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Tuesday, 21 March 2017

The South Pembrokeshire LGM - again

 Reddish Devensian till exposed in the wall of the Fopston Farm drainage reservoir, near Marloes. The cemented layer, stained with manganese oxide and iron oxide, may represent the position of the Holocene water table.

South Pembrokeshire really is a tough nut to crack.  I just wish that I had been around to keep an eye on all those elongated trenches that were dug across the area when they were installing the oil and LNG pipelines between the Milford Haven installations and the English Midlands.  Much will have been revealed..........  ah well, too late now......

When I looked at all the info from West Angle Bay over the past few weeks, I was reminded forcefully that there is abundant evidence of Devensian ice pressing into the mouth of Milford Haven from the west or north-west.  Along the coast to the north there are glacial and fluvioglacial deposits at Westdale Bay,  Mullock Bridge, Fopston (801096) and Fold near Marloes.   The Geological Survey field workers identified a sheet of "boulder clay" on the western side of the Dale Peninsula, and we are familiar with the evidence of Devensian ice affecting all of the coasts of St Bride's Bay.    We have mentioned Druidston and Sleek Stone (Broad Haven) on more than one occasion.  The surveyors identified strong evidence of glaciation on both Skokholm and Skomer Islands.  HH Thomas thought that there was a considerable expanse of "yellow loamy drift" with many foreign erratics to the west of Roch -- but very few patches to the east of the castle.  Cantrill said that the sheet of boulder clay on the coast between Nolton and Druidston extends "some little way inland."  He said there are good exposures of till at Madoc's Haven and Druidston, but that the drift keeps to the top of the cliff and thins out at Druidston Villa.  He said that to the east and north of the Villa there is a "small sheet of gravel" -- could this be a pro-glacial fluvioglacial deposit, laid down just beyond a static ice front?

Another spread of till with igneous erratics is described from the Talbenny area, and there is a giant erratic near the cliff edge at Mill Haven.  Small patches of gravel and sand occur at Rickeston Bridge and in the Walwyn's Castle Valley, and the surveyors seem to have thought that there was some glacial river diversion associated with a feature called "The Rock" c 300 yds NE of Rickeston Bridge.  Might this represent the maximum inland extent of Devensian ice in the SE corner of St Bride's Bay?  This might also be supported by the presence of a "conspicuous mound" of fine yellow sand, about 200 yds in diameter, to the NW of Orlandon, about 6 km to the west; this is linked to another smaller mound, and to exposures of gravel containing much ORS material observed to be about 15 ft thick.  OT Jones thought that these deposits might be linked with the kame terrace at Mullock Bridge (which I studied intensively during my doctorate fieldwork in 1962-65).   There is another patch of gravel on the edge of the cliff west of Ripperston Farm, with rounded ORS pebbles "probably picked up on the sea bottom".

The surveyors noted various small patches of reddish-yellow boulder clay (with striated pebbles) and also gravelly patches around Milford, Thornton and St Botolph's; but from the published descriptions it appears that these patches are well weathered and eroded, and that they lie stratigraphically beneath head.  Might they be the last remnants of a pre-Devensian cover of glacial and fluvioglacial deposits?  Much fresher ice-related deposits appear around the Dale Estuary, especially on the western flank.  On the eastern side of Gann Flat, almost a metre of gravelly and sandy head is seen in the cliff; this material seems to have originated in fluvioglacial sands and gravels like those at Mullock Bridge, but there has been "paraglacial rearrangement" similar to that at Westdale Bay.

My current impression is that the ice that came in from the NW across St Bride's Bay was not very thick, and that it found it very difficult to surmount the rampart of cliffs (which were of course at that time not sea cliffs, since sea-level was around 120m lower than it is now) between Newgale and the mouth of Milford Haven.  In many places these old cliffs or steep coastal slopes were in excess of 60 m high, and in places up to 80 m above present OD.  Beneath present sea-level the coastal slope continues to drop quite sharply down to -15m along most of the southern shore of the bay -- so incoming ice had to surmount an obstacle generally between 75 m and 100 m high between Little Haven and the western end of Skomer Island.  The stretch of territory to the north of Talbenny must have been a particularly prominent obstacle.  Lobes of ice may well have pushed inland both to the east of this rampart (towards Walwyn's Castle) and to the west (towards Orlandon and Mullock Bridge).  My impression is now that all of the land to the west of the St Bride's - Dale Estuary through valley was inundated by Devensian Irish Sea ice.



 Field sketch of the exposures at Westdale Bay, as recorded by Gillian Groom in 1957.  Note the till at the base of the exposure and the pseudo-stratified materials above.

Devensian glacier ice pressed into the mouth of Milford Haven, but what happened on the west and south coasts of the Castlemartin Peninsula?  There appear to be traces of massive clay till  beneath the beach at Freshwater West, and most of the coast between Frainslake and Broad Haven (South) is difficult to examine because of the presence of the Castlemartin Firing Range. Risking life and limb (there are unexploded objects lying around), I have looked at some of  the terrain and have seen scattered erratics but no expanses of fresh till.  


Dixon and his colleagues were sure of the presence of till in a "pipe" in the limestone at Catshole Quarry, Pembroke, capped by head and other deposits.  From the published description, it appears that this may be a very old deposit.  A fresher till may be present in the St Florence and West Jordanston area, and the GS field mappers recorded a thickness of 7 ft of gravelly reddish clay, with occasional igneous erratics.  They observed till overlying the Tertiary gravels and conglomerates in the Flimston clay-pits, and remarked on the presence of scratched and facetted stones and igneous fragments as well as large angular fragments of chert.  On page 199-200 of the 1921 Pembroke and Tenby GS Memoir there are abundant references to erratic boulders, many of which are derived from the St David's Peninsula and Ramsey Island.  Sandy reddish till with igneous erratics is also recorded in a deep fissure or pipe in the NE corner of Sandtop Bay on Caldey Island -- but when I visited this bay a couple of years ago no exposure could be seen.  On the other hand, I have observed fresh till at the other end of the island, in Ballum's Bay, which has to be of Devensian Age (it rests in a Carboniferous Limestone fissure, and if it was older it would certainly be solidly cemented....




Some of the faceted and rounded erratic stones taken from the fresh till in Ballum's Bay, 
Caldey Island.

The coast between Tenby and Pendine appears not to have been affected by Devensian ice, and DQ Bowen has presented evidence from Marros to support this contention. In my own research at Marros I did not observe any till or related deposits either.

The most parsimonious explanation of the distribution of ice-related deposits on the Dale Peninsula and Castlemartin Peninsula is that there are three zones:

1.  A zone close to the cliffline and a little way inland which appears to have been affected by Devensian ice pressing onshore.

2.  A zone further inland where erratic boulders are abundant but where glacial deposits appear to be scarce, heavily weathered and eroded, and mostly restricted to interfluves.

3.  A coastal strip running from Tenby to Pendine which appears to have escaped from any direct Devensian glacial effects.

As suggested by the GS surveyors in 1921 and by JC Griffiths in 1940, the last incursion by Irish Sea Glacier appears to have been by ice that was thin and not powerful enough to overcome the coastal slope and to push far inland.  My current thinking is portrayed on the map below.  I wonder how long this map will survive before it needs to be modified?  What was I saying the other day about falsification......? 


An attempt to portray the Devensian (LGM) limit for Pembrokeshire.  The ice from the north was powerful enough to progress inland as far as Mynydd Preseli, Wolfscastle and Roch; but further south it was nowhere powerful enough to progress far inland after encountering the coastal slope / old cliffline.









Saturday, 18 March 2017

The Last Glacial Maximum (LGM) on the Isles of Scilly


I've just done this new map of the proposed LGM (Last Glacial Maximum) limit on the isles of Scilly.  The map (with grateful thanks) is from James Scourse 1991 and 1998.  The solid black line is the limit as accepted by James and most other recent researchers.  The dashed line, showing the extent of the Hell Bay Gravel, does not in my view have any significance.  My ice limit for the Devensian, based upon observations in April 2016, is shown by the red line.

The biggest changes are the tongues of ice which I believe to have penetrated along the sounds between St Martin's and Tresco and Tresco and Bryher, and a much bigger lobe that pushed in from the west across St Mary's Road right up to the west coast of St Mary's.  St Agnes was certainly affected by Devensian ice, but I am uncertain about Annet, Samson and the Eastern Isles.  Maybe somebody else can gather evidence from those, over the coming summer season.........

It's interesting that the latest dating suggests that the maximum ice extent here was around 25,000 - 23,000 years ago -- earlier than previously supposed.

Thursday, 16 March 2017

Falsification rules, OK?





Much to my surprise, one of the most popular posts on this blog is this one, featuring the very eminent Prof Danny McCarroll, with almost a thousand page views:

The joy of being falsified

https://brian-mountainman.blogspot.co.uk/2016/11/the-joy-of-being-falsified.html

Falsification is obviously popular.............

Those eminent people......





Here is another choice extract from the Darvill / Wainwright chapter in the new Pembrokeshire County History, Vol 1:

Re the bluestones transport mechanism:  "In 1971, Kellaway revived an earlier suggestion that they were carried on glaciers which spread south before they melted and neatly deposited their bluestone burden on Salisbury Plain (Kellaway 1971)."  Kellaway did not say the stones were carried "on" glaciers -- he said they were probably carried in the body of the Irish Sea Glacier which was already known to have crossed Preseli and to have flowed up the Bristol Channel at least as far as the Somerset coast and probably some way inland from there.  The addition of the word "neatly" is a nice cynical touch, but we'll let that pass........ but Kellaway was not just dreaming up some mad idea, as is implied, but was making a statement well supported by a great deal of hard evidence on the ground.

To continue:  "Superficially attractive, the glacial theory flounders because of the lack of any evidence for glacial activity on Salisbury Plain, and no suitable blocks of bluestone have yet been found along the route they would have taken across south-east Wales."   Agreed that there is no unequivocal evidence of glaciation on Salisbury Plain, unless we count the presence of a great deal of debitage from a great range of sources, and a large assortment of bluestone boulders that are clearly not pillars and clearly not quarried.  But why would we expect to find suitable blocks of bluestone scattered  along some unknown route across south-east Wales?  Even if there really was a route (which there wasn't), and even if we know it (which we don't), why would we expect to find suitable lumps of bluestone along it?  Suitable for what?  I really don't know what the authors are talking about. I should have thought that a lack of dropped blocks along some hypothetical route would militate against a human transport hypothesis, and be completely irrelevant as far as the glacial transport hypothesis is concerned.

To continue:  "Although it still has vocal supporters, eminent geologists and glaciologists have dismissed the glacial theory (Bowen 2005; Green 1997; Scourse 1997) and concur with Thomas's original suggestion that the stones "were transported by human agency, in all probability by an overland route (Thomas, 1923, 259)."  So who are these eminent geologists and glaciologists?  David Bowen, Chris Green and James Scourse are not geologists and they are not glaciologists; they are geomorphologists, no more brilliant or eminent than those of us who believe that the glacial transport hypothesis is eminently reasonable.  I don't actually care very much about Messrs Bowen, Green and Scourse concurring with HH Thomas -- and I have argued many times on this blog that their arguments are deeply flawed.  I place much greater value on other papers (written by assorted specialists including real glaciologists) who have argued that the glacial transport hypothesis is a perfectly reasonable one.

It would have been helpful if Darvill and Wainwright had done some more careful research on this issue before going into print here and repeating the same points they have made many times before.......

If "lack of evidence" is the sticking point with respect to the glacial transport thesis, would somebody please present to me the evidence thus far accumulated which shows that human transport of the bluestones might have happened?  I wait breathlessly for a flood of responses.


Wednesday, 15 March 2017

Pembrokeshire megalithic culture and the erratic supply chain


 Carreg Samson, an erratic architectural statement.........

I have been reading the huge chapter (167 pp) by Tim Darvill and Geoff Wainwright in Volume 1 of the Pembrokeshire County History.  That chapter is almost a book in its own right, superbly illustrated and nicely laid out -- and packed with detail.  I'll leave a more considered review till a little later, since I still haven't quite finished the text.  But as I suspected, it really could have done with proper peer review (including scrutiny from the geologists!) and with some effective editing and pruning.......

But the thing that really strikes me, having now read the bulk of the text that deals with stones (monoliths, standing stones, stone circles, cromlechs, tools, weapons, etc) is the extraordinary lengths that the authors go to in order to avoid stating the obvious -- namely that the users of stone in the Mesolithic, Neolithic and Bronze Age were operating in a landscape littered with stone blocks or glacial erratics.  There is a mention somewhere of glacial till spread across the landscape, but I have only seen two fleeting mentions of glacial erratics in more than a hundred pages of analytical text.  This is quite extraordinary, given that the authors were perfectly familiar with my work on the glaciation of Pembrokeshire and with the work of Kellaway, Richard Thorpe, Olwen Williams-Thorpe and others:

https://brian-mountainman.blogspot.co.uk/2016/11/one-of-classic-stonehenge-papers.html

Even archaeologist Steve Burrow, in his book called "The Tomb Builders", argues that all of the cromlechs in Wales were simply built from glacial erratics or from loose bedrock slabs collected in the immediate vicinity:

http://brian-mountainman.blogspot.co.uk/2011/01/inside-neolithic-mind.html

http://brian-mountainman.blogspot.co.uk/2016/01/neolithic-bluestone-quarries-why-would.html

The authors of this big chapter discuss literally hundreds of megalithic structures in Pembrokeshire, and not one of them was built of stones transported from anywhere else by the builders.  Where there were glacial erratics, they used them by levering them up, wedging them and propping them, or sliding them into sockets.   They arranged them by lining them up or moving them into circles or ovals.  But in no case can it be shown that big stones have been moved a kilometre or more from a place or origin to a place of use.  Small stones were gathered from a radius of 100m or more in some instances, but I know of no large stone weighing a tonne or more being moved even 50m.  In fact, I would argue that we can forget about astronomical alignments, spring head locations, ley lines, auspicious positions and pretty views in the matter of monolith placements -- stone location was the prime determinant in deciding where the Pembrokeshire monoliths were put into the ground.

So why have Darvill and Wainwright apparently existed in a state of denial about this perfectly simple matter?  Well, we don't have to search too far for reasons.  For a start, one of their central theses is that big stones of particular lithologies (especially spotted dolerite) were inherently valuable, either because of their supposed healing properties (TD and GW) or because they were deemed to contain within them the spirits of the ancestors (MPP).  So if they were valuable, they had to be worth fetching and carrying.  That means they had to be worth quarrying.  And it also means they had to be worth carting all the way from Pembrokeshire to Stonehenge. So the Darvill / Wainwright text is full of references to quarrying, stone transport, stone veneration and so forth, as fantasy is built on fantasy.  What is completely lacking is evidence that withstands scrutiny.

So let's repeat the following points.

1.  There are abundant assertions, but there is no hard evidence in this long text of  any particular stone type being valued, or being accorded veneration, over and above any other stone type in prehistoric Pembrokeshire.  Stones of all lithologies, shapes and sizes were used wherever it was handy to use them. (That, by the way, is exactly the case at Stonehenge as well.)  If more spotted dolerite pillars and slabs appear to have been used in north Pembrokeshire, it is because there were simply more of them lying around as glacial erratics.

2.  There is no hard evidence, as far as I know, of any large stone in a Pembrokeshire monolithic setting being transported more than a few metres from its place of origin to its place of use.

3.  Because of the abundance of glacial erratics littered across the landscape, there was no need for any quarrying of stone from "bluestone quarries."  So there are no bluestone quarries, and the obsession with searching for them and "finding" them them is nothing more than a rather charming fantasy.

4.   Although I am a geographer who quite enjoys looking for patterns and arrangements in the landscape, I can see no "siting preferences" with respect to monolithic settings based on proximity to springs, views of the mountains or the sea, alignments, transition zones between boggy and and rocky land, or anything else.  The only thing I would concede is that some fortified sites and burial sites are located on hill summits.

5.  Through frequent mentions of other parts of Wales, the Irish Sea arena and Ireland, this chapter reinforces my view that the cultural associations in Mesolithic, Neolithic and early Bronze Age times were predominantly with other parts of the "Atlantic Fringe" and NOT with Salisbury Plain and the Stonehenge area.  There does not seem to be any cultural context for a situation in which people would suddenly want to start gathering up 80 bluestones and carting them off to Stonehenge.

6.  I know it's unromantic and unfashionable to say so, but I think that the prehistoric inhabitants of Pembrokeshire were a pretty pragmatic bunch.  They clearly had their reasons for making "statements" in stone, but they were also driven by utilitarian principles, and always used whichever handy stones were fit for purpose.  They may have been simple folk, but they were smart enough to know about cost / benefit analysis. 

Sunday, 12 March 2017

Prof Geoff Wainwright


I was sad to hear of the death of Prof Geoff Wainwright on 6th March 2017.  He had been seriously ill for some time.  Sincere condolences to his family.

This is a photo taken on the summit of Carningli, just a couple of weeks before he died.

Friday, 10 March 2017

Cilgwyn Cromlech


The photo above shows the site of the Cilgwyn Cromlech, according to the grid reference cited in the "Neolithic and Bronze Age Pembrokeshire" chapter in Vol 1 of the new County History.  The grid ref is SN 088362.   The site is due west of the southernmost of the rocks of Carnedd Meibion Owen.   There doesn't seem to be anything there -- so we must assume that it has been destroyed.

Thanks to Mark for drawing this to my attention.

On p 108 of the chapter, we see a pestle macehead made of 'preselite" (Group Xlll) from the late third and early second millennium BC -- apparently collected from a site c 100m east of the cromlech, in the ploughed field next door.  The reference is Burrow 2003, p 229 -- that's a catalogue of collections in the National Museum.

There is no Coflein record for this site.