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Monday, 14 May 2012

The Whin Sill Erratic

Grateful thanks to Daniel Bridge and the Essex Wildlife Trust for these photos.  They show the Whin Sill erratic which is now on show at the Wildlife Trust's Visitor centre at Bedford Park.

We look forward to seeing the interpretion which is currently being prepared.

The stone was found in a quarry in Havering, north of the Thames, at the Brett Lafarge Marks Warren quarry, approximate grid ref TQ 488 896. It weighs approximately 0.9 tonnes.  In an earlier post, dated 20 April, I published maps of the quarry site and the supposed glacial limits in SE England.

Several things strike one about this boulder.  The first is the damage done to it by heavy machinery -- not surprisingm since it was found in a gravel pit! The second is its blocky shape with sub-rounded edges -- typical of a boulder that has been carried a very long way -- maybe even over several different glacial phases.  The third is the dark and fine-grained nature of the rock -- in Whin Sill itself it appears almost black.  And lastly, I am very impressed by the extraordinary patina or crust which seems to have covered the whole of the boulder prior to it being damaged by those machines.  is this a genuine weathering crust?  I doubt it -- my initial thought is that this is a crust of iron oxides and other minerals precipitated onto it out of groundwater over a very long period of time.  More opinions please?

17 comments:

sciencebod said...

Why call it a Whin Sill erratic, Brian? Who's to say it's not a Havering non-erratic? In other words, intrusive rocks like dolerite are/were far more common than the geology textbooks would have us suppose. But as soon as they became exposed in thousands of years of Stone Age history, they were immediately harvested for tool manufacture.After hoiking out and back-filling the hole, one was left with what looked like an undisturbed soil surface.

This particular boulder has only come to light because someone decided to quarry at the precise spot where it was lurking. The encrustation is the result of being exposed to subterranean mineral-loaded water relatively near the surface.

(Anyone who has been following the comments on the earlier "maul and hammerstone" posting will know I am proposing a similar provenance for those so-called "Preseli bluestones". Nope, not Preseli, but freakish intrusions indigenous to Wiltshire that just happen to resemble some of the dolerite at Preseli. Ever heard of the "birthday problem" in probability theory?

OK, Brian will tell me I'm talking through my hat. Maybe, but here's a question for him. If glacially-transported dolerite etc, harder we are told than granite, had been littering the surface of Wiltshire etc for thousands of years since the end of the last Ice Age, then surely prehistoric man would have seized on it for tool manufacture long before entertaining megalomaniac ideas of contructing a megalithic necropolis. Is it not far more likely that the bluestones were first encountered by the ancients' grave-digging or other excavating activity below the surface, or through barely protruding above, and because of their seemingly exotic character were immediately accorded magical significance, and used to create a stone circle- not necessarily Stonehenge, but proto-Stonehenge so to speak. Stonehenge Mark 1 may have used just the bluestones, hauled maybe a few miles. Once confidence and know how had been gained, the much bigger sarsens could have been imported later.

Ooops, sorry, no glacial transport, Brian, and certainly no human mules either. Transport was in the form of liquid magma from plutons and basoliths, with freakish breakthroughs at or close to the surface where modern geologists would least expect it, e.g. central and/or southern England ;-)

Thanks for proving me with resource material for this punt, Brian.

Anonymous said...

It might be late-stage hydrothermal mineralisation along joint planes.
Much of the Whin Sill and other fine-grained doleritic rocks from the north of England show limonite-stained calcite along jointing and/or more exotic zeolite-like materials.
A little bit should be removed. I doubt that it is 'iron-pan'-it might be a heavily weathered skin.
But a thin section is the only way of cheaply being certain.
Small dolerite erratics are common in eastern and southern England(OWT has made a career from them!)
Nice lump though!!
Myris

Charlie said...

It would be interesting to see if the patina reacts to HCl.
Also, I've seen a very light tan weathered surface on mafic rocks (eg rhyolite) before.
Cheers,
Charlie

Charlie said...

My mistake: rhyolites are felsic not mafic. Petrology was a long time ago...

Anonymous said...

Feakish basilisks rising from their lairs with baleful stairs (sic) does it for me-everytime.
Myris (missing the relative wisdom of Kostas).
As Baldrick would say geophys.

chris johnson said...

Returning to some recent themes, I came across the work of Prof. Jane Evans and colleagues analyzing animal teeth from Durrington Walls and making a link to Precambrian or Palaeozoic rocks based on the strontium profile.

I wonder if anyone knows how this work has been progressing, and whether strontium levels can be used to provenance to particular rocks?

Brian mentioned Jane's work some time ago.

chris johnson said...

I did some homework to answer (partly) my own question. Likely this is known to several people on this blog already but in case anyone is interested ..

Many animals at Durrington did not grow up on the chalk. Some came over 100 kms to join the feast. As with the Boscombe Bowmen, here could be another link to Wales. At any rate, it seems some of the Stonehenge people were drovers.

BRIAN JOHN said...

Colin -- you cannot simply ignore the geological evidence and pretend that the "Preseli bluestones" are indigenous to Salisbury Plain. They are very varied, and their sources (or at least the sources of many of them) have been pretty reliably established. You may choose to ignore it for the sake of your latest theory, but nobody is going to take you seriously. As I said many times before to our friend Kostas, please go away and do some reading -- there are plenty of specialist papers for you to consult!

BRIAN JOHN said...

As for the patina / crust on that nice boulder, I still prefer to think it is a precipitate that has come from groundwater, following the transport of the stone and maybe while it was still embedded in the gravel. I have seem many such stones with that colour in old fluvioglacial deposits -- and in other places the stones have a pitch black crust, associated with a precipitate of manganese oxide.

sciencebod said...

Is that "PLEASE GO AWAY! (and do some reading)", Brian, or "Please go away (AND DO SOME READING!"? ;-)

BRIAN JOHN said...

A gentle encouragement, Colin, for you to go away and do some reading on the geology of the stones. No exclamation marks or capitals needed. Once you understand the geology a bit better, we might be able to have a meaningful debate.

sciencebod said...

From p138 of your own book, Brian:

"We have already shown that the bluestone monoliths at Stonehenge, and other rock fragments on the site, have come from as many as 15 different locations in West Wales, in the Brecon Beacons and in other areas as yet unidentified"... and further down:

"... they (senior academics who should know better) have also continued over the years to hunt for THE spotted dolerite quarry regardless of the fact that only some of the 43 known monoliths were made of that rock type..."

When the heterogeneity of Stonehenge bluestones is so great that a heterogeneity of West Wales and other sites is needed to account for each of them, then does it not rather call into question the entire idea, dare one say dogma, of each rock having a fingerprint, so to speak, that fits with one, and only one specific location?

Being a chemist/biochemist by training I tend to see rocks as reflecting the end-result of a process that is highly dependent on chemical and environmental factors like magma phase transitions, time and cooling rate, so it becomes possible that near-identical igneous rocks could come from very different locations, and conversely that different igneous rocks could come from much the same location. After all, a difference of a few tens of metres depth below surface of a new dyke could make the difference between years in rate of cooling, and consequently of the kind of inclusions one sees in the final rock or mineral.

In fact, my chief interest is not so much with the origin of the bluestones - while acknowledging it's an intriguing question. It's more to do with Stonehenge, and why those bluestones are there, possibly as the first monoliths on site, and why there is such a large accumulation of bluestone chippings.

I believe I know the answer - from thinking through the practicalities of moving from a hunter-gather existence in dense woodland to a settled agrarian existence on an open chalk plateau with poor thin top soil (with or without initial tree cover). I'll give you a clue to what I think is the answer.

Comminution/soil fertility.

I believe that Stonehenge went through various phases - and it was operating as a comminution plant that created the bluestone chippings...

Even now you can buy blood/bone fertilizer...

chris johnson said...

Brian,
I could have saved myself some time yesterday by simply rereading your book, p77. LOL. Still I met some interesting people.

Dan said...

Looking at that top image, the furthest point on the left hand side is darker (bare dolerite), and this is where a chunk was removed for analysis; that analysis identifying it as being from Whin Sill.

Dan

Anonymous said...

Is there a full description petrographical somewhere? I hope they did not just wave a PXRF over it.
Pity they did not do the material covering the joint surfaces as well-or did they?
M.

BRIAN JOHN said...

Agree it would be good if GeoEssex could publish the analysis somewhere instead of simply saying "we have determined that the boulder comes from Whin Sill." Dan, do you know if that is the intention?

Dan said...

I would assume so, but haven't had any further contact from them as yet. I'll update you as soon as I know anything further.

Dan