Rockfall near Sleek Stone

A gorgeous example of a cliff rockfall at Broad Haven, Pembs -- not far from the Sleek Stone.  The cliff face here is at extreme HWST -- which means that storm waves do hit this rock face occasionally.  The Coal Measures sandstones and shales are here intensely fractured, and the "solid" rock is riddled with fracture planes running in many different directions.  Groundwater has been percolating along these fractures and precipitating iron oxides, which explains the heavy "rust" staining.  This process of precipitation and water seepage might well have facilitated the breakdown of the cliff.

Within a few years, this iron oxide will have washed off the newly exposed rock face, the the scar left by the rockfall will not be so prominent.

I hope to goodness that no archaeologists visit this site, or before we know where we are, we will have another quarry on our hands.........

Pembrokeshire cromlechs

This is an interesting map, worth publishing here with due acknowledgement.

 It gives the lie to the idea that there was an especially sacred or spiritual area centred on the eastern end of Preseli -- but we should remember that this map simply represents the "high point" of Neolithic culture, when there was a lot of building of chambered tombs going on.  (This was about 5,000 years ago, when the Stonehenge bluestones are assumed by some to have been transported from here to there.....)  Later on, in the transition to the Bronze Age, smaller tombs were built and there was a move towards burying the cremated remains of the dear departed in smaller cists or stone-lined boxes buried beneath round barrows or mounds.  That might argue for a great expansion of settlement in the Bronze Age -- and of course many standing stones were put up at this time too.

Source:
Building the Great Dolmens Excavations at Garn Turne, Pembrokeshire, 2012
Data Structure Report November 2012
Univ of Central Lancashire
Site code: GT12 NGR: SM 979 272
Excavation directors: Vicki Cummings and Colin Richards

Crescentic gouges at Rhosyfelin

In glacial geomorphology we tend to put a lot of emphasis on roche moutonnees, troughs and cirques on a large scale and on striations at a smaller scale, but we hear much less often about crescentic gouges or chatter-marks.  But they are very widespread, and every summer I marvel at the thousands of crescentic gouges which have been created on the streamlined rock surfaces of the Stockholm Archipelago.  Often striae and crescentic gouges are found on the same rock surfaces -- sometimes suggestive of different episodes of erosion nebeath deep streaming ice.

At Rhosyfelin, I have not seen anything you which I would -- with conviction -- refer to as glacial striations.  But there are some features which do look suspiciously like crescentic gouges (see photos above).  These are different from the percussion fractures that you might get on a rock surface which has been struck with a maul or hammer stone.  The process is similar but different, if that does not sound too much like a contradiction. It is now widely assumed that crescentic gouges are created beneath very thick ice where bedrock surfaces are subjected to heavy pressure and stress, and where sharp-edged tools are frozen into the bed of a glacier and are pressed down onto the rock.  The tool tends to be under compression, which makes it relatively strong, whereas in some circumstances the rock surface might be under tension, which makes it relatively weak.  Thus we might find that even if the tool is of the same rock type as the bedrock, compression damage might occur, and a crescentic fracture might be formed.  In some cases, the tool itself might be damaged or even broken into smaller fragments during this process -- but we seldom find the hard evidence for this, because this process is hardly ever observed in the real world.  (Glacier beds are difficult and dangerous places for scientific observations......)  Where a tool is made of a very hard rock type (for example dolerite) and a bedrock surface is relatively soft (made, for example, of foliated rhyolite) then the creation of crescentic gouges might be commonplace.

Of course, the rock surface being subjected to pressure need not actually be bedrock.  It might be a large flattish slab of bedrock embedded into the ground on the glacier bed.

Two generalisations are made in the literature:

1.  The thicker the ice, the greater the pressure and the longer and deeper the crescentic gouge might be.  (Note that the fracture is always perpendicular to the direction of ice flow.)

2.  The right conditions for the creation of crescentic gouges seem to occur beneath ice which is below the pressure melting point or very close to it.  In contrast, abrasion (and the creation of striations) seems to occur where basal ice conditions are wetter, with ice at or above the pressure melting point and where bed materials are more fluid and hence more mobile.

I need to make more observations on this.....

Caged glacial erratic in New Jersey

 I came across this rather sad picture of a small glacial erratic in a cage.  Who on earth did that?  A bit over the top, don't you think?  Poor thing -- I feel sorry for it.

Caged nephelene syenite erratic found along the Glacial Geology Trail in Stokes State Forest, New Jersey. This erratic was glacially transported from a small body of syenite sometime during the last ice age, about 18,000 years ago. The syenite outcrop lies 1.5 to 3.5 miles northeast of the erratic on the east side of Kittatinny Mountain and about 200 to 330 feet below its summit.

Broad Haven: two generations of erratics?

Here is a little hypothesis, which seems to me to have some merit: 

At Broad Haven, and probably in the embayments to north and south, there are two generations of erratics, namely well-rounded boulders generally less than 1m in diameter and which may date from the Anglian Glaciation, and larger irregularly-shaped "giant erratics" which may date from the Devensian Glaciation.

Here are half a dozen photos of well-rounded boulders of many types, scattered about on the beach and on the rock platform in the vicinity of the Sleek Stone.

There is a huge contrast between these boulders and others including the bits and pieces of the "super-erratics" referred to in earlier posts.  Here are four photos of these "fresh" erratics, which have sub-rounded or sub-angular forms and where the abrasive processes of wave action are gradually smoothing off previously sharp edges.

Obviously I'm generalising a bot here, but the 4 photos above seem to me to show either sub-abgular glacial erratics delivered to this locality more or less as they appear today, with small amounts of recent rounding due to wave action, or else chunks that have recently been broken off one or other of the giant erratics which I have described.  Various lithologies are involved.

This all makes sense to me.  The Devensian erratics in this locality will be "fresh", having only had a few thousand years of tides, storms and wave action to modify them.  On the other hand, erratics emplaced here in the Anglian Glaciation, almost half a million years ago, will have been affected by hundreds of thousands of tides and many thousands of Atlantic storms, leading to breakages and substantial rounding through the abrasive processes at work in this high-energy coastal environment.

The last photo above -- of two of the giant erratics -- shows a complex pattern of fractures.  Both of these huge blocks will eventually be broken down into a multitude of boulders and cobbles as a result of the percussive effects of other large stones being thrown against them during severe storms.

Where did the Sleek Stone giant erratics come from?

Now I'm trying to work out where all these giant erratics found near the Sleek Stone, Broad Haven, have actually come from.......

I know that Rob says it's a mug's game, trying to work out provenances from photographs taken by amateurs like me, but I'm going to hazard a guess and suggest that the greenish and bluish rocks that dominate this erratic collection are quartz-feldspar porphyritic rhyolite with quartz and feldspar phenocrysts, of Ordovician age, from the south-western side of Ramsey Island.

Above I have juxtaposed the biggest close-up I could get from my photos of the erratics (on the left) with a photo of a porphyritic rhyolite from Mexico.  There seem to me to be considerable similarities.

Here are some more close-ups of the erratics:

The two lower photos are clearly different.  How would we describe these?  Quartz-diorite?  Porphyritic microtonalite?  These might come from outcrops on the northern part of the island -- expert advice welcomed!

More on the Sleek Stone Super-Erratics

 The two biggest chunks of the "super-erratic" near the Sleek Stone, resting on a rough platform of Coal Measures sandstones and washed by the sea at high tide.  The chunk in the foreground is the smaller of the two; the larger one is immediately beyond it.

 The hammer rests on a third chunk of this same erratic, broken up by the pounding of the waves over thousands of years.  There are visible fractures in all of these chunks -- so further breakages and diminution are inevitable.

Been over to the Sleek Stone at Broad Haven again today, hunting for more bits of that "super-erratic" which has come -- probably -- from Ramsey Island.  In addition to the two large chunks which I think have come from a single "super-erratic" weighing over 100 tonnes, I'm now pretty sure that there is a third chunk of the greenish volcanic rock about 10 m away, at a slightly lower level.  I calculate that this chunk weighs about 12 tonnes -- it's smaller than the other two.  There's a fourth boulder as well in the vicinity, about half that size and weighing maybe 5 tonnes.

About 50m towards the north there is a second very large erratic now broken into three chunks, one weighing about 17 tonnes, another about 23 tonnes, and another abouy 10 tonnes -- a total weight of about 50 tonnes.  Again the rock type is a bluish-green "quartz porphyry" presumably from Ramsey Island.

The second giant erratic about 50m to the north of the "super erratic" -- in the rock platform embayment north of Sleek Stone.  This erratic block has broken into three pieces.  The hammer rests on the largest of the three.

Then there is a third giant erratic, not far from the green drainage pipe that carries water down from the cliff top.  This is made of a similar rock type, and weighs about 21 tonnes.  It is shown in the photo below.

These are the three biggest erratics in the vicinity; I suspect there may be others that I have not yet spotted.  In addition, there are scores of boulders which are in excess of 1m in diameter and weighing around 3 tonnes each.  And many smaller cobbles as well, littered all over the place.  The commonest rock type by far is the greenish or bluish quartz porphyry.  There are literally thousands of tonnes of it, just scattered about in the small embayments in the vicinity of the Sleek Stone.  Some quite large chunks of Ramsey Island have clearly been transported across St Bride's Bay from Ramsey Island.  I am still unsure whether this transport was done in the Devensian glaciation, or in earlier glacial episodes. 

Geology is not dead....

I was at Broad Haven on St Bride's Bay this afternoon, taking advantage of an exceptionally low tide.  It was good to see almost 200 students on the beach -- first year field trips from the Geology Departments of Southampton and Royal Holloway Universities.  They were all studying sedimentology, structural features and fossils in the Coal Measures.  Good for them.  Fear not, Rob!  The future is in good hands -- we hope.

Pentre Ifan - Place of Stones

Above is a gallery of images from Pentre Ifan -- all photos taken within 100m of the cromlech itself.  The volcanic ash / agglomerate outcrops hereabouts are extensive, and because most of the fracture planes in the rock are parallel with the rough bedding in the ashes, the rock breaks up into slabs rather than pillars.  In many cases where we see "recumbent slabs" lying in the turf we may actually be looking at bedrock outcrops.  And almost always these exposed upper surfaces are heavily abraded, as a result of glacial action -- probably during the Devensian glaciation.

I'm not sure whether I should refer to many of these slabs and boulders as erratics, because they do not seem to have been moved far from their places of origin.  

There are embankments and terrace-like features on the slope below the cromlech, but it's hard to say whether these features are prehistoric or a result of modern field clearance.

Note that some of the rocks are "on edge" and it is possible that they have been hauled up into "standing stone" positions -- but it is also possible that the slabs have been tipped up by overriding ice.  Some of the slabs lying about in the fields and adjacent to the footpath leading to the cromlech are larger than the capstone itself -- generally assumed to weigh about 16 tonnes.

My conclusion from having a really good look around this site is that the chambered tomb was simply built here because of the sheer abundance of available slab-like stones.  I doubt that any of the stones which we now see in the cromlech has been moved more than 50m from the place in which the tomb builders found it. 

Pentre Ifan Geology

There is remarkably little in print relating to the geology of Pentre Ifan -- a pity, since it looks rather interesting. There is some variation in the stones but I would describe most of them (including the 16-tonne capstone) as agglomerates made of volcanic ashes or tuffs -- all belonging to the Fishguard Volcanic Series which outcrops widely in the vicinity.  Some of the rocks have a "flaky" appearance -- but they are not foliated.  The stone surfaces are often "knobbly" and rough, suggestive of quite large nodules or inclusions in a finer matrix of ash.  There are some traces of bedding which might relate to the accumulation of layer after layer during the volcanic eruptions of the time; and here and there fracture planes coincide with the alignments of these layers.  Some stones are quite an attractive dark blue colour where they are exposed as reasonably fresh or broken surfaces.  But such fresh exposures are few and far between -- the vast majority of exposed surfaces are very ancient indeed -- heavily abraded and weathered.  In a few places (as on the underside of the capstone) flattish slabs or flakes of rock have fallen off; my feeling is that such surfaces were face-down in the ground until these stones were erected into the current megalithic monumental setting.

Surging Glacier in the Pamirs

Nothing to do with Stonehenge, but it's a fantastic satellite image from NASA.

This is Bivachny Glacier in the Pamir Mountains of Tajikistan -- a nice example of a surging glacier.  The glaciers at the top of the image are normal enough, with smooth medial moraines running parallel with valley sides.  But the glacier in the lower part of the image has got strange loops in the medial moraine pattern, showing that there have been at least three "pulses" from the Bivachny Glacier which have disrupted the normally smooth junction with the MGU Glacier at the tip of the long spur.

Surging behaviour occurs all over the world, in just a few glaciers where special glaciological circumstances prevail -- but the mechanisms involved are still not fully understood.

Pentre Ifan today, with a solar eclipse off camera

Solar eclipse at Pentre Ifan

It was a pleasure, this very morning, to be one of a group of august senior scientists gathered at Pentre Ifan for the observation of the solar eclipse.  Those present, in perfect conditions, were happy to share with others their high-tech scientific implements designed to give an optimal view of the great event.

Over 650,000 page views

Much to my amazement, we have now gone over the 650,000 mark in terms of page views for this blog.  That's very gratifying -- some people must find it interesting and maybe even informative......

Other info:  I have put up 1470 posts since the blog started, and over 11,000 comments have been published.  Goodness knows how many more I have rejected, or have been intercepted and binned by my spam filter.

I keep on getting hassled by Google and Blogger to allow adverts on the blog, with a guarantee of vast earnings!  No way -- personally I get intensely irritated when I find adverts on other people's blogs, and I shall steer well clear of them on this one.  Hope you all approve!

Pembrokeshire's Neolithic tombs

 Above:  Llech y Dribedd cromlech, near Newport.  Below:  map of Neolithic chambered tombs in SW Wales (after Children and Nash.)

This post follows on from this one posted in February:

http://brian-mountainman.blogspot.co.uk/2015/02/erratic-boulder-shapes-and-megalithic.html

The idea that cromlechs / dolmens / megalithic burial sites were simply constructed where the stones were, on largely utilitarian grounds, is so simple that it must surely have been considered in some detail by archaeologists.  Steve Burrow makes the point in his book "The Tomb Builders", but I thought I'd check further by looking at the most widely-used book on the Pembrokeshire Neolithic -- namely "Neolithic Sites of Cardiganshire, Carmarthenshire and Pembrokeshire" by George Children and George Nash, published by Logaston Press in 1997 as part of their "Monuments in the Landscape" series.  Theirs is very much an anthropological take on things, with the use of frequent analogies from Papua New Guinea, Madagascar etc, and that's all fascinating enough; but what really interested me, on leafing through the book, was their explanation for the locations of the monuments which involved the use of large stones.

For a start, what is amazing is that there is not a single cromlech listed for Ceredigion, and that there are only six listed for Carmarthenshire. Against that, more than 30 are listed in Pembrokeshire, with a further 25 or so "lost monuments".  This is an extraordinary mismatch, but the authors to not speculate on what the reasons might have been.  Were those reasons related to social, cultural or economic factors?  Or might they have had something to do with the availability of big erratics and loose rocks in the landscape?  It's worth reminding ourselves that Ceredigion and Carmarthenshire were affected above all else (apart from the area around Cardigan and the lower Teifi valley) by Welsh ice streaming down from the uplands; but Pembrokeshire was affected quite dramatically by the Irish Sea Glacier, flowing across the landscape from the north and north-west.  And the litter of erratics and loosened bedrock slabs left behind when the ice melted is very extensive -- and would have been even more extensive in the Neolithic, before land clearance.

The two Georges (Children and Nash) spend most of their time, in the heart of the book, talking about social, religious, symbolic and political factors -- this is very much an anthropological approach, in keeping with other titles in this series.  They refer to design variations, and speculate that Pentre Ifan, Carreg Samson and Garn Turne were "super-monuments" with spacious chambers capable of holding human remains removed from smaller "satellite" tombs in the nearby landscape.  They say that these three have "panoramic views of the surrounding landscape."  I don't buy that one -- some of the mini-tombs have even better views, and I think that is pure fantasy.    The "double dolmens" and the five-chambered tomb called Cerrig y Gof, near Newport, are deemed to be later constructions than the simple single-chambered tombs.  "Earth fast" monuments (where one end of the capstone is embedded in the ground) are deemed to have been small and exposed, generally with room for just one corpse.  The authors ague that the siting of earth fast monuments (close to rock outcrops) was chosen so that they would be disguised in their surroundings.  They say: "This is important if a symbolic rather than a visual impact is to be created......."  When I read this, my heart started to sink even faster than it was doing already.  "For goodness sake, chaps," I thought. "Just forget the fantasies; earth fast monuments are close to rock outcrops because that is where the massive stones lay, and were available  to be propped up and used."

One interesting idea from the authors is that Pembrokeshire had a small and localised "passage grave" tradition in the Neolithic which was quite different from the tradition of building tombs of "monumental" proportions in Ireland and on Anglesey.  This brings into question MPP's idea that Pembrokeshire had a powerful clan with megalithic building traditions so well developed that they wanted to shift 80 or more bluestones from their home area to Stonehenge.  If power and status were that important to the Stonehenge people, why did they not do deals with more technically advanced and powerful tribes and choose stones from Anglesey and Ireland rather than a pathetic assortment of stones of all shapes and sizes (and variable durability) from north Pembrokeshire?

Most of the authors' consideration of the Neolithic tombs of Pembrokeshire is concerned with siting and spatial relationships, and here they get even more confused.  They say there is "an obvious affinity with the sea" in the siting of the monuments.  I disagree totally.  Only half a dozen or so are close to the sea -- the rest are inland, dotted all over the north of the county.  They say: "Many of the marine monuments in Wales... are locally-oriented in relation to the sea...."  For a start, I dispute the use of the term "marine monuments" -- and secondly, what on earth does that phrase about orientation actually mean?  They go on to refer to local orientations with respect to "certain topographic features" including mountain peaks, headlands and rivers.  Again, I fear that that is a completely meaningless assertion.  Every cromlech or grave site is of course located locally with respect to something or other -- a bog, a hillside, a valley, a river, a crag, an inlet, a cliff, a plateau........ That does not mean that there is a cause and effect relationship or that the landform in question was invested with some spiritual or mystical significance.

In a number of places in the book, the authors suggest that the upper surface of a cromlech (for example, Pentre Ifan) was carefully manufactured to mimic or mirror some distant prominent feature of the landscape.  The argument was, and is, unconvincing and fanciful, not least because many of the upper surfaces of cromlechs would have been covered by their mounds at the time when the builders were in a position to stand back and admire their handiwork.  Even with the mounds gone, and cromlech capstones now exposed in all their glory, I do not see the "landscape re-creations" that some archaeologists see.......  

After this, the authors really get going with a section on the "hidden architecture" of the tombs -- suggesting that tomb clusters possess territorial relationships.  It's a bit difficult to see what they are getting at, but the suggestion seems to be that each "cluster" of tombs (the Newport Group, the Fishguard Group, the St David's Group etc) contains different tombs with different spatial relationships -- ie one tomb related to a river, one related to the sea, one related to a prominent hill, and so forth.  "Each monument appears to take on a special role within the cluster."  Christopher Tilley (1993) is cited at length, and there is much about the "socio-political-symbolic nature of the body", about frames and skeletons and rib-cages replicated in the landscape, and even about the building of mounds as representations of flesh on the skeleton.    Oh dear.  I shall move swiftly on......

In another section on "Replicating the Landscape" the authors get even more confused and perhaps slightly deranged.  In seeking to show that each monument has its own unique morphology and spatial relationships, replicating or mirroring the essential features of the landscape round about, they end up in a frightful tangle, with some monuments deemed to point towards one particular topographic feature and others designed to point towards several different features at the same time.  Some are deemed to be at the peripheries of something or other, and some are deemed to be at the centre.  The ultimate in absurdity is in this sentence:  "The five chambers (of Cerrig y Gof), each possibly served by a small entrance, are aligned so as to ignore the rising and setting sun."

Before I totally lose the will to stay alive, here is a gigantic and earth-shattering hypothesis, based upon the above quote, and surely worthy of banner headlines around the globe:  "Stonehenge was cunningly built in such a fashion that all of the key alignments ignore solar solstice sunrises and everything else of astronomical importance, thus ensuring that the monument is of no significance whatsoever."

So far as I can see, there is no mention, anywhere in this book, of the possibility (I would call it a probability) that all of the chambered tombs in West Wales were built at locations where large suitable stones were readily available for use with minimal effort on the part of the builders.  Far too obvious, and far too simple?

Carreg Samson, near Mathry

Up on Bathampton Down

Bearing in mind the recent interest from Tony and others regarding the deposits at Bathampton Down and other localities near Bath, here is that Donovan map again -- based on field mapping by our old friend Geoff Kellaway,

Stonehenge - Mecca on stilts?

Thanks to Helen for drawing attention to this one.

Here we go again -- the latest dramatic, exciting and mind-blowingly earth-shattering theory about Stonehenge.  It seems that the more wacky the theory, the more chance it has of being published......

 =============

Circular thinking: Stonehenge's origin is subject of new theory

Wiltshire monument may have been equivalent of ‘an ancient Mecca on stilts’ according to an idea put forward by former museum director Julian Spalding

 

Dalya Alberge

http://www.theguardian.com/science/2015/mar/15/circular-thinking-stonehenges-origin-is-subject-to-new-theory 

 

Sunday 15 March 2015  

Whether it was a Druid temple, an astronomical calendar or a centre for healing, the mystery of Stonehenge has long been a source of speculation and debate. Now a dramatic new theory suggests that the prehistoric monument was in fact “an ancient Mecca on stilts”.
The megaliths would not have been used for ceremonies at ground level, but would instead have supported a circular wooden platform on which ceremonies were performed to the rotating heavens, the theory suggests.
Julian Spalding, an art critic and former director of some of the UK’s leading museums, argues that the stones were foundations for a vast platform, long since lost – “a great altar” raised up high towards the heavens and able to support the weight of hundreds of worshippers.
“It’s a totally different theory which has never been put forward before,” Spalding told the Guardian. “All the interpretations to date could be mistaken. We’ve been looking at Stonehenge the wrong way: from the earth, which is very much a 20th-century viewpoint. We haven’t been thinking about what they were thinking about.”
Since Geoffrey of Monmouth wrote in the 12th century that Merlin had flown the stones from Ireland, theories on Stonehenge, from plausible to absurd, have abounded. In the last decade alone, the monument has been interpreted as “the prehistoric Lourdes” where people brought the sick to be healed by the power of the magic bluestones from Wales and as a haunted place of the dead contrasting with seasonal feasts for the living at nearby Durrington Walls.
The site pored over by archaeologists for centuries still produces surprises, including the outline of stones now missing, which appeared in the parched ground in last summer’s drought and showed that the monument was not left unfinished as some had believed, but was once a perfect circle.

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Spalding, who is not an archaeologist, believes that other Stonehenge theorists have fallen into error by looking down instead of up. His evidence, he believes, lies in ancient civilisations worldwide. As far afield as China, Peru and Turkey, such sacred monuments were built high up, whether on manmade or natural sites, and in circular patterns possibly linked to celestial movements.
He said: “In early times, no spiritual ceremonies would have been performed on the ground. The Pharaoh of Egypt and the Emperor of China were always carried – as the Pope used to be. The feet of holy people were not allowed to touch the ground. We’ve been looking at Stonehenge from a modern, earth-bound perspective.”
“All the great raised altars of the past suggest that the people who built Stonehenge would never have performed celestial ceremonies on the lowly earth,” he went on. “That would have been unimaginably insulting to the immortal beings, for it would have brought them down from heaven to bite the dust and tread in the dung.”
Spalding’s theory has not met with universal approval. Prof Vincent Gaffney, principal investigator on the Stonehenge Hidden Landscapes Project at Bradford University, said he held “a fair degree of scepticism” and Sir Barry Cunliffe, a prehistorian and emeritus professor of European archaeology at Oxford University, said: “He could be right, but I know of no evidence to support it”.
The archaeologist Aubrey Burl, an authority on prehistoric stone circles, said: “There could be something in it. There is a possibility, of course. Anything new and worthwhile about Stonehenge is well worth looking into, but with care and consideration.”
On Monday Spalding publishes his theories in a new book, titled Realisation: From Seeing to Understanding – The Origins of Art. It explores our ancestors’ understanding of the world, offering new explanations of iconic works of art and monuments.
Stonehenge, built between 3000 and 2000BC, is England’s most famous prehistoric monument, a UNESCO World Heritage site on Salisbury Plain in Wiltshire that draws more than 1 million annual visitors. It began as a timber circle, later made permanent with massive blocks of stone, many somehow dragged from dolerite rock in the Welsh mountains. Spalding believes that ancient worshippers would have reached the giant altar by climbing curved wooden ramps or staircases.