More insights on the bluestone controversy

DFN: More insights on Stonehenge’s bluestone controversy, the controversy being where did the stones come from? Until recently, the commonly held belief was that the stones came from around Pembrokeshire. A previous article in this blog tees up an alternate theory, that the stones came from North Wales:

This new article, give more details surrounding why ‘they’ don’t think the stones came from Pembrokeshire:

Sunday, 1 November 2009

New Stonehenge Bluestone Mystery


Secrets of Stonehenge


Missing Stonehenge circle did not come from Preseli

British Archaeology has issued an important revision on its website (see below) to the news article on the Missing Stonehenge Circle that appearedin the latest edition of its magazine, BA 109 November/December 2009, the publication of the Council for British Archaeology (CBA).

The original full page news article, entitled “Missing Stonehenge circle did not come from Preselis” written by editor Mike Pitts, claimed that a new theory reveals that the dogma accepted by most archaeologists since first proposed in 1920, that almost all the Stonehenge bluestones came from the Preseli Hills in Pembrokeshire – is WRONG. According to the two geologists Rob Ixer and Richard Bevins, who have studied thousands of rock specimens from recent excavations at Stonehenge, concluded that many bluestones came not from Pembrokeshire, but from a far wider area, perhaps north Wales (Snowdonia, the Llyn Peninsula and Anglesey), or even beyond. The well-known spotted dolerite is a Preseli rock, they say – but the likely source was not Carnmenyn (where archaeologists have recently claimed to have found quarries) but nearby Carngoedog.
Outcropping rocks on Carn Menyn, Preseli.

Stonehenge’s megaliths are classed into two groups, ‘sarsen‘, and everything else classified as ‘bluestone’. The sarsens, the large lintelled circle and trilithons, are from a local sandstone, and estimated to have a total weight of about 1,800 tonnes. The total bluestones amass to about 250 tonnes and have been the cause of much debate as they are not a local stone; most bluestones derive from Preseli in Pembrokeshire in South East Wales and geologists and archaeologist fail to agree on how they arrived on Salisbury Plain. Most prehistorians favour human agency as the method of transportation following the geologist Herbert Thomas who first identified the Preselis as the geological provenance in 1923. This view has been endorsed by geologists such as Christopher Green and James Scourse and recently archaeologists Tomothy Darvill and Geoffrey Wainwright claimed to have identified quarry outcrops and “sacred springs” as the source of the bluestones around Carnmenyn in the Preseli Mountains.

Geologist Geoffrey Kellaway [incorrectly cited as George in the BA article] proposed in 1971 that the bluestones had been transported by glacier. The well known authority on stone circles Aubrey Burl also supports this view and refers to transportation of the bluestones by human agency as a “fairy tale”. In 1991 a team of geologists from the Open University, including Olwen Williams-Thorpe, produced a differing glacial interpretation but still favoured ice as the mode of transport.

The BA 109 article states that Rob Ixer (University of Leicester) and Richard Bevins (National Museum of Wales) are proposing a third option, quoted as saying many bluestones do not come from Pembrokeshire but from “a far wider and, as yet, unrecognised area or more likely areas” – perhaps North Wales (Snowdonia, the Llyn Peninsula and Anglesey) or even beyond. The well known spotted dolerite, is a Preseli rock, they say, – but Carngoedog was the likely source not Carnmenyn. These conclusions derive from a new study of thousands of Stonehenge rock specimens collected in 1947 from near the west end of the Cursus ( proposed as the site of a lost bluestone circle) and recently excavated by the Stonehenge Riverside Project in 2006/08 and from bluestone fragments retrieved from excavations at Stonehenge by Mike Pitts, 1979/80, and Darvill & Wainwright in 2008.

The geologists also found the Cursus bluestones, which are all rhyolitic and mainly tuffaceous (with no Stonehenge dolerites) has significant mineralogical differences from visually similar rocks at Stonehenge. The Darvill & Wainwright excavation produced significant quantities of a type of rhyolite or rhyolitic tuff “not recorded in north Pembrokeshire and noticeably absent in the Mynydd Preseli area”.

How the stones were moved, Ixer told BA, “is an archaeological problem” though he wondered if “different groups [of people] brought different stones?”

In 2006 Ixer, with Peter Turner, suggested that the Stonehenge Altar Stone [classified as the largest stone in the bluestone group although it is in fact micaceous sandstone] came from an unidentified source far from Milfrod Haven – the traditional attribution said to indicate where the Preseli stones were taken downriver for the sea-bound journey to Stonehenge.

Ixer and Bevins’s detailed study will be published in 2009 Wiltshire Studies.
Carn Menyn promontory identified as a specific Stonehenge bluestone source

An interim note by Ixer & Bevins on the latest developments since the BA 109 news item was published, has been posted on the British Archaeology website:

Important revision to Stonehenge bluestone theory

In the News pages of the Nov/Dec 2009 issue of British Archaeology, it is reported that new petrographical work by Rob Ixer (University of Leicester, Department of Geology) and Richard Bevins (National Museum of Wales) had suggested that some of the Stonehenge bluestones had not come from Pembrokeshire, but (in Ixer’s words) from "a far wider and, as yet, unrecognised area or more likely areas". As the magazine was being printed, however, Bevins was out in the field, and found an apparent source for the rocks in question north of the Preselis. Ixer and Bevins have kindly written this interim note on this latest development.

Stilpnomelane-bearing rhyolites/rhyolitic tuffs at Stonehenge are most probably from the Preseli Hills region
Field and petrographical work continues on new Stonehenge lithics and on in situ material from areas around the Preseli Hills. This includes excavated material from the Avenue at Stonehenge, and rocks from undistinguished outcrops in the low ground north of Mynydd Preseli, close to Pont Saeson.

The former, as expected, conformed to the range of lithologies seen throughout Stonehenge. But the latter had surprising results, and has led to our radically modifying our proposal that many of the bluestones do not have a Preseli Hill origin, but have an unknown and possibly non-southern Welsh origin.

In thin section the Pont Saeson fine-grained acidic rocks show most of the features of our class of Stonehenge rocks, informally called "rhyolite with fabric", including a lensoidal fabric and the presence of stilpnomelane. Despite nearly a century of collecting and analysis, this is the first record of this mineral in rhyolitic rocks in south Wales. The only previous recorded occurrences of stilpnomelane in acidic rocks in Wales are from the Cregenen granophyre in the Cadair Idris area of southern Snowdonia, and in granophyric rocks of the St David’s Head Intrusion, in north-west Pembrokeshire.

Although not an exact match for the Stonehenge rocks, the Pont Saeson lithics strongly suggest that the "flinty rhyolite/rhyolite with fabric" found in the excavations at Stonehenge has an origin in the Preseli region, and that there is no longer a need to look further north in Wales for this important class of Stonehenge debitage.

The other and more abundant unusual rock-type (carrying distinctive titanite-albite inter-growths) from the Great Cursus area (but not so far identified at Stonehenge) is still unprovenanced, and its petrography has still yet to be matched with rocks from south Wales, or indeed from the rest of Wales.

An interim summary of where we now believe the Stonehenge bluestones come from, and incorporating these new data, is:

* Spotted and unspotted dolerites, the flinty rhyolite/rhyolite tuffs and possibly the basaltic tuffs have a Preseli origin, but a search for their associated source rocks must no longer be restricted to the prominent outcrops on the Preseli Hills
* The Altar stone Devonian sandstone – the largest bluestone – cannot be from the Preseli region
* The rare other sandstone orthostats comprising a Palaeozoic sandstone are also not from the Preseli Hills, but may be southern Welsh in origin
* The titanite-albite-bearing rhyolitic rocks have yet to be sourced, but it is now anticipated that they too will have come from the Preseli region; only detailed and dedicated collecting and petrography will be able to prove that.

Rob Ixer & Richard Bevins

[Source: http://www.britarch.ac.uk/ba/ba109/interim.shtml]

So where does this leave the bluestone transportation debate?

The Stonehenge sarsens, the largest used in the Great Trilithon estimated at 50 tons, were brought 25 miles from the Marlborough Downs, which was a significant achievement in its own right. Transporting 4 ton bluestones 160 miles by human effort from South Wales to Sailisbury Plain certainly seems plausible. There were also a small number of limestone blocks and slabs used in the construction of Stonehenge brought to the site for the specific purpose of packing material to support the much larger sarsen uprights. The limestone quarries have been identified as Chilmark, 12 miles west, and 3 miles southeast at Hurdcot.

The number of different rock types found amongst the bluestone group at Stonehenge is cited as significant evidence of glacial deposits, with debate continuing for the location of the exact quarry sites seen as the basic flaw in the argument for human movement of the bluestones, in other words identifying the quarry site(s) will prove the human agency method. According to Ixer & Bevins’s revision statement were are still looking at Preseli as the geological provenance for the majority of bluestone. Anyone familiar with the Preseli mountains will be aware that there is ample loose bluestone over a number of peaks in the mountain range and would therefore not need to be quarried but pieces the appropriate size simply collected with minimal effort for use in the monument requiring later minimal dressing prior to erection. The bluestone constructions at Stonehenge were built and rebuilt maybe as many as five times over a 400 year period. We do not know if all the bluestones were brought at the same time, but it is quite conceivable that different working parties, possibly generations apart, collected from different sites in the Preseli mountains. As Rob Ixer told BA, had “different groups [of people] brought different stones?”
Preseli bluestone – abandoned because it cracked?

As Anthony Johnson states in his recent work on Stonehenge:

“…as there appear to be so relatively few bluestone finds outside Stonehenge and its immediate environs, with no extensive distribution across the Plain or its river valleys, a glacial derivation is considered unlikely. The glaciation theory has to address why the people building the earliest stone monument appear to have selected only exotic stones; if Salisbury Plain had been littered with a variety of rocks, including local sarsen, was the intention to gather material suitable to build the first stone circle, or primarily an exercise in prehistoric field geology?

It is far easier to envisage the bluestones collected at the source (i.e. where they outcrop), than to see them as having been selectively chosen from the surrounding landscape. There is a another important point to consider here: whilst a variety of large exotic rocks and even hammer-stones and mauls was used in the packing of the sarsen uprights, implying that stone for this purpose was in short supply, none was bluestone; had it been generally present within a local glacial assemblage it would undoubtedly have been collected and utilised.” [1]

It would appear the building materials for Stonehenge were carefully selected from various sources for specific purposes, far from being a “rag bag mix of glacial erratics”.

*

Reference:
1. Anthony Johnson, Solving Stonehenge, Thames & Hudson, 2008, p.127.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s