Haven.  The authors here seek to sort out some of the confusion of earlier researchers who have often assumed that assorted sandstone fragments (and those two stumps 40g and 42c) are related to the Altar Stone and therefore have the same provenance.  Rob Ixer and his colleagues demonstrate that nearly all of the "sandstone debitage" in the Stonehenge area is of Lower Palaeozoic age, not Devonian.  The use of palynology as well as detailed petrography (mostly thin section work) showing a "slight metamorphic fabric" is fascinating and quite convincing.

The authors argue that the samples examined from the Stonehenge collections have come from the northern or north-eastern corner of Pembrokeshire, to the north of the Preseli Hills.  The samples are not all identical, and seem to show two sandstone types.  Quote:  Although this might suggest two separate sandstone sources, the petrography of the two lithic samples (and indeed all of the other debitage samples) suggests that they are part of a single sourced lithology and the apparent age discrepancy is a sampling issue. It is hoped that further sampling of the Lower Palaeozoic sandstone will help to determine this.

I'm not sure what is meant here -- if the samples have come from "a single sourced lithology" they could of course have come from widely separated localities or provenances wherever that lithology outcrops -- and how can the "apparent age discrepancy" be a sampling issue?  If there is an age difference between one sample and another, does that not mean that one sample is older than the other, given that there are always some statistical / confidence issues?  Whatever the truth of the matter, it is clear that the debitage needs to be matched with sandstone stumps 40g and 42c -- if those two stones really have provided the debitage fragments, more progress will have been made.  And if they don't match, things get even more interesting.........

At the end of the article:
Wainwright et al. at the conclusion of the Cleal et al. (1995) monograph stated in their ‘future
directions’ for research at Stonehenge that ‘A detailed petrological description of each stone
and the identification of its source’ was needed (Cleal et al. 1995, 492). This paper completes the
initial petrographical descriptions of the major types of non-dolerite bluestone debitage from the
Stonehenge landscape begun in 2006. Now that there is enough petrography to enable a convincing match between buried Stonehenge orthostats, their surface debitage and their potential geographical sources to be realised, it only needs the buried stumps to be sampled.
I think we would all agree with that.......


Postscript

I'm intrigued by the mention of this unpublished paper, which precedes a lot of the detailed geological work undertake by Ixer and Bevins.  Given that it is now very easy to upload previously unpublished papers as PDF files on ResearchGate, might that not be a good option for Rob to pursue?  Better for it to be read than cited when nobody can see it!

IXER, R.A. 2007. Waiting by the river: Stonehenge and the Severn Estuary. Abstract. Stone artefacts as material and symbolic markers in cultural landscapes. An international perspective. Implement Petrology Group Meeting. York. September 2007

Citing things that are inaccessible is to take the route to notoriety........

Darvill and Wainwright on Neolithic and Bronze Age Pembrokeshire (review, part 2)

Tim Darvill and Geoffrey Wainwright (2016) "Neolithic and Bronze Age Pembrokeshire", Ch 2 in Pembrokeshire County History, Vol 1, pp 55-222

 

Preseli and Stonehenge 2500 - 1600 BC

This section of the chapter runs from p 156 to p 181.  As indicated on the first page, the authors make no attempt at a balanced discussion of pros and cons or at an even-handed assessment of conflicting theories.  This is simply a promotion effort on behalf of the human transport theory.  A pity -- in a chapter such as this we should have seen something more academically robust.

At the outset, Darvill and Wainwright refer to the research surrounding the bluestones and say it is their intention to dwell on “the landscape from which they were taken.”  They couldn’t be much clearer as to their particular belief system!  After a summary of the character of the Preseli uplands in which they flag up the “special” character of the area (for obvious reasons), the authors turn to the nature of the rocks.  I’m not sure what “geomorphic pressures “ are (p 159) but we’ll let that pass.  They then come to their SPACES project, designed to “provide an archaeological context for the sources of the Stonehenge bluestones.”

On the geology of the bluestones, there is an interesting and mostly accurate assessment of the various rock types represented at Stonehenge, including the work of HH Thomas, Olwen Williams-Thorpe and colleagues, and Richard Bevins and Rob Ixer.  The main types of bluestone are referred to following the Bevins / Ixer system of classification -- dolerites in Groups 1, 2 and 3 (spotted and unspotted dolerites from a number of different outcrops in eastern Preseli); six basic types of rhyolite including three that appear to have come from the Rhosyfelin - Pont Saeson area; two basic types of hard volcanic rock from unknown locations; and two types of sandstone also from unknown sources -- one of them (the Altar Stone sandstone) having come possibly from the Senni Beds of the Old Red Sandstone series.  The authors omit to mention the shapes and characteristics of the 43 known bluestones at Stonehenge, and they omit to flag up the fact that bluestone monoliths, slabs and boulders and recovered fragments have clearly come from at least fifteen (and possibly many more) different sources -- all of which militates against the idea that certain stone types were quarried because they were deemed to be “sacred”. 

In their assessment of the work of Parker Pearson et al at Rhosyfelin over a period of five years, the authors note that no standing pillars at Stonehenge have been sourced to this so-called “quarrying” site -- the evidence all comes from debris found during excavations on Salisbury Plain.  Then comes this very strange statement:  “...there appear to be conflicting views on what exactly was found in the excavations that critics suggest might have been opened up in the wrong place (cf John et al 2015a, 2015b).”  D & W appear to have completely misunderstood the point which I and my fellow authors were making -- namely that there is a rather interesting sequence of Ice Age deposits at the site, and that no matter how hard we searched, we could find no evidence at all for Neolithic or Bronze Age quarrying activity.    We never said that Parker Pearson was digging in the wrong place -- we simply said that he was determined from the outset to find a quarry, and claimed to have found it, in spite of the fact that it does not exist.

In referring to the work of the MPP team at Carn Goedog, the authors accept at face value all of the so-called man-made features (whatever happened to academic scrutiny?) while pointing out that the radiocarbon dates are extremely inconvenient......

There is a summary of the SPACES work led by the two authors of the chapter.  Their purpose is clearly to demonstrate from excavations at Carn Meini and Croesmihangel that these “special places” were the focal points of a great deal of activity in the later Neolithic and early Bronze age.  Over ten pages or more they repeat much of what is already in print.  The evidence of quarrying was not convincing when it was first published

https://brian-mountainman.blogspot.se/2014/12/the-carn-meini-bluestone-quarry-oh-no.html

and it is no more convincing today.  They refer several times to the “Carn Menyn Quarry” as having initially produced meta-mudstone for some unknown purpose, and then dolerites presumably destined for Stonehenge, and then finally meta-mudstones again.  The evidence from the excavations suggests occupation over a long time-span, and the authors take great pride in seeking to demonstrate, for the first time, late Mesolithic quarrying activity. However, no hard evidence is presented to suggest actual quarrying activity, and there is nothing anywhere in these pages to suggest that this SPACES research has anything at all to do with Stonehenge.  The radiocarbon dates do not fit, and the authors choose to ignore the work of Bevins and Ixer which suggests that none of the spotted dolerite bluestone monoliths at Stonehenge have come from Carn Menyn / Carn Meini. 

In the discussion on bluestone transport the authors demonstrate that they have little understanding of glaciers or glacial processes.  Kellaway did not “revive” an older idea that the bluestones “were carried on glaciers which spread south before they melted and neatly deposited their bluestone burden on Salisbury Plain.”  Kellaway was much more sophisticated than that. He did after all achieve publication in “Nature” - a journal always known for its strict refereeing standards.  He argued -- with much supporting evidence -- that the Irish Sea Glacier had travelled eastwards towards Somerset, carrying erratics from West Wales within it (not on it) before dumping them, not very neatly.  The glacial theory, as we have pointed out many times, does not “flounder” because of the lack of known evidence of glacial activity on Salisbury Plain or because there is no train of “suitable blocks of bluestone” across south-east Wales.  There are big erratics in many locations in SW England, and they do not have “trails” of similar rock types across SE Wales either.  And if the authors are so keen on “evidence”, why is it that they completely ignore the lack of any evidence for the human transport theory?  No sledges  or boats, no ropes, no abandoned monoliths, no evidence of strong cultural links between Preseli and Salisbury Plain, no evidence of long-distance stone haulage from anywhere else.  They want a cultural aberration, and come hell or high water, they intend to have it........

Then we have this statement:  “.......eminent geologists and glaciologists have dismissed the glacial theory (Bowen 2005; Green 1997; Scourse 1997).”   How many times do I have to point out that none of those three gentlemen is an eminent geologist or glaciologist;  all three of them are geomorphologists like myself, no more and no less liable to making serious errors of interpretation.  Where we place them on the scale of eminence is a matter of opinion.

Thus in a single brief paragraph, Darvill and Wainwright dispose, to their own satisfaction, of the glacial theory and return to the matter in hand.  We are instantly back into the realm of fantasy.  Sewn plank boats were apparently available “around 2000 BC” and could have been used on sea-faring journeys carrying heavy cargoes. The authors omit to tell us that the bluestones were already on Salisbury Plain at least a thousand years earlier than that.  Rafts were used on the Eastern Cleddau for carrying timber for the Pembrokeshire coal mines, and so they could equally well have been used in the Neolithic for carrying bluestones from Preseli. In my book, that’s special pleading.  I wonder whether the authors checked the nature of the Eastern Cleddau between its source and, say, Gelli?  They can take it from me that it’s not a good rafting river today, and was probably no better in the Neolithic.  Then we have the argument about overland transport -- and here the authors invoke the history of the “drover’s roads” and invoke the use of sleds or “slipes” back in the days of our Neolithic heroes.  This is all speculation, unsupported by any evidence.

In seeking to develop the theory that the “Ordovician rocks of the Preseli Hills” were somehow revered in the Neolithic, the authors refer to spotted dolerite medieval inscribed stones and the use of spotted dolerite in chapel building. They seek to demonstrate a continuous history of spotted dolerite use, and seem to think that the “extensive relatively modern quarries in this area” is somehow significant.  This is entirely disingenuous -- they are talking about commercial slate and aggregate quarries, not about dolerite quarrying.  And it is  a statement of the obvious that spotted dolerite boulders and pillars were used in the area in the Neolithic and the Bronze Age, since they littered the landscape.  Many other rock types were represented in the erratic litter, and they were used too.

In their final section (p 179), titled “Why were the bluestones brought to Stonehenge?” the authors again make multiple assumptions that are not underpinned by facts.  They ASSUME that around 80 bluestones were “transported to Salisbury Plain for exclusive use at Stonehenge.”  Pure speculation.  They claim that “the demonstrable antiquity of stone extraction on Carn Menyn ..... tells us something about the ancestral significance and power of the landscape from which the bluestones were taken.”  Pure speculation -- nobody has demonstrated Neolithic quarrying in an area where erratics were freely available for use, scattered across the landscape.  Mynydd Preseli as “the home of the gods”?  Nice thought, but........  Then to MPP and his stone embodiments of the spirits of the ancestors, with bluestones carted about the place and set up as memorials to the dead.  We won’t go there just now, and neither will we delve into the idea of political unification, of which Prof MPP is so fond. Already well covered on this blog.  The holy wells and healing springs of Mynydd Preseli?  There aren’t any.  The Stonehenge temple built in part with bluestones revered for their healing properties?  This idea was entirely fanciful a decade ago, and it is no better supported by fact today.

Overall, this section of the long chapter by Darvill and Wainwright is a profound disappointment, partly because it is so unbalanced.  The authors set out to dwell on the research and ideas surrounding the bluestones and their links with Stonehenge, but dismiss the glacial transport thesis out of hand, in just a few lines, while devoting 99% of the chapter to the promotion of the human transport thesis on the basis of special pleading and the flimsiest of evidence.  In addition, they fail to demonstrate that any of the archaeological features of the “right” age in the Preseli area have anything to do with Stonehenge.  The traces of intermittent human occupation at Rhosyfelin, Carn Meini and Carn Goedog are unexceptional, and have nothing to do with Stonehenge.  They just happen to be places where archaeologists have done some digging.  As far as one can see from the evidence, neither Croesmihangel nor any of the SPACES sites investigated has anything to do with Stonehenge. The Preseli upland springs have nothing to do with it either, and neither does Cana Chapel.  All in all, one has to admire the sheer effrontery of two authors who curtly dismiss the glacial transport thesis for a “lack of any evidence” while going on at great length about the human transport thesis which has, on balance, far less of a factual underpinning.

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

Postscript

Sadly, as we reported in an earlier post, Geoff Wainwright died on 6th March 2017:

https://brian-mountainman.blogspot.se/2017/03/prof-geoff-wainwright.html

Bluestone provenances - updated list

Tim Daw has kindly published this list on his blog:
http://www.sarsen.org/2017/01/table-of-stoneheng-bluestone-provenances.html
based on evidence supplied by Rob Ixer.

As we have said many times before, the geological work has confirmed the large number of bluestone (and fragment) sources -- at least 15, and probably many more.  And there are multiple questions about the list which are still unresolved -- in spite of the great advances made by Bevins and Ixer in recent years.  For example, Carn Goedog MIGHT be a source for some of the bluestone monoliths, but the stones listed could also have come from other parts of a rather extensive outcropping dolerite sill.  There is no certainty that stones 34, 42, 43 and 61 have the same provenance -- they might have come from 4 different locations.  The fragments identified as having come from the Rhosyfelin / Pont Saeson area have NOT been assigned with certainty to "within a few square metres" at Craig Rhosyfelin, as claimed by Rob Ixer.  And we do not know that stumps 32d and 32e have come from there -- that is pure speculation.  The rhyolites E, F, and G and the volcanic rock samples referred to as "volcanics groups A and B" have probably come from the Fishguard Volcanic Group, represented by outcrops over a very wide area.  The Lower Palaeozoic sandstone provenances are unknown, and the assignment of the Altar Stone to the Senni Beds is, according to rumour, not as certain as Ixer and Turner have claimed.

We look forward to further work -- but in the meantime we need to take any claims that "the geology is sorted" with a pinch of salt. There's plenty to keep Bevins, Ixer and others occupied for many years to come -- and I share their frustration that EH will not consent to a properly controlled sampling programme for every one of the 43 bluestones known at Stonehenge.

Deglaciation of Fennoscandia

I've been looking at a fascinating new paper on the deglaciation of Fennoscandia following the LGM (last glacial maximum).  Here is the source of the maps to follow:

Arjen P. Stroeven et al, 2016. Deglaciation of Fennoscandia. Quaternary Science Reviews, Volume 147, 1 September 2016, Pages 91-121
https://doi.org/10.1016/j.quascirev.2015.09.016

The images below are high definition, so it should be possible to enlarge them by clicking.

 Main retreat stages.  The most interesting thing about this map is the highly crenulated ice edge on the European mainland, south of the Baltic.  This is a much more accurate mapping than anything done earlier, and it is what one would expect -- an ice edge running out of steam (if you see what I mean) on undulating terrain, pushing into lowland valleys and held up by hills and ridges.  Also, this map shows for the first time the full extent and scale of eskers within the area being deglaciated.  They are incredibly accurate indicators of ice movement directions at the time when the ice was wasting.

 Map showing the main dating points used by the authors in their reconstructions.  In recent years luminescence dating, improved radiocarbon dating and cosmogenic isotope dating have provided a sound database from which ice edge positions can be calculated -- but interestingly enough, the old Swedish varve chronology (developed by Dr Geer and others) was not very far out.......

Main identified ice retreat stages.  The red lines show the retreating ice edge before the Younger Dryas.  The yellow lines show the retreat after 12,700 cal yrs BP.  Note that around 10,200 years ago the remnant of the ice sheet broke into two in the Scandinavian uplands.  Around many of the coasts of the Gulf of Bothnia the ice edge was afloat as it retreated, because the land surface was deeply depressed isostatically.

Darvill and Wainwright on Neolithic and Bronze Age Pembrokeshire (review, part 1)

 

Tim Darvill and Geoffrey Wainwright (2016) "Neolithic and Bronze Age Pembrokeshire", Ch 2 in Pembrokeshire County History, Vol 1, pp 55-222

This is an extremely long and detailed chapter, carrying the reader through from the time of the earliest farmers to the time of metalwork and the beginnings of hilltop defences around 700 BC.  For the most part it is well organized and easy to follow, as the authors systematically work through all of the key Neolithic and Bronze age phases and describe the main cultural features associated with each of them.  It's rather self-indulgent, since the authors place great stress on their own work, repeating in great detail the contents of assorted published papers -- it could have been more tightly edited and dramatically reduced in length.  That having been said, it is a very useful reference work, with many excellent new maps and diagrams which will no doubt be widely cited.

Interestingly enough, the authors choose to flag up the Stonehenge bluestone connection right from the beginning of the chapter.  This brings me great joy, but looking at it objectively, they would have been wiser to apply a more rigorous and fact-based approach rather than entering into the realms of speculation when that was not strictly necessary.  So on the second page of their text the authors state that the Preseli bluestones at Stonehenge "gave that monument its power and purpose."  Not in my book it didn't.  And on the fourth page we read that HH Thomas in 1923, with his famous paper on the bluestones, "established a robust cultural linkage between south-west Wales and Salisbury Plain."  Oh  no he didn't -- he showed that the bluestones were mostly from Preseli -- no more and no less.  The cultural linkage was and is fanciful, and ironically much of the evidence in this chapter militates against the idea that there were close contacts between west Wales and Stonehenge  at the time that the monument was built.  More of that anon.......

Let's get into some of the detailed points in the chapter. 

On page 62 the authors remind us that the distribution of recorded Neolithic and Bronze Age sites (as shown in the maps based on Dyfed records) "is not necessarily the same as the pattern of prehistoric activity."   I fully agree with that -- I have made those point many times on this blog, while pointing out that we must be very careful about assuming some sort of "special status" for NE Pembrokeshire simply because there are many prehistoric sites that have survived there today, mostly in the uplands.

On p 62, in describing the physical environment of the Neolithic in Pembrokeshire, the authors seem to think that the main valleys were carved by ice, which they patently were not.   They correctly state that there is " a smearing" of glacial drift in the north of the county, but they are not correct in stating that the south of the county is free of these deposits.  They should have looked at the geological maps.  They also omit to mention that one of the most striking of the legacies of the Ice Age is the scatter of many thousands of glacial erratics right across the Pembrokeshire landscape.  These erratics are the basic raw materials for Neolithic and Bronze Age monuments, and the authors appear not to have noticed them.  Maybe they have been too busy looking for quarries,

On page 67 the authors claim that they have found quarries for the exploitation of metamorphosed mudstone at Carn Menyn / Carn Meini.  Their 2014 paper does not support that contention.  Neither does it support the thesis that Carn Meini was one of the area's "persistent places" which was invested with special significance.  It's a little too obvious that this is one of the "persistent theses" of the authors -- but repeating it often does not make it true. It is a statement of the obvious to say that many locations will have traces of many different cultural phases.  Some of them have been dug up by archaeologists, but the vast majority have not.  The dug sites are not necessarily any more important than the undiscovered ones.

There is an interesting section on the early settlement enclosures, at Clegyr Boia, Rhos-y-Clegyrn, Carningli and Banc Du.

The section on megalithic tombs (p 77) is very useful, with a classification by shape and structure: propped stones, simple dolmens and portal dolmens.  There is a passing reference to glacial erratics being used in the case of simple dolmens, but the authors should have stated that almost ALL of the megalithic monuments in the county are made from glacial erratics, and they should have addressed the question of whether the occurrence of these erratics was a prime locational determinant.  Instead, they ponder on siting close to springs, streams and the heads of streams and rivers, and on a preference for views of the sea and of mountains.  This is all fanciful stuff -- one might as well say that the Neolithic tomb builders preferred sites that gave them nice views of the countryside.......

In a useful section on stone axes, the authors refer to the three main Pembrokeshire types, namely Group VIII (silicified tuff), Group XIII (spotted dolerite, sometimes labelled "Preselite"), and Group XXIII (other dolerites, including some that are spotted).  Within these groups of axes, found in many parts of Britain and Ireland, there is considerable variation, and the authors make the sound points that axes or rough-outs were moved about or traded very widely.  This attests to considerable links between Pembrokeshire and other parts of the country -- but there is no evidence of a preferential link with Salisbury Plain.  This is an important point.  The authors also say "The current position regarding the petrological study of stone axes is deeply unsatisfactory"  on the grounds that there is far too much petrological variation within groups and since no quarries have ever been found.  The authors contradict themselves by insisting that there must have been quarries, while also saying that any carn made of a suitable rock was a potential source of axe material.  No quarrying was needed.  They should have added that any convenient and suitable erratic boulder or slab must also have been used, including those located on Salisbury Plain or at any point beween Preseli and Stonehenge. This point has been made forcefully by Olwen Williams-Thorpe and her colleagues, but here it is ignored.  The quarrying obsession is never far away......

On page 112 the authors discuss the decline in the creation of megalithic monuments in the late Neolithic, and they make the point that Pembrokeshire seemingly lacks the spectacular passage graves that occur elsewhere -- for example in Ireland.  Does this mean that Pembrokeshire at this time was more "primitive" or isolated?  This is an issue which they also consider elsewhere in the chapter.

On page 113 Darvill and Wainwright refer to the Carn Menyn Cairn (more later) which they described in print in 2012.  They say the monument occupies  "a significant position on the site of an earlier standing stone and effectively encloses the source of the spring that fed a glaciofluvial channel now known as the Stone River........."   As already discussed on this blog, the idea that there is a "monument " here is really rather dubious; the position is not in the least "significant"; and  the standing stone connection is also dubious.  What on earth do they mean when they say that the site "effectively encloses the source of a spring"?!!  And there is no glaciofluvial channel here either; I have no idea where that statement might have come from.

When they move on to a consideration of the developments after 2500 BC / 4500 BP, the authors argue for a new stress being placed on the value of certain stone types, especially dolerites, that had earlier been used for axe making.  They argue, quite predictably, that dolerite was sufficiently valued for "more than 80 blocks of it to be transported to Stonehenge in Wiltshire".  Experienced academic authors should not make such dubious statements.  They do not know that more than 80 blocks of it were transported; that is pure speculation. In fact, they do not know that ANY chunks of Preseli rock were transported, at any stage in the Neolithic or Bronze Age.  In any case, only some of the bluestones at Stonehenge are made of dolerite.  As we all know, there are many rock types in the bluestone assemblage, including some that could hardly have been of any value in the making of axes. Here we go -- ruling hypothesis again......

In a long discussion of beaker culture, round barrows, cremations, ceremonial sites, stone circles and standing stones we find abundant useful information, although I find myself unconvinced by the assertion that the landscape was "heavily sectored" ( (p 129) with occupation mainly on the coastal plain and a "ceremonial focus" on the upland areas of the north.   I think there is special pleading or circular reasoning going on here, designed to flag up, in the mind of the reader, the idea that Preseli was "special" enough to justify a large-scale bluestone transport enterprise.  Nor am I convinced by the assertion that significance was attached to the removal and re-location of small fragments of stone from Preseli that were considered to be talismans -- leading up to the climax of the story involving the transport of big blocks of bluestone from Preseli to Stonehenge.  The time sequence is not established.  The finding of axes, fragments and big blocks of bluestone on Salisbury Plain is far easier to explain if we assume that the Neolithic and Bronze Age inhabitants of that area simply found them there and used them according to whim and circumstance, sometimes for tool making and sometimes for  making architectural statements. Of course, the idea that the standing or fallen bluestones at Stonehenge were used for tool-making (when the locals got fed up with the monument) has been around for a very long time.

Then comes a very long section (pp 156-181) on Preseli and Stonehenge, which deserves separate consideration.  To be continued.......

How not to move a bluestone monolith

How not to move  a bluestone.  I came across this old photo when sorting out my photos.  From the year 2000, on the occasion of the Millennium Bluestone Expedition which ended (as you will recall) with complete disaster.  On this rather gentle hill, the stone went completely out of control, and veered across the central reservation.  LUCKILY THERE WAS NO LOSS OF LIFE.

But the stone (which was rather a small one) ended up on the bed of Milford Haven almost as soon as it took to the water, and had to be rescued by the Royal Navy.  After being stored behind some sheds in Milford Haven docks for several years, it was taken off to the Welsh National Botanical Garden, where it still sits, rather forlornly, on its sledge.   I helped with the pull on land, and in spite of unlimited manpower, asphalt roads, modern ropes and harnesses, and rolls of low-friction netting, the whole thing was a shambles (and a scandal too, since over £100,000 of public money was expended on the experiment).  I had fun on my day of pulling, as did everybody else, but it proved to all of us involved that the bluestones were probably NOT pulled by human beings from Pembrokeshire to Stonehenge............

Washed moraines on the west coast of Sweden

 

 Washed moraine surfaces on the west side of the island of Rörö, in the archipelago to the north of Göteborg, Sweden.  The expanse of pebbles and boulders seen in the lower photo is still washed by extreme storm waves in the winter, and because of the very exposed position of this coast a vast quantity of flotsam and jetsam is thrown up every year.   Children have had considerable fun here, creating a structure which is part den and part work of art......

It's amazing what you find when you go for a walk.  We arrived in Sweden a couple of days ago for the annual hols, and our good friends Anita and Bengt suggested a trip to the little island of Rörö, which could be reached after a short ferry trip.  There were lots of interesting pebbles there, so they said.  Well, I can never resist looking at interesting pebbles, so we packed up a picnic and off we went.  

What an amazing landscape!  When we reached the western side of the island we found ourselves in a strange wilderness of gravel, pebbles and boulders of all sizes, in places quite difficult to walk over, interspersed with patches of scrubby west coast vegetation, rock outcrops,  and a few pools of water.  This "klappersten" terrain (meaning pebbly stone terrain) is up to 500m west-east, and stretches along the coast for maybe  1.5 km.  In places there are clear ridges of storm beach material, and in some of the inlets there are clear strandlines, as can be seen in the Bing image below.  The ice-moulded terrain of the greater part of the island (quite typical of the west coast archipelago) is shown grey in the image, and the pebbly terrain to the west shows up as white.  The ice moved across this area roughly from NE towards SW.

Most of the strandlines are less than 20m above present sea-level.  I reckon that big storm waves sometimes affect the surface up to a height of c 10m and maybe 50m inland from the shore.

I thought at first that this whole area was made up of a large suite of strandlines or raised beaches, but I quickly noticed that the degree of stone and boulder rounding was not sufficient to justify that interpretation, and the only possible alternative explanation is that this must be a vast expanse of morainic debris laid down during one of the big retreat stages of the Scandinavian Ice Sheet after 20,000 years BP.  Sea level must have been higher than it is today because of isostatic loading, but on a rising coast the morainic expanse was not affected by wave action for very long -- the result being a washing out of finer material and a survival of most of the sub-angular and faceted stone shapes typical of glacial moraine.

So -- this is a classic washed moraine landscape.  It's probably featured in all the Swedish glacial geomorphology text books -- but I have not had time to check!

Just before putting this post together, I checked out the maps of Scandinavian Ice Sheet retreat stages, and sure enough I see that the outer part of the archipelago in this area reveals fragments of the "Halland retreat moraine complex" dated at around 16,800 - 16,200 years BP.  So there we are then-  my observations slot very nicely into the established chronology.

On this map the Halland coastal moraines are shown, on the outermost islands of the western archipelago.

Source:  "Investigating the last deglaciation of the Scandinavian Ice Sheet in southwest Sweden with 10Be exposure dating", Journal of Quaternary Science 27(2):211-220 · February 2012, by Nicolaj Krog Larsen, Henriette Linge, Lena Håkansson, Derek Fabel