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Wednesday, 28 June 2017

More on quarrying

....... by which I mean glacial quarrying, the process by which chunks of bedrock are removed and entrained within overriding ice.

As a reminder, here is Prof Dave Evans's definition:

Quarrying (Prof David Evans)
Progress in Phys Geog 2004
 

Quarrying
Quarrying involves two separate processes: (1) the fracturing or crushing of bedrock
beneath the glacier;  and (2) the entrainment of this fractured or crushed rock.  Fracturing
of bedrock may take place where a glacier flowing over bedrock creates pressure
differences in the underlying rock, causing stress fields that may be sufficient to induce
rock fracture (Morland and Boulton, 1975; Morland and Morris, 1977). Fluctuations in
basal water pressure may also help to propagate bedrock fractures beneath a glacier
(Röthlisberger and Iken, 1981; Walder and Hallet, 1985; Iverson,1991a). Brepson(1979)
has successfully simulated the sliding of temperate ice over an obstacle in the
laboratory, and noted that large cavities form in the lee of obstacles, aiding quarrying.
Evacuation of rock fragments along joints in the bed is possible where localized basal
freezing occurs, for example as the result of the heat-pump effect proposed by Robin
(1976). Although Holmes (1944) originally argued that quarrying could occur beneath
both thick and thin ice, and outlined a theory based on pressure-controlled freezing of
meltwater in joints in bedrock, there is now general agreement that quarrying is
favoured beneath thin, fast-flowing ice (Hallet, 1996). Modelling studies indicate that
low effective basal pressures (0.1–1MPa) and high sliding velocities are the dominant
glaciological conditions required for quarrying because these conditions favour
extensive ice/bed separation (subglacial cavity formation)and also concentrate stresses
at points, such as the corners of bedrock ledges, where ice is in contact with the bed
(Iverson, 1991a; Hallet, 1996).


According to this theory, it may be that many of the features I have recently (again!) been looking at in the Stockholm Archipelago are very recent indeed -- formed just before deglaciation, which occurred here around 11,100 years ago.  Dave is picking up on a suggestion from many writers that when ice is very thick, movement on the bed might be negligible, with conditions effectively protecting rather than eroding the landscape beneath.  So for each glacial episode there might be two "erosional episodes" -- one at the outset of the glaciation, and the other at the end.  On the other hand there are other observations that suggest that thin ice may have a polar thermal regime on the bed, which means freezing-on and bedrock protection -- with erosion increased when the ice thickens because there may be a temperate thermal regime on the bed -- meaning high sliding velocities and considerable meltwater lubrication.  The truth is always more complicated than one would like it to be.........

Anyway, one thing that is blindingly obvious in the archipelago is that on the stoss (upglacier side) of all bedrock knolls and other roche moutonnees, abrasion and polishing features predominate, whereas on the lee side of all features plucking leaves rough and even jagged surfaces which show us where bedrock chunks have been dragged away.

Here are some photos of stoss-side slopes on RÖDLÖGA STORSKÄR:






Now here are some from the lee-sides or down-glacier sides of the same bedrock hillocks -- note how jagged surfaces, sharp edges and broken rock predominate.






 The only rounded or sub-rounded lumps of rock in environments like these are the glacial erratics dumped more or less at random during ice wastage.   It would be good to show some of these sites to certain British archaeologists, who would presumably (unless instructed otherwise) assume that all of them are Neolithic quarries littered with monoliths that were left behind by the builders of Stonehenge......





24 comments:

Myris of Alexandria said...

What are the lithologies?
There looks to be cross-cutting relationships. Are some gneissic?
Nice juicy rocks, real rocks none of your nambypamby chalk.
I always look forward to your Scandi voir(s. Very novel.
M

BRIAN JOHN said...

Myris -- this is a paradise for geologists interested in metamorphic processes. Yes, a great deal of pink and red granite, gneisses all over the place, and all sorts of rocks that I cannot give labels to. Almost all the colours of the rainbow! Because they are all Pre-Cambrian basement rocks there have been many phases of metamorphism, intrusions, fractures frequently crossing, dolerite dykes, quartz bands etc. Ice seems to affect all these different rock types in slightly different ways......

BRIAN JOHN said...

Been checking my photos against some of the local geology info -- so what's there? Banded gneiss in abundance — but also schist, red and pink granite, hornfels, mylonite, quartzite, reddish marble, dolerite, some sandstone and limestone — not seen any slate, but it is probably there somewhere……..

I think the most abundant rock types -- at least in the area I know -- are granites and banded gneisses. Some of the patterns in the latter are truly spectacular!

TonyH said...

Well, well, well! Nice to see Myris and Brian getting on.

Reminds me of that 'Oklahoma!' song,"The Cowboy and The Farmer Must Be Friends!"

There's Peace in the (Glacial Scandi) Valley. Amen!

BRIAN JOHN said...

Of course we get on, Tony! We might argue a bit now and then, and throw foul accusations at one another, but that doesn't mean we don't get on.... it's all in the spirit of academic debate. We even communicate frequently off the record on matters of mutual concern. Mind you, I'm a bit mystified by some of the musical exchanges that go on between you guys, but there we go. As the man said at the end of "Some Like it Hot". nobody's perfect......

Myris of Alexandria said...

Indeed Brian is completely correct, it is common academic practice when disputing to take extreme positions in order to hammer out the truth. It is one of the joys of academic meetings. Think of lawyers at trial (except neither Brian nor I are overpaid to lie for our clients).
In addition, being highly abrasive to someone who is incompetent and/or ignorant is cruel and worse, is unproductive and not done, silence is the accepted academic response to those people. Fun book reviews and dead dogs aside.
Brian has supplied help, good advice (as he is doing at present) and material/rocks for a few years now and if the Gods allow will continue we hope.
However when the righteous urge is in full flow, words will fly. Remember hate the sin but love the sinner. Brian is a long standing excellent observational scientist (and nice novelist)with some odd ideas.
M

TonyH said...

Nevertheless, Amigos, satire is definitely alive and well and the copies of "Private Eye" are flying onto the rails from the Salisbury rail station shop like hot peppers.

But blimey! .... Home, Home on the Range, you old geezers (I aint far behind you, mind!),

Where the deer and the antelope play
And seldom is heard(!)
A discouraging word
And bluestones are checked out each day
Ole!!

TonyH said...

You seem like two likeable, gneiss people, positively pink in your joy for life. R.E.M's "Shining, Happy People" even (but that has a deeper, darker, Chinese connotation!). But DO try not to overdo the slating and the mudstoning, otherwise you may both end up on "I'm a Celebrity, Get Me out of This So - Called Quarry!"

BRIAN JOHN said...

Thanks for kind words, Myris. I particularly like the bit about my "ODD IDEAS" -- well, I like to think that among those who know a bit of geomorphology (or quite a lot of it) my ideas are not odd at all, and are actually rather mainstream and conservative. What might be considered rather odd is a geologist who thinks like an archaeologist! Very strange indeed..... but I suppose we all have to go where the money is, or maybe where the publicity is...... Life is very hard.

Myris of Alexandria said...

Or the Gods forbid the TRUTH.

Glacial dumping in Salisbury Plain is not, I think, either mainstream or conservative more Whiggish/High Tory.

Stonehenge has not been a source, but rather a heavy drain, on monies (I have no book to sell)and thin sections/polished thin sections are expensive to have made -if you want well-made ones.

Publicity, ah you must be thinking of somebody else, I shun all forms of publicity and self-promotion, indeed all my many press releases say so.

No Brian you have some very odd (and incorrect) ideas with regard to Stonehenge that aside I am sure, and have been assured, you are pretty solid.

Life is indeed hard but the alternative is death.

M

BRIAN JOHN said...

Now this is more like it, Myris! Let's have a good argument. First of all, is the idea of glaciation on Salisbury Plain respectable or mainstream? Well, how many specialists have said, in print, that it is "impossible"? To the best of my knowledge, Herbert Thomas,Jim Scourse, Chris Green, and David Bowen. That's four earth scientists. How many have said on the record that it is possible? Judd, Kellaway, Jackson, John, Williams-Thorpe, Thorpe, Jenkins, Watson, Elis-Gruffydd, Hubbard, Sugden, and all the other authors of that big paper on modelling the ice cover of the LGM. I rest my case.

And what, pray, are my "incorrect" ideas with regard to Stonehenge? Chapter and verse, and supporting evidence please.......

TonyH said...

I sincerely do hope and trust that Messrs Mike Parker Pearson, Josh Pollard, Tim Darvill, Mike Pitts and Julian Richards, all of whom I have heard speak on various aspects of the prehistory of Stonehenge and/or met, take a look at this Post and, also, regularly, take an interest in this Blog.

They should ALL show some even - handedness, even if, at this stage, it is off - the - record, and merely for their own consumption at this stage.

It takes a big man to admit to the possibility that you may be mistaken in your beliefs.

TonyH said...

Further more, it would be interesting to ask David Jacques his considered opinion on the issue of the glacial transport hypothesis. Does he permit himself an open mind, or is he tied to the notion of human transportation of the bluestones ALL the way from Preseli to just west of modern Amesbury and Mesolithic Blick Mead?

Anyone feel like asking him politely, c/o his Academic address?

There must be SOME archaeologists who are not "under the Party whip"! If so, sen this Blog a comment or two. For instance, there must be some of you who studied Geography/Geomorphology at Degree level.

Myris of Alexandria said...

What would be more interesting would be to see something by Mr Jacques in the peer review press as opposed to the red tops. What does he know about geology/geomorphology? (check his first degree)
I wonder the worth of a man that gave us red magical red flint, votive flint ducks, the first house in Britain (maybe the world) Yorkshire running dogs and much else to amuse.
We have been promised scientific peer reviewed papers for a few years now. His worthy co-workers have also been surprisingly slow in publishing. He has been at the well-head for a decade now??
Let us see the worth of his work on his own material before asking for his opinion.
You have to admit he took his luck and has certainly run with it.

Brian, OWT bless her is no geologist, but an archy; her hubby (I once dragged him to see the hallowed Wigan all nighter)was an important geochemist and fine geologist, Dr Jenkins was a lovely man who confused the Lower Palaeozoic Sst with the Altar Stone (only OWT is still alive of the major authors of the OU 1991 paper) I am pleased to report both minor authors are still beloved of the Gods.

But a small drop of blood, nothing to be alarmed about, re the recent LGM paper. had forgotten that.Yes we must accept it did snow a little north of Salisbury mmmm

Oh where to begin. How close did the bluestone get to their present locality by natural means? Your best guess?
Mine guess Preseli.
M

TonyH said...

I note you mention D Jacques having been "at the well - head for a few years now?", Myris me ol' pal, me ol' beauty.

Do you have anything to tell us about a rumoured well in Amesbury town and its lining?

And what's this about 'the recent LGM paper, had forgotten that. Yes we must accept it did snow a little north of Salisbury'. Care to enlighten us poor ignorant uninitiated? A little less cryptic, a little more action please. Ta.

Myris of Alexandria said...

Enough of the Borchester Echo and its cub reporters.
I have nothing to tell you about the Amesbury well I suspect the info is confidential, you must ask others. What a veritable web of conceits exist in Wessex.
Do listen to Roger Macgough read his 'Mafia Cats' poem.

The LGM paper has been discussed at length on this blog I was conceding its presence and import. DKU!

M

Cryptic moi?

TonyH said...

Curiouser and curiouser, cryptic to the Nth degree! Now it's acronymns too... what on earth (or under) does the boy from Alexandria mean ? "A web of conceits in Wessex"? Hope he doesn't mean the Duke and Duchess.

Now, we COULD do WITH A Merseyside poet doing a Stonehenge Follies piece.

TonyH said...

".....something by [David] Jacques in the peer review[ed] press as opposed to the red tops..."


Wiltshire Archaeological & Natural History Museum Journal, Volume 107 (2014) pages 7 to 27. Mesolithic settlement near Stonehenge:excavations at Blick Mead, Vespasian's Camp, Amesbury. D Jacques and Tom Phillips.

BRIAN JOHN said...

Ah, that journal sounds familiar -- is that not the very one preferred by certain members of the geological fraternity>

TonyH said...

Yes, indeed, it is. But poor old (or perhaps part of his "problem" with his peers is that he is not so old!) David Jacques seems to be finding it difficult "getting his feet under the Mesolithic/ Neolithic 'experts' table. He's the New Kid On The Block.

But I expect the rather ordinary,sociable, everyday people of Amesbury quite like his contribution to extending the historical and prehistoric knowledge of their town.

TonyH said...

There is an excellent summary of the Stonehenge Mesolithic landscape written around 2015 in:-

www.silentearth.org/1000-year-old-stonehenge-monument/


Beyond that landscape, it also draws attention to a 1957 dig at Castle Meadow, Moot Lane, Downton, south of Salisbury; and also a 1970 dig near Christchurch Harbour and the mouth of the Avon, where Portland chert has been found. Portland chert has also been found in the Stonehenge landscape. Someone was moving this chert quite a distance, seemingly along the Dorset/Hampshire coast; and also inland from the mouth of the Avon to the vicinity of modern Amesbury.

Myris of Alexandria said...

There are quite a few mesolithic sites described, Pitts lists them.

It is surprising that for such a well publicised site like Blick Mead there is no Antiquity paper or series of papers, no PPS paper or series of papers, no JAS paper or series of papers. Where are the dozens of small specialist papers that normally accompany such a find. A decade is a long time for so much silence. It is difficult to think of any comparable British site that has been so 'neglected'. Just the one ferret club, welcome as that is, paper.
This might be another world first for Blick Mead.

Jacques and the established archy world. You may be correct and it could be about maturity. I wonder if it is counterproductive to write intemperate letters about established figures and then broadcast them widely (unless you don't care)as this often backfires and people tend to want to keep their distance.
M

TonyH said...

I believe the University of Reading, in relatively recent decades, has done a great deal of research/ excavation into Mesolithic activity broadly in the areas of the rivers Kennet and upper Thames. Others may be able to provide precise details.

TonyH said...

Blick Mead's David Jacques commented back to Mike Pitt on the latter's Digging Deeper site in the last seven months:-

https://mikepitts.wordpress.com/2016/10/18/the-strange-case-of-the-dog-in-the-stonehenge-tunnel/

His is one of 26 "Thoughts", dated 21 0ct 2016.