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Sunday, 23 May 2010

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

16 comments:

PeteG said...

Steve and I have been working on this for our forthcoming Avebury book and have started a research program to catalogue all the sarsens in and around Avebury.
This is throwing up some interesting discoveries...

Brian said...

Well done Pete! Sounds interesting. What interests me about the theory is the question of whether it is applicable to ALL of the sarsen discoveries all over S and SE England. Can all of these occurrences be explained in the same way?

PeteG said...

this is a question we hope to answer Brian, we will be studying the Kent and Dorset sarsens beds soon.

Flo Fflach said...

I must come and have a proper look here! bit busy today....

Kostas said...

Brian and all,

Very interesting idea. The Stonehenge sarsens may have 'marine origin'. These may have come from a shallow local seawater basin covering Salisbury Plain. Much like the very shallow sea bed where we see mangoes grow. This perhaps could explain the sarsen holes. Maybe! Since holes in large stones are ubiquitous and exist also in places where no mangoes grow. These could be made possibly from the action of sea water on the composition of the stones. Something like an absorbing 'sponch-like effect' of submerged surfaces.

One question, though. How does this theory conform with the chalky bedrock that covers all of Salisbury Plain? If the sarsen (made of sandstone) have local marine origin, shouldn't these then be made up of the same material as the existing bedrock?

Constantinos

Brian said...

Yes, the fact that the sarsens are essentially "concreted sandstones" with an abundance of sand grains argues for a very different sedimentological environment from the shallow clear tropical seas assumed to have been in place at the time of the creation of the chalk. So you have to find a source for the quartz grains -- that means a beach, or a river carrying sediment from an upland towards the lowlands and the sea.

Kostas said...

Brian, thank you for your response.

I assume then you too feel that the Stonehenge sarsens could not have local 'marine origin'.

One more question: What other geological conditions (aside from 'shallow clear tropical seas' can produce chalk? Could geothermal areas produce chalk? Under what circumstances?

Always very interested in your comments to this and all other posts!

Constantinos

Brian said...

No, I would't rule out a marine or coastal origin -- it's just that you would need plenty of sandy sediment available to be cemented by silica.

As far as I know, the chalk is so widespread, so thick and so consistent in its characteristics that the tropical sea idea is pretty well universally accepted. The marine creatures that make up the chalk would not, I assume, be likely to exist in geothermal areas which tend to be pretty hostile to life forms. And you can't postulate geothermal springs or outpourings on anything like the scale needed to explain the full geographical extent of the chalk across western Europe.

Kostas said...

Thanks for the post, Brian.

Couple of points of interest that relate to this. In the bottom of Lake Superior (I think) a stone circle has been found very similar to Stonehenge. Also, off the coast of Brittany, some stone alignments are submerged underwater. We can agree that these stone formations are not man made!

Constantinos

Brian said...

Stone circles beneath Lake Superior? I should like to see the evidence for that....

And of course there are man-made features of all sorts of ages beneath current sea-level. It all depends on what the relationship between isostatic adjustments and eustatic sea level changes might have been at any particular place......

Kostas said...

Good point, Brian! I thought of the very same thing just after I clicked the 'submit' button!

The photos that you post are really very helpful. Any photos of some excavated Aubrey Holes that show the pit bottoms? What is the evidence that these are postholes?

Anonymous said...

J.S.Ullyott, D.J.Nash and P.A. Shaw 1998 Proc. Geol Assoc. 109 255-270.
Recent advances in silcrete research and their implications for the origins and paleoenvironmental significance of sarsens.

Rob

Brian said...

..and another interesting paper, which refers to groundswater silification -- ie noithing to do with shorelines or sea-level.

Micromorphology and geochemistry of groundwater silcretes in
the eastern South Downs, UK
J. STEWART ULLYOTT and DAVID J. NASH
Sedimentology (2006) 53, pp 387-412.

Quote:
"Overall, the key factor influencing the
genesis of groundwater silcretes in north-west
Europe would appear to be the release of silica
from either within the host sediment or adjacent/
overlying strata following acid leaching. In the
South Downs, leaching resulted from the oxidation
of pyrite in the lignites and clays which overly the
host sands. This suggests that there was no strong
palaeoenvironmental control on groundwater silicification
in north-west Europe. Furthermore,
there was no specific ‘era’ in which silicification
occurred, with phased silcrete development
accompanying landscape evolution from at least
the Late Palaeogene to the Pleistocene."

Is this the last word? I doubt it....

Kostas said...

Brian, does it really matter how the sarsens were made? The real question is how they got to Stonehenge?

Are you aware of the 'Deep Freeze' in the Northern Hemisphere between 10,900 and 9,500 BC ? Below is a direct quote from an article in wikipedia with plenty of scientific references to serious geological evidence!

I think you will agree that this puts my article in a better light:

“The un-Henging of Stonehenge”
http://knol.google.com/k/constantinos-ragazas/the-un-henging-of-stonehenge/ql47o1qdr604/16#

quoting from http://en.wikipedia.org/wiki/Younger_Dryas :
“The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900–11,500 years before present (BP)[5] in sharp contrast to the warming of the preceding interstadial deglaciation. It has been believed that the transitions each occurred over a period of a decade or so,[6] but the onset may have been faster.[7] Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was ~15°C colder during the Younger Dryas[6] than today. In the UK, coleopteran fossil evidence (from beetles) suggests that mean annual temperature dropped to approximately 5°C,[8] and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas.[9] Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.[5]"

Constantinos

Brian said...

The formation of the sarsens does matter insofar as it will help to explain how widely distributed they are. For example, Chris Green thinks there are few sarsens in the Stonehenge area, and concludes that those used in the monument were therefore carried from a long way off. I say that is cockeyed reasoning, the believe that there are few sarsens in the area because they were all collected up!

Also, the new work on sarsens should help us eventually to distinguish the various types of sarsens accurately enough to know whether those at Stonehenge might be "local" or far travelled -- or a mixture of both.

As for the Older Dryas - warm interval - Younger Dryas - Holocene warming sequence is involved, that is well documented across the UK, and resulted in ice edge oscillations in those areas where ice masses were still present. Ice edge oscillations also occurred, for example, in Norway and the Alps.

Brian said...

Winding up on this one now -- time to move on...