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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Tuesday, 22 August 2017

Erratics from Newport beach

There have been some very low tides recently, and on a walk the other day I found a large exposure of rounded pebbles on an outcrop of till, very close to the outermost end of the river channel.  This was very close to the old Lifeboat Station at Cwm.  These pebbles are clearly derived from the till, but the rounding is modern -- every now and again this area is exposed and subjected to quite violent wave action.  This is a photo of some of the pebbles collected:


The bulk of the pebbles exposed just now are sandstones, gritstone and shales -- many of these are typical of the Ordovician sediments in this general area, and from the eastern part of Cardigan Bay -- in Ceredigion.  But there are a lot of these igneous pebbles too, which I refer to as porphyritic volcanics, and they sure as eggs are not local.  To my untrained eye they look as if they might have come from the Ramsey Island volcanic outcrops, but an origin there is very unlikely -- so I suggest that they have come from either the Harlech Dome, from the Snowdon volcanic zone, or maybe even from the Lake District.   Maybe from all of these areas...........

I'm intrigued by how similar this assortment is to the pebbles which Sid Howells is looking at from Flat Holm.  Some of those, of course, COULD have come from Ramsey Island, since an origin there would accord with what we know about directions of ice movement.

This is a photo which I published earlier, of some of the Flat Holm erratic pebbles:


All advice on what these latest pebbles from Newport are, and where they might have come from, gratefully received.......

Monday, 21 August 2017

Multiple bluestone rock types -- but how many provenances?



In January 2015 Myris said that the geologists accept around 10 different rock types in the bluestone assemblage.  But the number of rock types is self-evidently not a guide to the number of provenances. 

The dolerites, for example, which some might wish to count as belonging to a single rock type, have almost certainly come from multiple locations -- the samples reported on are all different, suggesting that they have NOT all come from the same place.  The same is true of the "rhyolites with fabric" -- one rock type maybe, but three different groups according to the analysis. So there are three (at least) different provenances for that rock type alone.  The same is true of the sandstones -- they are all sandstones, but of different ages and from three different places.

See also:
https://brian-mountainman.blogspot.co.uk/2014/08/the-vexed-question-of-stonehenge-stones.html 
https://brian-mountainman.blogspot.co.uk/2010/11/more-than-30-different-bluestone.html
https://brian-mountainman.blogspot.co.uk/2016/05/talk-to-chartered-institute-for.html

Here we go again, with an update of the list published in January 2015.  Maybe this will all become clearer with the publication of a new popular paper by Ixer and Bevins in Geology Today, in the autumn. There are at least 18 different rock types, and probably more than 20. That means multiple provenances, exactly as you would expect with a collection of glacial erratics.

** There are 31 dolerite orthostats, of which 14 were sampled in 1991 and 2008.  Some are standing stones and some are stumps.  Some are spotted and some are unspotted.  The spots are now thought to be not felspars but aggregations or clusters of low grade, secondary metamorphic minerals.

Bevins, Ixer and Pearce (2014) analysed 22 Stonehenge dolerite samples, and suggested that they were clustered into three groups, with one sample petrologically distinct from all three.  So there are three groups and one outlier -- four types. Every one of the 22 samples is unique, and the possibility must be entertained that every one has come from a different geographical location in eastern Preseli.

** There are four above-ground volcanic rocks in the orthostat collection (stones 38, 40, 46 and 48).  There seem to be four distinct types - two dacites and two rhyolites.  They are referred to by the geologists as rhyolitic tuffs, foliated rhyolitic tuffs, crystal-lithic-vitric tuffs, and argillaceous tuffs.  They have come from four different north Pembrokeshire locations. Stone 38 has an unusual mineralogy including graphitising carbon.  

** There is not much debris to match the 4 volcanic rock orthostats in the Stonehenge debitage, but similar fragments are found in the great cursus field.  In the debitage, unique volcanic material has been classified as belonging to two types -- Volcanic Group A and Volcanic group B.  None of the potential parent orthostats for Volcanic Group A (32c, 33e, 33f, 40c and 41d) have been petrographically examined, making it impossible to relate this debitage to any (or all) of the buried stones.  Ixer and Bevins (2016) say: ".........on present knowledge the origin(s) of the Volcanic Group A lithics is still expected to be found within the Ordovician volcanic sequences in the north Pembrokeshire area on the northern side of the Mynydd Preseli range probably amongst those outcrops examined by Evans (1945)."

** There are two micaceous sandstone stumps -- numbered 40g and 42c.  There are also lumps of Lower Palaeozoic sandstone scattered about in the debitage -- the largest lump weighs c 8.5 kg.  There appear to be two types, with possible sources in the Lower Palaeozoic rocks of north-west Pembrokeshire.

** The Altar Stone (stone 80) is a greenish calcareous sandstone, probably from the Senni Beds of Carmarthenshire or Powys. (It is not from Milford Haven.)  So just one rock type here. There is some debitage related to this stone, but it cannot be established that the fragments came from the Altar Stone itself (Ixer and Bevins, 2013).  The most feasible provenance for the stone is the Laugharne - Craig Ddu -- Llansteffan area of Carmerthenshire.

** In the debitage there are many fragments of rhyolites, some with planar fabrics -- and most matching closely (but not perfectly) the rock outcrops at Craig Rhosyfelin.  There appear to be three distinct groups of "rhyolites with fabric", all assumed to have come from the Pont Saeson area.  There may be a match with orthostat 32e or 32d (Ixer and Bevins, 2011), but the debris analyses does not match any of the known volcanic orthostats.

** There are also some basic tuffs in the collection of fragments from the debitage -- two lithologically different types (Ixer and Bevins, 2013). These are probably also from the Fishguard Volcanic Series.

** Other lithics in the stone collections from the debitage (eg. haematite (from the Reading Beds?), greensand, slate, limestones, Mesozoic sandstones and gabbros) appear genuine, and need further research.

To sum up: 

Ixer and Bevins (2014) state that there are “about ten types of bluestone” represented in the orthostat / debitage samples, but they also show that these have nonetheless come from at least 20 different locations.  It is estimated on the basis of the above points that there are at least 30 different rock types represented in the full "bluestone assemblage" -- especially when due respect is given in this count to cobbles as well as orthostats and flakes, for reasons frequently recited on this blog. 

It should also be noted that the majority of the 43 bluestone monoliths/stumps at Stonehenge have still not been sampled and analysed.  The new work reinforces the idea that the Stonehenge bluestone assemblage is made up entirely of rock types from the west, and that they have come mostly from north Pembrokeshire. 


Finally, a plea to the geologists -- if there is anything wrong with this list, please tell us about it, and give us your corrections and citations.  As ever, I'll publish them without fear or favour!

Friday, 18 August 2017

Horrid Happenings at Rhosyfelin


This is from the Oriel y Parc Facebook page, advertising a storytelling session which I have promised for them next week.  Goodness knows why they chose this pic of me at the Old Ruin.  Nothing to do with me.  That got me thinking.  Since I am supposed to be telling terrible tales and horrible histories and fantastical fables, I need the advice of experts.

Tell me, should I tell the terrible tale of the hairy monster who comes out of the dark woods to dig deep holes in the ground at Rhosyfelin, every year at the time of the harvest moon?  The kids could probably cope with it, but would it all be too scary for adults of a sensitive disposition?

Thursday, 17 August 2017

Bluestone recycling


What were we saying about the re-use of bluestones in many different setting at the old ruin?  Note that the artist has even accurately portrayed the many different bluestone lithologies..........

Wednesday, 16 August 2017

Spats are as old as the hills.......


Geologist and physician Henry Hicks, a good Pembrokeshire man who 
did not suffer fools lightly.

Sometimes one wonders whether our little academic spats demonstrate a lowering of academic standards, or a reduction in tolerance, or a sort of hypersensitivity on the part of researchers.  Surely it cannot have been so bad in the good old days?  Well, it's reassuring to know that it was just as bad, if not worse.......

Here is short extract from Geol Mag 1886, involving Henry Hicks (who seems to have been a crusty sort of fellow) and a certain Prof Hughes, who clearly had a habit of expressing strong views on places he had not actually visited and things he knew nothing about.........

Geol Mag Volume 3, Issue 12
December 1886 , pp. 566-571
V.—On the Ffynnon Beuno and Cae Gwyn Caves

Henry Hicks

DOI: https://doi.org/10.1017/S0016756800466926
Published online: 01 May 2009

Abstract

The best reply that I can make to Prof. Hughes’ remarks, in the Geological Magazine for November, on the Ffynnon Beuno Caves, is to publish the substance of the report presented to the British Association, especially as an opportunity will be given to those interested in the inquiry to examine the section during the further explorations to be carried on, probably in the month of June next year. Some of his statements—especially as regards the position of the fence, which is entirely at the opposite end of the cavern to that at which the flint flake was found, also as to the position of the flake, and. the nature of the deposits overlying it—are so entirely misleading, that I can only account for such statements being made by the fact that Prof. Hughes did not visit the section, though strongly urged by me to do so, until it had been almost entirely covered over, and work for the time suspended, and by his hasty survey of the surrounding conditions.

Erratic blocks of England and Wales



Thanks to Rob for drawing my attention to this wonderful article and map.  I had never seen this info before.  It has come to the surface thanks to the great project under way in the Birmingham area, in which Rob and Prof Ian Fairchild are involved.

Look at the date of this work: 1878.  That's rather a long time ago.  But our Victorian ancestors were great field workers and often pretty good geologists too -- even though their ideas on glaciation were rather primitive, to say the least.  No matter -- the observations formed the foundations of much of the glacial history as we understand it today, and the maps are classics of their kind.

I particularly love this bit:  "There are probably many boulders which I have not yet seen, and the positions of which, therefore, are not shown in the map."  Quite so.

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

Thanks to the wonders of OCR, here is a transcript of part of the paper:


ERRATIC BLOCKS OF ENGLAND AND WALES. p 425

RESULTS of a Systematic SURVEY, in 1878, of the DIRECTIONS and Limits of DISPERSION, MODE of OCCURRENCE,and RELATION to DRIFT-DEP0SlTS of the ERRATIC BLOCKS or BOULDERS of the West of ENGLAND and East of WALES, including  a REVISlON of many Years' previous 0BSERVATIONS.

By D. MACKINT0SH, Esq., F.G.S. (Read March 26, 1879.)


Extract
IX. :EXPLANATION OF MAP, PLATE XXlI.

As it is impossible to ascertain the precise routes taken by boulders, in a map it is perhaps least presumptuous to draw straight or slightly curved lines from their sources to their terminations. As most of the Kirkcudbrightshire granite blocks would appear to have been dispersed from Criffel mountain or the neighbourhood, to prevent complication I have represented that mountain as the centre of dispersion. The barrier offered by the westerly extension of the Cumberland mountains renders it necessary to assume a curve in the route taken by these boulders. As I have already described the Shapfell granite dispersion (see Geol. Mag. for Aug. 1870 ; and Quart. Journ. Geol. Soc. for Aug. 1873), and as it has been made the subject of papers by other authors, I have only mentioned it in the map. For the same reason the stream of large limestone boulders found along the east coast of Morecambe Bay is merely mentioned. The boulders of Silurian grit, felstone, &c. which went south from the mountain-front of Westmoreland are merely mentioned, as it is not certain that many of them found their way further south than Morecambe :Bay. The "greenstone" boulders are not inserted in the map. As the Arenig boulders which are believed to have wandered as far south as the neighbourhood of Bromsgrove would appear to have gone first in an easterly direction, it is obviously necessary that a curved route should be assigned to them in the middle part of their course. There are probably many boulders which I have not yet seen, and the positions of which, therefore, are not shown in the map.

An attempt to map the positions and courses of boulders is justified by the fact that most of them have been found more or less imbedded in clay or gravel, at all angles, often standing on end - in other words, in the positions in which they were left by the ice which carried them during the Glacial period.

Tuesday, 15 August 2017

Just see how the jolly bandwaggon rolls..........


 Image:  Colby College


I thought that with the departure of Phil Bennett, some common sense might break out in Pembs Coast National Park HQ, and that there might be rather more respect for the facts. Fat chance.  The opportunity of turning a dishonest quid or few has proved too tempting to miss, so here we go again.  The usual nonsense in the blurb, in spite of endless requests from me to moderate the purple prose. A guided bus tour on 20th Sept, at the end of the digging season, led by MPP himself, complete with barbecued burgers just as they used to be in the good ol' days at the Durrington raves. 

Then the usual evening talk at 7.30, for the ears of the faithful.   Title: The Welsh Origins of Stonehenge.  Depressing, isn't it?

If you want to join the whole rave, it will set you back almost £40. I think I might give it a miss.......

http://www.pembrokeshirecoast.org.uk/default.asp?PID=367&EO=2520