Banc Llwydlos Ancient Village

 

One of Hugh's drone images of the site

Somebody posted (on Facebook) the lovely image of the Carn Euny ancient village in Cornwall.  This is reputed to be of Iron Age / Romano British construction, possibly with  parts that go much further back.  It's on a very similar scale to the Banc Llwydlos "village" on the northern side of the main Preseli ridge, which Hugh 365 and I have been mentioning at frequent intervals over the past few years.

See this listing:

https://coflein.gov.uk/media/86/748/dat21_03.pdf

Surely it MUST be excavated?

Myth making and national trauma

In the recent interview which I did with Jacky Henderson, I referred to the "national context" in which HH Thomas proposed his theory of bluestone transport.  I referred to national trauma and the need to believe in the civilising influence of our ancestors.  Jacky and Coral illustrated the point by inserting a couple of images of women working in munitions factories.  That was fine, but more appropriate images might have been those pasted above, if we are to appreciate what the national mood might have been........

The Great War, the Twilight of Empire and the Supremacy of Man...........

Ten years ago, I posted this:

I have done a number of posts in the past about the socio-political climate that existed in 1920-21 when HH Thomas was formulating his ideas on the Stonehenge bluestones -- and preparing and presenting his lecture to the Society of Antiquaries in which he flagged up the heroic efforts of our Neolithic ancestors.

I have picked up on the fact that there was a great need, in Britain at that time, for reassurance and for a demonstration of the fact that Britain was a place of ancient wisdom and high civilization -- and having to cope with barbarians and the forces of darkness in various parts of the world. The trauma of the Great War was still in everybody's minds. The aspirations of the British Empire were of course never far away either....

Two authors who have found expression for this are David Keys and Stephen Briggs. David Keys, in the article copied above (from The Independent, 22nd April 1990), said: "But then came the Great War, twilight of Empire, and the supremacy of man. Out went natural explanations as to how Stonehenge's monoliths arrived on Salisbury Plain. In came a theory that made prehistoric engineers look, in their own Stone Age sort of way, every bit as capable as the ancient Egyptians............. The idea that the monument was constructed by ignorant savages directed by engineers from some superior civilisation struck a chord with 20th century Britons who lamented the passing of Empire, but cherished what they perceived to be Britain's civilizing role in the world."

Stephen Briggs, in an unpublished paper called "Preseli, Stonehenge and the Welsh Bronze Age", said this: "Because archaeology in the post-War years (ie after 1918) demanded our forebears to have been intrepid and sophisticated, and since it could be demonstrated that a bunch of schoolboys were able to devise a method to move the stones, therefore if it were possible, therefore it was probable........."

... and then this: ".........British prehistory has been anxious to own an important proof of early human prowess, but instead of being satisfied with the achievement represented by the erection of the stones at Stonehenge, we have cast Neolithic and Bronze Age man in our own mould, as a man of extensive geographical knowledge, a man of taste and one who left behind remains from which his political systems and trading routes could easily be traced."

That all feeds in very neatly to my comments about the lack of scrutiny of Thomas's ideas, and also into my post about the romance of the venturesome traders.

This is from another previous post:

A few months ago I spoke to the archaeologist and antiquarian Stephen Briggs about the idea (which I explore in the book) that sometimes an archaeological idea can be used for the promotion of the national interest. We only have to look at the manner in which the pyramids, the Easter Island heads, Angkor Wat, and the Great Wall of China are promoted as national icons or as symbols of great and ancient civilizations. Stonehenge is no different -- as journalist David Keys has pointed out in assorted newspaper articles. Stephen confirmed for me that after the First World War there was a strong emphasis -- during the rebuilding of a battered world -- on the triumph of civilization over the forces of darkness, and on the civilizing influence of the British Empire. Archaeologists and politicians were interested in flagging up the great achievements of our ancestors -- and when HH Thomas came up with his story of the great stone-hauling expeditions this was like manna from heaven! The media loved it, and I actually think that the lack of critical analysis and criticism from other academics was largely down to the fact they they thought any criticism would have been UNPATRIOTIC. There was also, says Stephen, an attempt to show that the Neolithic tribes of Britain were actually cleverer than the Neolithic tribes of Germany -- the defeated enemy. German archaeologists were, at the time, discovering that most of their megalithic monuments were built of stones collected from the immediate vicinity; what better way to show the "superiority" of British Neolithic tribes than to show that they were capable of collecting their stones from vast distances away? So the Stonehenge story was born -- as a way of flagging up to the world that the inhabitants of this small island were incredibly clever, at a time when others were still brutes who were incapable of organizing great civil engineering projects. "Anything you can do, we can do better!" This all sounds too crazy to be true? Indeed -- but you'd better believe it, since it's quite well authenticated.

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The more I think of it, the more convinced I am that in the years following WW1 people WANTED reassurance and national heroic myths -- and HHT was only too happy to oblige.  And to their eternal discredit, the archaeological and geological establishments went along with the myth-making without ever subjecting HHT's ideas to proper scrutiny.

The Lake House meteorite - in the news again

I'm intrigued by some of the key components in the story as it is told by Colin and Judith Pillinger -- ie the arrival on Planet Earth c 30,000 years ago, the "frozen preservation" for 20,000 years, the discovery by the Neolithic or Bronze Age inhabitants of Salisbury Plain, the burial in a ceremonial mound, and finally the rediscovery and extraction from the calcium-rich environment.  I am trying to track down the published papers which underpin this story...........

https://brian-mountainman.blogspot.com/2013/04/new-evidence-of-glaciation-on-salisbury.html

I tried to get hold of the research data more than 10 years ago, without much success.........

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The recent paper:

Pillinger, CT and Pillinger, JM. 2024 Grandfather's stone: the Lake House Meteorite, Britain's largest and earliest extraterrestrial sample. Wilts Arch & Nat Hist Magazine 117, pp 181-196.

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The record:

Lake House 51°8.98’N, 1°48.60’W

England, United Kingdom

Found: Early 20th century

Classification: Ordinary chondrite (H5)

History: Lake House is a large Elizabethan country house dating from 1578 located in the village of Lake in the county of Wiltshire, England. Photographic evidence demonstrates that the meteorite was located on the top step at the main entrance to Lake House at least as early as the first decade of the twentieth century. Robert Hutchison (Curator of meteorites, NHM) was notified of the existence of the meteorite in a letter from Robin Bailey dated 13 Nov 1991. A note written on a copy of this letter in Robert Hutchison’s hand writing and initialed "RH" and dated 16 Sept 1991 reads: "probably a chondrite Ol +Px +Ct …? metal with Ni …sulphides". Mr Bailey was unaware of a detailed history of the meteorite, which he described as being collected by his grandfather.

Physical characteristics: The single remaining mass can be recognized as the major portion of a larger meteorite. The existing fragment, measuring 55 × 38 × 35 cm, is dark brown, extremely weathered and deeply fractured, consistent with being exposed to the elements for a long period of time.

Petrography: Distinct chondrules are present, but these tend to have poorly defined boundaries. Porphyritic types predominate, but barred olivine and radial pyroxene textured chondrules are also common. Chondrule mesostasis is recrystallized, with grain sizes generally below 50 μm. The sample is cut by a network of veins, up to 2 mm thick, filled with secondary weathering products.

Geochemistry: The oxygen isotope composition of the meteorite was measured (after washing in EATG to remove weathering products) δ17O = 1.99 ± 0.05 (1σ); δ18O = 2.76 ± 0.09 (1σ); Δ17O = 0.55 ± 0.01 (1σ) (n=2) which is in the accepted range for H chondrites.

Classification: In thin section the sample is a heavily weathered (W5), moderately shocked (S4), equilibrated ordinary chondrite (H5).

Specimens: The owners of the main mass have agreed to loan it on a long term basis to the local county museum in Salisbury, where it will be on display to the general public. A 1 kg representative mass will remain at OU as the type specimen for research purposes.

https://www.lpi.usra.edu/meteor/metbull.php?code=56144
https://brian-mountainman.blogspot.com/2013/04/scientific-note-on-lake-house-meteorite.html
https://www.researchgate.net/publication/253308918_The_Meteorite_from_Lake_House

https://www.science.org/content/article/mystery-meteorite-house-sting

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From a BBC News write-up

"It's very unusual to find a meteorite this big in Britain," Prof Colin Pillinger said.

"They are very unstable, they contain a lot of metallic iron which oxidises and the meteorite falls to pieces.

"So the only logical explanation of how such a big meteorite may have survived being on Earth for 30,000 years is that it fell on or near a glacier and was in a deep freeze for 20,000 years."

Professor Pillinger, famed for his work on the Beagle II Mars explorer, said he believed the low-humidity and freezing conditions would have protected the rock from weathering.

"Then along came some druids, scavenging on Salisbury Plain for strange or interesting stones, and it was picked up and used in a chalk mound," he said.

"And the 'reducing environment' of chalk - the anaerobic environment - would have prevented the iron from oxidising."

The giant fragment of asteroid is then thought to have been unearthed by a previous occupant of Lake House, who is known to have excavated several nearby burial mounds.

"He was an archaeologist and was digging every barrow up in sight trying to find treasure," said Professor Pillinger.

"And we think he got it out of a barrow and added it to his collection."

The meteorite, known as a common chondrite, is due to go on display at the Salisbury and South Wiltshire Museum in autumn.

Adrian Green, the museum's director, said there was still "a lot of debate" about how the rock came to be on the doorstep of Lake House.

"But it's not uncommon for exotic rocks to be built into burial mounds," he added.

"And it's still covered in chalk which is the bedrock of the landscape.

"And it's colossal - it would take four people to lift it - and it's not aesthetically pleasing, so common sense dictates that this has not been shipped from abroad at ridiculous cost and significant effort, but that it came from the UK."

The Lake House bluestone boulder

Lake House, Wilsford

I have previously speculated on the matter of "other" bluestone fragments found in the Stonehenge landscape, apart from the mysterious Boles Barrow bluestone.  The Cunnington records relating to Boles Barrow mention "bluestones" in the plural as having been found there  -- so where are they now?  Are they all in Salisbury Museum, possibly unclassified and unloved?  According to the latest paper on the Lake House meteorite (Pillinger and Pillinger, 2024) various bluestone (meaning spotted dolerite) pieces were taken from Boles Barrow to various gardens, presumably including those at Heytesbury and Lake House..........  But Lake House is in Wilsford, a long way from Boles Barrow.  The very grand house is now occupied by the musician Sting and his wife.

https://en.wikipedia.org/wiki/Lake_House
http://www.insidewiltshire.co.uk/largest-meteorite-to-fall-in-the-uk-was-used-as-door-stop/

From an old post by Hobgoblin:

"In a letter of 1933, R S Newall, assistant to William Hawley in his excavations of Stonehenge between 1919-1926 and discoverer of the Aubrey Holes, stated he found a large piece of spotted dolerite in a cottage garden near Lake House. Newall described it as a rough cube of about 18 inches each way, which might have been broken off the top of a worked monolith of the bluestone horseshoe. The owner of Lake House, near Wilsford, south of Stonehenge, donated the bluestone to the Salisbury Museum."

Note the mention of a cottage garden on the estate.  If this large lump of rock had been donated by Cunnington and Colt Hoare to members of the landed gentry as an "'interesting stone", why was it not kept in a prominent place on the estate rather than in a cottage garden probably occupied by an estate worker?  Are we talking about the same stone?  Is it possible that the Lake House stone had nothing to do with either Boles Barrow or Stonehenge?

The mystery deepens.  Last year Julian Richards was pictured at the new Boles Barrow dig with a lump of spotted dolerite.  He was using it to show the diggers what they might need to look out for and what might just still exist in the depths of the barrow.  Where did that lump of rock come from?  Is it from the Salisbury Museum collection, and might it even be the very same lump of rock (or part of it?) that came from Lake House?

In the recent paper by Bevins et al (2023) mention is made of  ".........a dolerite block reputedly found in a cottage garden near Lake House, near Amesbury, which lacks a reliable context".  But the rock was not analysed by the geologists.

Bevins, R., Ixer, R. A., Pearce, N., Scourse, J., & Daw, T. (2023). Lithological description and provenancing of a collection of bluestones from excavations at Stonehenge by William Hawley in 1924 with implications for the human versus ice transport debate of the monument’s bluestone megaliths. Geoarchaeology: An International Journal, 38(6), 771-785. 

https://doi.org/10.1002/gea.21971

Does anybody have any light to shed on this issue?  All info gratefully received........

The myth of the shoreline erratics

I have published this map a number of times, assuming that it was a representation of the situation during the Anglian (MIS12) glaciation.  But maybe this was not the Anglian situation at all, but the  scenario that prevailed during the Wolstonian glacial episode (MIS6)??

One of the most frequently repeated myths about the erratics of the Bristol Channel is that they are all concentrated in the intertidal zone.  I have seen it in geomorphology and geology textbooks, conference proceedings and papers published in learned journals.  Over and again.  Let's get this straight.  This is simply untrue.

Of course, we all know about the famous "shoreline erratics" at Porthleven, Croyde, Saunton, Limeslade, Flat Holm and elsewhere, and the widespread assumption that they must have been emplaced by floating ice rather than by the ice of an active glacier.  As I have explained many times on this blog, I can see no realistic glaciological or isostatic scenario which would have allowed this "floating ice transport" to have happened.  In any case, the shore zone is special in that it is a "washed zone" in which wave action and tidal scour remove finer debris and leave large boulders behind.  Above it, inland, and below it, beneath the spring tide low water mark, sediments containing large foreign erratics survive, still holding their secrets.  So the "shore zone concentration" is more apparent than real, and it is extraordinary that one senior geologist or geomorphologist after another has failed to appreciate that fact.

The evidence of high-level erratics is hiding in plain sight, in the published records of Paul Berry, Peter Keene, Paul Madgett, Rosemary Inglis, Ann Inglis and others which are often ignored in the articles submitted to learned journals.   Here are some of the recorded  altitudes of erratics on or near the coasts of Cornwall, Devon and Somerset:

Lundy  138m

Shebbear  150m

Westonzoyland  10m

Baggy Point  80m, 60m and 45m

Ilfracombe  150m - 175m

Kenn  7m

Court Hill 68m  (ice surface was above 85m)

Nightingale Valley / Portishead Down  85m

These records leave us in no doubt that active glacier ice, on at least one occasion, crossed the Bristol Channel coasts of Cornwall, Devon and Somerset and pressed well inland.  

This brings us back to the recent paper by Gibson and Gibbard, which I find persuasive.  They argue that the most recent glaciation to affect the Bristol Channel coasts was the Wolstonian / Moreton Stadial / MIS-6 glaciation, which occurred around 150,000 years ago.  They argue that the Wolstonian ice rode over the limit of the Anglian ice in the Midlands and pushed far to the south in the Celtic Sea arena, making it the most extensive of all the glaciations in western Britain.  That is up for debate, since the green line shown on their map, based on Clark et al, 2018, appears to be very inaccurate and has already been questioned in a number of research articles.  

The suggested ice limits of Gibson and Gibbard, 2024. The suggested Devensian line across 

West Wales (green line) is unsupported by field evidence. The new Wolstonian line, in red, 

is more or less in the same position as the old "Anglian Glaciation" line, but it cannot be 

correct in the Somerset area.

An attempt to show where the Wolstonian limit of the Irish Sea Ice Stream might have been located -- based on a map by Gilbertson and Hawkins.  

More top Pembrokeshire erratics -- numbers 11 to 20

 11.  The Ogof Golchfa geocaching boulder.  This is a very large and well rounded erratic boulder, resting on the raised beach platform at Ogof Golchfa, near Porth Clais.  It's made of a coarse gabbro -- which means it has probably come from St David's Head.

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12.  The Lydstep Steps erratic, south Pembrokeshire.  I discovered this erratic close to the glacial deposits (Devensian?) where the path descends to the beach, near the famous Smugglers Cave.  It's an igneous boulder, probably made of gabbro, so maybe it has come from St Davids Head.  It is very different from the Carboniferous Limestone boulders that are prominent in the area.

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13.  The clifftop seat, St Bride's Haven.  An igneous erratic "pillar" on the clifftop a short distance to the east of St Brides Haven.  Probably from the west end of the St Davids Peninsula.  It has been utilised to make a pleasant seat for weary walkers on the Pembrokeshire Coast Path.  In another context, this one might be mistaken for one of the Stonehenge bluestones........

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14.  Igneous erratic boulder, Parc Mawr, near Bedd Morus, Newport.  This large well-rounded and severely weathered boulder rests on a surface of Ordovician shales.  It has probably not travelled far, but it may have travelled often....... it was moved a few years ago during some farming land clearance operations, and we can clearly see that part of it which was projecting through the ground surface.

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15.  The Abermawr clifftop boulder.  This is on the clifftop, adjacent to the Coast Path, very close to the turning area at the end of the road.  It is made of dolerite, in an area where all the outcropping rocks are Ordovician sedimentaries.

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16.  Newport Cattle Pound boulder, found adjacent to the Cattle Pound on the Bedd Morris road. It's made of a rock type I don't recognize -- a very hard, fine grained igneous rock that looks like a basalt.  I don't think it's from Pembrokeshire. I think the builders of the pound have tried to shape it for use in the building of the walls -- but they gave up because it was too hard and too resistant to break open........

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17.  Nevern Estuary boulder, found on the mud flats adjacent to the sand dunes.  It rests on a surface of clay-rich till, along with several other boulders of the same type.  It's made of a welded tuff which is difficult to provenance -- but it may have come from some of the igneous outcrops to the south of Newport.  That's a bit of a puzzle, since all the other signs are that the ice flow here was from north towards south.

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18.  The Whitesands Boulder Bed.  At the base of the Pleistocene sequence in Whitesands Bay there is a spectacular boulder bed with scores of large rounded igneous boulders visible as you walk along on the beach.  They are mostly made of dolerite and gabbro -- probably locally derived.  They rest on a raised beach platform, although they are not visibly embedded in raised beach deposits.  Some are so well rounded that they must have been subjected to wave attack on the beach platform at some stage -- but others are broken and faceted, so they must be derived from old glacial deposits.

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19.  Mynydd Dinas igneous erratics.  To the south of Carn Enoch and Garn Fawr there are abundant small rocky outcrops and large igneous erratics scattered across the landscape of Mynydd Dinas.  Most of the boulders have not travelled far, but many of them are faceted and abraded, and so they have clearly been affected by ice -- maybe during several glacial episodes.

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20.  Carreg Samson capstone, near Longhouse on the North Pembrokeshire coast.  This cromlech has the most spectacular location in Pembrokeshire, and is almost as popular with visitors as Pentre Ifan! The geology has not been studied in detail, as far as I know -- but at least three of the stones are made of Ordovician dolerite, and the capstone, like one of the uprights, is made of what appears to be a rough volcanic agglomerate or ignimbrite, probably derived from one of the local igneous outcrops. This might be the rock referred to by BGS as a pyroclastic "crystal tuff" belonging to the Llanrian Volcanic Formation.  A pit was discovered beneath the capstone, and it has been assumed the this was where the capstone was originally found before being lifted and propped up in its present position.

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Porth Meudwy, Llyn Peninsula -- an interesting erratic

 

Porth Meudwy -- the departure point for the boats to Bardsey Island

The interesting erratic, next to what looks like a chunk of concrete -- probably related to the slipway.

Thanks to Andrew for drawing attention to this erratic on the beach at Porth Meudwy--- and thanks for the photos. Andrew thought it may be a sandstone erratic, but on seeing his close-up shots I thought it was probably igneous. This is confirmed by two senior geologists whose opinions I value. One thought it was probably igneous because "it contains what appear to be euhedral phenocrysts that are probably feldspars."  The other thought it was probably a "porphyritic basalt, heavily altered. Rotted, prob originally zoned feldspars (plagioclase) set in heavily altered but dark, mafic-looking groundmass. Phenocrysts quite obvious (cream/brown, more or less euhedral) with cores most altered (rotted out)."

I thank them for their opinions -- qualified of course by the fact that they have not seen actual samples of the rock or thin sections.........

Anyway, it looks as if the rock has nothing to do with the Altar Stone!  Where has it come from?  Difficult to say --I don't know the geology of North Wales, but Anglesey?  Maybe further afield, in Galloway, Scotland?  Or even the inner Hebrides? 

Three of Andrew's close-up shots of the surface of the erratic boulder

 

The Stockholm Archipelago and areal scouring

 

This is a fantastic image posted on Facebook, showing a large chunk of the Stockholm Archipelago in March 2024.  The spring thaw is under way, but we can see a lot of sea ice still choking the straits, sounds and bays of the inner archipelago.

Although in detail we can see multiple traces in the landscape of glacial erosional processes, glacial debris transport and dumped glacial deposits, this is a classic landscape of "areal scouring" in which ice flow has been more or less uniform over the whole landscape, with only minor diversions into streaming flows in shallow troughs or valleys.  Ice flow here was almost exactly north to south -- away from the camera.

In previous posts I have described places where areal scouring, dead-ice conditions and maybe cold-based glaciers have combined to create undulating terrain and occasional areas of "knock and lochan" terrain. The main feature is always sluggish and diffuse ice flow. These rather beautiful wilderness areas can occur at high altitude, on plateau surfaces where ice cap generation takes place and where -- after ice cap growth -- the highest and most remote ice domes or ice-shed axes are located. The Glama and Dranga plateaux in Iceland are examples, as are the Teifi Pools area in mid Wales and the Hardangervidda in Norway. So these are essentially "ice source" areas where very specific glaciological conditions have applied, over and again during the Quaternary as one glaciation has followed another.

But the Stockholm Archipelago was not an ice source area -- it's an ancient PreCambrian Shield area close to sea level, with water that is not very deep and hills that are not very high.  The ice that originated far to the north extended far to the south.

There are obvious similarities between these plateau landscapes and the coastal landscapes that fringe some areas of intensive glaciation where fjords have been created. These are some of the most spectacular landscapes on earth -- for example the fjord landscapes of western Norway, East Greenland and parts of Arctic Canada. The general principle that seems to apply is that when ice-flow is concentrated within outlet troughs on the fringes of an ice sheet or an ice cap, erosion will make each trough deeper and deeper as long as there are supplements to discharge -- but as soon as the possibility of diffluence occurs (ie when the glaciers reach a pre-existing mountain front) ice-flow will spread sideways and erosive capacity will be suddenly diminished. Then instead of troughs and channels created by streaming ice, we will see the development of wide open plains of undulating bedrock under the influence of areal scouring processes. This is one of the most spectacular "process transformations" in nature, and when David Sugden and I were writing "Glaciers and Landscape" back in the stone age, we were very fascinated by it! I have done a number of related posts on this blog......

If you put "Stockholm Archipelago" into the search box you can see some of my earlier posts.

By the way, our summer cottage is very close to Granören, on the left edge of the photo -- many of the glacial features on the eastern edge of Blidö are described in other posts on this blog.

My contribution to experimental archaeology...........

 

I found this historic photo in the file --- taken on the day I helped out with the haulage of the Millennium Stone, back ion the year 2000.  It was a haulage exercise, but note that we were pushing rather than pulling -- because an adviser to the project said that this would minimise the risk of injury.  It was a jolly occasion, with lots of cameraderie amongst the pullers.  Philip Bowen, the organizer of the project, stands on the right in his yellow jacket.

Here we are crossing the Slebech Estate bridge over the Eastern Cleddau, at Blackpool Mill.  In the good old days of the Neolithic, of course, there would not have been a bridge -- just another horrible muddy tidal river with impenetrable jungle along its banks.

A couple of days after this photo was taken, the stone was taken off its sledge and transferred to two curraghs, with the help of a heavy lift crane.  Unfortunately the technology was not quite up to it, and the stone slid onto the bottom of the river and had to be recovered before starting its ill-fated journey to the docks at Milford Haven.

And the rest is history........

Top Ten Pembrokeshire erratics

1. Picrite boulder at Porthlysgi, near St Davids.  A very famous erratic made of "picrite" which appears very different from anything known in Wales -- therefore it is suggested that it might have come from Scotland.    It fell over the cliff and was lost for many years, but then it was hauled back up the cliff again in a heroic erratic rescue.  In May 2023 I found it again and recorded its precise position on the clifftop.

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2.  The Flimston boulders.  There are seven of them in the old churchyard, gathered up from the surrounding area and intended to be used as grave headstones.  Four of them stand in a row and have memorial tablets affixed -- the other three are lying unused in the long grass.  Origins are unknown, but the best bet at the moment is that the boulders have come from the St Davids Peninsula.

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3.  The Sleek Stone "super erratics" near Broad Haven.  There are several of them, the largest of which weighs c 75 tonnes.  It is a quartz porphyry, according to Cantrill -- possibly from Ramsey Island.  It rests on a broken platform of Coal Measures sandstones and shales.

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4.  The Ramsey Sound super erratic, made of grey or purple tuff.  Origin: possibly Ramsey Island.  It has probably not travelled far -- there are outcrops of the same rock type in the vicinity.

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5.  Druidston super-erratic.  An enormous erratic located in the valley upstream from the beach.  Difficult to get at, but it appears to be a dolerite, probably from the St David's Peninsula.  In the stream cutting there are multiple other large igneous erratics, associated with the thick Irish Sea till exposure at the head of the bay.

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6.  St Bride's Haven erratic pair.  These dolerite erratics are on the beach at the head of the bay -- you walk past them if you are walking on the coast path.  They rest on ORS bedrock -- the colour difference makes the erratics stand out prominently.

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7.  Llan spotted dolerite erratics, Lampeter Velfrey.  Spotted dolerite erratics incorporated into a ruined burial chamber on the lowlands of South Pembrokeshire (mostly underlain by sedimentary rocks).

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8.  Glan yr Afon erratic cluster, near Crosswell.  I'm mystified by these, because they are nearly all spotted dolerites -- but found to the NORTH of Mynydd Preseli within sight of the nearest known spotted dolerite tor called Carn Goedog.  But iceflow here was (or so we think) from the north towards the south. Were the boulders carried north by the ice of a small local ice cap?  Did they come from a spotted dolerite outcrop that we know nothing about?  Or are they not in a natural position at all -- were they collected up by people in historic time, looking for attractive building stones?

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9.  The Loving Stone, Loveston Farm, near Kilgetty.  This massive igneous erratic is clearly not in its original position.  It must have been found nearby, but it has been moved around in the farmyard, having been treated as a nuisance.  It has probably come from the St Davids Peninsula.

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10.  The Russia Stone, near Cwm Gwaun.  This massive monolith is one of a cluster of stones known as the Russia Stones. Some of the stones might have been used in Neolithic stone settings, but the farmer told me that this one was recumbent and was moved a short distance so that it could be used as a gatepost.  This stone has probably not travelled far -- there are a number of dolerite outcrops in the vicinity.

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Note:  I have resisted the temptation to include perched blocks in this list.  There are plenty of them -- but although they may have been dislodged from their original positions by overriding ice, most of them have only travelled a few metres, so they are not quite erratic enough to make the list.

The Cardigan Bay glaciers

From Patton et al, 2013, modified.  The three big outlet glaciers draining westwards from the Welsh Ice cap were the Tremadog, Mawddach and Dyfi Glaciers.  Separating them are the three sarns ++  long ridges of morainic debris exposed only at low tides.  They have much mythology attached ++ relating to Cantreär Gwaelod or the lost Lowland Hundred.  See other posts on this blog.  The confluence or contact between Welsh Ice and Irish Sea Ice shown on this map must have moved eastards and westwards as the relative strengths of the two ice streams changed over time within and between glaciations.  The survival of the sarns shows that the last significant glacial episode to affect this area must have involved an expansion of the Welsh outlet glaciers. They would not have survived an assault by Irish Sea Ice flowingv from the N and NW.

Rapid marine deglaciation : Asynchronous retreat dynamics between the Irish Sea Ice Stream and terrestrial outlet glaciers
December 2013
Earth Surface Dynamics 1(1)
DOI: 10.5194/esurf-1-53-2013
H. Patton et al

 

Outlet Glacier flowlines and surface velocities, after Patton et al.

The view from high above Pwllheli, as it might have been.....

Irish Sea ice and Tonfanau erratics

With ref to this interesting site report from John Mason:

https://geologywales.co.uk/storms/winter14c.htm

I was intrigued by the images of far-travelled metamorphic erratics found on the beach at Tonfanau -- on the northern coast of Cardigan Bay.  Thanks to John for allowing the use of the pics.

John says that these rocks are not local, and that they remind him of some of the rocks in the Lewisian / Torridonian / Moine sequence in northern Scotland. In the text of his blog, John says: 

"All sorts of other rock-types are to be found here (ie on the beach): perhaps the most exotic are the rare boulders and pebbles of high-grade metamorphic rock, reminiscent of the ancient, 2-3 billion year old rocks of NW Scotland. They consist of quartz, pink and white feldspars, glittering spangles of white and black mica and, in some cases, garnets - small examples of which are visible (small, intense red areas) in the photo with the 50p piece."

"There is one area on the beach, usually covered over by sand, where a number of large blocks of these metamorphic rocks lie embedded in the moraine, like the one in the image below (found in 1998), which is about half a metre long. Its more angular-looking underside is where it was embedded in the clay matrix of the moraine. The general scarcity of high-grade metamorphic rocks, and the occurrence of so many of them in the one spot, has led me to suspect that it all arrived together in one mass of ice - perhaps an iceberg, calved off some far-distant glacier and incorporated into the ice-sheet - that subsequently grounded here and, melting away, released its payload of rocks that it had brought from far away." 

Interesting stuff.  It's known that at Tonfanau, in the Devensian Glaciation, Welsh ice from the uplands of Snowdonia and Cader Idris flowed out into the bay across the coast -- but later this ice was displaced by the ice of the Irish Sea Ice Stream, which must by then have been immensely powerful.  I don't accept that these boulders can have been carried by floating ice -- sea level was far too low at the time, near the time of the glacial maximum..

Of course, we know of other assumed Scottish erratics in Pembrokeshire and on the Bristol Channel coasts, but if any of these have come from these ancient rock outcrops in NW Scotland, that means they must have come from north of the Highland Boundary Fault -- but this area was (according to all the text books and learned papers) affected by ice flowing west or north-west, ie on the northern flank of the ice shed.  On the southern flank of that same ice shed the ice fed the Irish Sea Ice Stream and flowed southwards.

This is an intriguing dilemma.  I wonder what the truth of the matter might be?  This might of course be of some relevance to the current Altar Stone debate.......

Patton, H., & Hambrey, M. J. (2009). Ice-marginal sedimentation associated with the Late Devensian Welsh Ice Cap and the Irish Sea Ice Stream: Tonfanau, West Wales. Proceedings of the Geologists' Association, 120(4), 256-274.

 https://doi.org/10.1016/j.pgeola.2009.10.004

BRITICE Devensian ice sheet animation: a model for the Wolstonian?

https://www.youtube.com/watch?v=oABxYza9ELM

This BRITICE animation, from 6 years ago, is suddenly relevant again because of the latest research showing that the pattern of glaciation assumed to be more or less correct for the Anglian glaciation may in fact accurately represent the ice extent of the Wolstonian British and Irish ice sheet.  In turn, the "extreme" model generated for the Devensian seems to fit rather well, with ice extent somewhat greater than that of the Late Devensian.

The model at 21 K shows approximate maximum ice extent. This is really interesting, as it shows rapidly streaming ice crossing western Pembrokeshire, with more sluggish ice in the east.  It shows the thin local ice caps of Exmoor, Dartmoor and Bodmin Moor incorporated into the main body of ice, with Irish Sea ice flowing between them and affecting the whole of the SW Peninsula.  That makes glaciological sense.  Then -- and this is really interesting -- the model throws up a stream of rapidly moving ice pushing far into the Somerset Levels depression almost as far as the English Channel coast.  I had not noticed this feature before -- but of course it backs up what I have suggested, and also the claim (made by Gilbertson and Hawkins many years ago) that glacier ice penetrated as far east as Salisbury Plain.

Watch this space......

The view from Pwllheli

There are always parallels.  I was looking for an illustration of what things might have looked like at the northern end of Cardigan Bay around the time of the LGM  and I found this splendid image of the Lower Skelton Glacier in Antarctica....

Just imagine that the photo was taken above Pwllheli, looking south.  On the right is Cardigan Bay, filled with the streaming ice of the Irish Sea Ice Stream.  On the left are the uplands of north Wales, with mountainous headlands separated by deep troughs carrying ice from the Welsh Ice Cap.  The junction between Welsh ice and Irish Sea ice runs along near the centre of the image, near the line of the present coast, with areas of dead ice separated by tributary streams of Welsh ice which are diverted southwards.  At first the Welsh ice dominates, with the contact zone pushed over towards the right (west) edge of the photo.  Any deposits laid on the coast are related to this Welsh ice.  But then the Irish Sea Ice dominates, pushing the contact zone in towards the mountain front.  Deposits laid along the position of the modern coast are nor related to this powerful ice stream carrying erratics from the north, including some from Scotland.......

There's nothing new under the sun.......

An erratic work of art.........

 

 

One of the family took this photo back in the jolly month of July, showing that erratics are not just of scientific interest, but have an artistic side to them as well.............

On comminution and giant erratics

Igneous geology of the west of Scotland (Wikipedia)

On a walk near the edge of the Nevern estuary (Parrog, Newport) yesterday, I found a couple of small pebbles which I think have come from Ailsa Craig.  They are microgranites with small bluish speckles and signs of some larger lighter coloured minerals, but the matrix is slightly pinkish -- rather than the pure white of genuine fresh Ailsa Crain riebeckite or micro granite. 

I'm not sure how unique the Ailsa Craig microgranite is, since related rocks also occur on the island of Arran -- no matter, since we can be reasonably sure that the pebbles have come from that general area of the Firth of Clyde.  There is nothing similar, as far as I know, on the Isle of Man or in North Wales.

The "curling stone granite" from Trefor Quarry in North Wales looks very different:

The blue, grey and pink granite used for curling stones -- from Trefor Quarry in North Wales.

Wikipedia:

Ailsa Craig is a spectacular, conical island in the Firth of Clyde about 20 km south of Arran (P914119). It is formed by a boss of peralkaline microgranite intruded into Triassic rocks. The microgranite is characterised by riebeckitic arfvedsonite and Zr-rich aegirine (Harding, 1983; Harrison et al., 1987); aenigmatite also occurs (Howie and Walsh, 1981). This distinctive rock-type is a widespread glacial marker southwards on either side of the Irish Sea (p. 160). It has traditionally been a favoured lithology for the manufacture of curling stones (p. 173).

Ailsa Craig curling stone quarry -- human being for scale

Anyway, the really interesting thing about these small pebbles with bluish spots is that they are all very small.  I have not seen one which is larger than a human fist.  Many thousands of tonnes of Ailsa Craig rock must have been removed  by overriding ice, and the original entrained blocks must have been of all shapes and sizes.  There is no reason to think that they were uniquely small before they started their journeys southwards towards Wales or westwards towards Donegal.

One of the assumptions in glacial geomorphology is that as large erratics travel within or under a glacier they are subjected to an assortment of processes which combine to cause comminution -- the gradual reduction in size as the rock mass is broken, broken again and then broken again until there is not much left apart from small pebbles.  These pebbles may be sub-angular, sub-rounded or rounded, and if water is involved towards the end of the journey they may even be well-rounded.

Different rules apply to supra-glacial transport because debris on a glacier surface is not subjected to abrasion or pressure-induced fracturing.  As Lionel Jackson and I explained many years ago in our article on the 930 km long Foothills Erratic Train in Alberta, Canada and the "Big Rocks" erratic cluster near Okatoks, huge rock masses that fall onto a glacier surface as a result of cliff collapse can be carried hundreds and even thousands of kilometres with relatively little modification.  The angularity of the giant erratics and their related debris may actually be increased as a result of frost (freeze-thaw) processes.

https://www.earthmagazine.org/article/stonehenges-mysterious-stones

The biggest erratic at Okatoks -- calculated to be 16,500 tonnes in weight.

Closer to home, we have other giant erratics on the shores of the Bristol Channel, including those at Limeslade, Lydstep, Freshwater Gut, Westonzoyland (now destroyed), Saunton and Shebbear, and on the tip of the South-West Peninsula at Porthleven.  

The famous Freshwater Gut (Baggy Point) erratic, made of granulite gneiss from Western Scotland (photo: Paul Berry).  It is reputed to weigh 50 tonnes.

So why is it that some clasts are comminuted down to pebble size over a glacial transport distance of 500 km, while other giant erratics survive?  Well, it has to be admitted that the great majority of clasts are broken down, while the giant erratic survivors are the great exceptions.  I have speculated before on this blog about the preferential survival of dolerite boulders in transport, and it seems that igneous boulders have a better chance of long-distance survival than sedimentary or metamorphic rocks.  If you look at a typical Pembrokeshire storm beach you will find that the great majority of pebbles and boulders have come from degraded or destroyed glacial and glaciofluvial deposits; maybe 90% of the clasts will not have travelled far, and maybe 10% will be from sources far away.

The clasts found in glacial deposits are typically of all shapes, sizes and lithologies, with variable surface characteristics as well.  Some will be polished and striated, and others will not be.  Some will have the "ideal" bullet shape, like the famous Newall Boulder found at Stonehenge, and others will be roughly rectilinear or even roughly rounded. 

 

So to answer the question raised above, I will have to say that we currently do not know why some giant erratics survive while others are broken down into small pebbles.  My best guess is that every clast undergoes a unique journey, related to its changing position in, or on, or under the ice; related to ice temperature and velocity and other glaciological conditions; related to rock type and internal structure; and related to distance travelled.  Is all of this random?  Well, not really -- the laws of physics apply, but as yet we do not fully understand them.  But "chance"factors come into play  -- for example when one clast in a vulnerable position is suddenly assaulted by something harder, sharper and heavier........

One final point.  Giant erratics are NOT restricted to the intertidal zone around British coasts.  That is a myth repeated over and again, even in learned publications.  So their distribution has nothing whatsoever to do with transport by floating ice.