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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Monday, 11 December 2017

Ice in the Bristol Channel -- the latest modelling

This is a comprehensive and important paper which draws together a vast amount of information on the Devensian Eurasian ice sheet complex.There is a huge reference list -- including many articles that can be open-accessed as PDFs.

Henry Patton, Alun Hubbard, Karin Andreassen, Amandine Auriac, Pippa L.Whitehouse, Arjen P. Stroeven, Calvin Shackleton, Monica Winsborrow, Jakob Heymane, Adrian M. Hall. (2017)
"Deglaciation of the Eurasian ice sheet complex"
Quaternary Science Reviews
Volume 169, 1 August 2017, Pages 148-172


Here is the Abstract:

Abstract

The Eurasian ice sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum with a span of over 4500 km and responsible for around 20 m of eustatic sea-level lowering. Whilst recent terrestrial and marine empirical insights have improved understanding of the chronology, pattern and rates of retreat of this vast ice sheet, a concerted attempt to model the deglaciation of the EISC honouring these new constraints is conspicuously lacking. Here, we apply a first-order, thermomechanical ice sheet model, validated against a diverse suite of empirical data, to investigate the retreat of the EISC after 23 ka BP, directly extending the work of Patton et al. (2016) who modelled the build-up to its maximum extent. Retreat of the ice sheet complex was highly asynchronous, reflecting contrasting regional sensitivities to climate forcing, oceanic influence, and internal dynamics. Most rapid retreat was experienced across the Barents Sea sector after 17.8 ka BP when this marine-based ice sheet disintegrated at a rate of ∼670 gigatonnes per year (Gt a−1) through enhanced calving and interior dynamic thinning, driven by oceanic/atmospheric warming and exacerbated by eustatic sea-level rise. From 14.9 to 12.9 ka BP the EISC lost on average 750 Gt a−1, peaking at rates >3000 Gt a−1, roughly equally partitioned between surface melt and dynamic losses, and potentially contributing up to 2.5 m to global sea-level rise during Meltwater Pulse 1A. Independent glacio-isostatic modelling constrained by an extensive inventory of relative sea-level change corroborates our ice sheet loading history of the Barents Sea sector. Subglacial conditions were predominately temperate during deglaciation, with over 6000 subglacial lakes predicted along with an extensive subglacial drainage network. Moreover, the maximum EISC and its isostatic footprint had a profound impact on the proglacial hydrological network, forming the Fleuve Manche mega-catchment which had an area of ∼2.5 × 106 km2 and drained the present day Vistula, Elbe, Rhine and Thames rivers through the Seine Estuary. During the Bølling/Allerød oscillation after c. 14.6 ka BP, two major proglacial lakes formed in the Baltic and White seas, buffering meltwater pulses from eastern Fennoscandia through to the Younger Dryas when these massive proglacial freshwater lakes flooded into the North Atlantic Ocean. Deglaciation temporarily abated during the Younger Dryas stadial at 12.9 ka BP, when remnant ice across Svalbard, Franz Josef Land, Novaya Zemlya, Fennoscandia and Scotland experienced a short-lived but dynamic re-advance. The final stage of deglaciation converged on present day ice cover around the Scandes mountains and the Barents Sea by 8.7 ka BP, although the phase-lagged isostatic recovery still continues today.

Most of this paper is concerned with the wastage phase of this vast ice sheet complex -- following on from the modelling done by Patton et al in 2016.  We have discussed that work before, here:

Henry Patton's early modelling was intriguing since it showed a Devensian ice margin far beyond the conventional one in the Celtic Sea arena -- with an ice-filled Bristol Channel, Irish Sea ice impinging on the coasts of Cornwall and Devon, and pressing well inland in Somerset too.  This was the 2016 published model for the Devensian maximum ice margin:

The modelled Devensian ice extent, as determined by Patton et al in 2016.  The red line is the older assumed ice limit.  Note that in the Midlands and in the Bristol Channel area, modelling suggests that ice could have progressed more than 100 km further south than assumed.  Ground truthing holds the key......

The modelling work published in 2017 is based on a vastly increased amount of data from many sources, and the segment of the ice sheet complex incorporating the British Isles (which the authors refer to as the "Celtic Ice Sheet") looks like this:

 
The extent of the Devensian glaciated area is very similar indeed, adding to our confidence that it fits well with glaciological, climatic, topographical, sea-level and many other parameters.    The garish green colour used here relates to the rate of ice edge retreat.  Solid green indicates rapid retreat, yellow suggests approximate stillstands or slow retreat, and red suggests a more prolonged stillstand. So the map shows a relatively prolonged stillstand of the ice edge along the coast of Cardigan bay and on the northern flank of Preseli -- I can live with that, since it is supported by rather a lot of evidence on the ground.

Now let's put some flowlines onto the map -- ice in ice sheets and ice caps always flows broadly perpendicular to the ice edge, so the patters will have been something like this.  (There will of course have been many internal irregularities caused by uplands, glacier discharge routes and changes in the surface topography and the bed conditions of the ice mass......as we have discussed many times regarding the interactions between ice from the Welsh ice cap and the ice of the Irish Sea Glacier or ice stream.)



This is in my view far more realistic than some of the maps published by the BRITICE team over recent years.  Since we are all interested in how the bluestones got to Stonehenge, note that the modelling shows -- yet again-- that Devensian ice COULD have reached Salisbury Plain.  I personally doubt that it did in reality, but since this modelling work will also hold pretty accurately for pre-Devensian glacial episodes, my view is reinforced that during the Anglian Glaciation (around 450,000 years ago) ice might well have carried those dearly beloved bluestones almost all the way from Preseli to Stonehenge.




Thursday, 7 December 2017

More on the Carn Goedog dolerite sill




This rather splendid oblique aerial photo has just been published in Pembrokeshire Life magazine -- it was taken in 2009 by Toby Driver of the Royal Commission.  It shows Carn Alw in the foreground and Carn Goedog in the distance. Because of the light snow cover there is tremendous detail in the image -- I have added a few pointers for those who do not know the area. Click to enlarge the image.

The photo shows just how extensive the outcrops of the Carn Goedog sill actually are.  As I have explained in previous posts, the assumption that there is a Neolithic bluestone quarry at Carn Goedog is very dodgy indeed, and is based in turn on the assumption that the spotted dolerites ar Stonehenge have been precisely provenanced to the Carn Goedog tor.  But these spotted dolerites outcrop all the way along the sill to the vicinity of Carn Alw, and I have seen nothing in the geological papers that demonstrates that any of the spotted dolerite monoliths actually come from the tor rather than from outcrops further to the east or indeed towards the west.  The sampling point density is just not great enough for any definitive conclusions to be drawn.  The geologists (Richard Bevins and Rob Ixer) sampled the tor because it is a prominent feature  and therefore "attracts" the sampling process.

http://brian-mountainman.blogspot.co.uk/2013/11/carn-goedog-and-stonehenge-new-work.html



https://brian-mountainman.blogspot.co.uk/2014/09/glacial-features-on-carn-goedog-sill.html

Confirmation bias yet again? I'm not sure I would put it as strongly as that, since I suppose many of the geological samples were taken years ago, long before Neolithic quarries were being thought of in this area.  But you must always be careful in this work not to attach "artificial significance" to places simply because they happen to be the places you have sampled........

Monday, 4 December 2017

Ice in the Bristol Channel



One of the figures from the article.   I do not believe that the shown "Late Devensian limit" is well supported by the published evidence, and I am inclined to accept that the so-called Early / Middle Devensian ice limit, drawn largely on the basis of the borehole evidence, is in 
fact of Late Devensian age.

I have been looking again at the 2017 article by Gibbard, Hughes and Rolfe, in which it is argued that Lundy Island was glaciated during the Middle Devensian -- at a time frequently assumed to have been characterised by a mild or interstadial climate. There is a big dispute going on about the dating of rock surface exposure on the island following a glacial transgression -- more of that anon. But what interests me more at the moment is the very strong evidence for thick glacial deposits in the middle section of the Bristol Channel. These deposits (for the most part assumed to be till) are up to 38m thick in logged boreholes -- they may of course be even thicker in other locations. They are not interpreted as glaciomarine sediments dropped from icebergs or from the underside of a flowing glacier; and so this points to heavy glacial incursion from the west. When did this occur? Because the sediments on top of the till are relatively thin (normally just a few metres) a Devensian age is most likely.

Here is an extract from the paper, dealing with the borehole evidence for the area to the east of Lundy.


New insights into the Quaternary evolution of the Bristol Channel, UKPHILIP L. GIBBARD, PHILIP D. HUGHES and CHRISTOPHER J. ROLFE.JOURNAL OF QUATERNARY SCIENCE (2017) ISSN 0267-8179.DOI: 10.1002/jqs.29513 April 2017


Extract:

Swansea Bay and Gower

Boreholes from Swansea Bay indicate tills associated with the southernmost extension of the Welsh Ice Cap which reached ca. 15 km south of Swansea (Bowen, 1970, 1973, 1974; Culver, 1976; Culver and Bull, 1979; BGS, 1986). Sedimentological and microfaunal analyses indicate that these glacial deposits were originally land-based and were progressively submerged by a marine transgression between 10 and 2.5 14C ka BP (Culver and Banner, 1978). A series of rock basins in Swansea Bay were occupied by glacial lakes before marine transgression and are likely to have been present from glacier retreat at ca. 16 14C a BP until marine transgression at ca. 10 14Cka BP (Culver and Bull, 1979). As noted earlier, a clear end moraine is also visible at the sea-bed surface in high-resolution bathymetric maps with a network of meltwater channels visible outside of this moraine (Fig. 3). The eastern part of the moraine arc is covered in thick sand accumulations at Scarweather Sands. Glacial deposits have also been identified beyond the limits of the Swansea Bay moraines by Blackley (1978, his fig. 11), south and south-east of the Scarweather Sands area based on continuous seismic profile records.

The Gower peninsula is well known for its Pleistocene sediment record, with glacial deposits and raised beach deposits recorded and reviewed in numerous publications (e.g. Bowen, 1974; Bowen and Sykes, 1988; Bowen et al., 1985). The Gower is important for glacial research because it marks the southern limit of the Devensian Welsh Ice Cap. A boulder associated with this limit has yielded a 36Cl exposure age of 23.2`2 ka (Bowen et al., 2002). On the south and west of Gower the Paviland Moraine, a marginal complex of meltwater gravels and sands, rests on a red clay till (Bowen, 1974, 1999), rich in Millstone Grit clasts derived from southern Welsh Carboniferous rocks. Raised-beach deposits date to the last interglacial and earlier. Uranium-series ages from a cave stalagmite interbedded with marine beds at Minchin Hole indicate an MIS 5 age (Stringer et al., 1986). Bowen et al. (1985) argued that at least three highstands of sea level are recorded, the older two of which are correlated to the interglacials of MIS 5e (Ipswichian, Eemian Stage) and MIS 7 (Late Wolstonian, Late Saalian Substage).

Atlantic Array and Lundy

Till is present throughout the Atlantic Array survey area (Channel Energy Limited, 2010). This till (unit 3) is overlain by sand and gravel deposits (units 1 and 2). The till is present throughout the Atlantic Array survey area and according to the Channel Energy Limited (2010) report by Gardline Geo- survey reached no more than 4 m thickness. However, a later borehole report (Channel Energy Limited, 2011) indicates a much thicker glaciogenic sequence (see below).

Three boreholes (BH 10, 12, 13), out of 15 proposed in a Senergy report for RWE npower renewables (2010), encoun- tered glacial sediments in the Atlantic Array survey area. They include clay and/or sands interpreted as glacial (Pleistocene) with a thickness of 2–8 m, underlying 2–18 m accumulations of sand. Data from four of the 15 proposed borehole locations (BH 6, 11, 12, 15) are presented in a later report by the Danish Geotechnical Institute (GEO) for Channel Energy Limited (2011) and in three of these bore- holes glacial deposits are reported (BH 6A, 12, 15A). The locations of the GEO boreholes are given in Fig. 1. Summa- ries of the borehole logs are illustrated in Fig. 5. The thickest glacial deposits in the Bristol Channel are present between Lundy and the mainland (BH 15A) where 37.9m of glacial deposits are logged (Fig. 5) (Channel Energy Limited, 2011). In this borehole the top 1.9m is composed of sand. This is underlain by three units of silt/clay deposits with occasional subangular and subrounded cobbles. The silt/clay units (1.9– 12.7, 17.2–23.0 and 24.5–39.0m depths) are separated by sand/silt and gravel units. The middle silt/clay units (17.2– 23.0m) and also the sand and gravels separating this unit from the upper silt/clay unit contain ‘rare seams of fine to medium gravel and lignite’. The upper silt/clay unit is predominantly grey although grades upwards into reddish and dark brown gravelly/sandy/silty clays. The lowermost silt/ clay unit is reddish grey and olive-grey in colour.


The borehole log from BH 15A records the entire sediment sequence below 1.9 m as from a glacial environment (Gl) and of glacial age (Gc) (Fig. 5) (Channel Energy Limited, 2011). Some units are logged exclusively as glacial (1.9–5 m), while below this the sediments are recorded as both glacial and marine (Ma) or freshwater (Fw), sometimes all three simultaneously. However, the lithostratigraphy of the borehole log is interpreted in this paper as representing three till units separated by sand/gravel units. The middle till unit and overlying sand and gravels contain organic materials (lignite). Neither the upper nor lower tills contain any references to lignite. While nogeochronology is available, it is possible that the lower till unit is Middle Pleistocene in age and the middle and upper tills are of Late Pleistocene (Devensian Stage) age. The middle till may be Early or Middle Devensian in age, with lignite seams being derived by reworking of interglacial deposits, possibly of freshwater origin. However, further research is required to determine the age and nature of these deposits. Nevertheless, it does appear that multiple till units are recorded in BH 15A.

In BH 12 the clay unit, described as exclusively glacial, is much less thick than in BH 15A and is present for nearly 4 m between 14.6 and 18.5m and is characterized by many colour variations (Channel Energy Limited, 2011). The clay unit is overlain by 14.6 m of sand with shell fragments. BH 12 is associated with a prominent asymmetrical ridge. This has smaller ridges superimposed onto it (ripples) but the main core of the ridge itself could be a moraine associated with ice coming from the west. The prominent ridges in the area of BH 12 (and to the east) look different from other large ripples and large sand waves of the area. The steep western slope of the ridge is consistent with a steeper ice-contact moraine slope.

BH 6A records just over 3 m of clays/silts between 3.5 and 7.6 m. They are described as mudstone and siltstone but are considered glacial in environment and age (with codes Gl and Gc), although codes indicating marine (Ma) and Jura[- ssic] (Ju) are also used (Channel Energy Limited, 2011). These fine-grained deposits are overlain by 3.5 m of sand and gravel mixed with cobbles and boulders. In the borehole log notes these deposits are considered marine/recent or glacial envi- ronment/age and the interpretation is ‘very uncertain’.

The upper contacts of glacigenic silt and clay deposits in all three cores are at similar elevations of 49.1, 48.9 and 48.6 for boreholes 6, 12 and 15A. Sand and gravel with shell fragments overlies this lower unit (Fig. 5) (Channel Energy Limited, 2011).

Geophysical surveys reported by Channel Energy Limited (2010) and RWE npower renewables (2010) have revealed the presence of more than 22 000 boulders (>0.3 m height) in the Atlantic Array survey areas. Each of these boulders has been identified and measured for height with data on each available in the report by Channel Energy Limited (2010, appendix D). This includes the cable routes through Barnstaple Bay. Two concentrated clusters of boulders occur in the Atlantic Array survey area, one in the west and a larger cluster in the east. The extent of the eastern boulder cluster can be extrapolated southwards since concentrations of boulders are also revealed in the three cable routes between Lundy and the mainland (Fig. 1). 

The origin of the boulder clusters can be attributed to either marine or glacial origin, or both. In the former case boulder concentrations may be related to transient shoreline positions. The boulders could also be glacially transported. The fact that till is reported in boreholes in areas where no surface boulders are present, whereas no till is reported in areas where boulders are found, suggests that either the boulders are not of glacial origin or that they represent boulder remnants of a pre-existing till layer, where the fine matrix has been winnowed out. Pleistocene tills are often relatively thin in the central Bristol Channel and no more than 2–8 m in the cores reported from the Atlantic Array survey area (although 16.1m thick between Lundy and mainland Devon – see above). Another possibility is that the boulders are ice-rafted, since erratic boulders are commonly found on shorelines along the coast of Devon and elsewhere in the Bristol Channel, including as far east as Flat Holm (see ‘Cardiff and the Vale of Glamorgan’ below).



The borehole records referred to in the text. There is clearly some variety in the glacial units revealed in the borehole logs -- there may be up to three till units, and some sands and gravels and freshwater deposits / marine deposits. How many glacial episodes are represented?

Boreholes from the BGS north and west of Lundy also report glacial deposits. These include reports from Borehole 72/49 (51 ̊0.860 N, 4 ̊54.820 W), Borehole 72/52 (51 ̊23.030 N, 4 ̊54.450 W) and Borehole 73/60 (51 ̊24.930 N, 4 ̊44.800 W). Borehole 72/49 is situated ca. 25km due west of Hartland Point in Devon. This borehole revealed sand and gravel (0–3 m) underlain by poorly sorted unconsolidated sediments (3–6m), including clay with numerous rounded, subangular and angular rock fragments. The latter includes ‘broken cobbles and pebbles of igneous material’. These unconsolidated sediments are considered ‘Recent and Pleistocene’, while underlying bedrock revealed in the borehole is Carboniferous mudstone. Boreholes 72/52 and 73/60 are situated west of the Atlantic Array survey area between Lundy and Pembrokeshire. In Borehole 72/52, deposits are entirely ‘Pleistocene and Recent’ (i.e. Pleistocene and Holocene), the top 0.5 m recorded as sand, underlain by 3 m of brown-grey ‘Boulder Clay’. In Borehole 73/60 the top 0–2.5 m is ‘Recent’ sand and gravel, below which ‘Pleistocene: Till, red-brown, stiff with small rounded pebbles’ is recorded between 2.5 and 9.5 m.

The offshore evidence clearly illustrates that the Outer Bristol Channel was glaciated during the Pleistocene (Fig. 6). As noted above, Lundy also shows clear evidence of glaciation (Mitchell, 1968; Rolfe et al., 2012, 2014; Fig. 1). The island has large areas of ice-scoured granite bedrock with perched boulders (local lithology) and extensive areas of erratic gravels. Rolfe et al. (2012) dated the granite bedrock surfaces using paired 10Be/26Al analyses, the results demonstrating that there was no evidence of long-term complex exposure (i.e. exposure, burial and then re-exposure etc). However, the island had been exposed for ca. 40ka, i.e. since the Middle Devensian.
=========================

Note that the text also refers to great concentrations of boulders in places. No less than 22,000 of them have been logged -- most of them NOT in conjunction with the glacial deposits found in the boreholes.
Note how similar this proposed ice edge position is to that proposed on this blog as the approx ice edge of the Devensian Celtic Sea piedmont glacier. Maybe things are coming together.......





Tuesday, 28 November 2017

BRITICE 2 -- the new map


Here is the new glacial map published by the BRITICE team.  Note that it is supposed to show only those features attributable to the Devensian glacial episode --  but many features are of course inherited from older glaciations and modified.  Hardly any erosional features can be assumed to belong to the Devensian glaciation alone.....

The map can be accessed here:

https://www.sheffield.ac.uk/polopoly_fs/1.706291!/file/Generalised_map.pdf

Sunday, 26 November 2017

BRITICE Glacial Map 2 -- a mixed blessing



I suppose we should welcome the publication of a big new paper, with multiple authors, describing the compilation of the latest version (No 2) of the BRITICE map of the British Isles.  The map is much more detailed than the first one, and that is to be welcomed, and the accompanying paper makes interesting reading.  The BRITICE project fieldwork is now wound up, but I dare say that there are many papers still to be published, from what has been a very large project involving large numbers of geomorphologists and other specialists.  They have done a vast amount of fieldwork, both on land and at sea..... or have they?

Here is a link to the paper:

http://onlinelibrary.wiley.com/doi/10.1111/bor.12273/full

Clark, C. D., Ely, J. C., Greenwood, S. L., Hughes, A. L. C., Meehan, R., Barr, I. D., Bateman, M. D., Bradwell, T., Doole, J., Evans, D. J. A., Jordan, C. J., Monteys, X., Pellicer, X. M. & Sheehy, M. 2017 : BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet. Boreas. https://doi.org/10.1111/bor.12273. ISSN 0300-9483.

Abstract



During the last glaciation, most of the British Isles and the surrounding continental shelf were covered by the British–Irish Ice Sheet (BIIS). An earlier compilation from the existing literature (BRITICE version 1) assembled the relevant glacial geomorphological evidence into a freely available GIS geodatabase and map (Clark et al. 2004: Boreas 33, 359). New high-resolution digital elevation models, of the land and seabed, have become available casting the glacial landform record of the British Isles in a new light and highlighting the shortcomings of the V.1 BRITICE compilation. Here we present a wholesale revision of the evidence, onshore and offshore, to produce BRITICE version 2, which now also includes Ireland. All published geomorphological evidence pertinent to the behaviour of the ice sheet is included, up to the census date of December 2015. The revised GIS database contains over 170 000 geospatially referenced and attributed elements – an eightfold increase in information from the previous version. The compiled data include: drumlins, ribbed moraine, crag-and-tails, mega-scale glacial lineations, glacially streamlined bedrock (grooves, roches moutonnées, whalebacks), glacial erratics, eskers, meltwater channels (subglacial, lateral, proglacial and tunnel valleys), moraines, trimlines, cirques, trough-mouth fans and evidence defining ice-dammed lakes. The increased volume of features necessitates different map/database products with varying levels of data generalization, namely: (i) an unfiltered GIS database containing all mapping; (ii) a filtered GIS database, resolving data conflicts and with edits to improve geo-locational accuracy (available as GIS data and PDF maps); and (iii) a cartographically generalized map to provide an overview of the distribution and types of features at the ice-sheet scale that can be printed at A0 paper size at a 1:1 250 000 scale. All GIS data, the maps (as PDFs) and a bibliography of all published sources are available for download from: https://www.sheffield.ac.uk/geography/staff/clark_chris/britice.

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



So is the new map reliable, and is it of any use to the rest of the world as a teaching and research aid? Yes, and yes again. But I have major reservations about the fact that the map is available as a "zoom in" version, rather like the BGS geology map available on their viewing facility. Clearly much of the data has been input on a very small scale, so that when we zoom in to look at a part of the countryside at a scale of 1:50,000 or maybe 1:10,000 the result becomes nonsensical, with straight edges to lakes and moraines (for example) that bear no relationship to the details of local topography.

We can only judge things on the basis of what we know, and so I have taken a long hard look at what the map tells us about that area around Preseli and Carningli (my home area). Even given that the map shows "last glaciation features" and nothing older, what we see makes no sense at all.

Glacial Lake Teifi and the surrounding area, from the BRITICE Glacial Map 2

Hypothetical "Glacial Lake Nevern" and other features in the Preseli - Carningli area, 


from BRITICE Glacial Map 2

The outlines of Glacial Lake Teifi more or less coincide with what we see in assorted published papers, but the other information shown on the map is very scanty indeed for the Cardigan - Moylgrove area. That must disappoint many of the authors who have published their research findings, with considerable local detail on the record. When we zoom in on Preseli and the hypothetical area of "Lake Nevern" we find even more problems.  Why is "Lake Nevern"  shown at all?  So far as I am aware, nobody since Charlesworth in 1929 has taken it at all seriously, and many papers by a variety of authors have shown that the "evidence" for it does not withstand scrutiny.  The paper authors site "George 1970"as their source of information, but George was simply citing (very uncritically) Charlesworth, and provided no evidence for this so-called lake.  Careless.   As readers of this blog will know, there are a few places where thin laminated (lake?) sediments appear to be present on the northern flank of Preseli, but they are better explained by short-lived ponding of meltwaters in complex terrain in a dead-ice environment.  I have called this "Lake Brynberian" -- a possible small lake of limited duration and extent.   There are other fluvioglacial landforms and moraines in this area which are in the literature.  They should have been mapped, but have not been.  And where did that impounded lake at Pontfaen come from?  I am aware of no evidence in support of it.

https://brian-mountainman.blogspot.co.uk/2015/05/lake-brynberian-further-thoughts.html

The impression is gained that this map has been produced by researchers accessing satellite and bibliographic data and sitting in front of computer screens, unlike the BGS bedrock and sedimentary map for the UK which has been based largely upon the field notes of surveyors working in the field. Black mark for BRITICE, and glowing praise for the BGS.  In the abstract to the paper, the authors state:  "All published geomorphological evidence pertinent to the behaviour of the ice sheet is included, up to the census date of December 2015."  That is patently not true.  There are abundant references in the literature (just a few of them by me, and many others as well) that have clearly not been consulted.

Then we come to the map showing the wider Late Devensian context, reproduced above from Figure 11 in the article text.  The authors still insist that there was a long "surge" lobe pushing far out into the south-western approaches.  I don't know of any sea-floor or sedimentary evidence that supports it,  and I do not know what glaciological theories have been summoned up in its creation, but it sure doesn't look like any other glacier ever seen on Planet Earth.  It was probably dreamed up by some guys as a way of explaining the short-lived advance onto the northern coast of the Isles of Scilly, but there cannot possibly have been an ice edge more or less at sea-level in western Pembrokeshire and simultaneously more or less at sea-level in the Isles of Scilly, more than 200 km to the south, with ice flow as shown on the map.  The ice must have come from the NW, not the NE, and in an unconstrained situation such as that of the outer Bristol Channel, the eastern ice edge must have been far to the east of that shown on the map.  The map also ignores published evidence of Devensian ice affecting the area around the mouth of Milford Haven.  You can do some searches on this blog for more information.



Overall, this new paper, and the map, are useful contributions from the BRITICE team.  But rubbish in, rubbish out.  Now we need some serious fieldwork, on the ground, involving people who know what they are looking at.  Apparently these days they call it "ground truthing" ---- and it is apparent that we all need more of it.



Friday, 24 November 2017

A week in the Azores




Just in case anybody wonders where we have been lately, we've enjoyed a week of walking in the Azores -- on the main island, Sao Miguel.  Thoroughly delightful.  Great walking territory, in spite of there being no glaciers or moraines to look at.  

Myris would have enjoyed it -- calderas, volcanic cones, ash beds, lava flows, volcanic bombs, fumaroles etc all over the place.  And a great orange-coloured swimming pool fed by geothermal springs, with a water temperature of 36 degrees C.  Now THAT was rather splendid.......

Apologies

Found the source of the problem -- some of the posts were going into junk and may have been deleted inadvertently.  Must be something to do with my screening settings -- will try and sort it out.  Meanwhile, if any important posts have been lost without trace, please try again........

UPDATE, Fri 24th Nov 2017

All sorted, I think.  My "mail rules" -- the settings that determine the mailboxes into which messages are sent -- had all been de-activated.  Goodness knows how that happened -- maybe there was a software update which I didn't notice.  So some messages were simply blocked, I think, and others went into the junk box.   And the sun is shining too........

Thursday, 23 November 2017

Stonehenge and the birds


I found this on the web the other day -- yet another illustration of the extraordinary use of the old ruin by birds, maybe particularly during the autumn flocking - roosting season.  Are they jackdaws or starlings? Whatever they are, the scale of droppings being dumped onto the stones is so vast that the actual damage to the stones has to be greater than the damage that might be done by allowing the geologists to take small cores from every one of the standing stones, recumbent stones and stumps.  We need to know what the provenances of all these stones are, and we need to know what the cosmogenic ages are for the exposed stone surfaces, in order to better understand how the stones got there and when they were erected.

Come on, Historic England -- you know it makes sense!  Let the geologists take their samples!

This is a great NASA image taken from the space station, showing Wales and Ireland, and showing too that the Earth is not flat.  That will come as a surprise to some people.

Because of a freakish cloud cover, we can see beautifully the Cheshire Plain - West Midlands depression along which the eastern branch of the Irish Sea Glacier penetrated to the neighbourhood of Birmingham on at least two or three occasions.

Is there a glitsch?




Just checking with faithful followers of this blog --  I have received a message from one person who has tried to post comments in the last few days, but I have received nothing through the normal Email channel. Sounds as if there is something wrong somewhere.

I'll appreciate it is one or two of you could post something so that we can do a clinical trial.......

Thanks!

Sunday, 19 November 2017

The Survival of Maiden Castle



Here is a great photo of Maiden Castle, a fragile tor exposing Pre-Cambrian (?) rhyolite adjacent to Trefgarn Gorge.  Thanks to Pete Storey and the Pembs Geology Group Facebook page.

As we have explained before, the prevailing view is that this tor is too delicate to have survived Devensian glaciation around 20,000 years ago, and so the maximum position of the ice edge has to be placed further north.  Trefgarn Gorge itself must have been a major meltwater route, carrying vast quantities of meltwater from north Pembrokeshire southwards towards Milford Haven.

Place names in the "Rocky Mountains"

The crags of Carn Meini / Caer Meini / the Ragged Rocks

Following my earlier post on "Caer Meini" and other place names in the eastern parts of Preseli, I have found another old book -- long since out or print --  by ET Lewis, called "Mynachlog-ddu -- a guide to its antiquities."  In it, he makes many interesting points, while displaying rather too much respect for the "expertise" of assorted "experts"......

Anyway, he says some very interesting things about place names. He refers to Carn Meini / Caer Meini as "the remarkable mountain called the Ragged Rocks" and also refers to the area as "the Rocky Mountains" --- maybe with tongue in cheek.  He cites a tourist called HP Wyndham (1774) as his source -- and maybe these labels were used informally by English-speaking visitors to the Preseli area.

Lewis speculates that the name "Caer Meini" arises from the fact that the summits "appear circular and like the stupendous ruins of a castle wall."  That's an imaginative interpretation, but it raises the issue of the AGE of some of these names.

Carn Goedog seen from the east.  A place of great slaughter?

In his text on the meanings of some of the other tor names, Lewis mentions that Carn Goedog means "the wooded carn" or "woodland carn" -- and I have no problem with that, since there is a copse of splendid mature trees not far away at Hafod Tydfil which suggests that without human interference / clearance and animal grazing, mature woodland would indeed be the climax vegetation here on the north flank of the mountain.  There is no reason why that should not also have applied in the Mesolithic and Neolithic, and right up to the time of the Celtic settlement and the naming of familiar places.  On the main ridge, and on the more exposed southern and western flanks of the uplands, the woodland might have been more scrubby.  There are bluebells growing in the Carn Goedog area, and as I have pointed out before, these plants are very good indicators of past wooded environments because they like heavy shade for the summer and autumn seasons.

The Welsh word for "forest" or "wood' is "coedwig" -- so "coedog" could simply be a mis-hearing, mis-spelling or corruption of that word.

More interesting by far is the suggestion by Lewis that "Goidog" or "Goidiog"may be related to an old Welsh word for "slaughter" or carnage. (The modern word is quite different.)  So, given the suggestion that the Battle of Mynydd Carn in 1081 might have occurred on the plateau of Talfynydd, less than a kilometre away from Carn Goedog, might this name have been given to the carn as a remembrance of that bloody conflict?  Indeed, might some phases of the battle have occurred around the carn itself?


Friday, 17 November 2017

Collapsing iceberg arch


I have been watching some of the videos on YouTube of collapsing ice fronts, rolling icebergs and iceberg arches giving way -- now that Arctic cruises are immensely popular, social media are full of snippets of film of "exciting" ice front events and of boats getting rather too close to the action.  There have been some close shaves -- members of the public (and even tour operators) seem to have little idea what natural forces are being unleashed when thousands of tonnes of ice fall into the water. The resultant displacement waves are quite high enough to wash people off rocks where they may be standing, or to turn boats over -- as I tried to demonstrate in my novel "Acts of God".

Anyway, people scream, shout and clap -- and gasp with relief when they manage to escape big waves that threaten to overtake their rapidly retreating boats. But one day there will be a big accident, because tidewater ice edges have become rather too popular with adrenalin junkies........

In principle, I don't have a problem with people getting close to glaciers and developing some understanding of how glaciers work.  And tour operators should certainly be informing as many people as possible about accelerating glacier retreat and the link with global warming.  Sadly, there is too much junk science around -- the fact that an iceberg rolls over, or an arch collapses, or a glacier front suffers a sudden catastrophic failure, does not in itself indicate that this is a "global warming event".   This is all perfectly normal glacier and iceberg behaviour -- and these things happen whether a glacier is advancing or retreating in a deep water situation.

The picture above is from one of the videos,  showing a big chunk of ice falling from an arch -- it would not have been a good idea to be beneath it in a kayak (or any other craft) at the time!  Over the next minute or two, after this photo was taken, the arch lost more and more of its mass in a welter of falls big and small, until it completely disappeared.  Then, of course, the remaining parts of the iceberg made major readjustments by rolling in the water, surrounded by a great apron of brash ice.

Monday, 6 November 2017

Stonehenge -- the authorised version (again)



Julian Richard's new book came out at the end of September.  It's called "Stonehenge -- the story so far", published by Historic England at £22.95 (or whatever), hardcover, 352 pp.

You can read bits of it if you look on the Amazon web site. It's very nicely packaged, with attractive photos and diagrams, and a modern page design.  But what about the text?  Well, from what one can see, we have the same old stuff as usual.  A friend very kindly sent me the text as it relates to the bluestones, in a section dealing with "sourcing and transporting the raw materials".  I am not impressed, since what we have --yet again -- is evidence of an author who is desperate not to rock the boat or to allow for any questioning of the fondly-held assumptions of decades.  Is this bland acceptance of the authorised version something that Historic England insists upon? Is it really true that it cannot admit to any disputes in an "official" publication? Does it really think that Joe Public cannot handle honest academic debate, in which there might be two (at least)  perfectly feasible explanations for one or another of the features at Stonehenge? I should have thought that honesty on this score would excite interest and enthusiasm for our historic heritage, rather then damping it down.........

So what does JR have to say?  Immediately we are into the ruling hypothesis, with confirmation bias flagged up for all to see. We are introduced to Carn Meini (Menyn) which is described as a place of "convenient slabs and pillars." JR continues excitedly: " In this showroom for monoliths tempting stones lie everywhere; some even look as if they have been propped up and are ready for loading onto a sledge for the start of their long journey - perhaps more stones intended for Stonehenge that never made it?" Oh dear. He forgets to tell us that there are at least a dozen other tors in the general area for which exactly the same words could be used.

He then goes on to talk of the geological provenancing, without any mention of the work of Ixer and Bevins, and he actually misrepresents their findings. They will surely not be amused.  Spotted dolerite does not just come from Carn Goedog and Cerrig Marchogion, as he implies. And the rhyolite at Rhosyfelin does NOT provide an "exact match" for bluestone fragments at Stonehenge. Quote: "At this outcrop excavations have shown where a pillar of stone was removed from the rock face, the quarrying dated by radiocarbon to c 3400 - 3300BC."  That weird "pillar" of stone and its "extraction point" pop up all the time, promoted by people who cannot be bothered to apply any scrutiny.  Has Julian Richards or any of the others who promote this nonsense ever been to the site and looked at the narrow natural fissure in the rock face that clearly has nothing whatsoever to do with monoliths or extraction points?  It was dreamed up by MPP in a moment of mad enthusiasm, simply because it was close to sampling point 8 used by Rob Ixer and Richard Bevins, and it has been re-imagined ever since by people who seem to have left their common sense behind.

And the radiocarbon evidence?  Most thoughtful human beings might think "It just doesn't fit.  Therefore, since there is no other evidence of quarrying anyway, we might as well give up on the quarrying thesis and think of something else instead."  But these archaeologists are made of sterner stuff.  "No, since we have already decided that this is a Neolithic quarry, we just have to shift the date back by a few centuries and work out what happened to all those nice bluestones before they were shifted to Stonehenge.  So we have to assume there was a convenient proto-Stonehenge somewhere in the vicinity....." Once these archaeologists have a nice ruling hypothesis to play with, there is no way they are going to abandon it.

And just as the geologists are going to be pretty angry about being ignored, we geomorphologists are also extremely displeased.  For Richards to trot out the quarrying story without question, in the full knowledge that there are two peer-reviewed papers in print which question every single bit of "evidence" presented by the archaeologists in one extremely dodgy "Antiquity" article, is not just careless but also disrespectful and deliberately misleading.  The author had two years to incorporate our findings into his text, and chose instead to ignore them.

Interestingly, having applied no scrutiny whatsoever to the quarrying hypothesis, Richards does devote considerable space to the glacial transport hypothesis.  So he does at least acknowledge that there are two competing theories.  But there is a complete lack of balance in the way he treats them.  He simply repeats the usual arguments about the Boles Barrow stone, the apparent lack of other erratics on Salisbury Plain, and the Christopher Green pebble counts. "Quite simply," says our intrepid author, "the theory of glacial transport does not stand up to scrutiny and should be dismissed."  And he concludes his section with a summary of the ideas of MPP, TD and others about highly prized or magic stones being brought to Stonehenge as a great sacred or symbolic gesture.

That's the authorised story, and Richards is sticking to it, come hell or high water.  If you are still tempted to buy this book, you will, I think, never find a better example of selective evidence citation and confirmation bias.






Friday, 3 November 2017

From the world-famous Rhosyfelin School of Archaeology.....



No comment needed........

Picton Point -- old glacial deposits?



Pembrokeshire geology map with the addition of a speculative flowline for the Irish Sea ice which might have affected the region around Picton Point

Picton Point is the south-facing headland that separates the western Cleddau and the Eastern Cleddau, in the tidal inner reaches of the Milford Haven waterway.  It's a delightful spot, with wide vistas to west, south and east.  My wife and I went over there for a walk the other day, and I was struck -- not for the first time -- by the frequency of erratic boulders and cobbles scattered along the shoreline.  These have not been carried here by longshore drift or by the tides -- there is very little wave action here.  So the erratics have dropped out of pre-existing glacial and fluvioglacial deposits as the low cliffs are nibbled away.

The main processes here (as at Mill Bay, the famous place from which the Altar Stone was supposed to have come) are biological and mechanical.  Tree roots are exposed as the Coal Measures sandstones and shales are sapped or undercut; then the trees tip over more and more until they fall down onto the beach; when that happens, loose rock debris on the cliffline ( already broken up to some degree by vast and expanding root systems) is dislodged and comes crashing down too, resulting in coastal retreat by a few more feet.  And so the process continues.  On the land surface above the cliffline there are glacial and fluvioglacial deposits, and these are dropping down onto the foreshore bit by bit as the coast retreats. The process is slow, but inexorable, and is essentially one of sudden or catastrophic cliff collapses in different locations as one big tree after another comes down-- usually during an extreme storm event like Storm Ophelia or Storm Brian in recent weeks.  A huge oak tree came down close to Picton Ferry during one or the other of those storms.

This tree will be down before too long, and when it falls a chunk of the 
cliff face will come with it.

(As it happens, similar processes have been at work at Craig Rhosyfelin, dislodging large blocks of rhyolite at irregular intervals and causing fallen and smashed-up rock debris to accumulate against the rock face.  There, however, the trees may have always been much smaller.)

Annotated image from the Geology of Britain Viewer (BGS).  Here we can see that glacial deposits are quite abundant in this area.


Erratics are scattered on the foreshore to the east of Picton Point, but they are far more numerous on the western side.  Most of them are less than 50 cms in diameter.  They are rather well rounded, and heavily stained.  This suggests two things to me -- one, that they have come from fluvioglacial sands and gravels rather than from till;  and two, that the deposits are old (Anglian or Wolstonian) rather than Devensian.

  
Two images from the foreshore to the west of Picton Point. Most of the rock fragments are quite angular, having come from recent rockfalls on the retreating cliffline; they are mostly Coal Measures sandstones.  But the rounded cobbles include rhyolite, dolerite, gabbro, volcanic ash, and Cambrian sandstones -- almost certainly from the St David's Peninsula

These are early days in my Picton Point investigations -- watch this space.....





Sunday, 29 October 2017

The Carn Meini mystery



Carn Meini, as we all know, is the place where it all started........ this set of four craggy outcrops of dolerite on the crest of the Preseli ridge was identified by HH Thomas back in 1921 as the place from which many of the spotted dolerites at Stonehenge had come.  He didn't think there was a quarry there, but Richard Atkinson did, and since the 1950's it has of course been promoted heavily by EH and Uncle Tom Cobbley and all as THE Neolithic quarrying site.  This idea was pushed very recently by Tim Darvill and the late Geoffrey Wainwright in their big chapter in the Pembrokeshire County History, even though the geologists now seem to be agreed that none of the spotted dolerites at Stonehenge actually came from here.  As they say, you don't want the truth to get in the way of a good story.......

For good measure, TD and GW were intent upon making the place even more famous, and in the literature it is now promoted as having the earliest stone quarry in the British Isles -- dating from the Mesolithic -- where "meta-mudstones" were extracted for reasons unknown.  Then there is that funny little enclosure, which TD and GW promoted as a sort of protective barrier to stop the locals from pinching valuable bluestones from the quarrying storage depot.  So there is quite a story there, which you can believe or poke fun at, as the case may be......

Now then.  We have a problem about the name.  "Carn Meini" means "stony crag".  On the older maps the locality is shown as "Carn Meini" or "Carn Menyn" more or less interchangeably -- but Carn Menyn is the preferred spelling on the modern OS maps.  That's a bit of a nonsense name, since "menyn" means "butter" -- and how can you name a craggy rock after a lump of butter?  Bertie Charles, in his book on the place names of Pembrokeshire, explains this as an indication of a fertile upland grazing area that ran up to the rocks, maybe involving cattle grazing and butter making in a "hafod" or summer settlement.  There is indeed a trace of a ruined dwelling with small enclosures about a kilometre from the rocks, and another just 500m from the rocks.


Was there a cottage here, where cattle were looked after on their summer grazing area, 
and where butter was made?

But then it gets interesting.  Last night I was reading the classic local history of Mynachlog-ddu parish, by ET Lewis.  With very rare exceptions, he refers to the rocks not as Carn Meini but as CAERMEINI.  In doing that, he must have been representing local verbal usage of the name, and he cites a lot of documentary evidence as well.  He was a fluent Welsh speaker, and he would not have confused "carn" with "caer".  The word means a fort, castle or citadel -- or some defended and strengthened place.  Interestingly enough, the three closest farms are Caermeini Isaf (lower), Ganol (middle) and Uchaf (upper) -- and those spellings are used on the current OS maps. Sometimes descriptive words are used in a picturesque sense (eg "castell" or "castle" is used for a crag that looks particularly romantic or ruinous!) but Lewis clearly did not think that this was the case here.  So we have a tradition locally of a fortified site at Carn Meini or Caer Meini...........

Lewis also mentions the local Welsh dialect in the Mynachlog-ddu area, in which the word "mynydd" (mountain) is pronounced "mini" -- so he infers that both "meini" and "menyn" might just be corruptions of the word for "mountain".   So the meaning might be "mountain fort" or "mountain crag" -- ie nothing to do with butter.

So here is an interesting thought -- could it be that there was a defended structure on Carn Meni which features in local traditions and local place names?  Is the "walled" enclosure described by Darvill and Wainwright a candidate?  It really is a pathetic little feature which can never have been very prominent, but who knows?

Now it gets even more interesting.  ET Lewis has a theory that the flattish plateau area above Talfynydd was the site of the Battle of Mynydd Carn in 1081, in which the forces of Rhys ap Tewdwr and Gruffydd ap Cynan (th princes of Gwynedd and Deheubarth) were involved in a terrible conflict, with huge slaughter, with the armies of the princes of Powys and Morgannwg.  Rhys and Gruffydd marched eastwards for a long day from St Davids, and Lewis thought that they marched along the old Golden Road on the Preseli ridge to meet the foe.  When they arrived in the vicinity of Talfynydd (just over 1 km west of Carn Meini) the armies of the princes of Powys and Morgannwg had already been in residence in the vicinity for about three weeks.  Could it be that that they had established their camp around the Carn Meini crags?  And could it be that the princes themselves had established their HQ in the "enclosure",  demarcated by a rough wall which could have been built in a day or two by a group of soldiers with nothing much else to do while they waited for battle?

Darvill and Wainwright were unable to establish that the "wall" had anything to do with quarrying, and did not demonstrate that it was a Neolithic or Bronze Age feature. I think it might have been built in prehistoric times as part of a simple animal enclosure -- but I am more and more attracted by the idea that it might not be prehistoric at all, but was built by some bored soldiers in the year 1081.



Maiden Castle near Trefgarn -- a tumbledown and very delicate tor which was presumably named because it looked like a ruined castle.








Saturday, 28 October 2017

On the redistribution of bluestone erratics



In the light of my previous post, on the "bluestone erratic train", I have been re-reading this interesting paper by Phil Gibbard et al:

New insights into the Quaternary evolution of the Bristol Channel, UK
PHILIP L. GIBBARD, PHILIP D. HUGHES and CHRISTOPHER J. ROLFE
JOURNAL OF QUATERNARY SCIENCE (2017)
ISSN 0267-8179.
DOI: 10.1002/jqs.2951
https://www.researchgate.net/publication/318198065

I covered it briefly in a previous post, here:


It's great to see that things are moving (so to speak) in the Bristol Channel arena at last, and a number of geomorphologists are realising that hereabouts we will find information crucial to our understanding of the Quaternary of south-west Britain.  Sounds obvious -- so it's all the more surprising that the area has been relatively neglected while research has been concentrated in other areas like north Wales, eastern England and the fringes of Scotland. 

For many years it has been known that during the Late Devensian glacial episode, ice from the South Wales outlet glaciers pushed across the coast in three main areas -- Carmarthen Bay (the Tywi Glacier), Swansea Bay (the Tawe and Neath Valley glaciers) and in the Cardiff-Newport area (the (Taf and Rhondda Glaciers).  The Usk Glacier is not thought to have reached the coast.  Most of the work on these features, as described by Prof David Bowen and others, has been land-based, and is summarised on this map:



A lot of work is now becoming available in the literature relating to the Bristol Channel sediments and bedforms, as repeated in other posts on this blog.  This is suggesting that Devensian ice did cross the coast around Cardiff and Newport, and that the above map needs to be corrected in that respect.  And the evidence for the inner and outer moraines in Swansea Bay is truly spectacular, as shown in the map at the head of this post, taken from the Gibbard et al paper.

What this all means is that any bluestone erratic train running between Preseli and Salisbury Plain would have been effectively disrupted or smashed to pieces by these Devensian advances involving ice from the Welsh ice cap, flowing broadly southwards from the Brecon Beacons and the uplands of the South Wales Coalfield. 

Here is another map from Gibbard et al, with my hypothetical bluestone erratics train superimposed upon it.


So if anybody asks me in future why there is no trail or train of large bluestone erratics running all the way from Preseli to Stonehenge, I will show them this map.  It demonstrates that there is NOWHERE in South Wales where the erratic train could have survived intact -- in an unmodified form.  In Carmarthen Bay, Swansea Bay and the Cardiff-Newport area old Irish Sea Glacier erratics might well have been flushed out beyond the present coastline.  In other areas any erratics lying about will have been incorporated and reworked into Devensian morainic deposits.  The erratics will still be there -- somewhere, and probably buried -- but the chances of finding them will be infinitesimally small.  In fact pre-Devensian glacial deposits are also very difficult to find in Glamorgan; we only know of one substantial survival, at Pencoed.

So that leaves two areas where we might find erratics -- immediately to the south of Preseli and in Somerset and Avon.  In the latter area, as we all know, no big bluestone erratics (demonstrably from Pembrokeshire) are known apart from those at Stonehenge.  But in eastern Pembrokeshire, down-glacier from the Preseli Hills, there are abundant bluestone erratics, as noticed by Thomas, Griffiths and other researchers.

By the way, ALL of the glacial limits on the above map are speculative.  I have argued over and again that the Late Devensian limit shown does not make any sense and I don't know why researchers keep on showing it on maps when it defies all the rules of glacier behaviour.  And the Early / Middle Devensian limit is even more controversial --  I'll report soon on a new paper that demonstrates that the assumed glacial episode at that time is just as fanciful as a bluestone quarry. (My words, not theirs.......)









Friday, 27 October 2017

The Bluestones Erratic Train


Over the years we have discussed erratic trains and trails on many occasions, and I have just prepared a new map showing the main ice streams affecting the Bristol Channel area in the Anglian Glaciation.  (Or maybe it was the Wolstonian?  Nothing is certain in this life......)

I'm fairly happy with the ice stream arrows for both the Irish Sea Glacier and the Welsh outlet glaciers affecting South Wales.  The red arrow is based on the paper I did with Lionel Jackson in 2009 in "Earth" magazine, suggesting a contact zone along which the two ice masses ran side by side at more or less the same speed, without much mingling.  (Ice acts in some ways like a fluid, but we must not carry that analogy too far.....)

Along the red line we might expect to find a train of bluestone erratics, but only if there was a continuous process of erosion and entrainment at the Presell end of the line, in the source area. As I have explained, I think the entrainment of Preseli erratics (spotted and unspotted dolerites, rhyolites, dacites, volcanic ashes and sandstones) might only have occurred on a substantial scale at the beginning of the glaciation concerned, with the supply cut off as the ice thickened.  So the route might be approximately OK, but the "erratic train" might just be a pipe dream.

There would also have been wobbles in the route, and in reality the "red route" would have had a lot of kinks in it, in response to waxing and waning ice pressure both from the southern flank and the northern one.

Then there comes the last complication -- the wastage of the Irish Sea Glacier, which would probably have been catastrophic and very rapid.  It appears to be normal towards the end of a glacial episode for "pulses" or readvances to occur around the fringes of smaller ice masses such as the Welsh ice cap -- and advances of the South Wales glaciers could well have pushed the debris associated with the Irish Sea Glacier (including quantities of erratics) southwards, beyond the present coast line and into the area now submerged beneath the sea.  There are signs of just such terminal and lateral moraines both in Swansea Bay and in Cardigan Bay, associated with Devonian glacier advances following Irish sea ice wastage.

More to be discovered -- of that I have no doubt.

Tuesday, 24 October 2017

Altar Stone, Thin Section 277, and the Senni Beds

Thin section of sample 277 -- courtesy Rob Ixer

Don't get me wrong. I just love the idea that the Altar Stone at Stonehenge has come from the ORS Senni Beds -- but since this is a scientific blog, every now and then we need to ask some inconvenient questions.

The current orthodoxy is that the Altar Stone is from the Senni Beds -- and not from the Cosheston Beds on the shore of Milford Haven. The foundation of all of that is the paper by Ixer and Turner-- much cited -- dating from 2006. That's quite a while ago, in geological terms.

Reference:  Ixer, R.A. and Turner, P. 2006. A detailed re-examination of the petrography of the Altar Stone and other non-sarsen sandstones form Stonehenge as a guide to their provenance. Wiltshire Archaeological & Natural History Magazine 99, 1–9.

Access: 

I have been looking at the paper again, and am forcefully reminded that there is actually a huge amount of doubt surrounding this identification. The centrepiece of the work by Ixer and Turner was a thin section from the Salisbury Museum collection, labelled "277 Altar Stone Stonehenge."  Who took the sample, and when?   Is sample 277 one of the Cunnington samples?  Quote: "Cunnington (1884) identified five fragments of the Altar Stone, that he assumed resulted from dressing of the stone, amongst his loose finds, although they are now missing."

Quote: "Therefore, the thin section labelled ‘277 Altar Stone Stonehenge’ in the Salisbury Museum Collection is likely to remain, for the foreseeable future, the only piece of the monolith available for investigation. It is imperative then that it should be described as fully as possible and that this description becomes widely available."

In Discussion (Quote):

This, paper represents the first detailed description of the Altar Stone for over eighty years and is in broad agreement with H.H. Thomas other than his identification of abundant garnet and glauconite. Glauconite is a green, chlorite-like mineral and so, if present, has been subsumed under chlorite in the present description. The disparity over the amount of garnet is more significant and puzzling. Thomas noted significant amounts of garnet in his ‘heavy residues’ (Thomas, 1923, 244) but did not report garnet in his thin section description of the Altar Stone. Although trace amounts of garnet can be overlooked/underestimated in thin section the present study could not confirm significant amounts of garnet microscopically. The presence and amount of garnet is important as Thomas was struck by the coincidence between the garnet-rich nature of his Altar Stone ‘heavy residues’ and the unusually garnetiferous nature of the Cosheston Beds and it was the presence of these unusual amounts of garnet in both, that led him to suggest the Cosheston Group might have been the origin of the Altar Stone. Without further sampling (this would require many grammes of Altar Stone to crush before separating the heavy minerals) the garnet problem must remain unresolved.

The big issue here is the amount of garnet among the heavy minerals in the rock.  Herbert Thomas and Richard Thomas have both stated that there are substantial amounts of garnet in the Altar Stone itself -- but garnet is missing from thin section 277.  There are also substantial amounts of garnet in the Cosheston Beds.  So was HH Thomas right all along?  And have Ixer and Turner simply assumed that thin Section 277 was correctly labelled, when it might have just come from a piece of debitage  assumed -- unreliably -- to have come from the Altar Stone?

A simple matter of a sample being mislabelled?  The mystery deepens.........


Monday, 23 October 2017

MPP: I have discovered a speculation!


I just came across this. I thought it worth sharing, especially since it came from an academic web site that prides itself on its academic rigour........

Mike Parker Pearson: "I led the team of researchers that discovered that Stonehenge was most likely to have been originally built in Pembrokeshire, Wales, before it was taken apart and transported some 180 miles to Wiltshire, England." 

Stonehenge isn’t the only prehistoric monument that’s been moved – but it’s still unique

http://theconversation.com/stonehenge-isnt-the-only-prehistoric-monument-thats-been-moved-but-its-still-unique-51962

(This was in the “science and technology” section of The Conversation…..Dec 11th, 2015. It prides itself on "Academic rigour, journalistic flair”………….)

Saturday, 21 October 2017

Green Bridge of Wales -- the beginning of the end?

Pics from Doug Reubens and Gareth Davies

 This has nothing to do with Stonehenge, bluestones or glaciers -- but since we enjoy talking of the forces of nature on this blog, this might be of interest.  Storm Ophelia has been causing some severe cliff falls in Pembrokeshire -- and one of the most spectacular rockfalls has been on the tip of the Green Bridge of Wales (one of the most famous arches in the British Isles).  These photos show the damage.

The outer "block" (which will become a stack when the arch goes) is now much reduced in size, and the state of near-equilibrium that existed there is gone.  Not sure how this will affect the stresses in the arch itself.  Depends how riddled with fractures it is.  Ironically, the compression on the arch may now be greater than it was before, so it may become stronger.......... we shall see........

As I write, Storm Brian is battering the coast, and lots of people are rushing down to the limestone cliffs and the Stack Rocks area with their cameras.........

I'm more interested in the submerged forest, and wonder if it will be exposed after this storm surge coinciding with spring tides.

 Pic: Beth McColl.  After the fall.........

Another photo, taken 22 Oct by Guy Candler.  It shows the fracture face in much more detail.  

POSTSCRIPT

We now know that the large fracture scar high up on the pillar is the result of the Storm Ophelia storm (16 October), and the lower (smaller) scar is the result of a second rockfall during Storm Brian on 21st October