Interim 2018 dig report from the "great circle" hunters.........

Courtesy of Bluestone Brewery, just up the road, the interim report from MPP et al on the 2018 dig is now in the public domain.  He confirms that there will be no dig this year, but expresses the hope that the diggers will be back in 2020.  Here we go again.........

I'm a bit pushed for time at the moment, but here is a link for those who want to take a look.  My initial impression is that MPP is as determined as ever to ignore the fact that much of his bluestone quarrying / Stonehenge / Waun Mawn  evidence is hotly disputed.  We have looked at these irregular indentations in the ground before, in earlier posts, and one has to admire the sheer effrontery of these guys in claiming without a shadow of a doubt that they are stone holes or sockets that once held monoliths!  I wonder if any other archaeologists have looked at them, and agreed?  Anyway, enjoy, and I shall come back to this in more detail in due course.

https://www.bluestonebrewing.co.uk/wp-content/uploads/2019/05/Waun-Mawn-2018-interim-report-lite.pdf

By the way, MPP is giving his annual jolly brewery talk early this year, as it happens on my birthday, 27th July.  Happily I shall be away, doing a spot of quiet kayaking in the Stockholm Archipelago.

Anglesey and Pembrokeshire -- both subjected to areal scouring?

A comment from a mysterious contributor the other day has got me thinking about the similarities and differences between Anglesey and Pembrokeshire with respect o the history of glaciation.  They are both lowlands located away from the bulk of the Welsh upland massif and its Welsh Ice Cap which waxed and waned during the Devensian glaciation and presumably during other glaciations as well.  In both regions superficial deposits are thin and scattered, and in both regions there are no marked uplands, and instead a sequence of undulating and gentle ridges and depressions which expose the "grain" of the country -- related in turn to the structures and textures of the underlying rock types.

Anglesey has quite prominent streamlined bedrock features, aligned broadly NE - SW, as noted in this article by Lee et al (2015):

Lee, J.R., Wakefield, O.J.W., Phillips, E. & Hughes, L. 2015. Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK. Quaternary Science Reviews 109, 88-105.

http://nora.nerc.ac.uk/id/eprint/509384/1/Lee_et_al_QSR_2015.pdf

Quote:

The Quaternary deposits of Anglesey form a thin and discontinuous veneer that mantles the bedrock. The most prominent superficial deposit is a diamicton containing far-travelled erratic lithologies sourced from bedrock strata within and surrounding Irish Sea Basin. This diamicton is synonymous with the ‘Irish Sea till’ (Greenly, 1919; Campbell & Bowen, 1989; Williams, 2003; Thomas & Chiverrell, 2007; Phillips et al., 2010) which forms part of the Irish Sea Coast Subgroup of the Caledonia Glacigenic Group (McMillan & Merritt, 2012). The till was deposited extensively around the Irish Sea Basin by the Irish Sea Ice Stream during the Late Weichselian (Devensian; MIS 2) glaciation (Figure 1c). The overriding of Anglesey and the adjacent offshore area by the Irish Sea Ice Stream led to the sculpting of the substrate and development of streamlined bedrock features (e.g., streamlined bedrock, roche moutonée, rock-cored drumlins) and sediment- to bedrock-cored (e.g. drumlins) subglacial bedforms (Hart 1995; Thomas & Chiverrell, 2007; van Landeghem et al., 2009; Phillips et al., 2010). These landforms and locally-preserved glacial striae record an overall ice-movement direction from northeast to southwest across Anglesey. The Irish Sea Ice Stream was one of several ice streams that drained the interior of the last British Irish Ice Sheet (Figure 1c) (Evans et al., 2005; Bradwell et al., 2008; Hubbard et al., 2009; Clark, C.D. et al., 2012; Clark, J. et al., 2012). At its maximum extent (c.24-23 ka BP) the ice stream occupied much of the Irish Sea Basin and was fed by glaciers emanating from western Britain and eastern Ireland (Knight et al., 1999; Clark & Meehan, 2001; Hiemstra et al., 2006; Roberts et al., 2007; Greenwood & Clark, 2009; Scourse et al., 2009; Rijsdijk et al., 2010; Clark, J. et al., 2012; Chiverrell et al., 2013).

In Pembrokeshire the "grain" of the country is not so easy to pick up, except in the south.  If you look at the following maps, this is apparent. (Google Earth satellite images are not so easy to interpret, because they have too much clutter on them........)

Low definition LANDSAT image of Pembrokeshire.  On this, the grain can be picked up, but it is much clearer in the south.

A simplified map of the structure of Pembrokeshire, with info extracted from BGS mapping material.

The grain can be tied in with the features of the geology map, but note that the strips of Ordovician igneous rocks in the core of the Preseli uplands are NOT easy to pick up in the satellite imagery.

Now let's tie these images in with what we know about directions of ice movement across Pembrokeshire.

This map incorporates information from many sources on ice movement directions, and can be taken as representing a "best fit" for the Anglian Glaciation, around 450,000 years ago.

This map by JC Griffiths (1940) is based on erratic distributions, taking no account of possible zig-zag routes and movements of single erratics across several glacial episodes.  Note that there are differences with the "Anglian" map above.  

Griffiths has Irish Sea ice coming across the north Pembrokeshire coast directly from the north.  He also shows central and southern Pembrokeshire affected by an ice stream flowing from NW towards SE. On the Anglian map we have a more westerly component, with ice moving from WNW towards ESE.

So what is the evidence for streamlining? I have to say that there is none in north Pembrokeshire, but that in south Pembrokeshire there is a degree of alignment between the "grain" of the country, the known structural trends, rock outcrops, and ice movement directions. Cause and effect?  Now that's very difficult to establish.............

Exposures of the submerged forest at Borth

Very extensive areas of the submerged forest at Borth have been exposed in the last month or so -- unusual because the stripping away of the covering sand has not been a result of extreme storm events.  There is much speculation as to what is going on -- there may be a link with coastal defences changing the characteristics of longshore drift and tidal currents, or the phenomenon may be entirely natural and cyclical.

More info here:

https://www.walesonline.co.uk/news/wales-news/stunning-ancient-forest-welsh-beach

Abandoned glacier troughs -- High Arctic

Devon Island

North Greenland (Peary Land)

 

NE Greenland

These are very cold and dry regions, and although there are many ice caps and outlet glaciers there are also very expansive areas of ice-free land where permafrost features predominate. 

But massive abandoned glacier troughs are to be found in many areas, attesting to a past much more expansive glaciation by the Greenland ice sheet and from independent ice caps on the islands of the Canadian Arctic archipelago and in north Greenland.  I must investigate further.......

PS.  You can see a few other posts on glacial troughs if you put "glacier troughs" or "glacial troughs" into the search box.   Shortly before leaving Durham University in 1977 I started some research on "brutalised trough patterns"  -- but I never finished it.  One of my regrets......

Anyway, a classic area of brutalised troughs is to be found around Akureyri in northern Iceland, as featured on the front cover of our text book:

Such troughs are created (maybe over the course of many glaciations)  where there is a very heavy discharge of ice from one source area, with very few supplementary discharges coming in from the flanks -- leading to a "brutal" truncation of many of the interfluves and subsidiary valleys that might have existed at some earlier stage.

Atlas contributions

It was a privilege to be invited by the learned academic panel responsible for this volume to write and illustrate the first part of the Atlas -- Part One: The Physical Setting, on pages 20-26.   It's good that some stalwart members of the academic establishment recognize my credentials and my ability to contribute something worthwhile -- even if certain archaeologists seem to think that I don't actually exist, except in nightmares.  So there we are then, as they say in these parts.

My four sections are entitled Geology and Structure, Landscape and Natural Regions, The Ice Age, and Superficial Deposits and Habitats.  With each map (or set of maps) I provided, explanatory text, notes and references.  Here are the maps.  Click to enlarge.

 

The Pembrokeshire Historical Atlas

This impressive historical atlas of Pembrokeshire has just been published as the culmination of the 5-volume official county history.  Edited by Prof DavidHowell, it contains 82 "topics" with maps and other illustrations on odd pages and explanatory texts on the facing (even) pages.  It's a large format book (28 cms x 28 cms) and runs to 205 pp.  References and notes are all gathered together at the end of the book.  The cover price is £30, and it's available via the Pembrokeshire County History Trust.

I was asked to provide the maps and text for Part Two of the book, on the physical setting.  More of that anon.

Topic 12, on Bronze Age Pembrokeshire (2500 - 800 BC) is by Tim Darvill.  There is a brief mention of Stonehenge and the "quarries" of Carn Menyn, Carn Goedog and Rhosyfelin -- and it's interesting that he considers them to be Bronze Age features rather then Neolithic ones.  Not at all sure how he squares that with the radiocarbon and other evidence from Stonehenge.  He also drops into the mix the idea that eastern Preseli was a significant or special area across several millennia -- and shows a number of features on Map 12C purporting to be Bronze Age "ceremonial centres".   Sadly, he does not tell us what a ceremonial centre is or was, and the precise locations are not named.  Does a supposed Neolithic quarry count as a "ceremonial centre"? Special pleading and ruling hypotheses come to mind.........

Darvill speculates as to why "more than 80 blocks of stone were carried from Mynydd Preseli to Salisbury Plain".   There is nothing new here -- and in the grand tradition of senior archaeologists thinking, talking and writing about bluestones, he fails to mention that there is a dispute going on,  fails to cite inconvenient research findings, and fails to give any mention at all of the glacial transport hypothesis.

It's a pity that a splendid volume like this should be marred by slapdash academic writing on some of its pages.

  

The jolly Stonehenge Quiz ---- what's the "right" answer?

Somebody complained to me the other day (why me, for goodness sake?) about the Stonehenge quiz published in the EH members magazine in October last.  They said, quite rightly, that the "correct" answer to the question about the bluestones was misleading, to say the least.

https://www.english-heritage.org.uk/members-area/members-magazine/october-2018-20-questions-quiz/

What do you think?

Neither the answer nor the added explanation should go unchallenged.

Not all of the bluestones are from the Preseli Hills -- but most of them almost certainly are.

The supporting wording is carefully chosen, but it is far from adequate.  It is thought by whom?  And ".... to bring the bluestones to Stonehenge"??  That assumes the correctness of the human transport theory.  EH should be honest enough to accept that there is a dispute going on.  The explanation should be this:

The mode of transport is disputed.  Some archaeologists believe that the stones were carried across land and sea by Neolithic tribes, but some earth scientists are convinced that the stones were moved to Salisbury Plain by glacier ice.  

To repeat -- it's really rather weird that EH seems to be incapable of using the word "dispute" -- maybe in the belief that an academic dispute would cause the public to become confused, or dilute the romance of the ancient stones.   Less emphasis on narrative and storytelling, and more on scientific accuracy, if you please......

Can cold-based glaciers pick up erratic blocks?

Wright Lower Glacier, Antarctica -- one of the coldest glaciers on earth.

From Bethan Davies’s Antarctic Glaciers web site:

http://www.antarcticglaciers.org/glacier-processes/glacier-flow-2/glacial-processes/

Cold-based glacial processes

This section is mostly from Hambrey and Fitzsimons 2010.

Despite a long history of papers arguing that cold glaciers do not erode or deposit glacial sediments, this paradigm is now being challenged, with a growing number of papers describing processes of debris entrainment, transportation and deposition at the margins of cold-based glaciers, where the ice at the ice-bed interface is not at pressure melting point [16, 24, 35]. Numerical ice sheet models have in the past assumed no movement where the glacier is cold-based [24], with geologists assuming little debris entrainment or movement, preserving delicate landforms and preglacial land surfaces [36].

However, there are a few studies challenging these views. The Dry Valleys, Antarctica, are in Southern Victoria Land near McMurdo station. They are the largest ice-free region in Antarctica [17], and are thought to be the closest place on Earth to Mars. In this polar desert, rainfall is unknown, and there is only 10 mm snow fall (water equivalent) per year. Mean annual air temperature is around -19.8°C, and the majority of the local glaciers are cold throughout [17]. These glaciers have basal temperatures of around -17°C [17, 37], and no free running water[38].

Wright Lower Glacier has a 3.5 km broad tongue that terminates as a degraded ice front in the frozen Lake Brownworth, which has an ice thickness of 9 m [17]. Next to the glacier is a sediment apron and there is a braid plain around the lake. The northern margin of the glacier has a 5-10 m high ice cliff, from which large blocks fall (dry calving )[17]. There are moraines within and beyond the lake, which have a similar plan view as the dry margin of Wright Lower Glacier.

The ice margin comprises pinnacles and gullies, with windblown sand-covered and clean ice parts melting at different rates. The ice margin is not very well defined and merges with the lake ice via debris-covered, ice-cored moraines parallel to the ice front with intervening ponds.

Hambrey and Fitzsimons (2010) found that the ice-contact debris apron was mostly made up of sand, and extended for several hundred meters towards the lake. It is dissected by several gullies, cut into the unconsolidated sand by streams (melting from the glacier surface is encouraged by the accumulation of dark wind-blown sand, which absorbs solar radiation) [17].

Hambrey and Fitzsimons (2010) argued that debris was entrained in Lower Wright Glacier by two mechanisms:
• Supraglacially, from windblown sand;
• Subglacially, where the basal ice layer is several meters thick.
The ice-proximal debris apron is similar to modern fluvial systems, with inclined bedding related to uplift of the region following rebound of the earth’s crust following the removal of glacier mass (isostatic uplift). All these glacigenic sediments have been reworked by flowing water and wind [17]. The debris apron has also been modified by glaciotectonic deformation. Deformation structures include angular bubbly ice blocks, boudin and thrust blocks in the northern margin of the debris apron. This range of structures indicates a heterogeneous deformation regime within the basal ice of Wright Lower Glacier in the Dry Valleys of Antarctica. Strain rates measured within the basal debris-laden ice indicate that simple shear is occurring, resulting in foliation and boudin formation. The clean and debris-rich ice is has brittle failure, resulting in landforms similar to thrust-block moraines.

The work of these author s[17] and others [16] indicates that cold-based glaciers can generate landforms, and erode, transport and deposit sediment. Bedrock erosion occurs through fracture and abrasion [16] as well as deposition. However, in comparison to glaciers in warmer climates, there is less abrasion at the ice-bed interface, resulting in coarser sediments and less clays and silts being produced. Sand is the dominant product [17]. Pre-existing sediments have been reworked without much modification. The lack of free-flowing water has resulted in this lack of modification.

In summary, the glaciers in the Dry Valleys of Antarctica represent the end-members in the glacier thermal spectrum, being the coldest glaciers on earth. However, these glaciers are capable of erosion and deposition. Debris entrainment encompasses the detachment of frozen blocks of sediment from the subglacial substrate, which is then folded and thrusted [17]. The geomorphological features that are created include sedimentary ridges and aprons with glaciotectonised sand and glacier ice, draped with a veneer of wind-blown sand. Supraglacial streams, which melted following increased albedos as a result of accumulations of wind-blown sand on the glacier surface, rework proglacial sediments, including the debris apron. All the glacial sediments bear little resemblance to their counterparts from warmer climates, and the preservation potential of these sediments is high [17].

Three more photos of Wright Lower Glacier

Abrasion beneath a cold-based glacier

This is an abraded rock surface overridden by the cold-based Fountain Glacier on Bylot Island.

https://www.swisseduc.ch/glaciers/bylot

It's argued that the rough surface and rather chaotic pattern of striae is typical of cold-based or polar glaciers -- and that the beautiful smoothed surfaces with regular and parallel striae are more typical of wet-based or warm-based glaciers.  I'm not sure about that.  Time is one factor, rock type is another, and sediment availability (and the occurrence of suitable "tools") is yet another.  I remain to be convinced......

Smooth striated surface, Rödlöga Storskär, Stockholm Archipelago. Signs of overriding 

wet-based ice?

Smoothed and striated gneiss bedrock on the west coast of Lewis, western Scotland.

The assumption is that beneath a cold-based glacier (such as those found in high latitudes) water cannot pass down to the bed from the surface, except in the immediate vicinity of the snout.  So there may be an oscillating wet-based zone near the snout, moving forward and back as the snout advances or retreats, and maybe expanding and contracting seasonally as well.  But the rest of the glacier -- including the accumulation area and the central "transport" section, should be cold-based and dry, with the ice either frozen to its bed or moving without any lubrication.  

So the difference in rock surface effects should in theory be similar to the difference between dry sandpaper abrasion and wet sandpaper abrasion.  In the latter case, as basal ice melts and lubricates the bed, not only is basal ice movement faster, but there is a continuous supply of abrasive materials as the glacier is gradually let down onto its bed.

The precise mechanics are of course seldom actually observed.......

Ice marginal channels

This is a fabulous image of  a marginal channel cut by meltwater near the edge of Fountain Glacier on Bylot Island.  When it was abandoned, the marginal stream started to cut another channel some distance downslope.  Probably this channel was used seasonally for a number of decades before being abandoned.......

https://www.swisseduc.ch/glaciers/bylot

Another abandoned ice marginal channel, this time cut in bedrock (to the right) and in glacial and glaciofluvial sediments (to the left)

The modern features shown above are similar in scale to a number of small channels on 

the flank and summit of the Mynydd Preseli upland ridge.

Bylot Island photo gallery

Mountain avens (Dryas octopetala)

Purple saxifrage

Fast sea ice in Eclipse Sound.  The greenish-blue areas are interconnected ponds of surface meltwater

Fast se ice breaking up.  The cracks are obvious -- and slabs are starting to drift apart, with patches of open water shown by the dark blue colour

A meltwater "fountain" on Fountain Glacier -- hence the name.  Here the meltwater is not being forced out of an englacial tunnel under hydrostatic pressure;  the fountain effect is created when surface meltwater flowing downslope on the glacier suddenly hits an obstacle and is forced into the air.


Abandoned meltwater tunnel near the snout of Stagnation Glacier.

Debris-stained rainwater which fell higher up the glacier and then found its way to the glacier bed, picking up sediment before issuing again at the snout.  Fountain Glacier.

A large erratic block of gneiss, stranded in the middle of Fountain Glacier.  Probably it has come down onto the glacier surface as part of a rockfall.

Courtesy the Glaciers Online web site.
https://www.swisseduc.ch/glaciers/bylot

Candle ice -- Fountain Glacier, Bylot Island

Candle ice crystals exposed in the "aufeis" area beyond the snout o Fountain Glacier on Bylot Island, far up in the Arctic off the NW coast of Greenland.  This is a real permafrost area where summer melting is very limited -- and ice crystals of this type seem to be restricted to this sort of environment.  The ice crystals form vertically, and may be just 2 cm across and up to a metre in length.

They seem to form where meltwater issuing from the glacier is unable to flow very far from the snout because this is a zone of continuous permafrost where temperatures seldom rise above freezing even in the middle of summer when the sun is shining.

From the superb Glaciers Online web site:

https://www.swisseduc.ch/glaciers/bylot/eclipse_sound/index-en.html

Larsen Ice Shelf

What a fabulous NASA image this is!  I shows the Larsen Ice Shelf (bottom left) and its junction with piedmont land ice beyond the mountain front.  The plateau edge is dissected, with groups of isolated peaks beyond.  Then on the plateau itself, in the interior of the Antarctic Peninsula, there are connected plateau ice caps.  These are linked through to the West Antarctic Ice Sheet at the base of the peninsula.

This image was taken in 2004.  Since 2000, there have been major concerns about the break-up of this particular ice shelf, with some huge calving episodes making global headlines -- all a consequence of global warming. The most dramatic event was the collapse of Larsen B, in the space of around a month in the years 2002.

PS
See this animation to appreciate how the Larsen Ice Shelf has been breaking up:

https://giphy.com/gifs/esa-rG7lb1rCQFwNG

What happened in 2002:

https://earthobservatory.nasa.gov/world-of-change/LarsenB

This is what happens when ice shelves collapse.  These are two images from the collapse of the Wilkins Ice Shelf (on the other side of the peninsula) in 2008.  The process starts with major fissures opening up, and then the breakup of vast slabs into smaller slabs, which in some cases tip over and in other cases just disintegrate into a chaos of small broken fragments covering the sea surface -- until dispersed by tides and currents:

Glacier entrainment

This is a very intriguing image from Nick Cobbing of the ice sheet edge somewhere in West Greenland.  You don't often see ice edges as clearly defined and as clean as this -- with an accumulated litter of morainic boulders that have simply dropped out of the ice front.  The stratification (probably old summer ablation surfaces) is unusually regular and tidy, with hardly any glacitectonic deformation.  And scattered through these layers of ice there are stones and boulders -- if these were in seafloor varved deposits they would be called dropstones.  Where did they come from?  They appear not to have been entrained in the usual way, by plucking of bedrock outcrops on the glacier bed, since there are no shear-planes or other structures.

Or have I got this all wrong? Are we looking at basal accretion of one regelation ice layer after another, with boulders and stones picked up in that way by freezing-on at the bed?  In other words, is this part of the ice front made up of ice layers that have accumulated from below, thickening the glacier from the bottom up?  That does happen, but I have never seen evidence of it on this scale before.

For the last 60 years or more, glaciologists have described and theorized on regelation ice layers, but generally it has been assumed that such layers accumulate beneath warm-based glaciers, with melting episodes and freezing-on episodes alternating, presumably on a seasonal or annual basis.  Here on Bylot Island the glaciers have to be polar or cold-based, making the matter more intriguing.

Maybe a glaciologist will give us a comment.....

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

PS. Just noticed that before today, I have published 2500 posts on this blog.  I hadn't realised that I have been working so hard or spent so much time thinking aloud.....

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Here are some more images from the Glaciers Online web site, all featuring basal ice features at the edge of Fountain Glacier on Bylot Island.  This is a cold-based "polar glacier":

https://www.swisseduc.ch/glaciers/bylot

These are all images of stratified or  layered basal ice with associated glacitectonic structures and enclosures of cobbles and boulders.  The exception is the lowest part of the bottom photo, which shows stratified basal ice overlying a bed of "meltout till".