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.

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

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.

Frost and the works of man

 Periglacial rockfall litter, assumed by archaeologists to be quarrying debris.  Time for the archaeologists to read something dating from 1973?

Well well -- an old colleague of mine reminded me that in 1973 Antiquity commissioned Dr Rendel Williams (one of my contemporaries) to write an article designed to warn archaeologists that they should be careful when looking at features that might be periglacial in origin and that they instinctively assumed to be man-made.  Sounds familiar?

Sadly, I have not been able to get at the article in full.  If anybody can get at it via the Antiquity web site, I'll be grateful for a copy........

R. B. G. Williams
Frost and the works of man
Antiquity, Volume 47, Issue 185
March 1973, pp. 19-31
DOI: https://doi.org/10.1017/S0003598X00034621
Published online: 02 January 2015

Here is the link to page one:

https://www.cambridge.org/core/journals/antiquity/article/div-classtitlefrost-and-the-works-of-mandiv/3BE49F036F48CED16EE089C055B60D20

Rocks and landscape: Pembrokeshire

Simplified geological map of Pembrokeshire, showing the clear split into two geological "provinces" -- Lower Palaeozoic in the north, and Upper Palaeozoic in the south

Structural trends -- generalised.  The trend lines do not coincide precisely with either fractures or anticlinal and synclinal axes

Topography of Pembrokeshire -- based upon satellite imagery and showing clearly the structural trends in south Pembrokeshire

I've been doing some work for the new Historical Atlas of Pembrokeshire, and came upon these three images which show rather nicely what a close relationship there is between landscape, coastal configuration, geology and structure.

Time to bring the Wolstonian in from the cold?

 Suggested ice cover for the Wolstonian / Riss / Saalian glaciation of Wales and adjoining areas -- showing relations between Welsh ice and ice flowing from other source areas.  The location of the Gwaun-Jordanston meltwater channel system is also shown.  The limits of the ice cap are taken from one of the models run by Henry Patton and colleagues, assuming a steep precipitation 
gradient from west to east.

 The subglacial meltwater channel complex to the south of Fishguard and the Carningli upland.  I have now come to the view that these channels cannot have been formed during the wastage of an Irish Sea Glacier that came in from the NW, flowing towards SE.  The orientations and characteristics of the channels are all wrong.  The channels must have been formed beneath ice flowing broadly from east to west -- in other words, under a segment of an expanded Welsh ice cap.

When  I was a research student working towards my doctorate, the prevailing view was theat there was a rather complex glacial history (involving many glacial phases) on the eastern side of Britain, but that there had been just two discernible glacial phases in the west.  These were called the "Older Drift" and "Newer Drift" glaciations, and most people in the 1960s and 1970s equated them with the Riss/Saalian / Wolstonian on the one hand and the Weichselian / Vistulian / Wisconsin / Wurm / Devensian on the other.  The assumption was that glacial activity had been much more concentrated and therefore vigorous in the west than in the east -- and this was an explanation for the lack of any coherent glacial deposits beneath the raised beach dated almost everywhere to the last interglacial (Ipswichian).  Older glacial deposits had simply been eroded away by succeeding advances (we thought), whereas in eastern England they has simply piled up like layers of a cake. Then things started to get a bit more complicated, as field workers started to find things that did not quite fit and as more and more dating evidence started to pour in from all over the UK and Ireland.  Eventually the most recent big glaciation (the LGM) was assigned to the Late Devensian (at least I got that bit right, in spite of much opposition) and the Older Drift Glaciation was reassigned to the Anglian glacial episode.  That pushed it back from c 250,000 years ago to around 500,000 years ago.  And that left a very big gap between 500,000 BP and 20,000 BP.........

  The Wolstonian glaciation was not abandoned, because there is clear evidence of it in East Anglia and south of Birmingham, for example -- but because many of its deposits were overridden by Devensian ice and because its outer limit lay within the greatest extent of Devensian ice here and there, researchers were very reluctant to portray its maximum extent on a map.  This is one suggestion, from Gibbard and Clark:

The dashed line on the map shows the tentative Wolstonian limit, but as I have suggested before, it is not sensible across south Wales.  There is no sound reason for assuming glaciation across mid Wales but not across the Brecon Beacons and the Coalfield uplands -- so the southern limit in Wales must have been quite close to that shown for the Devensian.  What about Pembrokeshire, the Bristol Channel and the Celtic Sea?  Difficulties galore........

There are several features in Pembrokeshire and Ceredigion which cause me now to think that there was a greatly expanded Welsh ice cap during Wolstonian times, around 250,000 years ago.

First of all, there is the matter of the orientation of the Gwaun-Jordanston meltwater channel system, as explained at the top of this post.  I think these channels might actually have been cut during the Wolstonian Glaciation, and later modified during the Devensian.  It's possible that some of the other coastal meltwater channels, and the spectacular channel in Ramsey Sound, are also of Wolstonian age.

Second, there are apparent double and triple till sequences in northern Cardigan Bay with -- in some cases -- weathering horizons between them, suggesting that interstadial or maybe interglacial conditions prevailed between lower and upper tills.

Third, I am still mystified by what went on in the Pleistocene on the coasts of Ceredigion, where thick diamictons are variously interpreted as till laid down under ice moving westwards from the Cambrian Mountains, as periglacial slope deposits (by Eddie and Sybil Watson) or as redeposited tills.  The glacial / periglacial sequence at New Quay is also a puzzle, as I explained in an earler post.  There is a reasonable chance that some of these deposits are Wolstonian in age, laid down by ice flowing from an expanded Welsh Ice Cap.

Fourth, I now think that the frequency of Silurian gritstone erratics in the Devensian till at Newport  must have come from previously deposited Welsh till far out in Cardigan Bay.  That till is most likely either Early Devensian or Wolstonian.

Fifth, we have the matter of the till patches of central and south Pembrokeshire.  They look old, but are they old enough to have been associated with the Anglian glacial episode, when almost all other coherent till deposits associated with it have been worn away?

 Extract from the BGS geology viewer, showing fluvioglacial sands and gravels (pink) and till (light blue) outside the supposed Devensian limit in SE Pembrokeshire

Sixth, the map of rock troughs and tunnel valleys (which we have discussed earlier) suggests formation at least in part beneath a big Welsh ice cap -- much bigger than the LGM ice cap modelled by Henry Patton and colleagues.

So here is a proposal.  During the Wolstonian Glaciation Wales was completely inundated by ice from the Welsh Ice Cap, which was powerful enough to flow right across Cardigan Bay and into St George's Channel. It may be that during this glaciation the Irish Sea Glacier was less powerful and less extensive than it was during the Anglian and Devensian glacial episodes.

There we are -- a working hypothesis........ comments please!  I would be rather interested to know how the evidence from coastal sections in North Wales and South Wales fits -- or does not fit.......

Putorana Piedmont Glaciers?

Now and then, when one is trundling cross-country, courtesy of Google Earth, one spots something rather peculiar.  Above, we see the northern edge of the Putorana plateau in Central Siberia.  The plateau edge is quite sharp, running tight to left across the photo.  In the mountain area we see the modified dendritic pattern of troughs that were transporting glacier ice northwards at various times during the Quaternary.  To the north we see the tundra of the North Siberian Plain, with patchy woodland and many small meltwater lakes in the permafrost.

But the really impressive features are the bulbous lobes pushing out from the mountain front onto the plain.  What on earth are they?  The most logical explanation is that they are morainic loops formed by piedmont glaciers pushing northwards from the ice sheet -- rather like the features we see in Greenland, Ellesmere Island, Alaska and elsewhere today.  The top photo is from Peary Land (Greenland), the middle one is the Malaspina Glacier in Alaska, and the lower one is from Ellesmere Island.

It's often stated that pedmont glaciers form most readily in high Arctic situations where conditions are quite arid, with continuous permafrost and very little glacier bed melting.  This means that basal melting and bed sliding are minimal, and that most glacier movement is byb internal deformation.  This does not alwats hold -- but it is a fair assumption that those conditions would have prevailed in the Weichselian (Devensian) glacial episode on the northern flank of the Putorana Plateau.

According to Svendsen and his colleagues (2004) there were some early glaciations during which both the plateau and the adjacent coastal plain were heavily inundated beneath the Eurasian Ice Sheet.  The Late Saalian Glaciation (160 ka - 140 ka) is the one about quite a lot is known.  During the Weichselian things got complicated.  There appear to have been three glacial episodes, each one less extensive than the last.  In the early Weichselian (related to the Onyoka Moraine) -- about 90 ka ago -- the Putorana ice sheet was incorporated into the Eurasian ice sheet on its western edge, but on the northern and north-eastern flanks of the plateau the local segment of the ice sheet terminated just beyond the plateau edge.  After an ice retreat there was another expansion -- referred to as the mid-Weichselian glacial phase -- where the glaciers in this area terminated in more or less the same positions.  This is called the "Norilsk Glaciation" by Russian workers.  After that there was another retreat, and during the LGM "last glacial maximum) dated to c 20,000 years ago, there was apparently no ice sheet on the Putorama Plateau -- just an assortment of small cirque glaciers, local ice caps and small valley glaciers.

Therefore it is possible that the piedmont glaciers decanting onto the North Siberian Plain were active twice, in the early Weichselian and again in the mid Weichselian. This means that the features seen in the satellite image at the top of this post might be composite in age.

Nonetheless, they are pretty impressive -- and I know of hardly any other examples of Weichselian piedmont glacier maraine loops that are so well exposed in an area currently devoid of glacier ice.  It will be interesting to see whether field observations tally with what the satellite image suggests!

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

Late Quaternary ice sheet history of Northern Eurasia
Quaternary Science Reviews 23 · January 2004, pp 1229-1271
John Inge Svendsen et al

https://www.researchgate.net/publication/223294900_Late_Quaternary_ice_sheet_history_of_Northern_Eurasia

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

This appears to be a crucial paper, if only one could get at it:

Isayeva, L.L., Kind, N.V., Kraush, M.A., Sulerzhitsky, L.D., 1976.
On the age and structure of the marginal formations along the northern foot of the Putorana Plateau. Bulleten Komissii poizucheniyu chevertichnogo perioda,
USSR Academy of Sciences
45, 117–123 (in Russian)
 

Large-scale glacial erosional features in Vestfirdir, Iceland

On looking back through some of my old research materials, I discovered that our short research notes written at the end of the Durham University Vestfirdir Project in 1975 have never been published digitally.  Here is one of them.  It's very short, but it does make some valid points that other researchers might wish to consider.  Here is the DOI if you want to take a look:

DOI: 10.13140/RG.2.2.14187.34080

And here are the maps which show quite well what the research was all about.

Brutalised glacial discharge dendritic patterns

For many years I have been fascinated by the difference between dendritic drainage patterns developed by rivers and those developed by glaciers.  Here is a past post:

https://brian-mountainman.blogspot.co.uk/2014/05/outlet-glacier-troughs-now-and-then.html

The illustration above is a satellite image from the northern coast of Iceland, in the uplands west of Akureyri.  (David Sugden and I used a similar image, with more snow, and false colours, on the front cover of our geomorphology text.)

On the main troughs in these images, there has been so much ice to be discharged that all of the smaller and more delicate fluvial valleys that existed in pre-glacial times have been "brutalised" out of existence, and many of the larger interfluves have also been removed.  So the troughs are wide and deep, with relatively few feeder valleys -- and even these latter features are very wide, with prominent trough heads.

Contrast this sort of landscape with that of Sognefjord:

This is one of the biggest fjords in the world (see recent posts), and yet it has retained much more of a delicate dendritic pattern which looks as if it has not changed all that much since the time when there was a fluvial landscape of hills and valleys on the western slope of the Scandinavian mountain range.   Some people use the term "fractals" when looking at an image like this.  Why the difference?  It's a bit of a puzzle.........

Now let's look at two segments of the Putorana Plateau, in central Siberia.  This one is from the NE segment of the plateau.  It was heavily glaciated at least twice, but there are no cirques in this landscape, and the trap plateauaway from the valleys appears almost unmodified by glacial processes.  The dendritic valley pattern is delicate -- and reminiscent of that of Sognefjord.

The plateau edge in the above image is just off the top edge of the photo.  So it appears that close to the point at which the glacier in question was decanting onto the adjacent lowland, the trough actually narrowed.  Was that because ice was spilling out of the trough and flowing across the slopes on either side?  Very intriguing.  My assumption is that in this part of the Putorana ice sheet the ice was cold-based and frozen toits bed -- except in the deep valleys where bottom melting occurred, and where glacial processes could operate.  Let s callthis a classic example of a "glacially protected" landscape......

If we then look at the NW segment of the plateau, only about 200 km away, we see a completely different type of glaciated landscape:

There is almost a rectilinear pattern here (maybe controlled to some degree by fractures in the plateau basalts) but the pattern is much more akin to that of the landscape west of Akureyri in Iceland.  The old river valley pattern has been simplified and brutalised, with smaller valleys and interfluves obliterated.  The ice that operated here is much more likely to have been warm-based, and presumably it flowed rapidly in these deep, wide troughs.

And if we zoom in we see a landscape dominated by glacial erosional features.  There are literally thousands of cirques, the majority of them opening northwards -- these must date from glacial phases before and after the episodes of fully-fledged ice sheet cover.  We are looking at more or less the centre of the photo above.

I must seek some glaciological opinion on all of this.........

History of Stonehenge digs

This is a short history of Stonehenge digs -- sent in by Garry Denke.  I'm not going to publish stuff about the hunt for coal on Salisbury Plain,  since those who are interested in such things can find it elsewhere.  But this is of broader interest, so here we are.

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

Complete History of Stonehenge Excavations

1611. King James I investigated Stonehenge "to see 'The stone which the builders refused.'"
King James Version, 1611

1616. Doctor William Harvey, Gilbert North, and Inigo Jones find horns of stags and oxen, coals, charcoals, batter-dashers, heads of arrows, pieces of rusted armour, rotten bones, thuribulum (censer) pottery, and a large nail.
Long, William, 1876, Stonehenge and its Barrows. The Wiltshire Archaeological and Natural History Magazine, Volume 16

1620. George Villiers, 1st Duke of Buckingham, dug a large hole in the ground at the center of Stonehenge looking for buried treasure. (Diary)

1633-52. Inigo Jones conducted the first 'scientific' surveys of Stonehenge.
Jones, I, and Webb, J, 1655, The most notable antiquity of Great Britain vulgarly called Stone-Heng on Salisbury plain. London: J Flesher for D Pakeman and L Chapman

1640. Sir Lawrence Washington, knight, owner of Stonehenge, fished around Bear's Stone (named after Washington's hound dog). Bear's Stone profile portrait a local 17th century attraction. (G-Diary)
The Wiltshire Archaeological and Natural History Magazine, Volumes 15-16

1652. Reverend Lawrence Washington, heir of Stonehenge, commissions Doctor Garry Denke to dig below Bear's Stone, reveals lion, calf (ox), face as a man, flying eagle, bear (dog), leopard, and hidden relics. Bear's Stone (96) renamed Hele 'to conceal, cover, hide'. (G-Diary)

1653-6. Doctor Garry Denke auger cored below Hele Stone 'The stone which the builders rejected' on various occasions. Gold, silver, brass, iron, wood, bone, concrete discovered at 1-1/3 'yardsticks' (under flying eagle). Elizabeth Washington, heir of Stonehenge.
Denke, G, 1699, G-Diary (German to English by Erodelphian Literary Society of Sigma Chi Fraternity). GDG, 1-666

1666. John Aubrey surveyed Stonehenge and made a 'Review'. Described the Avenue's prehistoric pits. (the 'Aubrey Holes' discovered by Hawley, not Aubrey).
Aubrey, J, 1693 (edited by J Fowles 1982), Monumenta Britannica. Sherborne, Dorset: Dorset Publishing Co

1716. Thomas Hayward, owner of Stonehenge, dug heads of oxen and other beasts. (Diary)

1721-4. William Stukeley surveyed and excavated Stonehenge and its field monuments. Surveyed the Avenue in 1721 extending beyond Stonehenge Bottom to King Barrow Ridge. Surveyed the Cursus in 1723 and excavated.
Stukeley, W, 1740, Stonehenge: a temple restor'd to the British druids. London: W Innys and R Manby

1757. Benjamin Franklin observes Bear's Stone (96) lion, calf (ox), face as a man, flying eagle, bear (dog), leopard, and Hele Stone 'hidden' relics below them. (Diary)

1798. Sir Richard Hoare and William Cunnington dug at Stonehenge under the fallen Slaughter Stone 95 and under fallen Stones 56 and 57.
The Ancient History of Wiltshire, Volume 1, 1812

1805-10. William Cunnington dug at Stonehenge on various occasions.
Cunnington, W, 1884, Guide to the stones of Stonehenge. Devizes: Bull Printer

1839. Captain Beamish excavated within Stonehenge. (Diary)

1874-7. Professor Flinders Petrie produced a plan of Stonehenge and numbered the stones.
Petrie, W M F, 1880, Stonehenge: plans, description, and theories. London: Edward Stanford

1877. Charles Darwin digs at Stonehenge to study 'Sinking of great Stones through the Action of Worms'.
Darwin, Charles,1881, The Formation of Vegetable Mould, Through the Action of Worms, with Observations on Their Habits. London: John Murray

1901. Professor William Gowland meticulously recorded and excavated around stone number 56 at Stonehenge.
Gowland, W, 1902, Recent excavations at Stonehenge. Archaeologia, 58, 37-82

1919-26. Colonel William Hawley extensively excavated in advance of restoration programmes at Stonehenge for the Office of Works and later for the Society of Antiquaries. Hawley excavated ditch sections of the Avenue, conducted an investigation of the Slaughter Stone and other stones at Stonehenge, and discovered the 'Aubrey Holes' (misnamed) through excavation.
Hawley, W, 1921, Stonehenge: interim report on the exploration.
Antiquaries Journal, 1, 19-41
Hawley, W, 1922, Second report on the excavations at Stonehenge.
Antiquaries Journal, 2, 36-52
Hawley, W, 1923, Third report on the excavations at Stonehenge.
Antiquaries Journal, 3, 13-20
Hawley, W, 1924, Fourth report on the excavations at Stonehenge, 1922.
Antiquaries Journal, 4, 30-9
Hawley, W, 1925, Report on the excavations at Stonehenge during the season of 1923.
Antiquaries Journal, 5, 21-50
Hawley, W, 1926, Report on the excavations at Stonehenge during the season of 1924.
Antiquaries Journal, 6, 1-25
Hawley, W, 1928, Report on the excavations at Stonehenge during 1925 and 1926.
Antiquaries Journal, 8, 149-76
(Diary)
Pitts, M, Bayliss, A, McKinley, J, Boylston, A, Budd, P, Evans, J, Chenery, C, Reynolds, A, and Semple, S, 2002, An Anglo-Saxon decapitation and burial at Stonehenge. Wiltshire Archaeological and Natural History Magazine, 95, 131-46

1929. Robert Newall excavated Stone 36.
Newall, R S, 1929, Stonehenge. Antiquity, 3, 75-88
Newall, R S, 1929, Stonehenge, the recent excavations. Wiltshire Archaeological and Natural History Magazine, 44, 348-59

1935. Young, W E V, The Stonehenge car park excavation. (Diary)

1950. Robert Newall excavated Stone 66.
Newall, R S, 1952, Stonehenge stone no. 66. Antiquaries Journal, 32, 65-7

1952. Robert Newall excavated Stones 71 and 72. (Diary)

1950-64. A major campaign of excavations by Richard Atkinson, Stuart Piggott, and Marcus Stone involving the re-excavation of some of Hawley’s trenches as well as previously undisturbed areas within Stonehenge.
Atkinson, R J C, Piggott, S, and Stone, J F S, 1952, The excavations of two additional holes at Stonehenge, and new evidence for the date of the monument. Antiquaries Journal, 32, 14-20
Atkinson, R J C, 1956, Stonehenge. London. Penguin Books in association with Hamish Hamilton. (second revised edition 1979: Penguin Books)

1966. Faith and Lance Vatcher excavated 3 Mesolithic Stonehenge postholes.
Vatcher, F de M and Vatcher, H L, 1973, Excavation of three postholes in Stonehenge car park. Wiltshire Archaeological and Natural History Magazine, 68, 57-63

1968. Faith and Lance Vatcher dug geophone and floodlight cable trenches. (Diary)

1974. Garry Denke and Ralph Ferdinand set out to confirm Sir Lawrence Washington, knight and Reverend Lawrence Washington's revelation (G-Diary). Auger cores 1.2m (4ft) below Heel Stone 96 (under face as a man). Gold, silver, brass, iron, wood, bone, concrete confirmed. No coal in cores. Stonehenge Free Festival.
Denke, G W, 1974, Stonehenge Phase I: An Open-pit Coalfield Model; The First Geologic Mining School (Indiana University of Pennsylvania). GDG, 74, 1-56

1978. John Evans re-excavated a 1954 cutting through the Stonehenge ditch and bank to take samples for snail analysis and radiocarbon dating. A well-preserved human burial lay within the ditch fill. Three fine flint arrowheads were found amongst the bones, with a fourth embedded in the sternum.
Atkinson, R J C and Evans, J G, 1978, Recent excavations at Stonehenge. Antiquity, 52, 235-6
Evans, J G, 1984, Stonehenge: the environment in the late Neolithic and early Bronze Age, and a Beaker burial. Wiltshire Archaeological and Natural History Magazine, 78, 7-30

1978. Alexander Thorn and Richard Atkinson. NE side of Station Stone 94. (Diary)

1979-80. George Smith excavated in the Stonehenge car park on behalf of the Central Excavation Unit.
Smith, G, 1980, Excavations in Stonehenge car park. Wiltshire Archaeological and Natural History Magazine, 74/75 (1979-80), 181

1979-80. Mike Pitts excavated along south side of A344 in advance of cable-laying and pipe-trenching. In 1979, discovered the Heel Stone 97 original pit (96 original Altar Stone pit). Survey along the Avenue course identified more pits. In 1980, excavated beside the A344 and discovered a stone floor (a complete prehistoric artifact assemblage retained from the monument).
Pitts, M W, 1982, On the road to Stonehenge: Report on investigations beside the A344 in 1968, 1979, and 1980. Proceedings of the Prehistoric Society, 48, 75-132

1981. The Central Excavation Unit excavated in advance of the construction of the footpath through Stonehenge.
Bond, D, 1983, An excavation at Stonehenge, 1981. Wiltshire Archaeological and Natural History Magazine, 77, 39-43.

1984. Garry Denke (and Hell's Angels) seismic survey. Auger cores 1.2m (4ft) below Heel Stone 96 (under lion head). Gold, silver, brass, iron, wood, bone, concrete reconfirmed. No coal in cores. Stonehenge Free Festival.
Denke, G, 1984, Magnetic and Electromagnetic Surveys at Heelstone, Stonehenge, United Kingdom (Indiana University of Pennsylvania). GDG, 84, 1-42

1990-6. A series of assessments and field evaluations in advance of the Stonehenge Conservation and Management Programme.
Darvill, T C, 1997, Stonehenge Conservation and Management Programme: a summary of archaeological assessments and field evaluations undertaken 1990-1996. London: English Heritage

1994. Wessex Archaeology. Limited Auger Survey.
Cleal, R M J, Walker, K E, and Montague, R, 1995, Stonehenge and its landscape: twentieth-century excavations (English Heritage Archaeological Report 10). London: English Heritage.

2008. Timothy Darvill and Geoffrey Wainwright set out to date the construction of the Double Bluestone Circle at Stonehenge and to chart the history of the Bluestones, and their use.
Darvill, T, and Wainwright, G, 2008, Stonehenge excavations 2008. The Antiquaries Journal, Volume 89, September 2009, 1-19
(Diary)
Mike Parker Pearson, Julian Richards, and Mike Pitts further the excavation of 'Aubrey Hole' 7 discovered by William Hawley, 1920.
Willis, C, Marshall, P, McKinley, J, Pitts, M, Pollard, J, Richards, C, Richards, J, Thomas, J, Waldron, T, Welham, K, and Parker Pearson, M, 2016, The dead of Stonehenge. Antiquity, Volume 90, Issue 350, April 2016, 337-356

2012-3. Stonehenge A344 road excavated and removed. (Diary)

https://archive.org/stream/wiltshirearchaeo16arch#page/n5/mode/2up
http://discovery.ucl.ac.uk/1474049/1/Dead%20of%20Stonehenge%20Antiquity%20final%20version.pdf
http://www.sarsen.org/2013/01/a-list-of-stonehenge-excavations.html

Complete History of Stonehenge Excavations

Any missing Digs?

Gallery -- Putorana and Vestfirdir

There are remarkable similarities between the "glaciated trap landscapes" of Putorana in Siberia and Vestfirdir in Iceland.  Flat-bedded basalt layers, flattish plateau surfaces, and deeply incised troughs cut by powerful ice streams.  The main difference is that Vestfirdir is at the coast, with many of the deepest troughs now containing fjords or arms of the sea.  Putorana is far inland, and the troughs are still dry, apart from a few which contain long finger lakes.

Five from Vestfirdir (thanks to Mats Wibe Lund and Murray Foote):

And four from Putorana:

More on closed subglacial troughs

This hasn't got anything to do directly with Stonehenge, but it is no doubt of interest to somebody or other, given the numbers of readers we get for posts on geomorphology and glaciology.  So here goes......

Back to the matter of closed sub-glacial troughs -- how they are eroded and how ice manages to extract billions of tonnes of rock from them and to transport all this debris uphill and over "exit thresholds" before releasing it somewhere near the outer coastline.

Several things have come up in my recent reading.  First, some troughs have beds which are divided up into a series of connected basins.  According to Julian Dowdeswell and others the bed of Nordvestfjord is like this, with a series of deep basins (over 1200m deep) separated by sills between 600m and 900m deep.  I have not seen the long profiles, and so we can but speculate as to whether the sills coincide with outcrops of highly resistant rocks (on the basis of lithology or structure) and whether the basins coincide with pulses or additions to glacier discharge derived from tributary glaciers.  In the case of Hardangerfjord, the deepest part of the biggest basin is more than 30 km downstream from the nearest tributary input, and the sharp sill which then follows (about 300m high) is apparently unrelated to any route suitable for diffluence or discharge reduction.  So something else is going on -- and a simple model will clearly not do.

Bedrock characteristics must have something to do with the very complex long profile of the bedrock floor of Hardangerfjord -- and the same must apply to the long profiles of many other outlet glacier troughs and fjords studied with the aid of new sounding techniques over the past decade or so.  Two other factors now come into play:  isostatic rebound and the relationship outlet glaciers to ice shelves.

1.  Isostatic rebound

Isostatic depression of a land mass in response to the loading imposed by an ice sheet or ice cap, and then isostatic recovery associated with ice melting, must play a considerable part in determining what processes will operate within a glacial trough used by an outlet glacier.  In Scandinavia, it appears that in the centre of the ice sheet there have been about 750m of isostatic depression, followed by almost the same amount of recovery, during the Devensian glaciation.  Possibly, isostatic responses were even greater in the Anglian and other glacial episodes.  In the fjord region of Norway, an isostatic rebound of approx 200m has been recorded in the inner reaches of the fjords, and about 100m in the outer reaches.

 Total postglacial isostatic rebound in Scandinavia since the onset of Devensian ice wastage.

 Because of the differential rates of isostatic response to crustal loading and unloading, stresses are exerted within the near-surface crust.  These stresses express themselves as seismic events such as small earthquakes; and many such have been recorded around the fringes of Scandinavia.  In practical terms, old faults are regenerated or re-activated, and rock fracture is a consequence.  On what scale does this occur?  And does it happen in some strata more than in others?  Whatever the answers are, it seems probable that over many millions of years, each glacial event will be followed by a "rock fracturing" episode which might facilitate easier erosion when the next glacial episode comes along.  Another factor, on the floor and flanks of troughs, will be pressure release and "rock bursting" in response to rock removal or the replacement of metres or tens of metres of rock by ice or even by water.

When David Sugden and I were working in Greenland and in the South Shetland Islands, we began to suspect (from our mapping of raised beaches) that some of the isostatic responses in recently deglaciated areas were very localised indeed. In some recent work by David and his colleagues around the Shackleton Range in Antarctica, it appears that isostatic adjustments to rock loss (by glacial erosion) can have major effects on the routes followed by ice discharging from the Antarctic ice sheet but also on the speed of flow and the capacity for enhanced erosive activity.  In other words, there is a positive feedback effect.

https://www.researchgate.net/publication/260114407

Sugden, D,E et al,  Emergence of the Shackleton Range from beneath the Antarctic Ice Sheet due to glacial erosion.  Geomorphology · March 2014
DOI: 10.1016/j.geomorph.2013.12.004

In this false-colour map of bedrock elevations we can see the Shackleton Range standing up as a series of nunataks, flanked by the outlet glacier troughs of the Slessor and Recovery Glaciers.  These are remarkably deep -- as shown by the blue colour.  Each trough has a base well over 2,000m beneath sea level, and each trough is closed, with a wide threshold (shown in green) in the pro-montane zone to the west of the trough exits. The threshold is particularly spectacular in the case of the Recovery Glacier trough, as we can see in the specked green and yellow markings on the map.  Also, if we examine the blue-coloured areas in detail, we can see that each trough is made up of a series of connected basins and sills -- as already described for Nordvestfjord and Hardangerfjord.  So glacier ice is clearly excavating vast amounts of bedrock and moving it uphill and away.  But it is also moving through one basin after another, excavating and then rising, doing it again, and then doing it again.  Is there any significance in this?  There must be, since ice always obeys the laws of physics.  There must be some very complex interactions going on, far too deep beneath the ice surface for anybody to observe them.

This is all very interesting, although of course it tells us nothing at all about the precise mechanics operating on the glacier bed.


What's going on here is by no means unusual.  If you look at this bedform map of Antarctica, and search for the blue-coloured areas, you will see that there are indeed closed troughs all over the place.  Rather a lot of material is being moved uphill.............

The Cryosphere, 7, 375–393, 2013
doi:10.5194/tc-7-375-2013 
© Author(s) 2013. CC Attribution 3.0 License.
Bedmap2: improved ice bed, surface and thickness datasets for Antarctica
P. Fretwell et al
www.the-cryosphere.net/7/375/2013/

2.  Interactions with ice shelves

Returning to Sognefjord and the other big fjords of western Norway,  the isobase map shows us that at the peak of the Devensian glaciation, the land surface was depressed by the weight of ice by something like 150m in the west coast zone.  That is approximately the same as the known eustatic depression of sea-level at the time.  So the relationships of land and sea around 20,000 year ago would have been approximately the same as they are today. That means that if you had taken away all the ice of the Sognefjord outlet glacier, the sea would have flooded in at more or less the same relative level as today. This is an important point, since it enables us to look at the current relationships between Antarctic ice shelves and outlet glaciers, and maybe learn some lessons from them.

It is widely assumed that at the peak of each Pleistocene glaciation the edge of the Scandinavian ice sheet was some distance to the west of the current Norwegian coastline.  Did all of the ice discharged westwards calve straight into deep water, or was there a fringing ice shelf?  Ice shelves thrive where a lot of glacier ice is disgorged into embayments or concavities in the coastline -- this provides anchoring points on the flanking headlands.  Out at sea, to the west of Norway, there would have been no anchoring headlands, but an ice shelf might have been supported on the skerries or strandflat which has exercised geomorphologists for many years.  What might the subglacial dynamics have been in that scenario?  With sea-level in more or less its current relative position,  there might have been a lifting of the shelf in places, a moving grounding line, and a consequent reduction in bed erosion.  If saltwater penetrated well inland of the ice shelf edge, there might have been enhanced melting on the base of the shelf from contact with warm oceanic water, and such water might well have penetrated intermittently over the thresholds and into the fjords.  Might there have been an "enhanced buoyancy" effect?  Maybe -- especially following the retreat of the ice edge to the vicinity of the present coast.

 The outlet glaciers flowing into the Ross Ice Shelf from the west.  The black line is the grounding line; its position is crucial in determining what processes can operate on the rock floor.  There are indications that sub-glacial rivers flow beneath these outlet glaciers, decanting into the sea at the grounding line.

There is a vast literature on Antarctic ice shelves and their dynamics -- and great work currently in progress, given the importance of shelf behaviour as an indicator of climate change.  There is no point, just now, in exploring this literature in detail.  But here is one interesting article:

Rapid submarine ice melting in the grounding zones of ice shelves in West Antarctica
Ala Khazendar et al,
Nature Communications 7, Article number: 13243 (2016)
doi:10.1038/ncomms13243
http://www.nature.com/articles/ncomms13243

The authors have studied  the Dotson and Crosson ice shelves and their main feeders, namely Smith Glacier, Pope Glacier and Kohler Glacier.   Their main interest is in discharge changes and accelerated melting, but their radar sounding work is of immediate relevance.  Here are the results of many radar sounding runs (mostly in 2002, 2004 and 2009):

There are considerable differences between the bedforms of the three glaciers, but the long profiles are very irregular.  In the case of the Kohler Glacier, there is an apparent transverse channel on the seaward side of the threshold; the threshold is much more marked in the case of the Smith Glacier, with a reverse slope rising from -2000m to -800m over a distance of c 30 km; and the bed of Pope Glacier shows no strong reverse slope at all, but instead a gradual rise of c 750m over a distance of c 25 km.  Note the different scales on the diagrams.  Another interesting feature of the Pope Glacier and Kohler Glacier profiles is the lateral variation in bedrock profiles, revealed when radar runs are parallel but slightly offset one from another.

 Clearly these and most other outlet glaciers around the Antarctic coast have overdeepened sections and thresholds as they approach their exit points. They also have rather irregular floors with various closed basins connected by sills.    The smooth bed of Sognefjord might be something of an anomaly......

Is most of the work of overdeepening done by ice, or by meltwater and slush working under very high hydrostatic pressure?  More to come......