The formation of the sarsens

Two of the rougher sarsens......

The sarsen stones have generally been interpreted as broken remnants of a duricrust layer. But there is an interesting new theory put together by Steve Marshall. Also, there is a fantastic new map. Read about the theory here:

http://www.stevemarshall.org.uk/sarsen_stone.htm

Bluestone 69

What an extraordinary shape there is on the base of this stone! Has there been any discussion in the literature about it? It is one of the bluestone horseshoe stones.

I have seen some mention of the TOP of the stone being shaped to support a lintel, at some stage, but it's the base that really interests me.....

Stumps near Bluestone 41

Stumps near Bluestone 33

Where the stumps are

This plan (acknowledgement to Anthony Johnson) shows where all of the standing stones and stumps are.

The numbers are easier to read if you double-click on the image to enlarge it.

More on Bluestone stumps

According to Atkinson, there are ten bluestone stumps. One (35) is above ground, and there are nine that are wholly buried beneath the surface: 32c, 32d, 32e, 33e, 33f, 40c,40g, 41d, 42c.

Of these, four (32c and 33e and 33f and 41d) are of "altered volcanic ash, dark olive-green in colour when unweathered, with a laminated fracture and a noticeably softer texture than the other bluestones." (Atkinson, p 48) Stone 32e is supposedly rhyolite, and stump 40c is a calcareous ash, referred to by Atkinson as a laminated blue rock. Stumps 40g and 42c are supposedly of Cosheston sandstone, "a blue-grey rock spangled with mica". This leaves 35 and 32d which are presumably dolerites, spotted or otherwise......

Another interesting point is that Hawley discovered four sockets and two stumps between stones 32 and 33, and Atkinson showed that there were five stones between 32 and 33, and no less than eight stones between 40 and 41. There are other big gaps between 33 and 34, and between 41 and 42. Did these gaps ever hold stones, or were they simply left empty while the builders of the monument searched for suitable stones and fiddled about with the ones they already had?

We already know that the "Cosheston sandstone" has been mis-identified -- and it will be interesting to see what else comes up when the new geology by Rob Ixer and Richard Bevins (arising from the 2008 Darvill-Wainwright dig) is published.

Fragments, stumps and layers

This old photo shows stumps 32c, 32d and 32e, together with standing stone 33. Note also the sockets or pits that presumably held different stones, or maybe the same stones in different settings.......... Note the large number of fragments beneath the turf, in the "Stonehenge layer."

Does it actually matter whether the debris in this litter is the same as, or different from, the rocks represented in the bluestone settings? Not sure that it matters very much, one way or another. As Rob Ixer reminds us, many of the standing stones themselves are not properly identified or sourced -- very few of them have actually been sampled and examined with the use of modern techniques, and the identification of the monoliths as dolerite, spotted dolerite, rhyolite and ash is based mostly on rather primitive visual identifications by Thomas, Atkinson and others.

On the whole, we know just a little about the makeup of the bluestone settings, about the stumps and about the precise lithologies represented in the Stonehenge layer. Most the the superficial layer within the stone settings is a complete mystery still -- as some have pointed out, the popular view that Stonehenge is thoroughly excavated and well known is actually far wide of the mark.

Back to Stonehenge

Have been exchanging some more thoughts with Ned on the site called "Armchair Prehistory" -- see here:
http://armchairprehistory.com/2010/03/25/bluestones-and-bell-curves/comment-page-1/#comment-49

Pondering on the impact of chalk on glacial and periglacial processes -- and on the mystery of why the long barrows of Salisbury Plain are apparently free of large stones...... except for Boles Barrow.

Outside the Limit

This is Maiden Castle tor, a rhyolite tor in the centre of Pembrokeshire, overlooking Trefgarn Gorge. It is so delicate, with pinnacles, crags and great broken slabs so precariously balanced that most of us who have examined the Pleistocene features of Pembrokeshire cannot imagine it having been affected by Devensian ice coming in from the NW. So I go with the flow on this one -- namely that the ice limit lay somewhere to the north of this point, leaving much of central and eastern Pembrokeshire ice-free.

Inside the Last Glaciation Limit

Ice-smoothed slabs on the northern face of Carningli. This sort of feature is difficult to date, except maybe with the aid of cosmogenic dating, but here the surface appears fresh, so I would hazard a guess that the smoothing was done by Devensian (Last Glaciation) ice -- thus matching this feature with the Pont Ceunant moraine described in the last couple of days.

Stonehenge and the Pont Ceunant Moraine

You might wonder what on Earth the Pont Ceunant Moraine has to do with Stonehenge. Well, it has quite a lot to do with it. If the moraine relates to something like the Last Glacial (Devensian) maximum ice extent of the Irish Sea Glacier, this means that the ice surface in this area was probably not higher than 300m. This means that the ice will have pushed well up the Teifi Valley and probably pressed against the north face of Preseli and the north face of Carningli, without overriding the Carningli upland. The line isn't that different in North Pembrokeshire from that of the famous "South Wales End Moraine" drawn by Charlesworth in 1929. The Devensian lake deposits and glacial deposits of the Teifi Estuary are within the line, as are the abundant sands and gravels (including eskers and kames) of North Pembrokeshire, as are the fresh tills of the North Pembs coast, as are the rounded tors between Fishguard and Cardigan.

The map above is based on Bowen and various others -- in many publications. I have crossed swords with DQB many times in the past, but I think I agree with him here -- the evidence does stack up.

I'm pretty sure that the ice crossed the mouth of Milford Haven -- there are ice-contact deposits at West Angle -- and then there must have been a lobe pressing into the Bristol Channel. How far did that lobe extend? Maybe as far as the coasts of Devon and Cornwall, but maybe not as far as Somerset. It must have been a pretty big lobe, if the ice did indeed reach the Scilly Isles in the Devensian.

Now we have to look at ice surface profiles -- which must have been remarkably low..... and that's another issue to be examined.

More about the moraine

Here it is -- a very broad area at grid ref SN046372 with a litter of sub-angular and rounded boulders of all sizes -- including many of fine-grained local dolerite but many more that are extremely coarse-grained. I don't think I have ever seen an outcrop of such rock anywhere in this area. I have taken samples.



This is an erratic of whitish rhyolite (probably from one of the outcrops to the N or NW) surrounded by much coarser dolerite boulders.



And this is the area with the hawthorn tree which is NOT moraine -- look at the different texture, colour and appearance of the boulders and rock slabs.

I think this moraine is plastered over a wide area in a broad depression above and below the 250m contour. It extends all the way down to Pont Ceunant, where the till is exposed in a river cutting. Higher up the slope from the area where the pics were taken, there is an area of hummocky terrain -- the vegetation is so thick that it is difficult to determine whether these hummocks are original depositional features, or created as a result of dissection and stream action.

The boulder surfaces on the moraine are very heavily weathered. That's a bit surprising -- if the moraine was placed here in the Devensian, around 20,000 years ago, I'm not sure I would expect quite such heavy weathering........

The Pont Ceunant Moraine

Been up on the mountain today -- and checked more carefully. The whitish area shown on my Google pic of the other day is NOT a moraine -- it's an area of whitish broken dolerite blocks -- coinciding with an igneous outcrop. But the area to the east of that -- shown as a rough brownish texture on the picture is really moraine. We geomorphologists seek always to refine our ideas.....

Bluestone fragments -- the plot thickens

The edge of the Irish Sea Glacier on Salisbury Plain, maybe 250,000 year ago..... (date very uncertain)

With reference to this earlier post:

http://brian-mountainman.blogspot.com/2010/02/bluestone-quarry-consigned-to-scrap.html

Rob Ixer has kindly confirmed that after studying around 7,000 fragments from the Stonehenge neighbourhood, he can say with some certainty that some of the bluestone monoliths are of rock types that are NOT represented in the "Stonehenge layer" or in the litter of fragments in the soil horizons; and that some frequently occurring non-dolerite rock fragments (ie rhyolites, ashes and sandstones) found in the "litter" are not represented in the 43 known bluestone monoliths / stumps known in the stone monument.

He also repeats this question: "Why are the geological origins of the non-dolerite bluestone so diverse and often from ‘insignificant’ outcrops?" He also says: "Detailed rock and mineral geochemistry plus statistical analysis of the ‘debitage’ may answer these and the more straightforward questions."

No doubt Rob and Richard Bevins have further publications in mind, and I look forward to seeing these. And I gather that further petrographic / geochemical work is under way on all those rhyolite samples collected during the OU project about 20 years ago.

At the risk of repeating myself, I suggest that what we are looking at here is an assemblage of glacial erratics, large and small. Exactly where they were emplaced or dumped by the glacier is another question.....

Strange marks at Carn Enoch

Further to my recent entry about polishing stones -- with the pic of the one on Fyfield Down -- I came across this photo in my archive of the strange grooves cut along a crack in the dolerite tor of Carn Enoch, not far from Dinas, Pembs. The grooves are clearly man-made -- but are they some sort of script? Something ornamental, done by some distant ancestor just to pass the time of day? Or are they "sharpening grooves" on a rock surface used for smoothing stone axes and improving their cutting edges?

I sent this photo ages ago to the Dyfed Archaeological Trust with a request for enlightenment, but they never bothered to reply.......

Addendum: just discovered that my friend Robin Heath thinks these marks are prehistoric "tally marks" used for recording the number of days in a year. Maybe there are 365 notches or grooves? Must go back and count them one day.

A new moraine

This is the "new" moraine -- I thought I knew the mountain well, so it was a great surprise to stumble upon it as I led a group of about 20 walkers on a guided walk. It's not far from Pont Ceunant, on the Bedd Morris road near Newport. Quite a prominent ridge, surrounded by boggy moorland.

Today I found a new moraine -- not at Stonehenge of Glastonbury, but on the northern flank of Carningli. Just thought you'd like to know........

I must go back and examine it in more detail -- but it has a mottley collection of stones and boulders of all sizes -- many of them quite well rounded and looking very different from the rocks of the periglacial blockfields that cover most of the mountain. Some of the rocks represented are VERY exotic -- I don't recognize them as having come from Pembrokeshire at all -- maybe they are from N Wales or Ireland?

If this is a genuine moraine, it will tell us more about the last glacial episode -- maybe not the one that transported the "bluestones" from West Wales to the environs of Stonehenge.