Distribution of rock types in the Stonehenge Layer -- or rather, in that part of the Stonehenge Layer recently investigated. There is an extensive area still not studied -- and there is no knowing what might pop up during future investigations.......
One of my moles has sent me some info from a lecture which has already been presented twice (I think) and which will be given again within the next fortnight by one of the key geologists involved. Since all of this is already in the public domain, I don't suppose I am breaching any confidences by putting some of it onto the blog.
The summary of recent research involves the following:
1. There are 31 dolerite orthostats, of which 14 have been sampled in 1991 and 2008. Some are standing stones and some are stumps. Some are spotted and some are unspotted. (I am a bit mystified as to why the unspotted dolerites do not appear on the diagrams above -- stones 45 and 62 are made of unspotted dolerite.)
2. There are five crystal vitric ash flow tuffs represented in the orthostat collection. (Stones 40, 48, 46, 38, 52c. (Four distinct types?) There is not much debris to match these in the Stonehenge debitage, but similar fragments are found in the great cursus field. Research is ongoing, but they may come from the Preseli area.
3. There are four volcanic ashes -- stumps 32c, 33e, 33f, 41d.
4. There is one calcareous volcanic ash stump -- number 40c
5. There are 2 micaceous sandstone stumps -- numbered 40g and 42c. (More info is eagerly awaited on these......) There are also lumps of Lower Palaeozoic sandstone scatterd about in the debitage -- the largest lump weighing c 8.5 kgs. From SW Wales?
6. There is another calcareous sandstone -- the Altar Stone (stone 80). sampled more than a hundred years ago, but not since. Probably from the Senni Beds of Carmarthenshire or Powys? (Not from Milford Haven) Interestingly, no debitage has been recognized in recent digs from this stone or from anything like it.
7. In the debitage there are lots of fragments of volcanics with sub-planar cleavage -- matching the Rhosyfelin rocks? The "rhyolite with fabric" is not all the same -- but most appears to be from the Pont Saeson area. There are NO matching orthostats.
8. There are also some basic tuffs in the collection of fragments from the debitage -- two lithologically different types. From the Fishguard Volcanics?
9. Other lithics in the stone collections from the debitage -- some stones are adventitious / introduced / modern, but some (eg haematite, greensand, Mesozoic sandstones and gabbros appear genuine, and need further research.
10. In the course of the recent geological research, 6,368 rock samples have been examined and classified -- and organized by archaeological context. the total weight of samples thus far is in excess of 70 kg. Most fragments are very small, weighing on average about 11 grams.
11. Almost half of the material in the debitage is sarsen -- I suppose we should not be surprised by that, but it would be good to know how many types of sarsen there are, and where they came from.....
12. This recent research matches pretty well with what I said in my post dated 3 December 2011:
http://brian-mountainman.blogspot.co.uk/2011/12/bluestone-rock-types.html
I reckoned then that there are about 30 different rock types represented in the "bluestone assemblage" -- and unlike Rob, I give significance to the small bits as well as the orthostats, since I am interested in glacial and other processes and want to know where they came from and how they got here.
5 comments:
Some indication of where the sampling was done would be useful .
If there is no unspotted dolerite in the layer and the area around stone 62 was sampled it suggests that the flake removal and light pick dressing (as noted in the laser survey ) may have been done elsewhere .
Brian,
Thanks for posting this! In my humble opinion, these 'facts in the ground' suggest natural agency. As does also the uniform distribution of stone fragments in the debitage (according to the Atkinson photos).
You write, “I give significance to the small bits as well as the orthostats, since I am interested in glacial and other processes and want to know where they came from and how they got here. “
What 'other [natural] processes' could account for all such 'facts on the ground'? Besides my 'meltwater retaining basin' hypothesis of Stonehenge? And if the Stonehenge Layer proves to be very distinct from the immediate area surrounding Stonehenge – with stone fragments that do not match any of the existing megaliths at Stonehenge but trace to geographically diverse and distant places as West Wales – wont that also support my working hypothesis? The Avenue and the 'periglacial stripes' will have a natural and sensible explanation also!
Kostas
Agree with that, Geo. No doubt the sampling locations will be published in due course. There is always a danger of assuming that your sampling points (which were selected for archaeological reasons, not geological ones) will give you a "typical" picture -- but you may of course be getting a skewed picture instead. So care has to be taken with extrapolations and generalisations.
Ah I am channelling strongly.
and here it comes……
It is always better to hear the master's voice some of this post is scrambled and although the diagrams remain, the text has evolved and some of the buried orthostats are now thought to be different. Many of the points are historically correct.
The diagrams and text are from a lecture given a couple of years ago in Devizes and things have moved on, more samples investigated (Craig Rhosyfelin was an unknown place then) and from more areas although these are concentrated about the Heel Stone/Avenue (not used in the above diagrams) and the May 2008 SH excavation- the data given.
The Stonehenge layer material examined was after some/?much of the sarsen material had been removed before the 80kgs of rock was delivered so the relative amounts of sarsen cf bluestones mean nothing.
The separation of spotted from unspotted is believed to be a chimera and so both are classed together. It is unwise to give any store to the differences. How do you tell if a 10cm flake of dolerite is spotted if the spots are 5cm in diameter, randomly distributed and not present in that flake.
The Stonehenge ONLY deals with debitage hence the composition of the orthostats, buried or alive, is irrelevant.
Treat these data as provisional. I assume that is why they are not published.
However, by and large, and with a fair wind in the face, the breakdown is more or less what is seen elsewhere but with much smaller numbers of lithics to look at.
The major point is that the amount of Brian’s exotica is trivial and take out the haematite, greensand and mica ssts almost totally disappears.
So Kostas you have a partial fulfilment of your wish.
The mists are clearing praise be to the great the sublime Apollo.
M
Thanks Myris -- useful comments. We look forward to seeing these diagrams updated when the most recent research is complete. There are still big questions -- over the dolerites (spotted or not), the other volcanic materials, the sandstones and the "exotica". Further research results eagerly awaited.....
I still find the dolerites interesting. There is certainly no chimera if you wander round in the eastern Preseli hills -- some of the outcrops have spotted dolerites and some do not -- and there are big differences in textures too. I don't think we can just lump them together -- it must surely still be possible to assign individual orthostats to individial outcrops?
Don't forget that there are some interesting records of sandstones etc in the hammer stone and maul collections -- these are too small to be counted as orthostats and too big to be counted as fragments in the debitage. But they are present -- and where did they come from?
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