THE BOOK
Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Saturday, 25 May 2019

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....... 

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.

 







Thursday, 23 May 2019

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......

Wednesday, 22 May 2019

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







Sunday, 19 May 2019

Abrasion beneath a cold-based glacier


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


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.......



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.