Breaking news: another igneous erratic on Salisbury Plain

One of the lumps of granidiorite found in the West Kennet excavation.  From Josh Pollard's talk.

https://www.wiltshiremuseum.org.uk/?event=online-event-connections-avebury-and-orkney&event_date=2021-11-26&platform=hootsuite&utm_campaign=Castle&fbclid=IwAR0tfR0Wtlt_dIA086d9VTMkan7bHy9lgRCt-RbuT1c5ug3kD6t1SrZHFaw

According to some posts on Facebook, quite a few people have known about this for quite a while, but now Josh Pollard has formally announced the finding of erratic material (a lot of it) from "Structure Five" at West Kennet, not far from Avebury.  The material appears to be a very distinctive form of granidiorite from the eastern edge of Cheviot.  According to Josh, the geologists (Ixer and Bevins) have provenanced the rock type to Cunyan Crags, near Dunmore.  It's very crumbly and heavily weathered, and from the lumps of rock collected from various parts of the excavation, there are at least 130 kg of it -- and probably a lot more.  Did the lumps of rock all come from a single erratic, or could there have been several in the vicinity?   Josh -- of course -- assumes that the boulder or boulders might have been "direct or indirect" imports, but then he's an archaeologist who has an established preference for the human transport of large lumps of rock, and a reluctance to believe that ice is capable of carrying large stones over great distances and dumping them anywhere near Salisbury Plain.   The suggestion in the talk is that these lumps of rock (found  mostly in post holes) are from a destroyed standing stone -- but there is currently no evidence to support that.  He does note that no lumps of the granidiorite have been found on the surface -- and he assumes that any that did exist have simply been weathered away.  Even the lumps that have been examined are more or less reduced to "grus" (the crumbly residue left when granitic rocks rot away) -- and that all suggests great age.

But we can rest assured that the southward transport of igneous material from the far north was not just possible but probable, during the Anglian and earlier glacial episodes.  In 1999 Olwen Williams-Thorpe and others described the occurrence of Whin sill quartz dolerite and many other far-travelled northern erratics in the glacial deposits of Hertfordshire and Buckinghamshire:

"Geochemical provenancing of igneous glacial erratics from Southern Britain, and implications for prehistoric stone implement distributions" by Olwen Williams-Thorpe, Don Aldiss, Ian J. Rigby, Richard S. Thorpe, 22 FEB 1999, Geoarchaeology, Volume 14, Issue 3, pages 209–246, March 1999

http://onlinelibrary.wiley.com/doi/10.1002/%28SICI%291520-6548%28199903%2914:3%3C209::AID-GEA1%3E3.0.CO;2-7/abstract

Abstract

Sixteen basic and intermediate composition igneous glacial erratics from Anglian (pre-423,000 years) deposits in Hertfordshire and Buckinghamshire, southern Britain, were selected for chemical and petrographic analysis in order to determine their original source outcrops. Major and trace element compositions suggest that seven samples (plus two uncertain) originated in the Lower Carboniferous volcanics of the Scottish Midland Valley (SMV), four came from the Upper Carboniferous quartz dolerite association which crops out in Scotland, northern England (Whin Sill) and extends to Norway, and one came from the northern England Cleveland Dyke. One sample of altered dolerite is ambiguous but has some similarity to the Old Red Sandstone (Devonian) age lavas of the SMV, and one meta-basalt sample may be from southwest Scotland or Scandinavia. These results identify specific outcrops which provided glacial erratics within currently accepted ice trails in the United Kingdom, and provide the first supporting evidence based on geochemistry, rather than petrography, for these ice movements. The distribution and provenance of glacial erratics are of importance in archaeological studies, because erratics provided a potential source of raw material for stone implement production. There is a marked geographical correlation between the distribution of prehistoric stone implements of quartz dolerite in the United Kingdom, and directions of ice movements from quartz dolerite outcrops within Britain. This correlation lends support to the hypothesis that prehistoric man made extensive use of glacial erratics for implement manufacture, as an alternative to quarrying at outcrops and subsequent long-distance trade.

In other papers Olwen and her colleagues have also noted the presence of other "inconvenient" erratics in archaeological contexts:

The Geological Sources and Transport of the Bluestones of Stonehenge, Wiltshire, UK
Proceedings of the Prehistoric Society , Volume 57 , Issue 2 , 1991, pp. 103 - 157
DOI: https://doi.org/10.1017/S0079497X00004527

Published online by Cambridge University Press: 18 February 2014
Richard S. Thorpe , Olwen Williams-Thorpe , D. Graham Jenkins , J. S. Watson , R. A. Ixer and R. G. Thomas

From the Abstract:

‘Bluestone’ fragments are frequently reported on and near Salisbury Plain in archaeological literature, and include a wide range of rock types from monuments of widely differing types and dates, and pieces not directly associated with archaeological structures. Examination of prehistoric stone monuments in south Wales shows no preference for bluestones in this area. The monoliths at Stonehenge include some structurally poor rock types, now completely eroded above ground. We conclude that the builders of the bluestone structures at Stonehenge utilized a heterogeneous deposit of glacial boulders readily available on Salisbury Plain. Remaining erratics are now seen as small fragments sometimes incorporated in a variety of archaeological sites, while others were destroyed and removed in the 18th century. The bluestones were transported to Salisbury Plain from varied sources in south Wales by a glacier rather than human activity.

This map shows Cheviot and the directions of ice movement during the Late Devensian glacial episode:

Was it possible that the erratic(s) from Cheviot were carried by ice all the way (c 480 km) from Cheviot to West Kennet and Avebury?  I wouldn't rule anything out.  The Anglian ice edge is most frequently shown as lying some distance north of Salisbury Plain,  but clearly much more research is needed, particularly on some of the deposits conveniently lumped into the "clay-with-flints" category -- and the discovery of this new erratic occurrence keeps the debate bubbling along nicely.

Rescuing the Millennium Stone: historic photo

 

I discovered this very rare and historic photo showing the rescue of the Millennium Stone after it had slipped into the Eastern Cleddau river in the year 2000.  This was taken a day or two after I had helped with the pulling of the stone on its sledge.  It fell into the river when assorted volunteers tried -- using authentic techniques -- to transfer the stone from the river bank into a curragh that was afloat, round about high tide.  Anyway, in order to recover the stone, authenticity had to be temporarily suspended, as it was on every single day of the ill-fated bluestone transport expedition.  The full and exciting story is related in detail in the pages of "The Stonehenge Bluestones". Available from all good bookshops!

Next time those enthusiastic experimental archaeology people show you one of their photos demonstrating how easy it is to pull a bluestone across a nice flat college lawn or a grassy London park, show them this photo as well..........

Patagonian brash ice

 

Came across this very unusual photo.  Brash ice (broken fragments of glacier ice from a floating ice edge) on the shore of an ice-dammed lake in Patagonia -- coming in very handy as cover for a hunting mountain lion. Otherwise known as a cougar, puma or panther.........

One of the chapters for my book "World of Ice" was about the wild life associated with glacial environments.    Some of the adaptations are really rather splendid.

Megaphone archaeology

 

Here we go again.  A big article in the Guardian, based on info fed to it by Vince Gaffney and relating to the supposed "mega-circle" of giant pits in the Durrington - Larkhill area.  

It's here, in all its glory:

https://www.theguardian.com/science/2021/nov/23/new-tests-show-neolithic-pits-near-stonehenge-were-humanmade

Other media outlets have picked up on the story as well, so as not to be outdone by the Guardian.

This is all based on third-hand information and speculation, issued now in order to drum up interest in a new programme to be shown on December 9th on Channel Five.  No data, no evidence -- just excited hogwash which we are all expected to believe because there is no way to scrutinize what has actually been discovered.    Maybe there is another paper somewhere in the pipeline, and maybe not.  This is real megaphone archaeology, using the media and press releases as a substitute for sound academic research and peer review.

Programme title: Stonehenge: The New Revelations. This is miles away from Stonehenge -- but that's the word that pulls the viewers in, so what the hell.......

I thought archaeology was in a bad way, having looked in detail at the working methods of our old friend MPP.  This is now confirmed.  And how.

Glacial wrapping

Starr Nunatak, Victoria Land, Antarctica.  This nunatak has been left "high and dry" as glacier ice has wrapped itself around its flanks, escaping in order to infill all the available low points in the landscape.

Mount Rea, Antarctica -- another nunatak ("lonely mountain" in the Inuit language).  The plateau on the nunatak is not extensive enough to have developed its own ice cap.

You saw it here first.  The term "glacial wrapping" does not appear in any textbook, and has never been used before, so far as I can ascertain -- but it's an important concept in glaciology.  It refers to the tendency of glacier ice to "wrap" itself around high obstacles rather than seeking to climb over them.  This is because glacier ice is essentially rather lazy, and will always take the route of least resistance.  It will always fill depressions and valleys before seeking to overwhelm ridges, interfluves and peaks;  that's because it FLOWS like a fluid medium, and it does not matter at all whether the ice is cold-based (polar) or warm-based (temperate).  It's only when lowlands are filled to overflowing that an ice surface may build up and overwhelm the highest points in the landscape.  In other words, if the ice cannot escape it will thicken until it can find another way of escaping.  But all glacial episodes are time-limited, and ice buildup that may have continued inexorably over tens of thousands of years may suddenly come to an end, with catastrophic ice wastage and surface lowering occurring so rapidly that it can be measured in centuries rather than millennia.  That's what appears to have happened at the end of the Late Devensian glaciation in the western British Isles.

And the result of glacial wrapping?  Nunataks and ice-free enclaves, in every glaciated territory.  Sometimes, plateaux may remain detached from ice-sheet glaciation, with the biggest outlet glaciers flowing around them in deep troughs, but if climatic conditions are cold and snowy enough for the biggest glaciers to be nourished and "kept alive", almost inevitably the plateaux will have ice caps of their own.  We can see this on a grand scale in East Greenland today, with abundant small ice caps ranged around the edges of the Greenland ice sheet.

In glaciated landscapes where we see a mixture of major drainage routes (used by outlet glaciers) and smaller troughs used by glaciers draining from plateau ice caps, the most likely places for nunataks or unglaciated enclaves are the tips of the interfluves, where confluent troughs come together.  I read an interesting article about this today, in a symposium on nunataks.  

East Greenland fjord country, with the ice sheet at the western edge of the image.  The fjords and other main ice sheet drainage routes are well shown -- but note the multitude of detached plateau ice caps which were (until very recently) perfectly "healthy".....

The Dyrafjordur area, NW Iceland, showing the plateau remnants after a long history of intensive glaciation along big outlet glacier routes and along shorter troughs used mostly by ice flowing from independent plateau ice caps.  After episodes of intensive glaciation the tips of the spurs are the first things to come out of the ice.

I have done a lot of posts already on nunataks and ice-free enclaves, and will not repeat them here.  But the main point I want to make is this -- there are still many professional glacial geomorphologists to draw their hypothesised ice limits as straight lines, or taking no account at all of local topography.  I have been on about this for years, criticising (for example) ice edges drawn across mid Wales regardless of the positions of mountain ridges and plateaux, and ice edges drawn in the Celtic Sea in positions that would have broken all the rules of physics.  The rules relating to glacial wrapping are just as relevant in open areas (such as the floor of the Celtic Sea) as they are in glaciated uplands where there may be high peaks and glaciated troughs.

Devensian ice limits on the Isles of Scilly.  Red line after Scourse and others.  The black line is mine -- based on glaciological principles and field observations.  I now think that the archipelago might have been a nunatak, completely surrounded by ice -- shown below.

Readers of this blog may remember the spat I had with Prof James Scourse over the glaciation and ice edge positions in the Isles of Scilly.  When I published my short paper on the glaciations of the islands, I provided evidence that Devensian ice had affected the west-facing coasts, as one would expect from glacier ice behaving normally.  In published correspondence, he accused me, in rather intemperate language, of being incompetent -- but then he had to agree that I was right and he was wrong.  Essentially, I was taking account of glacial wrapping, and he was not.

With regard to ice edge positions in and around Pembrokeshire, I have for years argued that ice edge positions cannot possibly have remained far off the western coast of the county if there was an ice edge pressing against the north face of Mynydd Preseli more than 350m above present sea level.  Again, this was all about glacial wrapping.  And at last, without any proper acknowledgement of my posts on this blog, the BRITICE team have acknowledged that I was right and they were wrong -- by showing on their latest maps that Devensian ice affected the coasts of south Pembrokeshire at least as far east as Caldey Island.  As I have been trying to tell them for years, that's what the observations on the ground show, and that's what glaciological theory must have predicted.

The revised BRITICE LGM ice edge for the Bristol Channel, showing glacier ice affecting the south coast of Pembrokeshire. That's more like it. I still think the line is in the wrong position in the centre of the Channel........

The article:
Maximum extent and readvance dynamics of the Irish Sea Ice Stream and Irish Sea Glacier since the Last Glacial Maximum
J. D. Scourse, R. C. Chiverrell, R. K. Smedley, D. Small, M. J. Burke, M. Saher, K. J. J. Van Landeghem, G. A. T. Duller, C. Ó Cofaigh, M. D. Bateman, S. Benetti, S. Bradley, L. Callard, D. J. A. Evans, D. Fabel, G. T. H. Jenkins, S. McCarron, A. Medialdea, S. Moreton, X. Ou, D. Praeg, D. H. Roberts, H. M. Roberts, C. D. Clark
Jnl of Quaternary Science, 7 May 2021 (special issue article)
https://onlinelibrary.wiley.com/doi/10.1002/jqs.3313?af=R
https://doi.org/10.1002/jqs.3313

We get there, gradually...........

===============

PS.  By the way, glacial wrapping has nothing whatsoever to do with this latest trend for the inhabitants of high glacierised mountains (as in the Alps and parts of China) to cover snow-covered surfaces with vast swathes of special fabric in a forlorn attempt to reduce the rate of ablation and glacier wastage.  I can understand why they are doing it, especially in areas where the skiing holiday industry is under threat -- but it is all really rather stupid.  There is no technical fix for global warming -- people should stop these stunts (costing millions of pounds) and listen to what they are being told by the climate scientists and by Greta Thunberg and the younger generation. 

Vatnajōkull ice edge

These three extraordinary images are from satellite imagery, showing parts of the northern edge (top two) and southern edge (lower pic) of the Vatnajōkull ice cap in Iceland.

The convoluted patterns are annual accumulation layers preserved in the ice and exposed as a result of surface ablation near the ice edge and further confused as a result of the internal deformation of ice as it moves from the ice centre out towards the edge.  What makes these patterns so clear is the presence of hundreds if not thousands of ash layers, each one relating to an eruption somewhere in the vicinity.  You can tell from the constitution and colour of the ash which eruption is responsible for each layer and when it occurred.  So it all works rather like the recognition and dating of tree rings.

Another reason for the complexity of these patterns is the complexity of glaciological processes in and under this ice cap -- because there is geothermal heating under the ice, causing intermittent excessive melting and the creation of water bodies on the glacier bed, which occasionally escape as "jōkulhlaups" or catastrophic floods.  So there are glacier surges too, on some of the outlet glaciers.

This is about as dynamic as a glacier gets.......

 

Shrinking glaciers

The BBC has just shown (for the 4th time) this excellent documentary dealing with ice on the surface of Planet Earth -- presented by Iain Stewart.  Worth watching again -- especially the section showing graphically -- and with the use of satellite imagery -- how glaciers have been shrinking.  I hate to think what has happened to some of the featured glaciers in the years since 2007..........

 https://www.bbc.co.uk/programmes/p00gbg92