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Tuesday 9 November 2010

More than 30 different bluestone sources

 "Oh dear -- this gets REALLY confusing.  Why don't we just fill the bloody hole in again and forget about those blasted stumps?"  By the way, they are stumps 32d (spotted dolerite, in the foreground) and 32e (rhyolite, to the left) ......

I have been convinced for some time that there are well over 30 different sources for the bluestones in the Stonehenge area.  Been looking through the records, and here is a starting list. It's a difficult list to compile, because many identifications are from visual examinations only, while others are from detailed petrography and geochemistry (methods used by the OU team in the late 1980's), and yet more through detailed analysis by Dr Rob Ixer, Dr Richard Bevins and others, using the latest methods, in the last few years.   Geological names also change over time, and (for example) what used to be called rhyolite now has many different names.  Some provenances are known (where stone numbers are given) but there are literally thousands of smaller stones and fragments in the various collections -- and it is not impossible that some of these samples have been given different names and maybe different numbers, leading to duplication. Even the numbering of stones is not consistent.

Here is a list for starters -- thanks are due to Dr Rob Ixer for keeping me informed of progress on the geology of the Stonehenge finds.  I have listed some of his publications on previous posts.  I'll welcome corrections, since there are bound to be mistakes.  But I wouldn't mind betting that when this list is corrected it will get even longer, since stones currently grouped together (for example the white spotted dolerites and the pink spotted dolerites) may well prove to incorporate stones from different provenances.

On the other hand there is considerable geological variation on the rocky outcrops of Preseli and along the exposures of Fishguard Volcanics -- so for example a dolerite and a rhyolite may occur right next to one another on a single tor.

Note -- not all of the Stonehenge monoliths are listed below -- many stones have not been examined and still have to be categorized, especially those from within the spotted dolerite and unspotted dolerite groups.

Stonehenge Bluestone Types

1.  Unspotted dolerite ---- monoliths  45 and 62.  Carn Ddafad-las?

2,  Spotted dolerite -- densely spotted.  Monolith 42  -- Carnbreseb? 43?

3.  Boles Barrow dolerite -- spotted?  But similar to stones 44 and 45? From Carnmeini / Carngyfrwy area?

4.  Rhyolite  -- stones 38, 40, ignimbrite character.  Ash-flow tuffs (dacitic). Not Carnalw ? May be from different sources?

5.  Rhyolite --  stones 46 and 48, rhyolitic ash-flow tuffs.  Carnalw area?  Same source?

6.  Rhyolite fragment from a different source from the above types

7.  Laminated calcareous ash -- stumps 40c, 33f,  41d

8.  Altered volcanic ash -- stump 32c, 33e?

9.  Rhyolite -- another type -- stump 32e.  Related to Pont Saeson samples?

10.  Micaceous sandstone -- stumps 42c, 40g (Palaeozoic -- South Wales origin?)

11.  Rhyolite -- lava -- stone 46

12.  Rhyolite -- flinty blue -- different lava?  stone 48

13.  Spotted dolerite with whitish spots --stones 33, 65, 68, stump 70a?, stump 71?, 72

14.  Spotted dolerite with few spots -- stone 31, 66?

15.  Spotted dolerite with pinkish spots -- stones 150, 32, 34, 35A, 35B (one stone), 39 (?), 47, 49, 64, 67, 69, 70

16. Spotted dolerite -- moderate spots -- stone 37, 61, 61a?

17.  Unspotted dolerite -- stone 44 -- different from stones 45 and 62

18.  Very fine-grained unspotted dolerite -- stone 62

19.  Silurian sandstone -- Cursus -- fragments

20.  Devonian sandstone -- Altar Stone -- Devonian Senni Beds -- Carmarthenshire or Powys

21.  Sarsen sandstones -- various types -- packing stones and mauls

22.  Jurassic oolitic ragstone -- Chilmark?

23.  Jurassic glauconitic sandstone -- Upper Greensand?

24.  Gritstone unspecified fragments (Maskelyne, Judd)

25.  Quartzite unspecified fragments (Maskelyne, Judd)

26.  Greywacke unspecified fragments (Maskelyne, Judd)

27.  Granidiorite -- Amesbury long barrow 39

28.  Quartz diorite -- ditto

29.  Hornblende diorite -- ditto

30  Flinty rhyolite -- fragments from Pont Saeson

31.  Rhyolite fragments -- with titanite-albite intergrowths (source unknown)


Anonymous said...

Good to see 32e in all its glory.
GCU In two minds.

BRIAN JOHN said...

Yes -- but in the picture it looks more like a laminated ash or even a lump of shale or mudstone! The shaly / flaky character of the rock is very noticeable. Don't remember the Pont Saeson outcrops looking quite like this........ is your sample very flaky too?

Anonymous said...

Yes some of the Stonehenge 'debitage' is very foliated and in extreme cases has been macroscopically described as slate!!!
After the Altar stone this is the first stone that I would sample.
Some of the PS material also has anextreme foliation.
GCU. In two minds

BRIAN JOHN said...

As far as I can make out, there are a lot of stumps in the bluestone settings that have only ever been described after visual inspection -- ie there has been no geology done on them. It would be fascinating to see that work done sooner rather than later -- and I wonder if any of them really are slates or other metamorphics? There are plenty of such things all over Wales -- and almost all of the igneous outcrops in the Fishguard Volcanic Series have baked / altered sediments in close proximity.