New dates confirm age of Scilly Isles LGM

New dates for rock surfaces and sediments within the assumed LGM ice margin.  

From Fig 2 of the new paper.

I've received a copy of a new paper on the Isles of Scilly, which concentrates on the dating of LGM-related sediments and also rock surfaces within the assumed LGM limit (parts of which are of course in the wrong position).  It's an interesting and detailed piece of work, and I go along with almost all of it.  Abstract and other info below.

The most interesting point to come out of the paper us that the LGM here was a bit earlier than previously supposed -- ie around 24,000 years ago rather than 20,000 years ago.  By 20,000 years BP, the authors assume that the retreat from the maximum ice edge was well under way.

I would have liked the authors to have done the following:

1.  Consider whether the ice margin shown on the map is indeed accurate, by testing some "control" rock surfaces and sediments outside the assumed glacial maximum.  It's rather extraordinary that they didn't do that.........

2.  Give greater consideration to the classification of the sediments above the Scilly Till.

3.  Give more thought to the shape and other characteristics of the "Celtic Sea surge lobe" which is shown pushing well to the south of the Isles of Scilly.  It seems to me to disobey the laws of physics in that there is hardly any lateral spreading shown on Figure 1, and no calving bay where the lobe hits the grounding line.  I would have liked more consideration to be given to a powerful ice stream pressing south-eastwards from Southern Ireland and coalescing with the Irish Sea ice stream.

Another interesting thing to come out of the paper is the presence of a quatrzite erratic on Tresco that appears to have come from Anglesey or the east coast of Ireland.

 All in all, another valuable contribution to our understanding of the late Devensian glaciation.

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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

ABSTRACT:    The southernmost terrestrial extent of the Irish Sea Ice Stream (ISIS), which drained a large proportion of the last British–Irish Ice Sheet, impinged on to the Isles of Scilly during Marine Isotope Stage 2. However, the age of this ice limit has been contested and the interpretation that this occurred during the Last Glacial Maximum (LGM) remains controversial. This study reports new ages using optically stimulated luminescence (OSL) dating of outwash sediments at Battery, Tresco (25.5+/-1.5ka), and terrestrial cosmogenic nuclide exposure dating of boulders overlying till on Scilly Rock (25.9 +/-1.6 ka), which confirm that the ISIS reached the Isles of Scilly during the LGM. The ages demonstrate this ice advance on to the northern Isles of Scilly occurred at c 26 ka around the time of increased ice-rafted debris in the adjacent marine record from the continental margin, which coincided with Heinrich Event 2 at c 24 ka. OSL dating (19.6 +/- 1.5 ka) of the post-glacial Hell Bay Gravel at Battery suggests there was then an  approx 5-ka delay between primary deposition and aeolian reworking of the glacigenic sediment, during a time when the ISIS ice front was oscillating on and around the Llyn Peninsula, c 390 km to the north. 

Copyright 2017 The Authors. Journal of Quaternary Science Published by John Wiley & Sons, Ltd.

Rotten rock in Pembrokeshire

I have been reading an interesting article about the evolution of the Pembrokeshire landscape, from the French researcher Yvonne Battiau-Queney.  She argues that the land surface is very old indeed, and she does not like at all the "traditional" view that the low-lying, undulating landscape is the result of a gradual and intermittent emergence of Pembrokeshire from the sea, with "peneplains" or fragments of marine-cut erosion surfaces separated by steeper slopes or steps.

This old view was nurtured in the days of "erosion cycles"and peneplains, after WM Davis, Wooldridge and Linton, and many others -- and I remember being brought up on the idea that in Pembrokeshire, if we looked hard enough, we could see a 100 ft raised marine platform, and another at 200 ft, and another at 400ft, and another at 600 ft.  If we were really observant, it was said that we could see traces of dissected or partly destroyed platforms at even higher levels. Built into the scenario were the old "islands" or monadnocks such as Carnllidi and Penbiri near St Davids and Garn Fawr on Pen Caer.  It was all quite convincing -- although I have to admit to having had difficulty, as a young student, in seeing the platforms and the steps in the places where more senior academics claimed them to be present.

Yvonne is having none of that.  She doesn't like the idea of a falling sea-level (or rising landmass) and prefers a scenario of very great stability over many millions of years, stretching back at least to the Cretaceous.  She says that the land surface has evolved very slowly as a "fundamental erosion surface" with gradual lowering by fluvial and other processes more or less in step with the gradual lowering of a "weathering front" as a result of deep rotting processes under a warm and wet climatic regime.  In other words, you strip off 10 m of material from the ground surface, and then the weathering front penetrates deeper, and then you strip off some more, and so the process continues........

The map above shows the contours of this "fundamental erosion surface" -- partly influenced by tectonic tilting and warping,  by the reactivation of ancient fault lines and the initiation of new faults as well, some initiated by unloading.  The map also shows the presence of exposures of rotted regolith on Millstone Grit and Ordovician sedimentary rocks, around the fringes of the Daugleddau Basin.  We can also see the so-called "inselbergs" such as those we see in Africa and Australia and other warm-climate -- and the tors on Preseli and elsewhere which have long been interpreted as the products of deep weathering and rock stripping.

 Carn Llidi near St David's -- monadnock or inselberg?  (Photo: Nathan Davis)

This is all "hard geology" which deserved to be thought about quite seriously.  I do have problems with some of it. For a start, we cannot assume that over tens of millions of years the climate has always been favourable for deep weathering processes to operate.  Secondly,  deep weathering products, deep regoliths and palaeosols occur all over the place, and their presence does not militate against coastal and marine processes operating on many parts of the Pembrokeshire coast beneath an altitude of 100m or so.  Then there is an old cliffline with things that look like stacks and old sea caves near Dinas --  if this looks like the position of an old coastline, maybe it is.  Yvonne also assumes that deep weathering and doline formation (for example) have caused many of the undulations on the ground surface -- which seems to deny the operation of other powerful forces including fluvial processes, periglacial slope processes and glacial processes during several glacial episodes.  They may of couse all be interrelated -- I am very happy to accept that deep rotted material is easier for a glacier or a river to remove than sold rock devoid of "weaknesses."

A very interesting article.


Battiau-Queney, Y. 1984. The pre-glacial evolution of Wales. Earth Surface Processes and Landforms 9, pp 229-252.

Bristol Channel in the Devensian

This is a nice map from the 2011 BGS Research Report, showing the accepted Devensian ice limits for South Wales at the time of publication. Things have changed -- the Swansea geomorphologists now think that the whole of Gower may well have been affected by glacier ice at the time of the LGM, and the ice limit shown off the Pembrokeshire coast has to be wrong; if the Devensian Irish Sea Glacier was powerful enough to reach the Isles of Scilly, it must certainly have pushed far up into the Bristol Channel as well. Also, there is fresh till on the south side of Milford Haven at West Angle and on Caldey Island, so the line as shown is at least 20 km away from its actual position....

Anyway, it's a nice map, showing the deep depression of the Somerset Levels. You need to take the coastline on this map as approximate +120m contour at the time of the LGM, since sea level was a great deal lower around 20,000 years ago.

Glaciation of Somerset

I was grubbing about (as one does occasionally, on horrible cold winter days) and came across a very bulky BGS volume on the lithostratigraphy of the British Pleistocene.  Pretty turgid stuff, but somebody has to do it...... anyway, it's all about the correlations of various stratigraphic units from one part of the country to another, bringing up-to-date material from a vast range of published sources.

Above is a re-drafting of a very old map from Gilbertson and Hawkins, giving a carefully considered ice margin for the Anglian Glaciation in Somerset and the surrounding areas.  It's not that different from the maps published by Kellaway, Williams-Thorpe and myself over the years -- based on topographic and glaciological considerations and on all known occurrences of glacigenic materials in Somerset.  You would expect ice to flow more or less as shown, because of the presence of a substantial depression here -- coinciding with the Somerset Levels.

There are now substantial Holocene deposits on the Levels, as shown in the map above.  Note that the ice tongue shown is bounded by the Quantocks, the Blackdown Hills and the Dorset Downs, and on its northern flank by the Mendips.  We have discussed the Mendips many times before on this blog....... I still think they might have been overridden by glacier ice.

I have also been interested to read recently that Campbell and Bowen, the editors of the Welsh GCR volume, consider it quite likely that the Cotswolds were overridden by ice during the GBG (greatest British Glaciation) -- which would push the ice limit of Gilbertson and Hawkins further to the east.

I'm amazed that certain archaeologists (who shall be nameless, for fear of upsetting Myris) seem to be blissfully unaware of this material, which they certainly should have found if they had bothered to do a search or if they had talked to academic colleagues from related disciplines.  They still insist that the ice never flowed eastwards, never flowed uphill, and never crossed the coasts on the eastern side of the Bristol Channel.

More to the point, I'm intrigued to see that the BGS personnel charged with writing this mammoth Report have apparently accepted that the Gilbertson / Hawkins line for the extent of Anglian ice is reliable -- or at least reasonably so.  This is not "fringe science"  -- some people need to get used to the idea that this is the state of current thinking in the geological / geomorphological community.

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British Geological Survey Research Report  RR/10/03
A lithostratigraphical framework for onshore Quaternary and Neogene (Tertiary) superficial deposits of Great Britain and the Isle of Man
A A McMillan, R J O Hamblin, J W Merritt
British Geological Survey    2011

nora.nerc.ac.uk/14531/1/RR10003.pdf

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Devensian ice edge in Pembrokeshire

I have been working on several new sections and maps for the forthcoming Pembrokeshire Historical Atlas, and in thinking about the Devensian or LGM glacial limits in Pembrokeshire this is the best I can come up with.  The line is partly based on BGS mapping (which seems to me to be pretty accurate) and partly on my own observations.  Note the following:

1.  The ice edge along the north face of Mynydd Preseli seems to me to be fine, as mapped by BGS, and I have written many times on this blog of an apparent trimline and marginal meltwater channels in the vicinity of Carn Goedog and Carn Alw.  But I now think that in places there was an ice contact here, between Irish Sea ice pressing in from the north (and completely covering Carningli and Dinas Mountain) and rather thin and stagnant ice belonging to a small Preseli ice cap.  That accords with the modelling by Henry Patton and others.

2.  I think that parts of eastern Pembrokeshire were affected by Welsh ice coming down from the north and north-east.  Some of it was related to discharge from the Teifi and Tywi Glaciers.  More work needs to be done on this.

3.  I have deliberately left out of the equation the most spectacular Ice Age features in Pembrokeshire -- namely the Gwaun-Jordanston meltwater channel system -- in the area south of Newport, Fishguard and Mathry.  That's because I am convinced the meltwater channels are not Devensian, having been formed in either the Anglian, or more likely the Wolstonian, glaciation.  See other posts on this.  The channels were used in the Devensian, but probably not altered very much.

4.  The map shows just a few of the till locations, moraines and flubvio-glacial accumulations in the county.  There are many more that could have been shown.

5.  The underlying thesis here is that although the Irish Sea glacier (or ice stream) affected western Pembrokeshire, the driving force came not so much from the north as from the west.  That means that there was very thick ice over Ireland during the LGN,  pushing eastwards.  Ice must have pressed far into the Bristol Channel and against the coasts of Devon and Cornwall at this time and also into the Scilly Islands archipelago.  Fresh till was deposited on Caldey Island.  So most of southern Pembrokeshire must have been ice covered -- although the evidence is scanty.

6.  The big question mark in all of this is the apparent ice-free enclave in SE Pembrokeshire.  I don't really like the look of it, but will go with it for the moment.  Of course, there are glacial and fluvioglacial deposits here, as identified by the BGS officers.  I have examined many of them myself in the past.  They are subdued, and look rather weathered, so for the moment I'll assume they are most likely to be of Anglian or Wolstonian age.  But I am aware of the opinion of Prof Danny McCarroll that areas like this, apparently beyond a recognisable ice edge or trimline, could well have been affected by thin cold-based ice.........

All up for discussion.  Opinions please.....

Some of the glacial (blue) and fluvioglacial (pink) deposits identified by BGS fieldworkers in SE Pembs -- are they Devensian, or are they older?

This is the limit of the Devensian as shown on the BRITICE map of the British Isles.  The cream-coloured area was supposedly ice free at the LGM.  The brown areas show the distribution of some -- but not all -- patches of fluvioglacial materials beyond the supposed ice limit.

Under the Greenland ice sheet

Here are two classic images from a recent video of the landscape around the edge of the Greenland Ice Sheet near Thule, on the west coast.  They are superb illustrations of the amount of debris transported on the bed of the ice sheet.  In the top photo, the scale is difficult to estimate, but I reckon the basal dirty (ie debris-rich) layer is about 50m thick, packed with entrained boulders and till. 

In the bottom photo the very dirty basal layer is thinner, but above it there is a great thickness of layered ice with till concentrated on the contacts between layers.  This may be because we are looking at ablation surfaces with debris concentrations -- or that the layers are accretion ice layers originating on the glacier bed during the process of basal melting, thawing and sliding.

Cerrig Marchogion on Mynydd Preseli

The rock outcrops on the Preseli ridge referred to as Cerrig Marchogion (the rocks of the knights) are difficult to photograph, but this free picture has just been posted on the Pembs CC web site as part of the "Year of Legends" campaign. I think it was posted in connection with the legends of King Arthur and the existence of the "Golden Road" trading route -- but these little crags (hardly any of them are more than 3m high) are of more interest to readers of this blog as possible sources for the spotted dolerites at Stonehenge.  Ixer and Bevins say that their preferred source is Carngoedog, but they say that the spotted dolerites here are geochemically and petrographically very similar, and they probably belong to the same sill or volcanic outcrop. Of course, chunks of this rock could have been entrained by over-riding ice from both locations.

There is other information on Cerrig Marchogion on this site -- just use the search box.