Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Friday 30 June 2023

The stones in Spain are very local indeed

Some of the quarried sites featured in the article, exploiting natural rock outcrops very close to the places where monoliths and filler stones were needed.

At last, some decent evidence of magalith stone quarrying in the Late Neolithic!  It comes from Huelva in Spain, thanks to Josė Antonio Linares-Catela and his colleagues.  It's a very detailed study of the rocks used in the construction of a dolmen complex in Southern Spain, in an area presumably outside the influence of Quaternary glaciation. (There were glaciers in the Sierra Nevada, but not, according to the latest studies, this far west......)

In their Introduction, surveying the European scene regarding Neolithic "quarries", the authors rather too blithely accept that there were quarries at places like Craig Rhosyfelin, Carn Goedog and Vestra Fiold, and they should be chastised for that.  But their work relating to the dolmen group is very careful indeed, and is well described.  They identify a number of different rock types used in the building of the dolmens, and show that they have all come from the immediate vicinity.  The most distant stone sources are about 500 m away, but most came from less than 100 m away from the dolmen sites.  Most of the stones were simply collected up from a stone scatter, but some were extracted from adjacent rocky outcrops, just as happened at Callanish.  

There is not much in the article about quarrying methods, but the presence of small river-rounded stones from a nearby river gorge are scattered about, and are assumed to have been used for the shaping of crude slabs and elongated pillars for the purposes of creating the dolmens.  it seems to have been assumed that the extracted monoliths were simple levered away and collected from rockface situations where they were already loose and accessible.  Needless  to say, there is no mention anywhere of  "soft rock wedges"........

I think this is an important article, showing that Neolithic megalithic sites were often (and maybe always) determined by the availability of convenient stone; that where possible big stones were simply collected up from where they lay; and that quarrying of monoliths was a last resort, used to supplement the supply of stones used "more or less where found".  

It was all very utilitarian and pragmatic, with the minimisation of effort clearly uppermost in the minds of the megalith or tomb builders.  This article does nothing whatsoever to enhance the fantasies about bluestone quarrying in West Wales; indeed, it demonstates that the quarrying of the bluestones hundreds of km from the place where they were used was vanishingly unlikely.

QUOTE:  "As Scarre (2020) noted, the choice of megalithic sites is determined by the presence of large rocky outcrops and abundant blocks scattered on the surface, and at the Los Llanetes group, there is a clear spatial association between the construction sites and the source areas."


Archaeological and Anthropological Sciences (2023) 15:101


Choosing the site, getting the stones, building the dolmens: local sourcing of andesites at the El Pozuelo megalithic complex (Huelva, Spain)

José Antonio Linares‐Catela· Teodosio Donaire Romero · Coronada Mora Molina · Luis Miguel Cáceres Puro


The geoarchaeological study focuses on the lithological characterization and provenance determination of the rocks of the El Pozuelo dolmens. The difficulty of identifying volcanic rocks in the intensely altered and deformed environment of the Iberian Pyrite Belt has required the implementation of a research methodology combining the archaeological and geological analysis of the megaliths and the area surrounding the Los Llanetes group. A total of 29 thin sections and 14 geochemical analyses (ICP-AES, ICP-MS and REE) have been carried out on samples from the dolmens and potential source areas, focusing on the chemical elements considered immobile during alteration processes. The petrological analyses confirm the identification of different andesite lithotypes and enable us to correlate the rocks used in the construction of the megaliths with source areas and quarries located within a 50–350 m radius. Several patterns are observed in the selection of the rocks, based on the material, visual and symbolic properties of the different lithologies. Foliated andesite is the most common stone used in the monuments, due to its excellent physical properties and technological suitability for extraction and transformation into megalithic supports. Other types of andesite (sheared, massive and amphibole-phyric), white quartz, ferruginous agglomer- ate and gabbro were also used for different architectural purposes. The results confirm the importance of locally available suitable rocks in determining site location, raw material procurement and monument construction during the Late Neolithic.

Tuesday 27 June 2023

The other bluestones

The Louth Stone -- one of the larger erratics described by Caitlin Green

This is an interesting study of the "bluestones" of eastern England -- particularly Lincolnshire and Norfolk.  The author looks at a group of glacial erratic boulders -- widely scattered, with all the characters of large glacially transported clasts including facets, abraded edges etc.  It's a nice study, concentrating on the questions of what the stones were used for and why they were called BLUEstones when many of them are not blue at all.  It would have been nice if Caitlin had investigated the sources of the bluestones, but there you go.........

The erratic in the photo is reputed to have come from the Whin Sill.

Sunday 4 June 2023

Dolerite erratics in the Nevern Estuary


This is a nice rounded dolerite erratic on the foreshore at Parrog, Newport.  It rests on a seaweed-covered wave-cut platform of dark Ordovician shales and mudstones. Dimensions approx 1m x 50 cm x 45 cm.  There are scores of others on the foreshore not far from this one, and large dolerite erratics are also used in the foundations for the sea walls, on the quays near the Boat Club, and on the other side of the river.  

The foreshore is littered with erratics of all shapes and sizes, sometimes associated with Devensian till exposures

I'm intrigued, because there are no known exposures of dolerite or gabbro anywhere near Newport.  The closest outcrops are to the south, a couple of km away, on the northern flank of Carningli. How did they get to the coast, and why in such abundance?  There is no reason to invoke human transport, even for building work or for ship's ballast, and it must be assumed that they are GLACIAL erratics.

Could these erratics have been carried northwards by ice from a Carningli or Preseli ice cap?  It's possible, but I suspect that when these small upland ice masses existed, they were small, cold-based and sluggish -- that means there was not much erosion, transport, and deposition going on.

Another possibility is that the erratics have come from unknown dolerite outcrops on the floor of Cardigan Bay.

A third possibility is that they have come from the dolerite outcrops on the high interior of Pen Caer, between 10 and 12 km away, carried by ice that was travelling pretty well directly from west to east........  Well, we know from crossing striations that there were big swings in ice movement directions during the Pembrokeshire glaciations.  Are these boulders, like many of those on the coast near St Davids, relics of an Anglian Glaciation?  The jury is still out.

Heroic quarrymen and soft wedges

Chris Johnson's photo of the Carn Goedog "soft wedges" as exhibited in a Stonehenge exhibition in Belgium in 2018.  All good fun......

The idea of soft mudstone and rhyolite wedges being used in the quarrying of large bluestone monoliths is so nonsensical (as pointed out by Tim Darvill and others) that I thought everybody had forgotten about it and moved on.....

But now on his blog, and following a jolly trip to Mynydd Preseli, fellow blogger Tim Daw is having a bit of fun on the subject of the wedges.  Apparently in all seriousness he cites assorted examples (from the collected writings of MPP, of course) of soft wedges used at Rhosyfelin and Carn Goedog and even claims to have found a few of his own, one of them complete with handy perched hammer stone.

He's having a laugh, of course, and is poking gentle fun at his more serious colleagues, but he really should be more careful.  There are a lot of gullible people around, and some of them will believe what he is telling us, tongue in cheek...........

As faithful bloggers will know, my colleagues and I have examined all of the claimed "quarrying wedges" and have found that not one of them makes any sense at all.  There are lumps and slivers of rock in cracks all over the place, and the ones cited at Rhosyfelin could not possibly have been of any use whatsoever for the extraction os useful monoliths.  Two are in a crack in a detached small block that was of no more use to anybody than scores of other blocks lying about the place, and two that were close to MPP's imaginary "monolith extraction point" could not have been effectively hammered with a hammer stone without serious injury, and in any case could not have provided a pillar that might have been desirable as a pillar or monolith.

Our Neolithic ancestors may or may not have been stupid, but whatever their level of intelligence they were certainly not so idiotic as to have hammered useless bits of soft rock into inaccessible crevices just to obtain lumps of bedrock that could have had no value at all for ceremonial or any other purposes.

Entering into the spirit of things, here are two photos of nice stones from my collection.........

A selection of shale and mudstone fragments that can be called "wedges" if we are so inclined.  They are from two newly-discovered Neolithic wedge factories in locations which I am not at liberty to divulge.

A selection of rounded cobbles which might, if we are so inclined, be referred to as "hammerstones".  Some of them have fractures and scars that might have been caused by percussion impacts.  It all depends on your system of belief......


Thursday 1 June 2023

The Ogof Golchfa sill -- a possible source for the Limeslade erratic?


At the eastern end of the Ogof Golchfa site (SM 742 236) there is a perfect example of a micro-tonalite sill, sitting conformably in a sequence of thin-bedded sandstones and shales belonging to the Cambrian Solva Beds.  The sill itself is assumed to be of Ordovician age, intruded along a bedding plane, joint or fracture.   It's a bit more complicated than that, because it varies in thickness between 5m and c 20m, and there are one or two offshoots which don't appear on the geological map at all.  These are difficult to spot because the sill and the argillaceous rocks have virtually the same colour -- a distinctive greenish-grey.

At the western end of the site the sill is c 20m thick.

Reading the features is made more difficult because the sedimentary beds are metamorphosed or baked close to the sill edges, so that they look more like slate. And in the sill itself, the geologists noted that the feldspar crystals are small close to the margins and larger in the middle as a consequence of differential cooling or solidification rates.

On those parts of the sill close to sea level, where marine processes operate, there are good fresh exposures of the rock surface.  The greenish colour is everywhere apparent, and there are many fractures with bands of quartz and other secondary minerals.

Fine grained micro-tonalite close to the edge of the sill

Larger crystals and a "coarser" or rougher texture some way in from the edge

A very rough texture close to the centre of the sill

Turning to the Limeslade boulder, the colour similarity is striking.  It has a greenish hue, unlike the Preseli dolerites which are predominantly blue or grey in colour.  Is this down to a different copper (Cu) content?  We know from recent work that the Limeslade boulder has a considerably higher copper content than most of the Preseli dolerites; the results will be published in due course. On the other hand a green colouration can also come from minerals containing iron, chromium or manganese, and other minerals can also be involved.

Visually, this photo taken on my visit to Ogof Golchfa yesterday reminds me of the Limeslade boulder.

Close up of a rock face at Ogof Golchfa where the feldspar crystals are of moderate size.

The Limeslade boulder, with a surface heavily abraded by shoreline processes.

Although the sill at Ogof Golchfa has been referred to as a dolerite / microgabbro / diabase sill by some geologists, the designation as a micro-tonalite is now preferred, with the BGS also using the labels "metagabbro" and "microgabbro" for this group of Ordovician intrusive rocks. 

According to Wikipedia, tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total feldspar content. Quartz (SiO2) is present as more than 20% of the total quartz-alkali feldspar-plagioclase-feldspathoid (QAPF) content of the rock. Amphiboles and biotite are common accessory minerals.  In older references tonalite is sometimes used as a synonym for quartz diorite.

I don't want to be carried away here, and I am not quite convinced that the rock at Ogof Golchfa is the same as that of the Limeslade boulder.  But there are other similar sills with a greenish colour in the St Davids area and along the North Pembrokeshire coast, and one obvious provenancing candidate is Carn Ysgubor on Ramsey Island, just a few km away, referred to by BGS as having a large micro-tonalite intrusion of approximately the same age as that of Ogof Golchfa.  As they say, watch this space........

The Ogof Golchfa giant erratic


While we are on about erratics, giant and otherwise, let's celebrate the Ogof Golchfa "giant erratic" which sits prominently on the raised beach rock platform, embedded in Devensian till.  This one is actually quite celebrated already, since it is used by the geocaching community as one of the key North Pembrokeshire locations.  Note the little box in the photo, which contains trinkets and other bits and pieces of memorabilia.........

The boulder, which is well rounded, has dimensions 2m x 1.40m x 1m, and it's made of a very coarse dark coloured gabbro -- almost certainly from St Davids Head,  around 5 km to the NNW.

Some of the old geologists who visited this site thought that the big well-rounded erratics on the rock platform are embedded in the raised beach, and indeed Leach thought that they are "cemented" into position.  Although I could see no unambiguously cemented material at the base of the raised beach, I have some sympathy with this view,  and on balance I agree with them that these boulders -- like those at Whitesands -- were already in position when they were incorporated into the raised beach shingle and pebbles.  So we have evidence of TWO glaciations at Ogof Golchfa.........