Sarsen provenancing talks cancelled

Another coronavirus casualty........ David Nash was planning a series of talks next month to coincide with the publication of the sarsen stone provenancing paper -- but I suspect that all will now be cancelled.  A pity........

Durham University Geography Department event:

Physical Geography Seminars: 'Geochemical Fingerprinting the Sarsen Stones at Stonehenge' - EVENT CANCELLED
7th May 2020, 14:00 to 15:30, W007, Main Geography Building, Professor David Nash, University of Brighton

Coming soon -- if you can afford it

I'm not sure if this is published yet -- volume one of the four-volume series in which will be reported all the research of the Stonehenge Riverside Project.  The paperback is a snip for £64.99.  I can't afford it, so somebody will probably send me a copy.......

https://www.sidestone.com/books/stonehenge-for-the-ancestors-part-1

Publication date was planned as 30th March 2020.

On the web site it appears that there is not much in Volume One that we do not know already -- and I am intrigued that "Bluestonehenge" features quite prominently -- in spite of the criticism directed at the name and the lack of evidence in support of it.  But MPP and his research colleagues are nothing if not determined that their hypotheses, once proposed, are bound to be correct......

Now here's a fine thing........

Have you heard the one about this great gang of archaeologists who turned up at this remote spot in west Wales, did a lot of quarrying, at vast expense, and then proudly announced to the world that they had found a quarry?

The ice edge on the Scilly Nunatak

In the literature, much attention has been devoted to the problem of nunataks close to ice accumulation centres, to mountain top trimlines, and to ice-free enclaves or plant refugia.  But there is relatively little in the literature about low-level nunataks close to ice margins.

The above illustrations, showing two nunataks of very similar size, one in the Isles of Scilly at the time of the LGM, and the other near the eastern edge of the Greenland Icesheet today, prompt me to wonder what the nature of glacial deposits might be on up-glacier and down-glacier edges of the nunatak, and on its flanks.  

For a start, we have to say that the nature of the glacial and other deposits will be very different on a rounded or gently undulating nunatak such as those shown above, and on a pinnacle or mountain peak projecting through the ice.  In the former case, glacier ice in contact with the nunatak will be capable of re-working or incorporating pre-existing brecciated slope deposits on steeper surfaces, weathered bedrock and materials such as old beach deposits (in the case of islands or coastlines that have experienced oscillating sea-levels and shoreline processes).  In the latter case, there will be a continuous supply of frost-shattered debris down onto the ice surface during the course of a glacial episode, and some of this -- or maybe all of it -- will be incorporated into glacial deposits.  Quite prominent moraines may be the result.  

A cluster of nunataks near the edge of the ice sheet in West Greenland.  These are not very spectacular nunataks, but a great deal of debris has been eroded and picked up by the ice flowing around them.  The alignments and details of the morainic ridges are very confusing, showing that the pattern and history of ice-flow have been very complex.  Glacier ice is adept at filling in depressions, with minor ice streams sometimes flowing opposite to the regional direction of ice flow.

An isolated nunatak acting as a "blocking ridge" near the edge of the Greenland ice sheet, SW Greenland, inland of Nuuk. The ice from the ice sheet is flowing broadly from NE to SW here, and the ridge is more or less transverse to this direction of flow -- and is thus acting as a barrier. The pattern of crevasses is highly complex, but it is almost certain that the moraine on the northern side of the ridge is supplied by thrusting within the ice and the upward carriage of debris. If you look at the western end of the ridge, and then at the ice surface to the north, you can see a series of long linear crevasses transverse to the direction of ice flow -- almost inevitably these will be the surface expression of thrust planes within the ice.



Another small nunatak near the western edge of the Greenland Ice Sheet, showing massive arcuate or looped moraines on the up-glacier side and a single medial moraine on the down-glacier side separating the ice streams that have diverged and then converged again.  Note the meltwater ponds sealed within the nunatak's bounding morainic ridges.



Another west Greenland nunatak, showing the typical down-glacier medial moraine and huge morainic ridges that cover almost the whole of the nunatak.  Note the large impounded meltwater lake.



A group of four nunataks near the coast of SW Greenland.  Here there is a good vegetation cover on the largest nunatak, which measures approx 4 kms x 5 kms.  The moraines indicate very clearly what the ice movement directions are.  Here the ice is streaming between the nunataks, and the regional ice movement direction (east-west)  is disrupted by ice movement from SE towards NW, between the two largest nunataks.  Movement is maintained, although the ice thickness may well be less than 100m.

So what can we learn about nunataks close to existing ice edges?

1.  Where possible, if the ice surface level is high enough, flowing glacier ice will find its way into depressions between the landscape high points -- for example old river valleys or the straits between islands -- even if that means that it will be flowing in a direction vastly different from the regional ice flow direction.

2.  If the ice is too thin or sluggish to overwhelm a nunatak, some valleys and depressions on its surface will remain ice-free, and may be occupied by perennial or seasonal snowfields, or by meltwater lakes.

3.  On the up-glacier or proximal flank of a nunatak, one or more ridge of frontal moraine may be formed (we will not call these terminal moraines because the glacier terminus may actually be many kilometres away, down-glacier).

4.  Because there is no other source of debris supply, all of the material in the frontal moraine will have been entrained from the up-glacier bed and then brought to the surface by shearing and other processes.  Some of the till associated may be lodgement till laid down under thin ice, and we would expect to find evidence of glacitectonics and rearrangement or redeposition of diamictons. 

5.  It would not be surprising, in a situation like the Isles of Scilly, to find ancient raised beach deposits incorporated into till or morainic materials as ice has moved onshore.

6.  Neither would it be surprising to find glaciofluvial or lacustrine materials laid down on the nunatak, especially in valleys and other depressions.

7.  Further marginal moraines may be formed on the flanks of the nunatak, including some material eroded from the nunatak itself, if the flowing ice has sufficient power.

8.  The formation of moraines, and the deposition of till, on the down-glacier flank will depend partly on sediment supply and partly on the extent to which a dead -ice zone provides some protection.  In the ice, complex shear structures and crevassed zones may be found some distance away from the nunatak itself.

9.  The up-glacier glacial deposits will always be at a higher level than those in down-glacier locations.    The gradient of the deposits on the nunatak flanks may however not be regular, and there may be icefalls or sudden changes in ice gradient in response to underlying topography or changes in iceflow characteristics.

10.  The time factor.  It is clear from looking at the morainic ridges on current nunataks that some are very prominent and abundant, suggesting that the nunatak has existed in its current configuration for a long period of time.  Nunataks that are effectively covered with moraine ridges suggest a gradual lowering of the ice surface and a rather dynamic glacier regime with an abundant sediment supply.  A nunatak almost devoid of prominent moraine ridges may indicate a very rapid lowering of the ice surface and / or a surrounding ice mass that is sluggish or even dead.  If moraines are absent or poorly developed on the up-glacier flank, this may indicate that the nunatak was affected by glacier ice for just a few centuries or decades. 

It should be emphasised that there is no prominent moraine ridge anywhere on the Isles of Scilly today; the established (or disputed!) ice limit has been fixed by reference to glacial and glaciofluvial deposits found at many locations around the northern and western coasts.  This suggests that the ice was present for a relatively short period of time -- measurable in decades or centuries rather than millennia.

Virtually all of the geomorphology research on the Isles of Scilly in the last 20 years has been underpinned by the assumption that the ice edge recorded on the northern coasts marked the maximum extent of Late Devensian ice in this part of the Celtic Sea.   There has been much discussion on the nature of the deposits, the mechanisms of emplacement and deformation, the Quaternary stratigraphy and the classification and naming of the various deposits.  As explained in my 2018 paper, I am not entirely happy with the nomenclature proposed by Scourse, on the basis that it is unnecessarily prescriptive and too complex.  Be that as it may, there is nothing in any of the papers cited below which suggests that the Nunatak hypothesis is at fault.

From an examination of the modern analogies mentioned above, we would expect to find the highest glacial traces (around 40m above sea-level) along the northern coasts, with traces reducing in altitude southwards.  On the west coasts of St Marys and St Agnes the traces are close to sea-level.  There might still be undiscovered traces around sea-level on the east coasts as well.  On the south coasts of the southern islands I would expect all traces of the Devensian ice cover to be located below present sea-level. 

========================= 

Key references:

Brian John, 2018. EVIDENCE FOR EXTENSIVE ICE COVER ON THE ISLES OF SCILLY. Quaternary Newsletter Vol. 146, October 2018, pp 3-27. 

https://www.researchgate.net/publication/328413421_Evidence_for_extensive_ice_cover_on_the_Isles_of_Scilly/figures?lo=1

Late Pleistocene Stratigraphy and Palaeobotany of the Isles of Scilly

J. D. Scourse

Phil Trans Roy Soc B, December 1991

Volume: 334 Issue: 1271

Published 30 December 1991.

DOI: 10.1098/rstb.1991.0125

https://www.researchgate.net/publication/339106826_Glacial_sculpting_and_post-glacial_drowning_of_the_Celtic_Sea_2019_PhD_Thesis?channel=doi&linkId=5e3dae0892851c7f7f25f0c1&showFulltext=true

Glacial sculpting and post-glacial drowning of the Celtic Sea.

Thesis submitted in accordance with the requirements of Bangor University for the degree of Doctor of Philosophy 

By Edward Alan Lockhart

School of Ocean Sciences Bangor University, Wales 

August 2019(PDF)

New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly

R. K. SMEDLEY, J. D. SCOURSE, D. SMALL, J. F. HIEMSTRA, G. A. T. DULLER, M. D. BATEMAN, M. J. BURKE, R. C. CHIVERRELL, C. D. CLARK, S. M. DAVIES, D. FABEL, D. M. GHEORGHIU, D. MCCARROLL, A. MEDIALDEA and S. XU 

JOURNAL OF QUATERNARY SCIENCE (2017) 32(1) 48–62 

DOI: 10.1002/jqs.29224

November 2016

http://onlinelibrary.wiley.com/doi/10.1002/jqs.2922/epdf

Trimline Trauma: The Wider Implications of a Paradigm Shift in Recognising and Interpreting Glacial Limits

Danny McCarroll

Scottish Geographical Journal, 2016

Published 27 Feb 2016 

http://dx.doi.org/10.1080/14702541.2016.1157203

James Scourse et al. 2019. Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the Last Glacial: Timing and maximum extent. Marine Geology, Volume 412, June 2019, pp 53-68

https://www.researchgate.net/publication/328413421_Evidence_for_extensive_ice_cover_on_the_Isles_of_Scilly 

https://www.researchgate.net/publication/331465702_Glaciation_of_the_Isles_of_Scilly_Discussion

Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the Last Glacial: Timing and maximum extent

James Scourse et al, Marine Geology, Vol 412, June 2019, pp 53-68

https://www.sciencedirect.com/science/article/pii/S0025322718303372?via%3Dihub

Ice sheet extension to the Celtic Sea shelf edge at the Last Glacial Maximum (2015)

by Daniel Praeg, Stephen McCarron, Dayton Dove, Colm O Cofaigh, Gill Scott, Xavier Monteys,

Lorenzo Facchin, Roberto Romeo, Peter Coxon

Quaternary Science Reviews 111 (2015) 107e112 

http://dx.doi.org/10.1016/j.quascirev.2014.12.010 

Scourse, J.D. (1998) The Quaternary History of the Isles of Scilly (Chapter 8 in "Quaternary of South-West England", edited by S. Campbell et al.) Joint Nature Conservation Committee; Springer Science.

 

McCarroll, D. and others (2010).  Exposure-age constraints on the extent, timing and rate of retreat of the last Irish Sea ice stream. Quaternary Science Reviews, Volume 29, Pages 1844-1852.

Hiemstra, J.F. and others (2006) New evidence for a grounded Irish Sea glaciation of the Isles of Scilly, UK. Quaternary Science Reviews, Volume 25, Pages 299-309.

The Celtic Sea piedmont glacier lobe: the eastern terminus

Following on from previous posts inspired by Ed Lockhart's doctorate thesis and some other recent articles by James Scourse and others, I have modified Ed's final map of ice directions (with many thanks!) to bring it more into line with the field evidence on the coasts of Pembrokeshire, Devon, Cornwall and the Isles of Scilly.  See my previous post on the Scilly Nunatak.

I am now convinced that the ice edge along much of this eastern terminus (I refuse to call it a lateral ice margin) was buttressed agains the cliffed coasts of south Pembrokeshire, Devon and Cornwall and was nowhere quite powerful enough or thick enough to surmount them and press inland -- except in a few estuaries.

On the above map I have left most of Ed's  ice direction arrows in place, and added my own in the east.  I have also left in place Ed's suggested eastern ice limit (the white line running NE from the Isles of Scilly) -- but as already explained, I now think that this line has no significance.

The Scilly Nunatak

Satellite image of the Isles of Scilly.  Maybe at the time of the Last Glacial Maximum (LGM) the scene did not look so different?  Just replace the blue with white........ 

Enough of intellectual contortions.  For the last few years all of the geomorphologists (including me!) who have been mulling over the significance of the Late Devensian ice limit on the north coast of the Isles of Scilly have been trying to work out how an ISIS  terminus (or lateral ice edge) at this position could be squared with an ice edge far to the south, on the continental shelf edge.  Most people have shrugged their shoulders and tried to explain that on the eastern flank of the ice stream the ice really did stop here; and some have become so wedded to this idea that they have become really rather angry that I should have had the temerity to say that there was evidence of active ice further south, on the coasts of St Agnes and St Mary's islands.

My map of proposed ice extent, published in Quaternary Newsletter, caused considerable anger in some quarters..........

Well, in the last few days I have been reading Ed Lockhart's excellent PhD thesis, and have once again been involved in intellectual convolutions as I tried to work out whether his evidence really does support his own conclusions about the "eastern ice edge".........  

This morning, while having my morning shower, I realised that the only way out of this impasse is to propose that the Scilly island group was a single nunatak at the time of the LGM, completely surrounded by flowing glacier ice; and that the "eastern ice edge" was some distance to the south and east.  If there were any moraines associated with this limit, they are on the sea bed.

I propose that the ice edge on the northern islands, on the up-glacier nunatak flank, was above present sea-level; as suggested by Scourse, Hiemstra and others, active ice affected the islands up to an altitude of c 40m.  Further south, on St Mary's and St Agnes, the "glaciation limit" was around present sea-level.  And on the SE or down-glacier flank it was below present sea-level, explaining why there appear to be no fresh glacial deposits on the SE-facing coasts.

The map below updates the map published with my QN article (2018) by adding an ice edge along the eastern and south-eastern margins of the archipelago.

A reconstruction of what the Scilly nunatak might have looked like.  The areas coloured green and blue on the bathymetric chart are the shallows -- probably, at the LGM, these areas were covered by extensive snowfields, while the island summits were exposed and would have been visible as rocky hill masses during the summer season.

Here are some "nunatak analogies" -- many illustrations show very spectacular nunataks with steep cliffs and rocky pinnacles -- but most of them are in reality rather boring in appearance......

I have been taking a look at some of the ice sheet fringes (on Google Earth) to find analogies, and this one hit me between the eyes.  It's an image from East Greenland, not far from the ice sheet edge.  These are not jagged mountain summits projecting through the ice, but a series of broad and gentle hill summits which must have been overridden by the ice sheet in the past but which are now in the ablation zone -- the ice is not thick enough or active enough to overtop the summits.   The scale is very similar to that of the Scilly Nunatak -- the up-glacier nunatak edge is between 6 km and 8 km long.  Along this edge the ice is blocked, and we can see that a sinuous frontal moraine has been formed -- just as happened in the Scilly Isles in the Late Devensian.  The unimpeded ice to left and right of the nunatak continues out towards the coast.  There are no traces of moraines above the ice surface on the nunatak flanks, or on the down-glacier edge, which will be at a lower altitude than the up-glacier edge.  This is a rather spectacular analogy.........

It's interesting that some of the models by the BRITICE group and others have also shown the Scilly archipelago as a nunatak, surrounded by streaming ice.  Another factor that feeds into the discussion is the "accepted wisdom" that the ISIS terminated at the northern coast of the archipelago, meaning that sea-bed studies of sediments and bedforms have been largely ignored to the south and east of the islands, with effort (quite understandably) concentrated further to the west and south-west, in the centre of the perceived ice stream.

So if evidence is missing, it is because hardly anybody has tried to collect it......

I believe that there is nothing in the literature re sea floor sediments and bedforms to contradict this "nunatak" hypothesis.  As ever, comments and additional information are welcome.

Bourbonhenge

Thanks to Dave Maynard for drawing our attention to this important development.  From a Twitter feed by Reading Museum, purportedly from 1921.  April 1st, 1921, I dare say.

Not very good science in the text, but we'll let that pass.

Anyway, as I said to Dave, I have happy memories of making trilithons from these exotic biscuits when I was a kid.  Now that we are all in lockdown because of the Corona virus, we can all while away the hours by building models of Stonehenge and then eating them.