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Monday, 5 October 2015

Red sarsens


On the matter of red sarsens, Pete Glastonbury has kindly given permission for me to use this interesting pic -- of red sarsens in a cottage wall. What strikes me is the distinctly foxy red colour -- typical of the iron oxide staining which can occur on rock surfaces when they are underground.

What intrigues me rather more is the pinky colour that sometimes appears on rocks -- presumably that must also be due to oxidation processes, involving different mineral combinations?

10 comments:

ND Wiseman said...

I've always found this process fascinating. I had always assumed that Sarsen was a silicate formed on limestone - so where'd the iron come from?

But then we have the Slaughter Stone and several other examples, so beats me ...

Neil

Myris of Alexandria said...

Flint chert and sarsen are all silica rocks made of cryptocrystalline silica/quartz and quartz but they are all very different rocks.
Only chert is associated with limestone. Flint is associated with chalk and sarsen is a
Not associated with either especially.
M

chris johnson said...

I read that the sarsens at stonehenge might have been buried before use - the argument being that sarsen is easier to work before being exposed to the elements when it become very hard indeed. I am not sure whether this assertion - by an archaeologist - is true but presumably he had the facts from a geologist.

I am therefore surprised to hear that buried sarsen will take on colour, e.g. from iron because then one might expect the stonehenge worked sarsens to be coloured. Actually all sarsens in the monuments at Avebury and Stonehenge seem to be the same colour - boneyard white.

More questions, as if there were not enough

BRIAN JOHN said...

Interesting thoughts -- I am most used to areas where soils are very acid and where podsols are common, with gleying everywhere beneath peaty layers. This is really a matter of pedogenesis -- we need some comments from a soil scientist. On Salisbury Plain, of course, we are dealing with highly calcareous environments, and I am not sure how iron oxides would be mobilised and precipitated on rock surfaces.......

Steve said...

The pink colouration on sarsen stone is ‘rock varnish’ – a mineral coating, only microns thick, composed of iron, manganese and clay, that is bonded to the stone by the action of bacteria. Unlike the wind-blown ‘desert varnish’ that can take millennia to accumulate, rock varnish may form in only decades, in the splash zones of rivers: http://alliance.la.asu.edu/dorn/NewYorkVarnish.pdf

Rock varnish can produce a wide range of colours from honey-yellow, through orange, red and purple, to dark brown. All of these colours may be seen in and around Avebury, on some of the megaliths and particularly on the smaller stones used in walls and buildings. When broken, the rock-varnished stones reveal light-coloured sarsen beneath the coating.

The Avebury stones are not a uniform grey, but come in a range of colours – pink, blue and orange. The colours may be seen easily with a magnifying glass. Cove Stone I, for instance, is blue; its partner, Cove Stone II, is pink. The colour of the stones, as well as morphological features, may be used to group the stones into several ‘batches’ that were presumambly formed in different areas of the surrounding landscape.

This is all explored in more detail in my forthcoming book 'Exploring Avebury: The Essential Guide' which will be published next spring by The History Press.

The Stonehenge sarsens also come in different colours: stones 53 and 54, for instance, are uprights of the same trilithon, yet one is blue, the other pink.

Steve Marshall

BRIAN JOHN said...

Thank you Steve -- very useful info!

chris johnson said...

I finally booked my visit to SH this sunday and will be looking out for the colours!

Any tips from locals on making the visit are most welcome - I intend to be there first thing.

BRIAN JOHN said...

Chris -- if you have a moment, you might check what books they have on sale at the Visitor Centre...... thanks! Nobody I contact seems to know anything, and book supplies are apparently always somebody else's business. Quite extraordinary.

The only advice I have from those who have been there recently is -- WALK to the stones and politely decline the offer of a lift!

chris johnson said...

Thanks for the tip. It is a good walk I expect, and I will enquire for your book.

TonyH said...

Well, Chris, I do hope you did enjoy your visit to the Stonehenge Landscape today. Had I not been out of action owing to an ancient (but non - Celtic) injury, I might have considered a 30 - mile trip down there from my abode in West Wiltshire to see t'Old Ruin from various viewpoints. By all accounts the weather should have been pretty good all day, and fine and dry by p.m.

On the topic of Brian's enigmatic book, it will be interesting, to say the least, to discover whether it is INDEED hidden in some niche or cranny, or beneath some granny or grandad (possibly from Latin America) within the Stonehenge Visitor Centre. If not, I have in mind another avenue (no pun intended) we might yet use in our efforts to discover WHO (if anyone) has RESPONSIBILITY for book selection and book stocks within the S.V.C.