Entrainment events

This is rather an important topic -- and interestingly enough, it is not a topic covered properly even in the glacial geomorphology literature.

Let's see if we can encourage a debate on this.

Beneath the ice

It stands to reason that when a glacier is moving across a landscape, eroding material and entraining it into the body of the ice,  the process of entrainment is not continuous but intermittent.  I have talked a lot in previous posts about the processes that operate on the glacier bed, on its flanks and above its flanks on mountainsides and rocky peaks.  When abrasion us under way under a fairly steady basal thermal regime, small fragments of gravel, sand and even cobbles might be entrained on a small scale every day or even every hour.  But where larger chunks of rock are involved, fractures and entrainment events may only occur two or three times a year, when the relationships between ice and underlying bedrock (we are talking glaciology and rock mechanics) are just right.  The process referred to as "quarrying" or "plucking" kick in, and on the downstream side of rock fractures massive slabs of rock can be dragged away from their places of origin.  There may be a debate about how rapidly this "extraction" process takes place; we can probably assume that rocks on the glacier bed will not be moved away from their source positions at anything like the ice velocity as measured at the surface.  So entrained erratic blocks or slabs may only be moved at a rate of 50 - 100m per year, even if the ice is surging or flowing at a rate of 1km / yr.

Here's that quote again, from Prof Dave Evans of Durham University:

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).

The Sleek Stone super-erratics, for example, appear to have been entrained by ice flowing over Ramsey Island and intermittently incorporating large blocks of bedrock initially made available by fracturing on the glacier bed, possibly in association with shearing within the ice mass.

Part of the biggest Ramsey Island super-erratic at Sleek Stone near Broad Haven -- estimated to have weighed over 100 tonnes before it broke into two.

http://brian-mountainman.blogspot.co.uk/2015/03/more-on-sleek-stone-super-erratics.html

In one of the classic examples in the literature, the roche moutonnee on the island of Rodloga Storskar in the Stockholm Archipelago,  we can actually see how a series of massive slabs have been sheared off the down-glacier side of a whaleback feature and dragged away by the overriding ice.  First, there comes the  fracturing, and then the extraction -- and these events are clearly intermittent.

See also this post about a huge fracture observed on the same island:

http://brian-mountainman.blogspot.co.uk/2015/07/genesis-of-giant-erratic.html

In this case, the glaciation came to an end before the down-glacier mass of rock could be broken up or dragged away.

On the ice surface

The intermittency of input or entrainment events is much easier to understand here, because the processes operating on glacier surfaces are much easier to observe.  In the case of the Foothills Erratic Train in North America, the material dumped onto the glacier surface has come from intermittent landslides or rockfalls high in the mountains, in one rather small area with a recogniseable rock type.  Once dumped onto the glacier surface, the glacier has simply carried the debris away, acting more or less like a conveyor belt:

http://brian-mountainman.blogspot.co.uk/2011/10/super-erratics-picture-gallery.html
http://www.earthmagazine.org/article/stonehenges-mysterious-stones  
http://brian-mountainman.blogspot.co.uk/2011/05/foothills-erratic-train.html 

We are talking here not about continuous small rockfalls every spring (which might well have been happening too, of course) but about intermittent and widely-spaced catastrophic events, each one of which dumped thousands of tonnes of broken rock onto the glacier surface below.  The landslide that dumped the vast group of erratics later emplaced at Big Rocks was not the first to occur, but it must have been one of the biggest.

These rockwall collapses might not have been as spectacular as the earthquake-induced landslides that have affected the Lamplugh Glacier and the Sherman Glacier, but they must have been pretty impressive nonetheless.

 The effect of the Lamplugh Glacier landslide, 2016

The effect of the Sherman Glacier landslide of 1964

Something similar has happened in Tierra del Fuego, in the case of Darwin's Boulders.  They appear to have been dumped onto a glacier surface during a series of "pulses" or events which have given rise not to a long and continuous erratic train but to elongated clusters of erratics in various locations which cannot be traced all the way back to their places of origin.  So each cluster is discrete, and each one owes its origin to either a single landslide event of a series of events within a limited time frame.

Two of the discrete clusters of Darwin's Boulders

http://www.geosociety.org/gsatoday/archive/19/12/article/i1052-5173-19-12-4.htm

When we refer to mountain earthquakes and major landslides in the mountains, we must bear in mind that these might be directly glacier-induced.  The vast ice load of a large glacier, ice cap or ice sheet on a small part of the earth's crust can in itself induce earth tremors; and rockwalls adjacent to glaciers are especially vulnerable when the ice begins to melt at the end of a glacial episode.  A process called "pressure release" occurs, as the crust adjusts to a reduction in its surface load -- and catastrophic rockfalls or landslides can occur as a consequence.   This process of isostatic readjustment and unloading can continue for thousands of years after complete deglaciation.    But that's another story.........

The lessons to be derived from all of this?  Glaciers do not continuously pick up erratics from the bed during a glacial episode.  Sometimes they entrain bog blocks, and sometimes they do not.  You should never assume that there "should be" a continuous erratic train leading from A to B.  And you should not necessarily be surprised if there was and is a cluster of old and very weathered erratics from Pembrokeshire in the vicinity of Stonehenge, with not very much to be found between the source area and the final destination. 

I keep on trying to explain all of this to the archaeologists, but they are not very good at listening.

Outcropping layers and foliations

I came across these wonderful images of landscapes created by some crazy sculptor who likes to carve landscapes out of collections of old bound volumes.   I suppose the motive is to demonstrate that all the best novels have a strong sense of place.......

Putting frivolity to one side, they do illustrate exactly the point which I and my geomorphology colleagues have been making about outcropping foliations and layers in near-vertical strata appearing across large tracts of countryside.

In their big paper on Rhosyfelin, Ixer and Bevins say this about "locality 8" -- before going on to say that they had provenanced most of the Stonehenge foliated rhyolite debitage to "within a few square metres":

The majority of the lithic comprises alternating fine-grained quartz and slightly coarser grained chlorite-bearing layers. In thin section the rock is banded with a pronounced planar foliation and a lensoidal fabric giving the ‘Jovian’ fabric. Very fine-grained quartz layers that look streaky alternate with slightly coarser grained quartz/feldspar-chlorite-rich layers. The fine-grained quartz carries rare, small quartz/feldspar-chlorite lenses......... 

OK -- that's all very fine.  Now, let's imagine that locality 8 is exposed on one of the dark-coloured hard covers of one of the books shown on the right hand side of the top image.  Can the geologists give us any information that might serve to disprove the thesis that anything found at locality 8 might well also outcrop at multiple locations cross-country, just as you can trace the individual book jackets across country in the lower image?

If they try to tell us that the fabric or "signature" revealed in thin section at locality 8 is completely unique to that locality, I will not believe them, since they do not have a dense enough grid of sampling points to support that contention.

RB to talk at Aberystwyth

Richard Bevins is giving a talk in Aberystwyth tomorrow, 8th Nov, about the geological work relating to the bluestones.  The info is below.  If anybody attends and wants to send us a report of the proceedings, feel free........

The blurb does not exactly inspire confidence.

Richard and his team have NOT "identified the exact source of Stonehenge’s spotted dolerite “bluestones” as Carn Goedog" -- they have proposed that Carn Goedog is the most likely source for some of the spotted dolerite bluestones.    Neither have they  "identified the source of another of Stonehenge’s “bluestones”, its rhyolites, as Craig Rhos-y-Felin."  What they have done is to provenance some of the rhyolitic debitage at Stonehenge to the area around Rhosyfelin and Pont Saeson. None of the bluestone monoliths has been sourced to Rhosyfelin.

Too much in the way of unsupportable generalisations, chaps, and too little care with words....... must do better.
 

Chips off the old block - sourcing the Stonehenge Bluestones


Pembrokeshire’s bluestones and the link with Stonehenge will be the focus of a public lecture at Aberystwyth University on Tuesday 8 November.

"Chips off the old block - sourcing the Stonehenge Bluestones" will be delivered by eminent geologist and Keeper of Natural Sciences at the National Museum of Wales Dr Richard Bevins.

In 2015 Dr Bevins was a leading member of the team that identified the exact source of Stonehenge’s spotted dolerite “bluestones” as Carn Goedog in the Preseli hills in Pembrokeshire.

Three years earlier, in 2011, his team identified the source of another of Stonehenge’s “bluestones”, its rhyolites, as Craig Rhos-y-Felin, a site also in the Preseli hills in Pembrokeshire just three kilometres away from Carn Goedog.

A graduate of Aberystwyth University (Geology, 1974) and Keele where he completed his PhD, Dr Bevins’ research has focused on the Caledonian igneous volcanic rock of Wales.

Between 1998 and 2006 he was Project Leader for the £33.5m project to build the Waterfront Museum Swansea.

Professor Neil Glasser, Director of the Institute of Geography, History, Politics & Psychology said: “We are delighted to welcome back Dr Richard Bevins to give what will be the first in a series of lectures to mark a century of teaching and research in Geography and Earth Sciences here at Aberystwyth University.

“As an Aber alumnus Dr Bevins has maintained his geological research interests and collaborated with members of staff in the Department on the study of the Stonehenge Bluestones in the Preseli mountains, and provided valuable input to the University’s ambitious plans to redevelop the Old College. We look forward to what will undoubtedly be a fascinating lecture.”

Dr Bevins is active in a number of academic circles related to minerology and geology and is a Fellow and Member of the External Relations Committee of the Geological Society of London, Chair of the National Geological Repository Advisory Committee, a Fellow of the Mineralogical Society of Great Britain, a Fellow of the Society of Antiquaries of London, a Chartered Geologist and an Honorary Lecturer in the School of Earth & Ocean Sciences at Cardiff University.

The lecture takes place at 6.30pm on Tuesday 8 November in A6 lecture theatre, Llandinam Building on Penglais Campus, preceded by a drinks reception in the Think Tank (Concourse) in the Llandinam building at 6.00pm. All welcome.

Contacts

Jackie Sayce
Institute of Geography, History, Politics & Psychology
Aberystwyth University
jqs@aber.ac.uk / 01970 622212

Arthur Dafis
Communications and Public Affairs
Aberystwyth University
aid@aber.ac.uk / 07841979452 / 01970 621763

The Carn Enoch grooves -- for sharpening axes?

 On several occasions we have discussed the origins of those strange rock grooves at Carn Enoch, a tor high up on the mountain above Dinas Cross.  This is one previous post:

http://brian-mountainman.blogspot.co.uk/2010/05/strange-marks-at-carn-enoch.html

and I have published this photo before:

Well, today I found this photo on Twitter, with no mention of its location:

It's labelled as a site where the sharpening and smoothing of Neolithic stone axes has occurred.  Sounds reasonable to me.  Any observations?

Strange things near Devil's Den...

Click to enlarge these photos -- they are very high quality.

Many thanks to Neil and Pete for these three photos -- just three out of a very big sequence from the area around the Vale of Pewsey, where sarsens are still abundant.  Clearly many big blocks have been cleared away, but the thing that interests Neil is the "sarsen drift" as he calls it -- made up of a great concentration of sarsens "sloshed" up against a hillside.  I just love that description........  but the top and middle photos show the context beautifully.  I'm not sure about the "ridgeline" which in some places seems to mark a steepening of the slope, and in others not.  Maybe Neil or Pete can explain further?

Can they also explain the labelling in Photo 3?

Neil says he is at a loss to explain the physical process that might have led to the situation shown particularly well in Photo 2.  I too am a bit mystified.  The accumulation of sarsens does seem to have a ridge-like or embanked form,  and although we know that some sarsen or duricrust formations seem tohave coincided with very ancient stream-bed situations, this does not seem to be the right place for such a block or slab that might later have been broken up over many millions of years.  Neither does the context look right for an accumulation of sarsens that have simply slid downslope, maybe under periglacial conditions during the Ice Age.  If you were to show Photo 2 to me and say it was a photo taken somewhere in Wales, I would immediately think "trim line" and "moraine"..........

This deserves careful thought.  What do others think?

Postscript

More images, courtesy Chris Heaton and Andrew Smith.

 These give a slightly different perspective, suggesting that the dense litter of sarsen stones lies on the base of a long gradual slope, with the lowest point in this asymmetrical valley lying where a slight path can be seen, at the foot of the chalk scarp.  There doesn't seem to be any mound or ridge in these two photos.  Needs to be investigated on the ground......

The myth machine in good working order -- and who needs facts anyway?

 Photo: English Heritage

 

Here is the latest weird and wacky story relating to Stonehenge -- courtesy the Telegraph and Katy Whitaker, who is currently a research student at Reading University.  At one level it's all good fun, since research students who are worth their salt are required to push boundaries, test exciting ideas and (preferably) say something new.  So it's almost acceptable, I suppose, for her to suggest that the sarsens used in the Stonehenge stone settings might have come from very far away, and not from Salisbury Plain at all.  She will have a jolly time presenting this idea to her peer group, and it will all be debated at the research student's conference with gusto. 

But at another level this illustrates the obsession with archaeological myth-making, and the manner in which the scientific method has been subverted or even abandoned.  There is, as far as I know, no evidence whatsoever (to do with petrology, surface characteristics or morphology) that the big sarsens have come from far distant territories,  and what Katy is doing here is throwing out an hypothesis which will presumably, at some stage, be tested by a search for evidence.  Create the story first, shunt it off to the media -- and no matter how wacky it is, is then becomes invested with a degree of respectability.  Look at the Telegraph headline, and look how "supporting information" is pulled in to suggest that the thesis has some credibility.......... and from now on Katy will no doubt be looking at field evidence through her rose-tinted spectacles and citing it in support of a wildly premature hypothesis.

I venture to suggest that a geology or a geomorphology research student would NEVER operate in this way -- and indeed would not be allowed to by his / her supervisor.  It all tells us a lot about archaeology, and helps to explain why MPP,  Josh Pollard and the rest of them thought it would be OK to tell the world that there had to be Neolithic bluestone quarries in Pembrokeshire, to announce within a few days of starting their first dig that they had found one at Rhosyfelin, and then to spend six digging seasons looking for and describing "evidence" which does not actually exist.

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Massive 25 ton stones of Stonehenge may have come from further afield

by StonehengeNews
http://www.telegraph.co.uk/science/2016/10/28/massive-25-ton-stones-of-stonehenge-may-have-come-from-further-a/

The builders of Stonehenge are known to have sourced the smaller bluestones used in the 5000-year-old monument from Wales.

But a new theory suggests that the entire monument might have come from elsewhere, even the huge 25 ton Sarsen stones which make up the large circle of the Wiltshire megalith.

The huge sarsens at Stonehenge could have come from elsewhere

Katy Whitaker, of the University of Reading, will present a new paper at symposium at University College London next month suggesting that the sarsens could have come from sites as far away as Ken. (Kent?)

“Most people are aware that some of Stonehenge’s stones came all the way from south-west Wales,” she said.

“The really huge sarsen stones at Stonehenge are assumed to have come from sources on the Marlborough Downs in Wiltshire, about 30km to the north of Stonehenge. Sarsen stone, however, is found in other locations across southern England.

“There are sarsens in Dorset, spread about dry chalk valleys similar to the locations on the Marlborough Downs in Wiltshire, and as well as locations in Berkshire, Oxfordshire, Hampshire and Sussex, there are even sarsens in Kent.

“The distribution is quite broad, there are sarsens in Buckinghamshire and even across to Norfolk.”

People in the Neolithic are known for trading stone across large areas, including from the Lake District to the East of England.

Huge Sarsen boulders from outside of Wiltshire are known to have been used in other prehistoric monuments including Kits Coty House in Kent, and Wayland’s Smithy, a burial mound, in Oxfordshire.

“People were clearly aware of, and using, these stones in prehistory.” said Miss Whitaker. “Why not think about the possibility that sarsens came from further-afield too?”

The idea could also challenge that Stonehenge represents a peak of monument construction which could only have been achieved through organisation by a hierarchical leadership.

Instead, it may show that smaller groups had banded together to bring meaningful stones to a central area.

“Maybe it wasn’t a large group of people under the control of a tribal leader ‘cracking the whip’ to move all the rocks from one location down to Stonehenge as has been suggested before,” added Miss Whitaker.

“What about groups of people related in different ways, working collaboratively to move a special stone from one area to another? “

The source of the Stonehenge stones was first determined in the early 1920s by H.H. Thomas, an officer with the Geological Survey of England and Wales.

He determined that the so-called ‘spotted dolerites’ matched a small number of outcrops in the Mynydd Preseli district in south-west Wales

Latest theories about Stonehenge also suggest it was once an impressive Welsh tomb which was dismantled and shipped to Wiltshire.

An experiment this summer by University College London found that mounting huge stones on a sycamore sleigh and dragging it along timbers required far less effort than was expected.

They discovered that a  one tonne stone could be pulled on a raft by just 10 people at around one mile per hour, far faster than experts believed.

MS Whitaker is presenting her work at the Neolithic and Early Bronze Age Research Student Symposium at University College London from 18th to 19th of November.
================

Katy Whitaker (k.a.whitaker@pgr.reading.ac.uk)
What is the impact of the historical exploitation of sarsen stone on the understanding and interpretation of the prehistoric archaeology of southern England?
Supervised by: Dr Jim Leary ,  Josh Pollard (Southampton University) 


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ANARCHY IN THE UK?


We are happy to announce that the 2016 Neolithic and Early Bronze Age Research Student Symposium (NEBARSS) will take place at the Institute of Archaeology, University College London on Friday 18th and Saturday 19th November 2016.

NEBARSS is an annual symposium organised entirely by postgraduate students, to showcase innovative research by postgraduate and early career researchers.

-------------------------

11:20 Katy Whitaker, University of Reading
What if…none of the building stones at Stonehenge came from Wiltshire?

Over the past 20 years archaeologists have been exploring the idea that Neolithic monument construction provided conditions in which social differentiation could develop. This is in contrast to earlier interpretations of cursus, barrow, enclosure, mound, henge, and stone circle building in which perceived growing complexity of construction through time, and thus of inferred complexity of resource-management, were seen to indicate an increasing centralisation of prehistoric political authority. The henge earthworks, stone settings, and avenue at Stonehenge (Wiltshire, UK) play a prominent role in these contrasting interpretations.

This paper presents a discussion, by means of a thought experiment, of the role of Stonehenge’s stones in some 60 years of debate about Neolithic and early Bronze Age social structure. The paper starts with the revolutionary proposition that not only the Welsh bluestones, but all of Stonehenge’s building stones are ‘foreign’ to the monument’s locality. It goes on to explore the implications of this proposition by examining those of Stonehenge’s rocks that have in general been taken for granted, geologically-speaking, in the archaeological literature – sarsen stones: and others that have been almost completely ignored – packing stones to the sarsen settings.
Drawing in particular on work by Colin Richards, and Mark Gillings and Josh Pollard, to interpret these unsung components of the internationally-important monument, the paper suggests that an alternative to the dominant twentieth-century discourse in which Stonehenge represents the culmination of Neolithic social evolution, is possible.

Archaeology and the fog of corporate delusion

Over the past week or so there has been vast coverage in the media for the latest big archaeology story -- namely the decision to axe Archaeology as an A level subject.  Tweets galore from the great and the good, TV personalities up in arms, petitions, letters of protest, and statements from professors galore to the press and the broadcasters.   Of course it is an absurd move by the government and the one examination board that was offering an Archaeology A level exam course --  but a part of me thinks that archaeology has had it coming, since it clearly does not know what sort of subject it really is. 

There are hordes of excellent archaeologists out there, doing meticulous and highly specialised work -- but the obsession with the media that we see in some quarters, and the propensity of senior archaeologists (those who are featured heavily on this blog) to be more concerned with storytelling than with careful evidence-based science has really brought into question the academic standards of archeology departments in our universities.  Over and again on this blog I have asked the question "Whatever happened to scholarship, and whatever happened to the scientific method?"  One can argue that both have gone down the drain because of the activities of a few very high profile individuals whose fantasies seem to get more colourful with every year that passes. Too many wild goose chases and too little quiet, systematic fieldwork.

The spat over the so-called "bluestone quarries" is a case in point, and over the past six years we have seen one fantastical story after another thrown into the public domain, on the basis of "evidence" which simply does not withstand scrutiny.  Not only does the quarrying hypothesis fail to stand up under pressure, but those who are proposing it and selling it to the media completely refuse to admit that there are alternative explanations for the things which they consider to be "engineering features".  By all accounts, Prof MPP does not even mention the glacial transport theory, or the criticisms of the quarrying theory, when he gives his "bluestone quarry" talks.  That is disrespectful and unscientific -- and I am frankly surprised that he gets away with it as often as he does.  The whole world knows that there are two theories about bluestone transport, but MPP apparently does not............

If I was a Government minister responsible for education, I think I might well consider that the version of archaeology apparently being practised and promoted by these senior academics (with the aid of substantial research grants) is not really worth bothering about, since it seems to have more to do with fairy tales, myths and creative writing than it does with scholarship and education.  It's all very well for the members of the archaeology establishment (and thousands of professional archaeologists) to rage against a short-sighted government and a philistine examination board, but until they call certain senior figures into line and stop all this storytelling nonsense, and all the premature ejaculations, their subject will not get much respect from anybody.

Another of the problems faced by Archaeology is that its high-profile individuals are lauded not just by the media but by local authorities including the Pembs Coast National Park Authority (PCNPA).  Every year MPP tells his wonderful tales to an adoring audience at the annual Archaeology Day lectures -- and he is doing it again this year on 26th Nov, with the title "Stonehenge's bluestone quarries at Craig Rhosyfelin and Carn Goedog."  I wouldn't mind betting that he won't even mention the glacial transport theory, let alone consder the detailed criticisms made by John, Dyfed and me in the peer-reviewed literature.  PCNPA knows perfectly well that there is a dispute going on, but it prefers to ignore it too, since it is intent on flagging up the message that Pembrokeshire's prehistoric heritage is second to none.  It's called destination marketing...........

It's all a right old mess.  So here's my message to the Education Minister -- please bring back Archaeology as an A level subject, but not before its most prominent spokesmen can demonstrate that they have more respect for scholarship than they do for fantastical stories, press releases and media impact.