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Monday, 21 August 2017

Multiple bluestone rock types -- but how many provenances?

In January 2015 Myris said that the geologists accept around 10 different rock types in the bluestone assemblage.  But the number of rock types is self-evidently not a guide to the number of provenances. 

The dolerites, for example, which some might wish to count as belonging to a single rock type, have almost certainly come from multiple locations -- the samples reported on are all different, suggesting that they have NOT all come from the same place.  The same is true of the "rhyolites with fabric" -- one rock type maybe, but three different groups according to the analysis. So there are three (at least) different provenances for that rock type alone.  The same is true of the sandstones -- they are all sandstones, but of different ages and from three different places.

See also:

Here we go again, with an update of the list published in January 2015.  Maybe this will all become clearer with the publication of a new popular paper by Ixer and Bevins in Geology Today, in the autumn. There are at least 18 different rock types, and probably more than 20. That means multiple provenances, exactly as you would expect with a collection of glacial erratics.

** There are 31 dolerite orthostats, of which 14 were sampled in 1991 and 2008.  Some are standing stones and some are stumps.  Some are spotted and some are unspotted.  The spots are now thought to be not felspars but aggregations or clusters of low grade, secondary metamorphic minerals.

Bevins, Ixer and Pearce (2014) analysed 22 Stonehenge dolerite samples, and suggested that they were clustered into three groups, with one sample petrologically distinct from all three.  So there are three groups and one outlier -- four types. Every one of the 22 samples is unique, and the possibility must be entertained that every one has come from a different geographical location in eastern Preseli.

** There are four above-ground volcanic rocks in the orthostat collection (stones 38, 40, 46 and 48).  There seem to be four distinct types - two dacites and two rhyolites.  They are referred to by the geologists as rhyolitic tuffs, foliated rhyolitic tuffs, crystal-lithic-vitric tuffs, and argillaceous tuffs.  They have come from four different north Pembrokeshire locations. Stone 38 has an unusual mineralogy including graphitising carbon.  

** There is not much debris to match the 4 volcanic rock orthostats in the Stonehenge debitage, but similar fragments are found in the great cursus field.  In the debitage, unique volcanic material has been classified as belonging to two types -- Volcanic Group A and Volcanic group B.  None of the potential parent orthostats for Volcanic Group A (32c, 33e, 33f, 40c and 41d) have been petrographically examined, making it impossible to relate this debitage to any (or all) of the buried stones.  Ixer and Bevins (2016) say: ".........on present knowledge the origin(s) of the Volcanic Group A lithics is still expected to be found within the Ordovician volcanic sequences in the north Pembrokeshire area on the northern side of the Mynydd Preseli range probably amongst those outcrops examined by Evans (1945)."

** There are two micaceous sandstone stumps -- numbered 40g and 42c.  There are also lumps of Lower Palaeozoic sandstone scattered about in the debitage -- the largest lump weighs c 8.5 kg.  There appear to be two types, with possible sources in the Lower Palaeozoic rocks of north-west Pembrokeshire.

** The Altar Stone (stone 80) is a greenish calcareous sandstone, probably from the Senni Beds of Carmarthenshire or Powys. (It is not from Milford Haven.)  So just one rock type here. There is some debitage related to this stone, but it cannot be established that the fragments came from the Altar Stone itself (Ixer and Bevins, 2013).  The most feasible provenance for the stone is the Laugharne - Craig Ddu -- Llansteffan area of Carmerthenshire.

** In the debitage there are many fragments of rhyolites, some with planar fabrics -- and most matching closely (but not perfectly) the rock outcrops at Craig Rhosyfelin.  There appear to be three distinct groups of "rhyolites with fabric", all assumed to have come from the Pont Saeson area.  There may be a match with orthostat 32e or 32d (Ixer and Bevins, 2011), but the debris analyses does not match any of the known volcanic orthostats.

** There are also some basic tuffs in the collection of fragments from the debitage -- two lithologically different types (Ixer and Bevins, 2013). These are probably also from the Fishguard Volcanic Series.

** Other lithics in the stone collections from the debitage (eg. haematite (from the Reading Beds?), greensand, slate, limestones, Mesozoic sandstones and gabbros) appear genuine, and need further research.

To sum up: 

Ixer and Bevins (2014) state that there are “about ten types of bluestone” represented in the orthostat / debitage samples, but they also show that these have nonetheless come from at least 20 different locations.  It is estimated on the basis of the above points that there are at least 30 different rock types represented in the full "bluestone assemblage" -- especially when due respect is given in this count to cobbles as well as orthostats and flakes, for reasons frequently recited on this blog. 

It should also be noted that the majority of the 43 bluestone monoliths/stumps at Stonehenge have still not been sampled and analysed.  The new work reinforces the idea that the Stonehenge bluestone assemblage is made up entirely of rock types from the west, and that they have come mostly from north Pembrokeshire. 

Finally, a plea to the geologists -- if there is anything wrong with this list, please tell us about it, and give us your corrections and citations.  As ever, I'll publish them without fear or favour!


chris johnson said...

I do not take issue with the facts you present but perhaps the presentation might lead an innocent reader to a false conclusion.
The traced and supposed sources in Preseli are close together. A modern normally desk bound homo sapiens sapiens (us) could get up latish, go for a stroll, stop for lunch and a chocolate biscuit, and still get home in time for tea after visiting ALL the known sites.

One presumes that our ancestors might have been able to get round all the localities between breakfast and lunch.

The coincidence of so many of these Prescelli stones on Salisbury plain and nowhere else is remarkable. No?

BRIAN JOHN said...

Well the Altar Stone isn't from north Preseli, and neither are those Palaeozoic sandstones -- and I also doubt that the provenancing is as accurate as the geologists would have us believe. But yes, it is intriguing that most of the apparent provenances are quite close together. But they do sit very nicely on one of the flowlines of the Irish Sea Glacier, as pointed out by Kellaway many years ago. If the monoliths and bits and pieces were picked up by our Neolithic ancestors, it's even more wonderful, don't you think, that they had such a sophisticated knowledge of glaciology?

chris johnson said...

Very droll.
I was struck by a recent map you published of the terrain at the end of the last ice age. The Prescellis were free of ice as was the area around Stonehenge/Avebury. We know both areas had an abundance of fresh spring water and warmish too in the case of Wiltshire - likely spots for early hunter-gathers to enjoy and well within the range they might roam between summer and winter quarters. Just saying.
For sure they would have recognised the Preselli volcanics - after all the Victorians did.

TonyH said...

Haven't some of the so - called bluestones got potential provenances further away than Preseli, e.g The Strumble, Brian, or am I barking up the wrong trees, metaphorically speaking?

BRIAN JOHN said...

As Myris will confirm, there are still some "unprovenanced" orthostats and rock fragments at Stonehenge. Strumble Head / Pen Caer is one possibility for igneous materials, and the Palaeozoic sandstones have probably come from north of Preseli. Richard is no doubt hoofing about and looking for the spots from which they might have come......

TonyH said...

So if some "unprovenanced" igneous orthostats and rock fragments at Stonehenge were in due course pinpointed to Strumble Head, then that would be quite an extended "stroll" away, by Chris Johnson or even early hunter - gatherer standards, from any perceived Preseli point of human collection.

The likelihood of a glacial dynamic to the arrival of West Welsh geological items somewhere in Wessex would thus have increased probability.

Neil's conviction that Prehistoric Man intrepidly shifted his beloved "blues" remarkably long distances might even be on shaky ground with this scientific discovery.