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Monday, 20 February 2017

Llansteffan and the Altar Stone



Thanks to Geraint Owen for further information on the Senni Beds.  He has studied the Craig Ddu section in detail, and from his descriptions, and the work of  Ixer, Bevins and Turner, I am now more than ever convinced that the Altar Stone might well have come from Craig Ddu near Llansteffan. Below I reproduce some of the key info on the Senni Beds from the big report by Barclay et al 2015.
 It's clear from the published work that the Altar Stone has significant differences from the Cosheston Sandstones around Mill Bay (Milford Haven) and significant similarities with the sandstones described from Craig Ddu.  The samples are not identical, but that is not surprising;  we do not know the precise positions from which all the samples have been taken, along a rock exposure c 400m long and 20m high.

For the moment, until further work is done, I think we can assume that the Altar Stone has probably come from the Llansteffan Peninsula and maybe even from the Craig Ddu exposures. That makes perfect sense, since the eastern Preseli hills and the Llansteffan Peninsula are on pretty well the same Irish Sea Glacier flowline.  All of the speculation about the Altar Stone having come from somewhere along the A40 road between Llandeilo and Sennybridge becomes redundant.

  
Flow lines and erratic trails -- reconstruction by Olwen Williams-Thorpe, after Kellaway.  Note that the pinkish coloured flowline crosses both Preseli and the Llansteffan Peninsula.

 ==========================

Big BGS report on ORS stratigraphy.  It provides a more formal correlation of Cosheston Group and Senni Formation:

BARCLAY, W J, DAVIES, J R, HILLIER, R D, AND WATERS, R A. 2015. Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin. British Geological Survey Research Report, RR/14/02. 96pp.
It's a free download from the BGS website.

Extract:

4.2.5     Senni Formation

Name
Derived from the Senni Valley (Glyn Senni), Powys, south-central Wales. The name Senni Formation supersedes the traditional Geological Survey name Senni Beds (Cantrill in Strahan et al., 1904). In the Clee Hills, the formation was previously named the Clee Sandstone Formation.

Type area
Senni Valley [SN 930 209]

Partial type section
Waterfall exposures [SN 930 209] in a tributary of the Afon Senni (Nant Ystwyth) above Tyleglas.

Reference sections

1.    Heol Senni Quarry [SN 9154 2210], Powys provides a well documented reference section. It exposes about 40 m of grey-green sandstones, with minor siltstones, mudstones and intraformational conglomerates at the top of the formation (e.g. Edwards et al., 1978; Loeffler and Thomas, 1980; Dineley, 1999b; Barclay, 2005e).

2.    Cliffs at Craig Ddu on the western side of the Llansteffan peninsula, Carmarthenshire [SN 32441015](Owen, 1995). These reach a height of 20 m and have continuous exposure for 400 m of beds near the top of the formation.

3.    The stream draining north-westwards across Clee Liberty [SO 583 848–587 844] in the Clee Hills exposes about 140m of the formation (Allen, 1961) and provides a reference section for this area.

Lithology
Mainly of green and green-grey (locally red-brown and purplish green), very fine to medium- grained, micaceous sandstones, mainly channelised, cross-bedded and parallel-laminated, with green and red-brown siltstone and mudstone interbeds, some calcretes and intraformational conglomerates; the formation is characterised by the presence of vascular fossil plant remains and some soft sediment deformation is also present. In the Clee Hills, pale green sandstones are mainly arranged in fining-upwards conglomerate–sandstone–siltstone cycles, with subordinate red or green, sporadically calcretised mudstone/siltstone interbeds (Ball and Dineley, 1952, 1961; Allen, 1961; Greig et al., 1968).

Lower and upper boundaries
The lower boundary is placed at the base of green sandstones, which overlie red-brown sandstones and mudstones of the underlying Freshwater West (formerly St Maughans) Formation. Where mature calcretes at the top of the Freshwater West Formation are present (the Ffynnon/Abdon Limestones), the base of the formation is placed at the top of the uppermost calcrete. The upper boundary is placed where red-brown sandstones of the Brownstones Formation overlie the green sandstones of the Senni Formation, the junction being gradational. In the Clee Hills, Here, the lower boundary of the formation is placed at an erosion surface cut in the uppermost calcrete of the Upper Abdon Limestone, where it is sharply overlain by the basal green sandstones. The upper boundary is a gradational passage into generally coarser-grained
strata lacking in argillaceous beds and in which the sandstones are more variably coloured, these being assigned to the Brownstones (previously Monkeys Fold) Formation.

Thickness
300 to 450 m in the Brecon Beacons and Black Mountain, 150 to 200 m in the Black Mountains and 152 to 167 m in the Clee Hills.

Distribution
From Carmarthen Bay eastwards to the Black Mountain, Brecon Beacons and Black Mountains, and from there southwards to Abergavenny, wedging out north of Pontypool.

Depositional environment
High-discharge, mixed-bedload, sand-dominated, braided stream systems with relatively high sedimentation rates and water-table levels; although the formation is dominated by in-channel deposits, overbank floodplain silt and mud deposition also occurred.

Age
Latest Lochkovian to latest Pragian or Emsian in age.

REFERENCES

OWEN, G. 1995. Senni Beds of the Devonian Old Red Sandstone, Dyfed, Wales: anatomy of a semi-arid floodplain. Sedimentary Geology, Vol. 95, 221–235.

see also:

Also, RG Thomas's opus magnus on the Cosheston Group, which includes petrographic data:
THOMAS, R G, BARCLAY, W J, MORRISSEY, L, WILLIAMS, B P J, and ALLEN, K C. 2006. Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south-central Pembrokeshire, Wales. Geological Journal, Vol. 41, 481-536.


23 comments:

Myris of Alexandria said...

Add the 2008 Ixer and Turner paper on the Altar Stone (FREE on Ixer's academia edu site)They used the same section that Gerait Owen used -he lent them to I and T.

All of this will be lovingly and accurately discussed in Ixer et al 2017 in the legendary Ferret Club Newsletter

The petrography, geological age and distribution
of the Lower Palaeozoic Sandstone debitage from
the Stonehenge Landscape
by Rob Ixer, Peter Turner, Stewart Molyneux, and Richard
Bevins

Wiltshire Archaeological & Natural History Magazine, vol. 110 (2017), pp. 1–16

The provenance of the Altar Stone can/will only be determined by field studies as Dr Ixer has been saying for the last 10 years Bedding jointing studies.

Herefordshire ten floating down the Severn anyone???
M

TonyH said...

Those who wish to will be able to buy the above - mentioned Wiltshire Archaeological Magazine from WANHS at Devizes in due course. Go to the Wiltshire Museum website.

Alex Gee said...

How exciting! One presumes that the outcrop mentioned shares its location with the

quarry that the Altar stone was excavated from?

Wither the hypothesis of those damned awkward glaciologists/geomorphologists!

Surely we can all get along without awkward facts getting in the way?

After all we are truly living in a "post truth" age!



Sent from my laptop, whilst led in bed (watching M.A.S.H.) awaiting the arrival of

pro-bono proctologists from other star systems!

P.S. don't forget that the scientific opinion of earth scientists who work for oil/mining companies Re:groundwater pollution/climate change etc is solely dependent on the thickness of the brown envelope; containing the required data!



BRIAN JOHN said...

The Craig Ddu quarry? Well, maybe our learned friend Myris has already given MPP and his friends a prod in that direction. They have run out of steam in Pembrokeshire, and will probably be at a loose end next September. A Craig Ddu quarry hunt would really be a wizard wheeze!! The quarry hunters need a good hostelry in the vicinity, and just over the river is the famous Brown's Hotel where Dylan Thomas once propped up the bar -- the National Geographic can probably get the lads and lasses a nice inflatable craft for ferrying them back and forth across the muddy waters of the Afon Taf....... So that's all sorted then.

Myris of Alexandria said...

Oh dear too much Welsh Spring Lamb? Jiggled Hare.
Too early in the year for ergot poisoning.

Although starting this wild goose chase I have no idea what is going on now.
Perhaps were I too to indulge in White Lightening I might become enlightened and wish for cosmic anal probing. Which soap opera was that from Dynasty, Prisoner Cell Block H?? I guess it will be FREE.

I have no idea from whence the Altar Stone came, however, it must have come from an outcrop whose combined bedding-jointing characteristics could create a joint block of the correct size. BIG. But quarried it will have been.

The petrography and I suspect the geochemistry of the Altar Stone will not be sufficiently distinctive to help in provenancing. That is the point, field work is the key.

The next Ferret Club News paper discusses possible origins for the Lower Palaeozoic Sst not certain it uses the feared Q word.

With a gun to my head (a genuinely unpleasant experience -especially if the young druggie? gunman is shaking with nerves) I would hope the Altar Stone comes from Herefordshire. I suspect it will never be known, too ordinary a rock.

MPP does not need help in finding suitable sites to excavate. Is he done with Pembrokeshire.

M

chris johnson said...

Well Myris, would it not be elegant should a quarry enter the story somehow.

Why do you think the Altar Stone was quarried?

BRIAN JOHN said...

I'll let Myris answer that question. But since Llansteffan / Craig Ddu is on the flow-line of ice travelling from Preseli towards Stonehenge, the default position for any self-respecting earth scientist is that the Altar Stone probably came from here -- unless somebody can prove otherwise.

Myris's point about big blocks and appropriate jointing patterns is a fair one. Just look at the illustration published on this blog on 30 December -- the scrub-covered slope at Craig Ddu is littered with very large slabs and boulders, many of them far larger than the Altar Stone. I rest my case, but I shall go over there one day and take a look.

Alex Gee said...

Myris: It is a source of stubborn pride to me, that despite my habit of

posting on this site after indulging in a tincture or three; we all have to get our

relaxation somewhere.

I've yet to posit a hypothesis as unscientific and batshit crazy as

the "professionals" "Rhosyfelin was a Quarry" nonsense.

I don't think that's bad going for a loaded Builder?

I'd like to know what substance Tim Darvill was on when he posited his "Stonehenge

was a giant Xylophone" Hypothesis?

I'm afraid the rigours of the temperance league will never be for me

Bottoms Up!

Alex Gee

P.S. I presume some of my evidential contributions have been worthwhile?

Phil Morgan said...

What is Alex Gee on about?





BRIAN JOHN said...

Partly, I suppose, he is on about Myris's claims (in other threads) that the highly skilled geologists who have looked at Rhosyfelin have declared it to be a quarry, demonstrating how little they know about geomorphology. Apart from that, I'll leave it to AG to elaborate....

Phil Morgan said...

According to the British Geological Survey Memoir for the 'Geology of the country between Hereford and Leominster' the maximum extent of the Devensian glaciation in Herefordshire forms a curved line from Ludlow south through Leominster and on to Hereford before bearing south-west to Ewyas Harold.
If Myris is correct regarding the source of the Altar Stone being in the Herefordshire area, would this Devensian maximum rule out glacial transport, and is this the reason for Craig Ddu entering the debate?

BRIAN JOHN said...

Wrong glaciation, Phil. Nobody says that the Devensian ice carried erratics towards Stonehenge, but plenty say that the Anglian ice did. That's why I always try to differentiate between the LGM (Last Glacial Maximum) and the GBG (Greatest British Glaciation) -- there are some who think that Anglian ice from the N or NW overrode the Cotswolds, and those who think that it did not!

Myris has not, as far as I am aware, claimed that the Altar Stone came from the Herefordshire area -- he has just said that he would quite like that to be the case..... Different matter entirely....

Phil Morgan said...

Possibly.

Myris of Alexandria said...

Very true.
Herefordshire would be nice but who knows from whence.

Brian of course is being naughty claiming that Craig Ddu as the default position for the Altar Stone. There is no decent provenance for it yet -it awaits the correct quarry site (I include taking loose litter as quarrying- for me quarrying is the removal by man of stone for a purpose). Recognising ancient quarrying sites I leave to the experts in the field-not me- in preference to those with vague icy thoughts. I suspect that the quarry site hence the provenance is unlikely to be recognised.

Incidentally sandstones such as the Devonian Altar stone make extremely poor erratics more likely to be rock flour than big boulders. Where are the erratic on Salisbury Plain or in the river sediments draining the Plain or as blessed Kostas would have it, the mountins???
M
who leads a pure tictureless life (begone white lightening and dead dogs and those weak-willed creatures that need them)) and whose ONLY vice is self-righteousness.

BRIAN JOHN said...

I'm not being at all naughty in suggesting that Craig Ddu might be the default source for the Altar Stone. Makes perfect sense -- since we know pretty well what the flow lines of the Anglian Glacier were across West Wales, and since Preseli and the Llansteffan Peninsula are on the same flow line. So with any further arguments we just get into the realms of speculation.

Quarrying for me involves physical intervention in either the ground surface or in upland slopes, crags or cliffs. So traces must be left. Simply collecting or gathering stones off the ground surface is not quarrying -- that is stone collecting. Different thing entirely. MPP obviously works to the same definition as I do -- which is why he was so desperate to find "traces" or "evidence" at Rhosyfelin to support the hypothesis -- and why he was so disappointed when nothing of any use was turned up over 5 seasons of fieldwork.

Sandstones as bad erratics? Are foliated and flaky rhyolites any better? Hmmm.....

Phil Morgan said...

It now seems that not only was the glacier capable of selecting the C80 dolerites and rhyolites required for constructing the monument, but it could also select and transport a single large micaceous sandstone (plus 2 small lumps Stones 40 and 40g) whilst totally ignoring my old favorites, the Blue Pennant Sandstones of the Welsh Ice. Damn clever this glacier, and on St David's Day.

Didn't Myris once say that the source of the Altar Stone "will be far to the East", you can't get much farther East in the Senni Formation than Herefordshire.

BRIAN JOHN said...

Phil -- you shouldn't believe all you read. There are 43 bluestones, not 80, and they come in many shapes and sizes, and lithologies. I know you would love to have Pennant Sandstone at Stonehenge, just as I would love to have Carb Limestone, St Davids Head gabbro, Cambrian basal conglomerate, ORS from Dale, and all sorts of other alsolutely terrific rocks. Sadly, ice is never quite as accommodating as we would like.

Myris of Alexandria said...

Brian I do so agree
"Sandstones as bad erratics? Are foliated and flaky rhyolites any better? Hmmm.....""
Very very wise words.

No they are not any better especially the Volcanics A.
That is why their presence exclusively within the Stonehenge landscape(absent from Salisbury Plain) must be due to their being brought there by hand. They too would be fine grained material in any glacial deposits.

See Brian there is more rejoicing in Heaven of one sinner returning to the light than etc. etc. Turn away and sin no more.
Shall I book you for a tour of the Neolithic quarries of Pembroke? Shall throw in a cream tea.
M

BRIAN JOHN said...

Don't get too excited, Myris. Get the irony......

I have seen all over the place glacial erratics made of all sorts of rocks that should be far too soft to survive entrainment, transport and deposition. Sandstones, shales, ashes, rhyolites, agglomerates and so forth. And some of them, when left in the open, subject to normal weathering processes, proceed to crumble away. The laws of physics are indeed very wonderful.......

Alex Gee said...

"Self-righteousness" That's very funny! The Quarryists don't do irony do they?

A.G.

Tylwyth Teg said...

Was 'irony' first established in the Iron Age?

BRIAN JOHN said...

Quite possibly. Before that they probably just maintained a stony silence.

Myris of Alexandria said...

Oh really Splendid. You should try writing for a living!

I doff my cap and bells to you.

I have a wonderful tee-shirt with Irony printed on it surrounded by its atomic number, weight, electron configuration, etc etc. Nearly as good as my "Its only a pile of rocks in a field" I have two of those.

Marshall's fluffy pamphlet is fine, it is exactly what Pitkin press is about, but it tries to offend nobody and goes New Age weird at the end. mmmmmmmmm. Follow-ups to highly successful albums are always difficult, ask the Troggs. "Split y're Hands"

Were speed cameras attached to ley lines what would be a fine currency?
Where are the ley line motorways??
Cunningham in his book has pan-global ley lines (one going through Shangri La (I kid you not) that are 361.375miles wide.
M