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

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.

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


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:!/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:

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. ISSN 0300-9483.


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 moutonnĂ©es, 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:


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.

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.......


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!