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....
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Saturday, 31 December 2016

The New Quay Glacial Episode


 New Quay Bay and Little Quay Bay -- where interesting things may be seen.......



There are remarkably few analyses in the literature of the Pleistocene sedimentary sequence at New Quay, on the Cardigan Bay coast between Cardigan and Aberaeron.  I'm increasingly convinced that it is rather important........

I examined the coastal sections of New Quay Bay, Llanina Point and Little Quay Bay in 1964 and noted, like other field workers, that the ice of the Irish Sea Glacier had impinged upon the Cardigan Bay coast in many locations to the south of Aberystwyth, but that this ice had seldom managed to force its way far inland.  This was probably because of the blocking presence of glacier ice from the Welsh Ice Cap.

Etienne et al, in Ch 7 of "The Glaciations of Wales and Adjacent Areas", refer to Irish Sea tills at New Quay, but I am not sure why they refer to them in the plural.  There is but one Irish Sea till, visible in multiple locations along the coast.  It has all of the "normal" Irish Sea till characteristics -- a stiff calcareous clay matrix derived from sea floor sediments, contained broken marine molluscs, faceted pebbles and cobbles from a wide range of northerly locations, and a decalcified and reddish upper surface where it is in contact with overlying fluvioglacial sediments.  In places there are signs of deformation, and at Llanina (the headland between the two bays) a detached mass of the till is underlain and overlain by fluvioglacial sands and gravels.  There are thus indications of complex ice wastage conditions with detached masses of dead ice and considerable debris flowage and redeposition.  But the Irish Sea Till clearly all belongs to one glacial episode which affected a great length of coastline.

What is more intriguing is the occurrence at New Quay West, about 300m from the Lifeboat Station, of a thick layer of "diamicton" containing mostly locally derived rock fragments and also many foreign faceted cobbles and pebbles.  It is very different from the "lower head" which we see at Poppit West above the raised beach and beneath the Irish Sea till, and different from the stratigraphically equivalent lower head in many Pembrokeshire coastal sites.  The New Quay deposit is up to 15m thick, and it varies in colour from purple to grey and grey-brown.  There is an irregular contact with the underlying lower head.  There are signs of pseudo-stratification, some distinct clay bands and other layers of concentrated stones with fines washed out.   There are lenses of sands and gravels, and here and there slumps and other deformational structures can be seen.

Workers have puzzled over this deposit for many years.  Frank Mitchell thought it was a local till, Sybil Watson thought it was an ancient soliflucted glacial deposit, and Etienne et al (2005) seemed to be rather unsure what to make of it.  In 1965, after making heavy weather of a protracted discussion, I came to the view that it is a true till laid down by Welsh ice which flowed across the coastline, perhaps moving from SE towards NW.  I am now more than ever convinced of the correctness of that interpretation, and think that very little time might have elapsed between this NEW QUAY GLACIAL EPISODE and the Late Devensian incursion by Irish Sea ice which has been well documented.

The chart below is from my doctorate thesis, in which I seek to make some sense of the local stratigraphy.  I suggest a short periglacial or non-glacial phase between the two glacial episodes, but I would like to get back to New Quay so as to re-examine the evidence.


It's interesting that Etienne et al (2005) refer to another exposure at Llannon where a shelly Irish Sea till overlies "subglacial tills of Welsh provenance".   This site is also described in the "Quaternary of Wales" volume edited by Campbell and Bowen, on p 65.  They refer to a Welsh till overlain by an Irish Sea till, and argue for a complex interaction between Welsh Ice and Irish Sea ice along the coast of Cardigan Bay.  They do not quite resolve the issue of the intense periglacial disturbance of the surface of the Welsh till, which seems to have taken place prior to the laying down of the Irish Sea till. Was there a prolonged periglacial episode with deep permafrost between the two glacial phases?

Somebody, one day, will sort all this out.  But today the Cardigan Bay Late Devensian glacial episode looks a bit more complicated than it did yesterday.........

The Cardigan Bay Conundrum


 Fresh surface of Welsh till exposed beneath HWM on the east side of the Nevern estuary, Newport

Washed surface of Welsh till in the same area -- incorporating fluvioglacial pebbles and cobbles.  The lithologies are dominated by grey and grey-brown gritstones and coarse sandstones that appear to have come from the Upper Ordovician or Silurian rocks of Ceredigion

 Close-up of a washed till surface nearby, with a more typical variety of stone shapes, including many faceted cobbles.  Again the grey and grey-brown gritstones and coarse sandstones dominate

Every time I go for a walk on the estuary in Newport,or even on the beach, I come across exposures of till that do not quite tie in with the view that this coast was overwhelmed, on two occasions at least, by Irish Sea Ice that came in from the N or NW.   Actually, the evidence does not contradict the idea that the Irish Sea Glacier was dominant here -- rather, it argues for a situation that was more complicated, with phases during which Welsh ice was dominant.

I have not done any stone counts or other careful forms of monitoring, but my impression is that 70% - 80% by weight of the erratic content in the exposed till consists of grey and grey-brown gritstones and sandstones which are quite unlike the mudstones, shales and fine sandstones that outcrop in the cliffs on either side of Newport Bay.  These rocks belong to the Cwm-yr-Eglwys Formation of Caradog age, about 450 million years old.  They overlie the igneous rocks of the Fishguard Volcanic Group which outcrop a little further inland.  On the outer headlands the outcropping rocks belong to the Dinas Island Formation, of Upper Caradoc age.  The point is that all of these sedimentary rocks are deep-water sediments -- quite unlike the shallow-water facies (including coarse gritstones) of the Lower Silurian rocks which outcrop between Llangranog and Tywyn and which underlie much of the Ceredigion landscape.

The assumption has to be that the till in the Newport Estuary has been deposited by Welsh ice flowing across the coast from the NE, or (more likely) by Welsh ice pushed far out into Cardigan Bay and then diverted southwards by a more powerful Irish Sea ice stream.  A third possibility is that thick glacial deposits far out in Cardigan Bay have been re-worked or picked up and incorporated into the load of the Irish Sea Glacier as it woved across the North Pembrokeshire coast from the north and north-west.

Whatever the truth of the matter, things were certainly more complicated than suggested by the models of the Devensian Welsh Ice Cap created by Henry Patton and colleagues:



Henry and his colleagues of course admit that their models do not take any account at all of the interactions that occurred along the margins of the Welsh Ice Cap between Welsh Ice and Irish Sea Ice, and we hope that that is something covered in the next stage of the modelling work.

But what is now clear is that at some stage in the Devensian the ice flowing westwards from the Welsh Ice Cap was much more extensive than shown in the models, probably by at least 20 km.  That would have sufficed for Silurian gritstones in vast quantities to be carried onto the North Pembrokeshire coast in Irish Sea till or in a special "hybrid facies" of the till.

So the maximum extent of the Welsh Ice Cap must have pre-dated the maximum extent of the Irish Sea glacier.  That makes sense.  We have some rather mysterious exposures of tills that seem to pre-date the Devensian LGM -- at Criccieth,  Glanllynnau,  Llanystumdwy,  Moranedd, and New Quay.  These tills might well date from an early expansion of ice from the Welsh Ice Cap prior to the arrival of the Irish Sea Glacier.  (Big glaciers always take longer to react to climate change and then to build towards their maximum extent than smaller glaciers do.)  On the other hand, was there an interval between these two glacial "events" which coincided with the Middle Devensian interstadial episode conventionally labelled as O18 stage 3?

As I have pointed out in another post, the "weathering layer" (assumed to have been formed during a warmer interlude)  seen above the Criccieth Till in the cliffs at Glanllynnau is overlain by fluvioglacial sands and gravels, with another till (called the Llanystumdwy Till) above that.  The weathered layer is a yellow-brown horizon on top of a blue-grey clay-rich till -- but we need much more evidence than a simple colour change before we can be sure that the top surface of the till has genuinely been affected by surface weathering under a warmer climate.  The upper surface of the Irish Sea till in Pembrokeshire is often decalcified and weathered in this way, simply because ground-water passes through overlying fluvioglacial deposits quite easily and then penetrates into the till beneath.  The colour change has nothing to do with surface exposure and nothing to do with a warmer climatic interlude -- it is simply a matter of chemistry.......

Then we come to the matter of the North Pembrokeshire meltwater channels, which we have always assumed to be associated with Irish Sea Ice coming in from the N and NW.  That doesn't make sense any longer -- but I'll deal with that issue in another post.









Friday, 30 December 2016

The Altar Stone is not an aberration


Over the years, on this blog, I have tried, over and again, to reconstruct the ice streams of the Irish Sea Glacier during the Anglian Glaciation.  I've also published many other reconstructions from glacial geomorphologists who have an interest in the matter.

Look at the two maps above.  I am increasingly convinced that Geoffrey Kellaway, who was lampooned by a rather snooty archaeological establishment during his lifetime, was more sensible than the rest of them put together when it came to understanding erratic transport.  The top map is a reconstruction of the Kellaway map, by Olwen Williams-Thorpe, and on it I have highlighted in pink the crucial streamline.  Note some of the key locations on it -- Mynydd Preseli, the Llansteffan Peninsula, Pencoed, Flatholm, Boles Barrow, and Stonehenge...............

Lionel Jackson and myself, in our "Earth" paper in 2009, argued along similar lines that the bluestone erratics were carried along a contact zone between two converging ice streams -- Welsh ice to the north and Irish Sea ice to the south -- on essentially the same route.

https://www.researchgate.net/publication/270162075_Stonehenge's_mysterious_stones 

So if the Altar Stone erratic did indeed come from Craig Ddu, as I am now suggesting, it is not an aberration at all, but an erratic picked up on exactly the right ice stream route.  I have always argued that the Stonehenge bluestones look like an erratic assemblage (from the physical appearance of the stones) -- and the recent geological work by Rob Ixer, Richard Bevins and others suggests that the stones have come from a rather narrow band of countryside on the north flank of Preseli, with probable source locations including Carn Goedog, Cerrig Marchogion and Craig Rhosyfelin.  Other igneous sources are still being looked for, as are sources for the Palaeozoic sandstones.  I now think there is a very strong chance that these sandstones are Ordovician in age, and that they have come from the coastal zone between Fishguard and Ceibwr.

There's my hypothesis.  Let's see how it works out as the geological work proceeds.  Quarries?  Who needs quarries?

Dylan Thomas and the Altar Stone

This gets more and more interesting.  It's a very long shot, but what the hell........

I followed up the grid reference and the text of the BGS Research report RR/14/02:

One of the reference sections of the Senni Formation is at Llansteffan:
Cliffs at Craig Ddu on the western side of the Llansteffan peninsula, Carmarthenshire [SN 32441015](Owen, 1995). These reach a height of 20 m and have continuous exposure for 400 m of beds near the top of the formation.
Lithology:  Mainly of green and green-grey (locally red-brown and purplish green), very fine to medium- grained, micaceous sandstones, mainly channelised, cross-bedded and parallel-laminated, with green and red-brown siltstone and mudstone interbeds, some calcretes and intraformational conglomerates; the formation is characterised by the presence of vascular fossil plant remains and some soft sediment deformation is also present. 
Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin
BGS Research Report RR/14/02


I suggested in the previous post that a  prominent ridge such as that surmounted by the castle would be well suited for thrusting in the basal layers of glacier ice, and hence for the entrainment of erratics from outcropping beds of sandstone.  But now that I have checked out the location of Craig Ddu, this is the way it looks, thanks to "Where's the Path3":




Bingo!  Craig Ddu is about as perfect as it gets, as a glacial entrainment site.  It's a prominent spur,  50-60m high, with a steep scarp face on its western side.  Climbers refer to it as an excellent "bouldering spot" -- meaning there are big slabs that can be scrambled over.  Its in heavy shadow on the satellite image, and is currently thickly wooded.  But the Senni Beds outcrop along the shoreline for 400m. In fact, an enlarged image shows that the whole of the wooded slope is littered with grey boulders.  Glacier ice coming in from the WNW would have hit this scarp and would have done its best to get rid of it.

 In this enlarged image, we can see that the scrubby woodland on the slope partly masks a great litter of grey-coloured boulders -- presumably all derived from the Senni Beds.


For an extra touch of romance, the Dylan Thomas Boathouse is just across the estuary, about a kilometre away.  The beloved fellow, as he sat there seeking inspiration, would have had the Craig Ddu scarp in his line of sight..........


In the lower image, we see the Boathouse and the wooded scarp of Craig Ddu across 
the estuary, top right.


So here is a message for all  geologists.  Ixer and Turner have examined thin sections for Craig Ddu and presumably from the Kidwelly area as well, although their paper does not list the samples used for comparisons with the material from Stonehenge  (apart from sample 86.31).  It looks as if at least four different lithologies from the site have been examined.  Please give us some more info from the Senni Beds and especially from Craig Ddu -- it would be interesting to see how much variation there is through the Senni Beds sequence, and how much change there is across country as one follows the outcrop northwards and eastwards.

Postscript:

The Ixer and Turner description:

A detailed re-examination of the petrography of the Altar Stone and other non-sarsen sandstones from Stonehenge as a guide to their provenance.
by R.A.Ixer and P.Turner


Thin sections of typical sandstone, siltstone and cornstone lithologies present within outcrops of the Senni Beds from a coastal exposure on the estuary of the River Taf at Craig Ddu were investigated in order to compare them with the Stonehenge samples. The outcrops are at the western end of the Senni Beds outcrop, closest to Milford Haven. The lithologies range from micritic limestones with minor amounts of muscovite and quartz (cornstones) to carbonate-poor, matrix-supported siltstones and carbonate-poor, clast- supported, very fine-grained sandstones and to fine- to medium-grained carbonate- cemented sandstones. Characteristically, all of the lithologies lack any significant signs of a tectonic fabric.
 

Although the overall mineralogy including the heavy minerals suite is similar, there are significant differences between the petrography of the siltstones and sandstones from Craig Ddu. Only the coarser grained sandstones have significant carbonate (>10% by volume) and long muscovite laths lying along bedding planes, whereas only the fine- grained lithologies carry significant interlayered chlorite-muscovite grains.
 

The fine- to medium-grained carbonate-cemented sandstones, as typified by thin section 86.31, resemble the Altar Stone and so are described in detail. Macroscopically the rocks are grey, micaceous, fine- to medium-grained lithic arenites with a calcareous cement (Owen 1995, 223), but are pale grey-green in thin section. 

Detailed petrographical description of Senni Beds sandstone 86.31
 

Microscopically the rock is a fine-grained, well-sorted sandstone with a mean grain size of 0.15mm and a maximum grain size of approximately 0.30mm. The clastic grains are angular to subrounded and the modal group is subangular. The grains are predominantly equant in shape, but a number of prolate grains are also present.
 

The rock has a homogeneous fabric but thin, planar bedding is observed by less than 0.5mm wide, phyllosilicate-rich laminae that are muscovite- and green and brown chlorite-rich. There is very little detrital, fine-grained, interstitial material/clay and the main matrix is a carbonate cement. Quartz and feldspar grains float in this cement and show both pristine and corroded grain boundaries.
Mineralogically, visual inspection shows quartz to be the most abundant detrital mineral accompanied by lesser amounts (in order of decreasing abundance) of fine-grained rock clasts, alkali and plagioclase feldspars and phyllosilicates including white mica and a number of optically different chlorites. Accessory minerals are dominated by equant, opaque mineral grains accompanied by zircon, tourmaline and rutile plus very rare amounts of garnet, sphene and anatase.


Angular to subangular, single quartz grains showing uniform extinction and no authigenic overgrowths are present within the carbonate matrix. Polycrystalline quartz clasts include quartzite and stretched quartz suggesting a metamorphic origin for them. Untwinned or simply twinned, tabular, alkali feldspar, including some perthite, shows slight alteration; this is seen as pale brown/orange, turbid cores. Microcline, if present, is rare. Lath-shaped, multiple twinned, sodic plagioclase is predominately fresh and shows very rare, thin, authigenic overgrowths.
Long muscovite laths lie parallel to the bedding and many show fine-scale kinking about quartz or feldspar grains. Pale to dark green or brown-green chlorite laths show a range of interference colours and are widespread. They too lie parallel to bedding and show fine-scale kinking. Biotite, if present, is very rare. A single interlayered muscovite- chlorite intergrowth was identified.
 

Accessory minerals are concentrated in poorly defined heavy mineral bands. These bands are dominated by opaque iron-titanium oxides and TiO2 minerals including orange-brown rutile. Lesser amounts of rounded, zoned zircon, dark green-brown tourmaline, sphene and subhedral garnet accompany the opaque minerals. A little authigenic anatase associated with carbonate infills void spaces.
 

Rock clasts are the same size as the mineral clasts.  Many are fine-grained siliceous "cherts" that include acid volcanic lavas but minor amounts of quartz-white mica phyllite, graphic granite and foliated feldspathic lava (trachyte) are also present.  The matrix/cement is untwinned carbonate, probably calcite.
 

The sandstone is unmetamorphosed and lacks any tectonic fabric. It lacks significant, authigenic overgrowths on its detrital quartz and feldspar clasts or any interlayered chlorite-muscovite grains. Its rock fragments and heavy mineral suite suggest that its source area comprised eroding acid igneous rocks rather than high-grade metamorphics. Lithologically, it is clear that this rock shares many characteristics with the Altar Stone and since carbonate-cemented sandstones are common in the Senni Beds, one of them may well be the source of the Altar Stone.

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

Comment:  I take this to mean that the match is pretty close, and that the Altar Stone could well have come from part of the Craig Ddu outcrop.













Thursday, 29 December 2016

Could the Altar Stone have come from Ferryside or Llansteffan?


 The lower Old Red Sandstone outcrops of South Wales.  Note that this map shows ALL of the lower or older ORS beds exposed -- the Senni Beds (now called the Senni Formation) are widely exposed in the Brecon Beacons, on the base of the north scarp of the mountains, around Crickhowell, in the Black Mountains, around Ross on Wye, on the flanks of the Wye Valley, and in narrow strips near Newport and Cardiff, and on the eastern edge of the South Wales Coalfield.

This map shows ALL of the ORS outcrops in SW Wales.  The "triple estuary" of the Taf, Tywi and Gwendraeth is in the centre of the map.  The Llansteffan Peninsula is between the rivers Taf and Tywi.

Text revised 30.12.16

I have been looking again at the admirable paper by Rob Ixer and Peter Turner on the provenancing of the Altar Stone (or rather, a sample of rock purported to have come from the Altar Stone).  They present convincing evidence against the old theory that the stone came from the Cosheston Beds of South Pembrokeshire -- and their preferred designation is to the Senni Beds (Lower Old Red Sandstone / Devonian) which outcrop across a large area of South Wales.  The Senni Beds strata comprise fluvially deposited fine- to coarse-grained sandstones with associated calcrete deposits.   The rocks are grey of greenish-grey in colour.

We have speculated on this blog before  about where, precisely, along the Senni Beds outcrop the Altar Stone might have originated.  It's clear from the above map that there are multiple possible locations, and Ixer and Turner are circumspect about suggesting what their preferences might be.  But on the assumption that the most likely locations are withing the Brecon Beacons National Park, Prof MPP and others have suggested that "the A40 land route" was the most likely land route for any Neolithic tribesmen who were in stone-gathering mode.  We have discussed this before, without much sympathy!

 Outcrop of Senni Beds at Craig y Fro quarry, in the Brecon Beacons.  Photo:  UK Fossils

 In our previous discussions of glacial transport routes for the erratics that have ended up at Stonehenge, we have suggested that glaciers carrying erratics from the uplands of Mid and South Wales did indeed flow southwards towards the Bristol Channel coast, and could well have carried Senni Beds erratics southwards into the path of the Irish Sea ice stream that flowed (probably on more than one occasion) towards the coasts of Devon and Somerset.  We now know, from studies in Tremadoc Bay and in North Wales, that the advances and retreats of Welsh Ice and Irish Sea ice were not synchronous, and indeed it is safe to assume that in South Wales (and maybe in Pembrokeshire as well) glaciers flowing from the Welsh ice cap reached their maximum positions prior to the arrival of Irish Sea ice and were then overwhelmed or displaced by its superior power.  I will provide more evidence on that matter in another post.

So erratics carried in the glaciers decanting southwards from the Brecon Beacons and the South Wales Coalfield could well have been turned through 90 degrees and later carried broadly eastwards by the Irish Sea Glacier in the same glacial episode.

Another possibility is that there was no need for a 90 degree turn at all in the course of erratics such as the Altar Stone.  Perusal of the British Regional Geology for South Wales shows that there are narrow outcrops of Senni Beds sandstone on the Carmarthen Bay coast between Ferryside and Kidwelly and in the area to the west of Cardiff, in the Vale of Glamorgan.  Without any need for a "two-phase" transport route, erratics from these outcrops could well have been picked up by the Irish Sea Glacier during the Anglian Glaciation.  The BGS volume refers to Senni Beds at Pengau (Pen-y-gau) Farm near Ferryside, and Ixer and Turner refer to "representative samples of Old Red Sandstone siliciclastics from the Senni Beds near Kidwelly loaned by Dr G. Owen."  Erratics from both of these sites could well have been picked up by overriding Irish Sea Glacier ice:


 Text added:  Llansteffan now also comes into the frame.  On looking at the Carmarthem Memoir (1909) by Strahan et al (including HH Thomas!) it's clear that there are a number of outcrops of the Senni Beds on the coast at Llansteffan, in the vicinity of the Llandyfaelog Fault.  These exposures are also described by Cope (1982) in the volume edited by MG Bassett called "Geological Excursions in Dyfed, SW Wales" (Ch 14, p 268).  No doubt there are many mentions in the specialist literature as well.

http://www.ebooksread.com/authors-eng/geological-survey-of-great-britain/the-geology-of-the-south-wales-coal-field--volume-10-hci/1-the-geology-of-the-south-wales-coal-field--volume-10-hci.shtml



 One of the reference sections of the Senni Formation is at Llansteffan:
Cliffs at Craig Ddu on the western side of the Llansteffan peninsula, Carmarthenshire [SN 32441015](Owen, 1995). These reach a height of 20 m and have continuous exposure for 400 m of beds near the top of the formation.
Lithology:  Mainly of green and green-grey (locally red-brown and purplish green), very fine to medium- grained, micaceous sandstones, mainly channelised, cross-bedded and parallel-laminated, with green and red-brown siltstone and mudstone interbeds, some calcretes and intraformational conglomerates; the formation is characterised by the presence of vascular fossil plant remains and some soft sediment deformation is also present. 
Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin
BGS Research Report RR/14/02


A prominent ridge such as that surmounted by the castle would be well suited for thrusting in the basal layers of glacier ice, and hence for the entrainment of erratics from outcropping beds of sandstone.

So the premise is this:  if you know that the Altar Stone came from the Senni Beds of the ORS series, and you want to know where the precise provenance might be, you might as well start by looking at the outcrops that are located on the known route of the Irish Sea Glacier.

There is one further question, for which I would appreciate an answer.  How much variation is there within the Senni Beds sandstones between the western outcrops in the Tywi Estuary and the eastern outcrops in the Brecon Beacons and in Gwent?  If somebody can advise us on that matter, it may be possible to further narrow down this particular provenancing dilemma.  Myris, over to you.......

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

As an aside:  it would be great fun if the Altar Stone should ever be shown to have come from the Llansteffan - Ferryside - Kidwelly area, since that's where some of my roots are. My parents lived for a while in Ferryside, and that's where my sister was born.  My grandparents lived in Ferryside and Pembrey, just along the coast.  I had relatives (a couple of ancient great aunts dressed in black and smelling of camphor balls) who lived in a grand house in Llansteffan -- we used to visit them occasionally.  And my mother's family farm was called Coedybrain, near Llandyfaelog, close to the famous fault of that name.  I spent many happy holidays there as a small boy.




Wednesday, 28 December 2016

How did the Devensian BIIS collapse?

I never did like the look of this strange purge / surge lobe running from St Georges Channel all the way south into the Celtic Sea approaches, with no lateral spreading.  Does this new paper from Stokes et al question the "binge-purge" model?  It suggests that as deglaciation proceeds, ice streams are switched OFF and are less and less significant as vehicles for ice evacuation.

I'm a bit slow catching up on articles these days, but it was interesting to see this article and its findings.

Ice stream activity scaled to ice sheet volume during Laurentide Ice Sheet deglaciation
C. R. Stokes,  M. Margold,  C. D. Clark & L. Tarasov
Nature 530,322–326(18 February 2016)
doi:10.1038/nature16947
Published online 17 February 2016

ABSTRACT
The contribution of the Greenland and West Antarctic ice sheets to sea level has increased in recent decades, largely owing to the thinning and retreat of outlet glaciers and ice streams1, 2, 3, 4. This dynamic loss is a serious concern, with some modelling studies suggesting that the collapse of a major ice sheet could be imminent5, 6 or potentially underway7 in West Antarctica, but others predicting a more limited response8. A major problem is that observations used to initialize and calibrate models typically span only a few decades, and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves over longer timescales. This represents one of the largest sources of uncertainty when predicting the contributions of ice sheets to sea-level rise8, 9, 10. A key question is whether ice streams might increase and sustain rates of mass loss over centuries or millennia, beyond those expected for a given ocean–climate forcing5, 6, 7, 8, 9, 10. Here we reconstruct the activity of 117 ice streams that operated at various times during deglaciation of the Laurentide Ice Sheet (from about 22,000 to 7,000 years ago) and show that as they activated and deactivated in different locations, their overall number decreased, they occupied a progressively smaller percentage of the ice sheet perimeter and their total discharge decreased. The underlying geology and topography clearly influenced ice stream activity, but—at the ice-sheet scale—their drainage network adjusted and was linked to changes in ice sheet volume. It is unclear whether these findings can be directly translated to modern ice sheets. However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet.

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

The authors admit that there are big differences between the Laurentide Ice Sheet and the modern ice sheets of Antarctica and Greenland -- both of which evacuate large quantities of ice via calving into the sea.  But there are similarities with the British and Irish Ice Sheet or BIIS (which Henry Patton is now calling the Celtic Ice Sheet) which was of course diminishing at the same time as the Laurentide Ice Sheet.  I'm particularly interested in the extent to which this new work calls into question the "binge-purge" model of ice sheet collapse as described by Hubbard et al in their seminal paper of 2009. 

This is from one of my earlier posts, in 2011 (seems a long time ago):

The British - Irish ice sheet almost melted away around 27,000 BP, and then (because there was a relatively prolonged period of increased accumulation without any major interruptions) the ice sheet grew and grew until it reached its maximum volume and extent around 20,000 BP.  There was a phenomenal rate of growth over 7,000 years, followed by a catastrophic collapse.  The authors refer, over the whole lifetime of the ice sheet, to "binge-purge cycles" of gradual accumulations followed by rapid ice evacuations.....................

Quaternary Science Reviews Volume 28, Issues 7-8, April 2009, Pages 758-776
Quaternary Glaciodynamics
Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet

Alun Hubbard, Tom Bradwell, Nicholas Golledge, Adrian Hall, Henry Patton, David Sugden, Rhys Cooper and Martyn Stoker

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

I have always been rather sceptical about the strange "surge lobe" described in a number of papers as occurring at the peak of the Devensian glaciation.  The assumption seems to be that this lobe represented the final "purge" which evacuated so much ice from the centre of the ice sheet that its demise was then inevitable.  Catastrophic ice sheet collapse then followed, with the whole ice sheet disappearing in just a few thousand years.  I have argued that this lobe, which supposedly brought ice from the NE all the way south the the Isles of Scilly, is glaciologically improbable, since according to the model it penetrated for 400 km southwards with hardly any lateral spreading, in a landscape devoid of any lateral constraints.  It was not flowing in a well defined trough, and there were no bounding hill masses or mountain walls such as those seen in Norwegian fjords or even on the flanks of current Antarctic and Greenland ice streams.

So does this new work question the "binge-purge" hypothesis at a fundamental level?  Could it be that the BIIS actually disintegrated through surface melting, with rapid ice stream evacuations switched OFF?  All comments welcome!

See also:

https://brian-mountainman.blogspot.co.uk/2011/02/irish-sea-ice-stream.html






Wednesday, 21 December 2016

Sarsens and the solution hollow dilemma



A few months ago I did a post on the 'honeycomb' characteristics of the solid chalk surface beneath Stonehenge -- or at least, beneath the bits that we know about.   I referred to the pits, hollows, sockets and solution features which are almost universally interpreted by archaeologists as being man-made -- the assumption being that the bluestones -- and maybe the sarsens as well -- have been moved about from one setting to another during the various "stone phases" of the monument.   This has been remarked upon by Anthony Johnson in his book on Stonehenge, and by many others as well.

Anthony Johnson says on page 157:  "... the maze of holes, hollows, abandoned settings and residual stone fragments in the centre of the monument has left the underlying chalk resembling a very badly mauled Swiss cheese which, compounded by extensive disturbance from rabbit burrows, has made the disentangling of the various periods of activity almost impossible, leading Hawley to conclude:  "I frankly confess that I have no explanation to offer in elucidation of this tangle, and I doubt whether anybody will ever be able to explain it satisfactorily."

I wondered whether some of the pits might actually be "extraction pits" from which embedded monoliths had been collected -- and I cited Nick Snashall, who is quite convinced that she can tell exactly which pits are man-made and which ones are not........  

https://brian-mountainman.blogspot.co.uk/2016/08/extraction-pits-solution-hollows-post.html

We have discussed the nature of the  chalk bedrock surface in other posts, including many on the so-called "periglacial stripes" within the Avenue near Stonehenge.  Here is another of my posts:

https://brian-mountainman.blogspot.co.uk/2013/04/the-chalk-surface-grooves-everywhere.html

I would hazard a guess that at least a part of the pitted surface we see in the image above is natural, revealed when the archaeologists carefully scraped away everything rotten or loose.

Some more guidance on the nature of the bedrock / regolith interface is found in the booklet on Fyfield Down written by Mike Clark and published by NCC in 1976.  This is a revealing and perhaps surprising diagram from page 16:

 It shows a section cut alongside one of the recumbent large sarsens found in Clatford Bottom.  It shows that the sarsen is embedded in a layer of brown flinty  loam which extends for about a metre beneath the stone base, with combe rock beneath that, and then with almost a metre of strongly weathered chalk above largely unaltered chalk bedrock.  The brown loam is presumably the periglacial material that has moved downslope, maybe carrying or rafting the sarsen along as it accumulated.  But what interested Clark and his colleague was the evidence that in ten pits examined under and adjacent to recumbent sarsens, there was increased soil acidity as compared with soils where no sarsens were present.  There was also a tendency for solution pipes to occur beneath sarsens of various sizes in the combe rock, penetrating into the weathered chalk beneath.  One of these pipes can be seen in the illustration above.  Conclusion:  the presence of sarsens in one position for many thousands of years leads to enhanced solution in the regolith and rotten chalk beneath.
  
Result: a highly irregular and pitted bedrock surface.

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

Fast forward to another paper, kindly brought to my attention by Alex.  This one is from 1996, by Julian Murton.  Details:

Julian B. Murton:  Near-Surface Brecciation of Chalk, Isle of Thanet, South-East England - a Comparison with Ice-Rich Brecciated Bedrocks in Canada and Spitsbergen
Permafrost and Periglacial Processes,Vol. 7: 153-164 (1996)

ABSTRACT

Chalk on the Isle of Thanet, Kent, is brecciated to depths of a few metres beneath the ground surface. The brecciation commonly comprises (i) an undeformed layer of angular, platy blocks more or less parallel to the surface overlain by (ii) a deformed layer containing small open folds, typically with vertical axial planes. Above the brecciated chalk is an involuted layer (-0.5 to 2.0 m thick) of chalk diamicton and brickearth.
By analogy with brecciated ice-rich limestones, arkoses and shales in areas of continuous permafrost in Arctic Canada and Spitsbergen, it is suggested that brecciation of the Chalk resulted primarily from ice segregation in perennially frozen bedrock, and repeated segrega- tion formed an ice-rich layer just beneath the former permafrost table. Subsequent thaw consolidation of this layer is thought to have formed an involuted layer through soft-sediment deformation.
Three implications arise from this study: (i) near-surface brecciation of the Chalk probably took place during conditions of continuous permafrost; (ii) the growth and thaw of the ice-rich layer in chalk was probably an important element in the geomorphological evolution of the English Chalklands, heaving and brecciating the Chalk during permafrost conditions, and deforming or redepositing the overburden during periods of active layer deepening; and (iii) repeated ice segregation near the top of permafrost may have brecciated other bedrocks in the British Isles.

The paper illustrates how, under conditions of continuous permafrost such as those which prevailed for lengthy periods on Salisbury Plain, brecciation of the bedrock combined with frost-heave processes led to the creation of both involutions and a highly complex bedrock - combe rock interface.
  
Result: a highly irregular and pitted bedrock surface.

-------------------------------- 



Fast forward to another paper brought to our attention by Alex. This one is from a group of Polish researchers, published in 2012:

Radosław Dobrowolski, Andrzej Bieganowski, Przemysław Mroczek and Magdalena Ryżak:
Role of Periglacial Processes in Epikarst Morphogenesis: A Case Study from Chełm Chalk Quarry, Lublin Upland, Eastern Poland
Permafrost and Periglac. Process., 23: 251–266 (2012) 
DOI: 10.1002/ppp.1750

ABSTRACT

Pocket forms several decimetres in diameter, 0.5–1.5 m deep and infilled mainly with glaciogenic sands, silts and clays of Saalian age are commonly developed on the top of the karstified chalk massif of the Lublin Upland, eastern Poland. Analysis of lithofacies, particle-size distribution and micromorphology of three pocket infills in the Chełm chalk quarry reveals a prominent clay cortex between the host chalk and the glaciogenic infill and suggests that periglacial processes have played a considerable role in the formation of the pockets and in the redistribution of their primary glaciogenic infill. A conceptual model for epikarst morphogenesis for the chalk karst of Lublin Upland is proposed, involving three stages. Stage I: In the absence of permafrost, precipitation water infiltrates unconsolidated glaciogenic deposits. Stage II: Periglacial transformation with underlying permafrost. Primary cryoturbation structures became protokarst forms, and then epikarst forms. Stage III: Degradation of the permafrost, with increased carbonate dissolution and development of a clay karst cortex.

This is a highly technical paper, and a convincing one, arguing that karstification and the creation of deep "pockets" can occur in chalk landscapes under a periglacial / permafrost regime, with physical and chemical (solutional) processes operating together.  We are talking about pockets here, and not solutional rills or runnels.

Result: a highly irregular and pitted bedrock surface.

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

 I suspect that if you were to let loose groups of archaeologists with brushes, pans, buckets and trowels in any of the areas described in these three research publications, they would eventually reveal a chalk surface looking very much like the one shown from Stonehenge, at the top of this post.

So Newgrange Neolithic "solstice sunbeam" engineering is a con?



Well, this is interesting.  Apparently the most striking feature of Newgrange, namely the winter solstice "sunbeam slot" that allows the light of the rising sun to penetrate to the back of the chamber, is not a Neolithic feature after all, buy a modern con.  Is nothing sacred?

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

Newgrange sun trap may be only 50 years old, says archaeologist 

Evidence solstice monument was Iron Age ‘Hiberno-Roman cult site’ was ‘underplayed’


Lorna Siggins
http://www.irishtimes.com/news/ireland/irish-news/newgrange-sun-trap-may-be-only-50-years-old-says-archaeologist-1.2913483
   
Trapping the winter solstice sun at Newgrange in Co Meath is not a 5,000-year-old phenomenon, but a 50-year-old “construct”, according to a former State archaeologist.

Our Stone Age ancestors were not as clever as we thought, and the significance of Newgrange as a “Hiberno-Roman” cult site in the late Iron Age has been deliberately  underplayed, Michael Gibbons, co-author of a paper on the subject, argues.

Newgrange’s alignment, which captures the rising sun during the winter solstice period around December 21st, has made it one of the world’s best known megalithic tombs.

If skies are clear during sunrises from December 19th to 23rd, a narrow beam of light penetrates its “roof-box”, reaches the chamber floor and extends gradually to illuminate the entire chamber over 1 7-minute period – marking new life at the turn of the year.

However, in an article published in archaeological journal Emania, Mr Gibbons and his nephew Myles take issue with the excavation and reconstruction work carried out on the passage tomb half a century ago by the late Prof Michael O’Kelly, including the famous “roof box” for trapping the sunlight – said to be 5,000 years old.

Mr Gibbons, an independent archaeologist based in Connemara, is a former co-director of the Office of Public Works (OPW) national sites and monuments record office.

Roman artefacts

Discovery of “high value” Roman artefacts inside and outside the tomb, along with remains of dogs and horses, point to an Iron Age burial site dedicated to an “Irish elite” with links to Roman Britain, the Gibbons paper argues.

It takes issue with Prof O’Kelly’s contention that the tomb was largely unaltered from the Neolithic period, and says that Iron Age activities may have included construction of an enclosure or “barrow” on the summit and alterations to the mound’s profile.

Mr Gibbons says the “roof box” which was central to capturing the winter light has “not a shred of authenticity”, and was “fabricated” during reconstruction in the 1960s.

He says that it is “no wonder” that Newgrange was included in a list of the “world’s worst archaeological reconstructions” by Durham university archaeologist Prof Chris Scarre and colleagues in 2006.

Newgrange has been the subject of much archaeological debate since Prof Michael O’Kelly made the first observation of the mid-winter “solstice phenomenon” at Newgrange in 1967.

Questions have been raised about the quartz wall surrounding the passage tomb, which was erected between 1967 and 1974 on the basis of Prof O’Kelly’s interpretation.

Dogs and horses

The paper by the Gibbons brothers analyses the various challenges to Prof O’Kelly’s thesis, and says Newgrange’s real significance as an Iron Age “Hiberno-Roman cult site” is supported by discoveries at the Boyne valley location which he “underplayed”.

Valuable Roman material includes coins, two gold torcs, a brooch and gold ring, pendants and a bracelet, and there was also evidence of dogs and horses, and stags or elks were uncovered from the late 17th century on.

Asked to comment, Dr Richard Hensey, author of the recently published First Light: the Origins of Newgrange, said he would agree that there was significant Iron Age activity, and said Prof O’Kelly placed “less stress” on this period.

However, Dr Hensey said it was “not correct” to state that the roof-box was a “construct”, and a series of studies indicated it was 5,000 years old.

The box’s “elevation” during reconstruction work, supervised by Prof O’Kelly, was due to the fact that orthostats or upright stones in the tomb’s outer passage required straightening, he said.

“Prof O’Kelly worked at a different time, when the criteria and guidelines we have for reconstruction did not exist,”Dr Hensey said.

“Bord Fáilte Éireann, as the tourist board was known then, gave the initial impetus for the project to attract visitors, and it has been enormously successful,” he said.

Dr Hensey will be attending the solstice at Newgrange on Wednesday on behalf of the OPW.

Tuesday, 20 December 2016

Linear concentrations of sarsens

 We have talked about this before, particularly with respect to Fyfield Down.  We also had a bit of fun speculating on the situation at Devil's Den, where there is a similar linear arrangement of sarsens at the food of a steep valley side:

https://brian-mountainman.blogspot.co.uk/2013/08/where-did-stonehenge-sarsens-come-from.html

https://brian-mountainman.blogspot.co.uk/2016/11/strange-things-in-pewsey-vale.html 

In the latter post we referred to these linear concentrations of sarsens (which are quite common) as "sarsen drifts."

There is of course much more on Tim Daw's site called Sarsen.org:

http://www.sarsen.org/2011_08_01_archive.html

I come back to this because I discovered in my pamphlet collection a little geomorphological trail called "Fyfield Down NNR Geomorphological Trail, by MJ Clark.  It was published by NCC in 1976.  A long time ago, but the observations are still valid, and it is mercifully free of speculations about the collection and transport of sarsens destined for Stonehenge!  It simply refers to a sarsen collecting "industry" that has been gong on for a very long time.



Anyway, the key discussion in the booklet is to do with the manner in which, in an asymmetrical chalk valley like Clatford Bottom (with one gently sloping flank and one steep one) the greatest concentration of sarsens is in a line along the foot of the steepest slope.  Could concentrations like these really be morainic remnants?  Could the concentrations be more apparent than real, because the fines on these valley floors have been washed away by intermittent stream flow, leaving sarsens exposed, whereas similar concentrations on the valley sides and interfluves  may still be buried in chalk debris and slope deposits?  Or could it be that the linear concentrations are still there because elsewhere so many have been removed over the centuries for land clearance and building use? (Because it involved more hard work to remove large quantities of stones from valley bottoms....)


Clark's analysis of the situation is summarised in the diagram above.  In phase 1 he suggests that in the early Pleistocene there is a slightly flexed land surface with a fragmented duricrust and with gentle valleys here and there, with sarsens mostly concentrated on the interfluves where they have best been able to survive erosional attack.  From then on, the story is mostly to do with extended phases of periglacial activity.  Permafrost and snowpatches?  Yes.  Glaciers? No.

Gradually (phase 2) the valley becomes more asymmetrical, as a result of more shade on the N or NE-facing slope and more frost action and snowpatch sapping.  On the long gentle slope solifluxion becomes a dominant process, and debris begins to accumulate on the valley floor.  In phase 3, as this process goes on, more and more of the sarsens from the  right-hand interfluve are transported downslope, gradually accumulating in the valley bottom.  Intermittent meltwater action in the valley floor (assisted by the presence of permafrost which prevents infiltration and effectively makes the chalk impermeable) removes the finer sediments in which the sarsens have been carried.

In phase 4, presumably coinciding with the Holocene, periglacial activity comes to an end and there is continued intermittent stream action and downcutting, and continued removal of finer material, leaving the alignment concentration of sarsens more or less as we see them today.

I would concur with Mike Clark that this is essentially a periglacial landscape, affected by hundreds of thousands of years of periglacial conditions during the Quaternary.  For much of that time there would have been permafrost on Salisbury Plain.  But on some of his other points I am not convinced.

For a start, I am not sure why he thinks there would have been greater concentrations of sarsens (and more duricrust or silcrete remnants) on the interfluves rather than in the early Pliestocene valleys.  I am more convinced by the argument (as discussed by Ed Pegler and David Field) that the duricrust (and hence the concentration of sarsens) would have been greater in the valleys, since that is where moisture would have been concentrated in pre-Ice Age times and where silcrete formation would have been accelerated.  I am also not sure why he thinks that periglacial processes and debris accumulation would have been more active on the warmer, gentle slopes of the valleys than on the steeper more shady slopes.  (Because the active layer was mobile for a greater part of the year and because more moisture was available rather than being locked in perennial snowpatches?  Might go with that........)  I am also not very convinced by the idea that downcutting and the washing out of fines was accelerated after the last removal of permafrost.  Surely, once the full permeability of the chalk had been restored, river erosion in these dry valleys would have been reduced, not enhanced?

Anyway, it's a stimulating little booklet, which encourages debate!  And that is what, as a teaching aid, it was intended to do.

Monday, 19 December 2016

Brecon Beacons glacier breeding ground


Couldn't resist adding this one too -- it shows the locations along the lee flank of the Brecon Beacons main ridge, and the shady conditions in which little glaciers must have developed, over and again, during the Quaternary.  The lake in the distance is Llyn y Fan Fach -- which was the location for a beautiful small cirque glacier.  These armchair-shaped  hollows were not quite so well developed elsewhere -- but we can see where snowfields have accumulated in the shady areas.  Some of these will probably have been thick and persistent enough to have given rise to niche glaciers.  At the peak of the main glacial episodes this whole landscape will have been submerged beneath the ice of the Welsh ice cap.

A Toby Driver image, reproduced on the RCAHMW web site.

Foel Fawr Quarry Complex


Now this is what I call a real quarry -- the Foel Fawr quarry complex in the Black Mountains, Carmarthenshire.  A fantastic Toby Driver image from the RCAHMW collection.

Apparently these quarries were used for the extraction of limestone, mostly for lime burning.  Some of the cuttings are less than a hundred years old.  It may be that the more rectangular cuttings are younger than the more irregular ones in the distance.  I'm intrigued by the little circular pits all over the place -- what were they?

Sunday, 18 December 2016

Another 100,000 page views

In less than 4 months since we went through the "million page views" barrier, we have now chalked up another 100,000.  I've been looking at the statistics, and there is a now a truly global reach. This is difficult to prove, but Google searches suggest that a lot of the page views are coming from people who are not looking up Stonehenge topics, but glacial geomorphology topics. As a crusty old geomorphologist, I really like the idea that geomorphology and geology students are using the blog as a source of information and debate.  Carry on, guys, and feel free to let us have your comments and questions!

An Insecure Archaeological Context



I came across this rather entertaining (and sad, too) photo on the web the other day -- posted by somebody who wanted to demonstrate what damage is being done to our oceans and coastal habitats by plastic flotsam and jetsam. This Yoplait yogurt pot has been on the coast for 40 years, and is virtually undamaged. Would anybody suggest that the coastal cliffs and caves just off the photo to the right are also 40 years old, because this yogurt pot dates them accurately? Er, probably not, but this is a nice illustration of what is meant by an insecure archaeological context.

Sadly, this dating problem is illustrated perfectly in the case of Rhosyfelin. The archaeologists persist in their claim that the famous "monolith excavation point" on the rock face towards the tip of the spur is accurately dated, and that a lump of rock was quarried from there in the Neolithic, because they have radiocarbon dates from not far away:

"The most probable dates associated with the removal of the rhyolite pillar from its recess are c 4590 BP and c 4667 BP, provided by carbonised hazelnut shells from the small occupation layer just 1.5 m away from it."
Source: p 1344 of the Antiquity article by Parker Pearson et al.


 There is no logic in that statement, and nothing to link any stone removal with any particular dated phase of occupation.  This is a classic case of an insecure archaeological context, since there is no independent evidence of any monolith being removed from here by human agency, and nothing to suggest any link between the rock face and the sediments and organic materials found nearby.  The rock face as we see it today may be many thousands of years older than the sediments found in the vicinity; and there is no more sense in the claim by MPP and his colleagues than there is in the claim that the cliffs and caves in the vicinity of that Yoplait pot are 40 years old.

Sorry, but this is yet another illustration of just how dodgy the archaeology is at Rhosyfelin, and how misleading that Antiquity paper is.  MPP has apparently changed his mind about there being a quarry at Rhosyfelin, if reports of his latest talk the the recent NPA Archaeology Day are to be believed.  Would it not make good sense, therefore, for the authors to write to the editor of Antiquity and simply retract the paper?  It does nobody any good for it to remain in print, and indeed it does most of those involved a great deal of harm.............

Saturday, 17 December 2016

Images Restored!



This may be of interest to other Blogger users.  For some reason which I still cannot fathom (and neither can all the Blogger experts) some of my images disappeared from this blog -- on my computer at least.  Very strange -- because other users of the blog could apparently see all the images as they were originally posted.

I could not work out whether this was a Blogger or Google issue, or something to do with my computer's installed web protection / security software, or something to do with BT's web protect software which users can switch on or off. (I have had problems with the latter before, since it apparently does not like the Weebly web design site......)

At last, after much helpful advice on one of those chat pages from assorted whizzo people, I discovered that all images used on a blog are not necessarily stored on my computer, but by Blogger itself (in other words Google) in a number of Albums or picture galleries which take a bit of finding.  If you hover with the cursor over an image you can see where it is located -- and in my case I discovered that Blogger has four image collections labelled as 1.bp.blogspot.com, 2.bp.blogspot.com, 3.bp.blogspot.com, 4.bp.blogspot.com.  Images seem to get allocated more or less at random into one or other of those collections.  For some reason the last of those was the one giving the trouble, and I thought at first that BT was blocking it because of some security / authentication issue.

I was about to complain to BT when somebody mentioned that the images should show up on my blog so long as they exist on my own computer -- and in a flash of inspiration I realised that all the missing images must have been imported straight from one web site to another.  In other words, they had been dragged from one web page and inserted directly into a blog post, without first being stored in iPhoto or somewhere else on my computer.

Then I realised that there is a download facility on each of the Blogger albums.  Bingo!  I downloaded each of them and unzipped the downloads on my desktop, and once 4.bp.blogspot.com was safely installed on my computer, all the missing images reappeared.

So there we are then.......  the message is that if you are working on a blog, and want to use some images in it, make sure you use images from your own computer, and not images just dragged from another web site.......

Thursday, 15 December 2016

Archived images (3)



This is the most recent archive, bringing things up to date:

https://get.google.com/albumarchive/112312986974547323674/album/AF1QipMJYzO3F9DgmDGyOZHqfxtzpprXVUHZop4sDZA7?hl=en_GB&authKey=CNG5xcDg7pvwBQ

On the label, it says it was archived on 10 Sept 2015 -- that's a bit strange, since there is also more modern material on it.  Weird and wonderful are the ways of Google.......

Archived images (2)



These are the archived images for the re-named blog, up to 20 December 2011.  Enjoy!

https://get.google.com/albumarchive/112312986974547323674/album/AF1QipNV5mE3dCft5nRsxbmwbyjAhwFuJGUjZyg7KNin?hl=en_GB&authKey=COn01qvAu-jVDw

Archived images (1)



If anybody wants to search for an old image used on this blog, or just look at all the pictures used over the years, here is a good way to do it.  Just scan through the images in the appropriate Google Image Archive.  Here is the link to the old blog, before May 2009, when it was called "Stonehenge Thoughts" :


https://get.google.com/albumarchive/112312986974547323674/album/AF1QipMY7RRcW-iVjciiEhc0cYjjI1v_hWxlsINkTNkZ?hl=en_GB&authKey=COCPz-q3-amrxwE

Wednesday, 14 December 2016

Disappearing images

Hi folks
A lot of my past posts suddenly appear to have gaps where images should be -- more or less at random.  Are others also seeing gaps or blank spaces?  Please let me know.  Thanks!

Thursday, 8 December 2016

The aurochs and snow sledge theory



Thanks to geologist Paul Sanday for reminding me of this article which he wrote about a year ago:
"To Stonehenge by snow sledge", Pembrokeshire Life magazine, Dec 2015, pp 4-8.

Paul said he won't be upset if I say it's nonsense, so here we go.  It's nonsense. (I suspect he wrote it with tongue firmly in cheek anyway.........)

He goes through the various theories of bluestone transport -- by boat, glacier ice, by land, aliens (ancient astronauts) and air (using magical powers).  He dismisses the boat or raft transport theory pretty quickly, and I agree with most of his points on that.  Then he comes to glacial transport, and goes seriously adrift -- a pity, but as we all know geologists do not necessarily do much homework on glaciology or geomorphology.

Paul's points re "problems" with the theory:

1.  Ice in the last glaciation didn't go further south than South Wales. He cites "Cambridge University" as his source.  Sadly, Cambridge University does not do research -- people do research, and some of them happen to be based at Cambridge. In any case, the assertion is incorrect.  Devensian ice extended as far south as the Isles of Scilly.  In any case, we are not talking about Devensian ice here, but Anglian ice.

2.  The ice was moving towards SW and not towards SE.  "Cambridge University" is again cited as the source.  Where on earth did this idea come from?  Of course the ice in the Bristol Channel was moving towards the E and SE -- that has been well known for over a century.

3.  Stonehenge is just one location -- erratics should be widely spread.  Maybe, and maybe not.  Lots on this blog on just this topic.  and we still do not know how far the Stonehenge builders needed to range across country during their efforts tomgather up stones.

4.  No other erratics found, such as Gower ORS or quartz conglomerate.  How many erratics do we want?  One might as well say that there "should" be erratics from Ailsa Craig at Haverfordwest or erratics from Anglesey at Milford Haven...... Glaciers tend to be rather unaccommodating for most of the time.

5.  The size and shape of the Stonehenge bluestones are "pretty consistent", rather than involving a range of sizes.  Not true.  The bluestones at Stonehenge are a pretty mottley collection of slabs, pillars, boulders and stumps.  The best collection of glacial erratics that you are likely to find anywhere in southern Britain.

6.  Bluestones at Stonehenge are either monoliths or flakes.  Not so.  There are stumps and lumps of bluestone too, and also all sorts of stones classified as "packing stones, hammer stones and mauls".

7.  Very specific rock types have been selected -- eg Preseli spotted dolerite.  Not so.  There are many rock types represented in the bluestone assemblage, including some that are not really very suitable for use as monoliths.

After all of this, Paul considers land transport according to MPP et al, and finds the theory wanting.  Similarly, he does not sound too impressed with aliens, space ships, giants, fairies and wizards.

And so he comes to his central hypothesis, partly based upon all sorts of things, including a new Cambridge University map of the Devensian glaciation.  I haven't got a clue what he's talking about there, or what its relevance is, so let's move on.  Paul starts off by saying that a lot of things happened earlier than previously thought.  We can live with that.  He then proposes that the bluestones were moved to Stonehenge "at the end of the last Ice Age"  around 12,000 -10,000 years ago, when the Severn Estuary would have been "non-tidal and frozen."  Well, we know that it wasn't frozen in the Younger Dryas, and it certainly wasn't non-tidal.  Sure, there was intermittent permafrost on land, but I have never seen any evidence of coastal ice action at this time.  Then he says that the Bristol Channel and Severn Estuary area beyond the glacier front was a permafrost plain, just right for hauling sledges with bluestones on them. So that assumes a much lower sea level.  The trouble is that the low sea level associated with the last glaciation was around 20,000 years ago, and that by Younger Dryas times it had risen to about -20m.  Quite low, but not low enough to leave the centre of the Bristol Channel as dry land. Sadly, Paul has got his glaciations, his sea levels and his permafrost episodes all screwed up........

Essentially, the thesis is that tame aurochs were used to haul bluestones from Pembrokeshire in the Palaeolithic or the Mesolithic, across a frozen landscape in the Bristol Channel, in nice snowy weather when sledges could be made to slide nicely.  How?  Why? When?    It's a very jolly image  -- why let lots of specifics get in the way of a delightful generalisation?  Why, indeed, let the truth get in the way of yet another jolly story?

Garn Fawr ring cairn


I found this wonderful photo the other day, showing Garn Fawr, a spectacular crag on Pen Caer, looking out over the coast at Pwll Deri towards St David's Head in the distance.  In this photo, Pwll Deri YHA is just off the photo to the left.

Normally the centre of attention is the Iron Age hillfort or fortified settlement site on the summit, but very prominent in this photo is a circular embanked feature on the hillslope below.  There are also traces of other curved or looped embankments below it.

In the Coflein and Archwilio records there are no mentions of this circular feature, which I suppose should be dated to the Bronze age.  Or could it be an Iron Age feature, connected to the fort?  On these RCAHM photos we can see the "ring cairn" quite clearly.  Does anybody know anything about it?




Monday, 5 December 2016

The Preseli Tors

This is a small glossary of the main tors on the Presely upland ridge -- extracted from the manuscript of a book that I never did get round to publishing........


I will enhance this post by adding photos as appropriate.

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Up Among the Presely Crags


Carn Afr (093302)

This is a very beautiful tor on the southern flank of the Presely ridge, only a couple of kilometres away from the hamlet of Rosebush and almost due south from Foelcwmcerwyn. Its name can be translated as "the cairn of the goat". It is a hillside tor (rather than a summit tor) at an altitude of 375m, and it looks down upon a shallow depression and then over the "step" of Mynydd Crwn. The main feature of the tor is a prominent pinnacle of rock which is visible from a great distance. Around it there are small vertical cliffs, grassy steps and clean slabs of rock, with a litter of massive boulders especially to the west. There are several perched boulders near the pinnacle. The rock is for the most part dolerite, but there are also foliated or layered volcanic rocks similar to those of Carn Alw. There is only a little scree, at the foot of the lowest of the cliffs. Just beneath the pinnacle there is a clean grassy bank which is perfect for a summer picnic or even an overnight camping stop. Beneath the tor there is a wonderful old sheepfold, and to the east a long-since abandoned rectangular enclosure with six small fields and a ruined cottage and garden. Nothing is known about its history. The tor is best approached across the common from the bridle path which runs along the south-eastern flank of Pantmaenog Forest.

Carn Alw (138338)

The largest and most spectacular tor in the whole of the Presely area, standing in glorious isolation above the sweeping grasslands on the northern flank of the main upland ridge. Translated, its name is "echoing rock" or "calling rock". Its summit is only about 240m above sea-level. It is most easily reached from the roadside south of Glynmaen Farm. Its northern and eastern faces are high, shadowy and forbidding, but the approach is easy from the west and south. It is made of light-coloured volcanic rocks including dolerite, rhyolite, felsite and various others with complicated names. Look at the glassy texture and "flow structures" in some of the rock outcrops, formed while the rock was cooling from the molten state. About 400 million years ago this was clearly a major centre of volcanic activity; perhaps it developed as a subsidiary eruption cone on the flank of the much larger Foeldrigarn volcano. Some pieces of rock from here were assumed by HH Thomas to have found their way to the "bluestone circle" at Stonehenge; probably they were eroded by ice moving along shear-planes in the Irish Sea Glacier as it crossed the upland ridge, and were later dumped when the ice melted not far from the famous and enigmatic monument. (Note: Thomas's work has been questioned by more recent geological research, reported on this blog.)  The most interesting feature of Carn Alw today is the small Iron Age fort located on its western flank and summit. Like some of the Norman castles of Pembrokeshire it has an "outer bailey" and an "inner bailey"; the former was the main settlement site, with traces of old hut circles still to be seen; and the latter, just beneath the summit, was the last defensive position that could be occupied if the outer defensive walls were breached by an attacking tribal army. The old defensive ramparts and portal are still visible, as is a wonderful "chevaux de frise" beyond them. This feature, very rare in Britain, was built to prevent cavalry attack and was made of pointed stones (and probably sharp wooden stakes) angled into the ground and set close together. Out on the moorland to the south-east there is a fine walled enclosure (now somewhat ruinous) probably used by the Carn Alw Iron Age community, and also other cairns, walls and man-made stream channels of similar age. There is a charming fairy story associated with this carn and a nearby farm (now lost) called Llech-y-deri. The hero of the story is a young man who was taken to fairyland after succumbing to the "enchanted music" of the fairies which he heard echoing around the rock; perhaps this is where the name "Carn Alw" comes from.




Carn Arthur (135323)

This tor is perched on the mountain side at 300m, looking down on the open depression of Cors Tewgyll and across to the crags of Carnmeini. The local rock here is coarse dolerite, and there are some outcrops which show the large while felspar crystals which are characteristic of spotted dolerite or bluestone. The rocky outcrop is easily approached from above, where there is a grassy platform close to a huge perched boulder. This massive block -- and possibly one or two others lower down on the outcrop -- were assumed by the locals to have been placed there by King Arthur or some other mighty hero. Less romantically, we now know these rocks to be nothing more than large glacial erratics left by retreating ice. The tor is in an advanced state of decay, with a great litter of broken blocks and scree banks on its lower part. Not far away is Bedd Arthur, reputed to be the burial place of the mysterious king.




Carn Bica (129326)

This little tor would be called "pointed carn" or "peaked carn" in English, in recognition of its conical shape. Its altitude is about 380m. It is not a very prominent feature, and has suffered from a great deal of frost shattering in the Ice Age, and also from comprehensive rearrangement by the Bronze Age folk who built a large burial mound on its flank. The mound of loose stones has since been further rearranged by various members of "Dad's Army" who manned a lookout post here during the Second World War; and by assorted boy scouts and cadets trying to make shelters from the wind and the rain in this bleak and exposed spot. Note that the rock here is classic spotted dolerite -- the rough surface texture of boulders is characteristic, and on broken fresh surfaces the large white feldspar crystals are very noticeable. This cairn is close to the route of the Bronze Age "Golden Road", and about 100m to the east you can see the strange stone arrangement referred to by the locals as Bedd Arthur or "the grave of King Arthur." The wooden posts close to the carn mark part of the route of Ras Beca, the tough fell trace which takes place in these hills in August each year.



Carn Breseb (137333)

This carn (translated as "manger rock") is a substantial outcrop of spotted dolerite on the northern flank of the upland ridge, at an altitude of about 300m. In reality there are several stepped outcrops here, all of them very much degraded because of frost shattering and the downslope movement of blocks and scree. The tor does not look much like a manger, but the name may indicate that the local farmers once used to feed their animals here. The most prominent feature of the carn to be seen from a distance is the flat slab which looks like a gigantic needle when seen end-on. There is a large area of broken rock (and some solid rock) here, and it is fascinating to explore the nooks, crannies and crevices and to discover the sheltered and nutrient-rich "micro-environments" which provide homes for lichens, liverworts, and higher plants in this otherwise hostile and open terrain. There are traces of very ancient stone walls on the north-eastern flank of the carn, suggesting that it may have been used by the Iron Age inhabitants of nearby Carn Alw.

Carn Ddafad-las (147330)

There is not much left of this little tor, which can be named "the cairn of the blue sheep" in English. Goodness knows where its name comes from; perhaps there was once a prehistoric sheep farmer here who marked his sheep with blue dye made from bilberries! It is one of a group of little tors at about 325m just to the north of the Golden Road track and close to Carn Gyfrwy. Frost shattering, and possibly the action of overriding glacier ice, have damaged the tor so badly that it now consists simply of a rickety pile of massive boulders. One feels that the whole thing could collapse at any moment, so if you explore this tor and climb on it, please employ great caution.

Carn Gaseg (160328)

This is a small tor south of Foeldrigarn and the Golden Road. It is on private land and like Carn Sarn is therefore difficult to visit; but there are similar small tors (Carn Goi, Carn Pant-teg, and Carn Bwdcyn being the only named ones) in the coniferous woodland to the west. They are made of dolerite and other volcanic rocks. Before the woodland was planted there were two smallholdings in the neighbourhood, now reduced (like many others around the flanks of the uplands) to tumbled cottage walls and a few traces of paddocks and kitchen gardens. When you approach Carn Gaseg from the north you may feel that it does not look very interesting; but you would be mistaken, for when you drop down over the brow of the hill you encounter several splendid rock bastions up to 6m high. When seen from downslope the tor is very spectacular, and very beautiful. Most of the dolerite rock faces overlook a grassy terrace of bedrock slabs with patches of gorse and heather. Some of the slabs appear to have been smoothed by the action of overriding ice. The dolerite here is fine-grained and in places flaky. There are a few elongated stones which have been broken from the solid rock by frost action, and the tor is still breaking down, with long deep cracks, gullies and perched blocks.

Carn Goedog (128333)

This is one of my favourite Presely tors, partly because of its impressive size and partly because it is a tranquil place, seldom visited by hill walkers. Located on the northern flank of the upland ridge, it is adjacent to the old drover's route used by many thousands of animals en route from Pembrokeshire farms to the meat markets of the Midlands and London in the 1700's and early 1800's. The up-slope flank of the tor (at about 300m) is not very prominent, but on the downslope side there is a wilderness of huge dolerite blocks, banks of scree, little grassy platforms, solid rock outcrops, crevices and caves. The rock is very coarse, with some spotted dolerite. One or two small trees have managed to survive here in places inaccessible to grazing animals; but the name ("woodland cairn" in English) indicates that there must once have been an extensive woodland here after most of the upland ridge had been cleared by burning, felling and grazing. Recently the tor has attracted attention following the suggestion by geologists Richard Bevins an Rob Ixer that many of the Stonehenge spotted dolerite monoliths have come from here.  Prof Mike Parker Pearson and colleagues claim that there is a Neolithic "bluestone quarry" here; but intensive investigations (reported on this blog) have revealed no unequivocal signs of quarrying in prehistoric times.




Carn Gwr (142330)

This name means "husband's cairn" or something similar. There are really five separate small tors here, scattered about on a broad saddle or col in the upland ridge at an altitude of c 315m. There are no steep slopes, and it is easy to stroll between one tor and another on the heather and bilberry moorland. Here you are right at the heart of "bluestone country", with tors all around you. Carn Bica, Carn Breseb, Carn Alw, Card Ddafad-las, Carn Gyfrwy, Carn Meini and Carn Arthur are all within easy walking range. As you walk around this area, with skylarks above and sheep and mountain ponies grazing on the moorland, you can feel the timeless serenity of Presely; and you can also see traces everywhere of human occupation of this landscape over 3,500 years or more. Slight ridges in the turf, pits, elongated hollows, fallen stone walls and small stone cairns may date back to Bronze Age or Iron Age times. An old cart track runs across the moorland here, linking the rock outcrops with the farming community to the north which used the area as a handy quarry for gateposts and other "special stones". The cart track, in places deeply entrenched, runs downslope between Carn Breseb and Carn Alw and then to the west of Carn Alw down towards Mirianog Ganol.

Carn Gyfrwy (147327)

One of the most prominent features on the Presely ridge, this cairn is more of a rock bastion than a tor. It is made of spotted dolerite or bluestone, and has a steep cliff face at its northern end, immediately overlooking the route of the ancient Golden Road. Its summit is about 365 m above sea-level. On the OS 1:25,000 Outdoor Leisure Map the place-name is incorrectly located; the name means "saddle cairn" in English, and there is no doubt which cairn has the saddle-like appearance. There is another error in the labelling of the so-called "sheep-fold" beneath the cliff face. This crude stone-walled enclosure has also been referred to in the literature as a "drover's hut." However, it is far too small to contain sheep, and it is some way from the old drover's route. It is more likely to be a simple shepherd's shelter used during bad weather. It is still used for this purpose by weary hill walkers caught out in the wind and rain.

Carn Meini (144325)

Carn Meini or Carn Menyn is the one Presely cairn whose name is known to people from all over the world, since it is reputed to be the source of some of the famous Stonehenge spotted dolerite bluestones. (Its reputation is largely undeserved, since recent geological work suggests that the said bluestones have probably come from other locations.) The single name is used to describe a cluster of seven or eight distinct tors arranged in an arc around a spur on the upland ridge. They are very spectacular indeed, and from a distance they give the skyline a jagged or "cock's comb" appearance. The name is interesting. In English the name Carn Menyn (used on recent OS maps) would be "butter cairn", but this is a nonsense name in this geographical setting, and it is much more likely that the alternative Carn Meini ("cairn of the stones") has the better historical claim. Also, the words "maen" (singular) and "meini" (plural) are used in the context of shaped or dressed stone, as in "maenhir" ("long stone") used by the old archaeologists as a word for a standing stone. We may speculate therefore that the name was first used to describe a place of special elongated stones; and indeed there are many elongated stones here as a result of columnar jointing in the spotted dolerite. The massive tumbled blocks which are characteristic of this area indicate that there has been large-scale frost shattering here during periods of prolonged cold climate; but there are also some wonderful ice-smoothed surfaces at the southernmost edge of the rocky spur, with deep gullies and cliffs which indicate that over-riding glacier ice has "plucked" or dragged away large quantities of broken rock. The rock fragments, some of them weighing several tonnes, were carried up into the body of the glacier along shear-planes as it flowed southwards and south-eastwards, to be dumped hundreds or even thousands of years later on the fringes of Salisbury Plain. There are a number of possible man-made features among the Carn Meini tors. For example the flat stone referred to as the "altar stone" may be a sub-Neolithic burial chamber or cromlech, and there are a number of traces of very old stone walls that may date back to the Iron Age. An ancient cart-track is cut into the hillside between two of the eastern tors; it runs down to the road near Glanrhyd, and was used by farmers who were collecting convenient elongated stones from the mountain for use as gate-posts, lintels, steps or sills.


 


Carn Sarn (159327)

Literally this means "causeway cairn", and indicates an alignment of rocks. This little tor is almost due south of Foeldrygarn and is on private land to the south of the Golden Road. The causeway or route remembered in the name may be the Golden Road itself, since it has been used by traders and local inhabitants over a period of at least 4,000 years. The tor is part of the same volcanic complex as Carn Meini and Carn Gyfrwy; and other small tors are now lost in the coniferous plantation of Llethyr-mawr. The tor is not very spectacular, consisting of little more than a few dolerite outcrops on the hillside and a few small pinnacles up to 3m high. There are broken boulders scattered across the slope, over an area of about 50m x 50m. There are grassy banks and patches of gorse and heather, and this is a pleasant sunny place sheltered from westerly and northerly winds.

Carn Sian (128322)

This little dolerite tor, almost in the centre of the Talfynydd upland spur, is named after someone called Sian or Jane. It is unremarkable as tors go, but well worth a visit since the walking is easy hereabouts, and there are fine views towards Carn Meini and Foeldrygarn. The land surface in the vicinity of the tor has been greatly modified by the hand of man. If you look carefully you will see the traces of old stone walls and enclosures which probably date back to the Bronze Age or Iron Age. There are many elongated hollows which show us the routes of ancient trackways. And the turf is pock-marked with long-abandoned peat cuttings, reminding us that large parts of the upland were used as turbaries by the commoners who owned the farms and smallholdings on the flanks of the mountain.

Cerrig Marchogion (112323)

Here, on the crest of the upland ridge at an altitude of about 400m, the strange craggy outcrops of Cerrig Marchogion make a lasting impression on all who pass by on the nearby Golden Road. They are really small tors and rock outcrops made of dolerite, surrounded by a litter of broken boulders affected by frost action and moverd about by glacier ice. There is a magical quality about this place, especially when it is raining, or when low cloud or mist is drifting across the mountain ridge. Literally, the name means "the stones of the knights". The explanation for the name can be found in the ancient book called "The Mabinogion", in the story of Culhwch and Olwen. This is adjacent to Cwm Cerwyn where, according to legend, King Arthur and his knights fought a pitched battle with the wicked black boar Twrch Trwyth and his cohorts. So bloody was the battle that eight of Arthur's knights were killed, and the rocks are supposed to be their petrified remains.




Craig Talfynydd (130314)

This is a series of rock outcrops with banks of scree and broken slabs on the eastern flank of the Talfynydd spur. Literally, the name means "crag on the end of the mountain". This is a beautiful sheltered spot when the wind is in the west or north-west, with many gullies, dolerite outcrops and grassy banks to explore. Some small trees have managed to survive in the rocky wilderness. Because the slope is quite steep in places, some of the rock slabs and scree banks are still moving periodically but inexorably downslope; they are therefore quite unstable, and great care is needed when you clamber across them. There is a very large rabbit warren here, and you are certain to see rabbits of all ages scuttling about among the rocks. The spring below the crags is used to supply water to the cottage of Dan-y-garn in the copse of trees.

Craig y Cwm (096313)

This rocky outcrop high above the hollow of Cwm Cerwyn is not really a tor, but a series of crags and cliffs created by a combination of glacial action and frost shattering. The name means "the rock of the valley". Located quite close to the Presely summit of Foelcwmcerwyn, the altitude here is about 490m. The crags are made of slates and other rocks metamorphosed close to the dolerite intrusion which outcrops on the mountain summit. Down below there is a small and very old slate quarry with pits, cuttings, a trial tunnel, spoil heaps and a cart track running southwards around the end of the spur. The quarry excavations have made Craig y Cwm quite dangerous, especially when the grassy slopes are wet or when visibility is bad; this is one of the few places on Presely where a serious accident can happen if you are lost in the mist. A few years ago there was a fatal air crash here when a light aircraft en route from Swansea to Dublin flew into the side of the mountain in poor visibility.  The cwm was probably the site of the last small glacier in Pembrokeshire;  morainic traces and exposures in stream cuttings suggest that there was a small glacier here around 10,500 years ago.


Foel Drygarn (158336 )

This prominent hill mass towards the eastern end of the Preseli upland ridge stands in glorious isolation, and the summit can only be reached via a stiff climb from the nearly "Golden Road" footpath.  The name means "the bare hill with three cairns" -- and the three massive Bronze Age burial mounds on the summit are the most spectacular features of this age in Pembrokeshire.  They lie within the confines of an Iron Age hillfort which contained both an animal enclosure and a substantial settlement site.  Scores of hut circles can still be made out in the turf.  The defensive ramparts are prominent.  A gorgeous location, with spectacular views in all directions.  Maybe we shouldn't classify this as a "Preseli tor" but on balance I have included it in this list because there are indeed spectacular crags here towards the western end of the summit.  The rocks are rhyolites rather than spotted dolerites, and as with the other high tors the dominant process which has shaped the crags in recent millennia is frost shattering and the downslope movement of detached blocks under the influence of gravity.  The jury is still out on whether the summit of Foel Drygarn was affected by glacier ice during the last glaciation.