Monday, April 18, 2016

Why the need for all those Neolithic standing stones at Stonehenge and Avebury? Why were igneous bluestones required from the distant Welsh mountains?

Here's a little experiment I did earlier today with some spotted dolerite, one of the distinctive stone monoliths from which Stonehenge was constructed some 4,500 years ago on Salisbury Plain in Wiltshire, England.

I'd have preferred to have done it with a chunk of the as-is material, not from Stonehenge obviously but from the outcrop of rock in the Preseli mountains of west Wales from which it has recently been confirmed as coming from originally (we can leave the question of how for another day).

Regrettably, all I had to hand was a polished specimen of spotted dolerite, purchased at the tourist gift shop at Stonehenge. Never mind. one has to start somewhere.

What I wanted to do was test its resistance to penetration by water, i.e. porosity, for reasons I'll explain in a minute or two.
Fig. 1:  That's the specimen next to my watch, before adding anything. That's a metal ring, top left, for attaching to a chain, in case you were wondering.

Fig. 2: Here I'm adding a few drops of water from a plastic pipette to form a puddle on the surface

Fig. 3:  Here's a close-up view, showing how the water puddles on the surface, which straightaway looks as if it's non-porous, non-absorbent.

Fig.4: Here we are again, just over 30 minutes later. The puddle is still there. the water has not soaked into the rock.

So what's the relevance of this to Stonehenge one may ask?

When or if you visit Stonehenge, you could be forgiven for thinking that the  standing stones are all made of the same kind of rock. One might even get that impression from looking at photographs from the internet:

Fig 5: At first glance the standing stones of Stonehenge may all look much the same, differing only in size. In fact the tall ones are local sedimentary sandstone ("sarsens") , while the dwarf ones to the right ar  igneous Welsh-derived bluestone.

In fact there are two main types in the above photograph.  The taller ones are the local so-called sarsens, which are made of a particular kind of sedimentary rock (sandstone) which has been subject to further chemical action by acidic water to make "silicified sandstone". But they are still classified as a  "sedimentary rock". But note the smaller stones.They are made of the same stuff as my gift shop "bluestone" which could  be spotted dolerite or something else (rhyolite) but there's an important difference between the small and large ones. The small ones are igneous/metamorphic rock that  was originally from the Preseli mountains or maybe other locations far away in west Wales. Ignoring how the stones got to the Salisbury Plain, why was that igneous rock used for the smaller standing stones, which some believe to have been the first ones to be erected on site?

No one knows for sure, nor does this blogger. But I'm not persuaded by the argument that our Neolithic ancestors regarded spotted dolerite etc as having "curative properties". Why not? Because apart from being entirely conjectural and not amenable to testing, it skips the question as to why one would go the trouble of setting up these standing stones in the first place. One thing's for certain. "Bluestone", despite its name, is not noticeably
 more attractive than sarsen sandstone, at least in its natural unpolished state. But it's water-repellent. Might that be part of the answer, for reasons we'll see in a minute?

So what made this blogger decide to do the simple test with water?

Answer: it was prompted by what he read as regards the local sarsen stone and its response to water. Here's what William Stukely said, writing a few hundred years ago when attempts had been made to build homes from broken-up sarsen stones.

From wiki:

William Stukeley (1687-1765) wrote that sarsen is "always moist and dewy in winter which proves damp and unwholesome, and rots the furniture". In the case of Avebury, the investors who backed a scheme to recycle the stone were bankrupted when the houses they built proved to be unsaleable...

Perhaps not surprisingly, sarsen stones, being a sedimentary rock formed by compression and cementation between solid particles, has a porous open structure that allows damp and water to penetrate.

Unfortunately,I have not been able as yet to do the control experiment with sarsen sandstone, for direct confirmation that it is porous, taking up water, in contrast to the bluestone which is non-porous. But their different origins as sedimentary versus igneous rock make that difference highly predictable. An igneous rock, formed with slow solifidification of molten magma beneath the blanketing Earth's crust, with no rapid outgassing of volcanic gases, would be expected to have a non-porous water- impermeable structure, and would thus be water-repellent, as seen above.

Relevance to Stonehenge? No obvious relevance if starting from scratch. But this blogger had a prior model in needing of testing, and the test with the bluestone and water confirmed a prediction. What then was the model?

Answer: it was flagged up earlier today in comments posted to two other internet sites. I'll simply paste them here for starters, in the belief they are self-explanatory, and maybe return later to see if there are any details that need further explanation.

First, my comment on the ancient-origins site:

  • Reply to: The Monumental and Mysterious Silbury Hill   5 hours 25 min ago
    Comment Author: Colin Berry
    I put up a post yesterday on my own Neolithic site, recently re-activated after a longish break, hardening up on a theory some 4 years in the making, one that offers an explanation for the close proximity of Avebury, Silbury Hill and the Long Barrow at West Kennet.
    Very briefly, it’s as follows. The reason for creating standing stones, e.g. like those at Avebury, was to provide places on which scavenging birds (crows, ravens, seagulls) could perch between meals. The meals were the recently dead, laid out for excarnation (aka defleshing aka ‘sky burial’). Silbury was the home of a ‘token gesture’ interment, probably the heart only – harvested before excarnation, probably intended to assist release of the soul from mortal remains. The excarnated bones were then taken home by the relatives, or for more important folk, deposited in the nearby Long Barrow.
    I’m working on a similar narrative for Stonehenge/Durrington Walls and hundreds of barrows with the cross-piece lintels arriving finally as a superior bird perch. I think I can explain the initial preference for the Welsh bluestones over local sarsen sandstone, prior to lintels, apart from them being smaller. Clue: they’re easier to keep clean, igneous rock being more liquid-repellent than sedimentary sandstone...  (go figure).
    Yes, it can make somewhat grisly reading, but the alternative options were difficult too (thin soil on chalk bedrock making grave-digging difficult, especially with pre-Bronze age antler picks, and shortage of firewood for cremation via funeral pyres).
Second: my comment on Brian John's Stonehenge/ Ice Age site:

Blogger sciencebod said...
Whether quarried, or simply picked up and carried away, whether by ice or by the unmentionable, there's another possibility re the use of that igneous, non-sedimentary rock. I've just flagged it up on the ancient-origins site.

A full posting will follow in a day or two. Suffice it to say I reckon there's a simple rationale for all those standing stones, without or without lintels.

They served simply as perches for scavenger birds (crows, ravens, seagulls). What were they scavenging, within a Neolithic funereal context (excarnation, sky burial). Go figure.

The ideal material for a bird perch is one that is easily cleaned (non-porous rock...).

Colin Berry
18 April 2016 at 13:27


Yes. I consider that the standing stones of Stonehenge, Avebury, maybe further afield (Carnac,Brittany etc) were intended be perches for scavenger birds, the latter required and indeed encouraged to visit in flocks for the purpose of passive excarnation (de-fleshing, aka sky burial) of the newly dead. Welsh bluestone was valued more highly than sarsen stone when the standing stones were the height of a man or less, ie. the tops clearly visible, since being non-porous it was easier to periodically rinse-off with water as and when they started to look unsightly (food remnants, fouling etc) as inevitably they would.

1 comment:

sciencebod said...

Have just this minute installed a new feature on my site - a widget for Latest Comments, and so far it works, inasmuch as it's listing the most recent ones in correct chronological order.

I'll now check to see that this one heads the list. If any other Blogger Blogspot users are reading this and fancy the same for their site, then click on the blue link that appears beneath the listed comments. It takes one to the heaven-sent site that provides the HTML code. Installing it couldn't be simpler, using Design, Layout, Add a gadget, HTML etc, paste in the code, replace dummy URL with one's own, click Save, and hey presto the job's done.

Why doesn't Google provide the necessary code as part of the default software? Why does one have to go AWOL to find such an essential feature on any blogsite? How else would readers know that a new comment has been placed on an older posting if not prominently flagged up on the Home Page? Ours is not to reason why...