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Sunday, 10 July 2011

More on glacial erosion by ice sheets

I have just made a new visit to the outer islands of the Stockholm Archipelago in Sweden, where many detailed observations were made more than 40 years ago prior to the publication of the text called Glaciers and Landscape.  Once again, I have been mightily impressed by the vast range of glacial erosional features in an area that lay right at the heart of the Scandinavian ice sheet every time it expanded and moved southwards towards Denmark, Germany, Poland and the other southern Baltic lands.


The archipelago is basically an area of ancient shield rocks including granites and gneisses and metamorphosed rocks that were originally shales, sandstones, greywackes, lavas and ashes -- over 3,000 milliion years ago.  The complexity of these rocks is striking, and many of them have had such complicated histories of deformation and alteration that they are difficult to classify.  At any rate, they are extremely beautiful, with colours ranging from grey and blue to green, black and brown, with some rocks bright red or in shades of pink.  Some rocks, predominantly made of quartz fragments, are white or buff-coloured.  There has been such a long history of erosion here that the landscape is essentially one of gently undulating plateau surfaces not much above present sea-level, with deep "tunnel valleys" incised into these surfaces and coinciding for the most part with faults or other lines of weakness.

There is an incredible micro-morphology of glacial erosional features which can be seen around the coasts on thousands of small islands to the north and south of Stockholm.  Most of these features date from the Devensian glacial episode, and they are remarkably fresh.  They are easy to examine because the whole of this area was deeply depressed isostatically by the weight of overlying glacier ice, and was submerged to a depth of about 150m following ice wastage.  Since that time (about 9,000 years ago) the land has been rising as a result of isostatic recovery, which has gradually slowed to the current rate of about 5 mm per year.  The islands are still "growing" -- at a rate which humans can appreciate over the span of a single lifetime.  On the higher parts of the islands vegetation has had time to establish itself and to create thin soils, but in many coastal areas virtually all soft sediments from till, lake and sea floor deposits have been washed away during the process of emergence from the sea, leaving behind clean rock surfaces with occasional erratic boulders and raised beaches of cobbles and rounded boulders.  The ice here has moved from the north towards the south, with minor deviations which can usually be attributed to the "control" exerted by the shape of the rock surface.  On the clean rock surfaces one finds wonderful examples of virtually every kind of glacial erosional feature:  roches moutonnees (large and small), striations and gouged grooves, chatter-marks, crescentic cracks and gouges with the ends sometimes pointing up-glacier and sometimes down-glacier, and sinuous P-forms which appear to have been caused by water at the base of the glacier or by heavily saturated sediments moving under pressure like toothpaste being squeezed out of a tube.

Below are some of my recent photos, which can be looked at in conjunction with my posts on glacial erosion and entrainment. The photos are all from Rodloga Storskar, in the outer part of the Stockholm Archipelago.

Chatter-marks on a smoothed and abraded pinkish granite rock surface.  These transverse fractures are almost at right angles to the direction of striations, and are probably caused by massive forces on the glacier bed, at a time of high basal friction when traction forces were applied to the rock surface -- literally causing it to tear.  This is a early phase in rock breakup.  The ice has moved from bottom R to top L.
Striations, deep parallel gouges, tensional cracks and striations, all on a rock surface that has been moulded and polished.  These features probably relate to several different ice movement directions -- most probably all during the Devensian.
Here the ice has moved from top L towards bottom R --notice the fine polishing and abrasion on the smooth rock surface.  Notice too that there is something that looks like a graze or a flesh wound in the centre of the photo.  The colour is slightly more pinkish than on the smooth area.  Here a process of tearing and gouging has just commenced -- look at the fractures and roughened surface.  This process has also affected the area towards the top R of the photo.  This is the first phase in the formation of a roche moutonnee, with different processes affecting the up-glacier and down-glacier faces.  Pressure differences have something to do with it, but what we see here is clearly not the result of a single large tool being pressed on to the glacier bed -- if tools have been involved, there must have been many of them........

22 comments:

Constantinos Ragazas said...

Welcome back Brian! How was your annual trip to Sweden? We missed you. Had only Robert Langdon around to argue a bit!

Very interesting photo-souvenirs from Sweden you brought back! I am especially intrigued by the one of a granite mount with a small round pool of water at the very top and a water drain channel down along the side. This reminds me of the 'dimpled depressions' I saw on many of the round mounts and barrows around Stonehenge.

Robert argues that these 'dimpled depressions' are evidence of Mesolithic lighthouses built on top of these 'man-made' round mounts to guide boat traffic at night through his Mesolithic waterways.

What do you think? How did these round depressions on the top of these mounts form?

Kostas

BRIAN JOHN said...

Hi Kostas

Still here -- and experiencing some Email problems. But I'm occasionally able to get at a broadband link that works! Sorry to disappoint you, but the "dimpled depression" with water in it has no significance whatsoever -- over thousands of square kilometres the rock surface is moulded and undulating, and wherever there is a little depression rainwater collects in it. Some of these pools are temporary, and some are more or less permanent. All shapes and sizes, and in many different positions.....

Constantinos Ragazas said...

Brian,

Thanks for the explanation. Is that the same process then that explains the “dimpled depressions” on otherwise flat tops of the round mounts at Salisbury Plain?

I even saw one such round mount on top of a hill by the sea coast south of Stonehenge. Though relatively small, it even had the very characteristic circular ditch in segments which did not completely connect.

What is your explanation for all these geomorphological features?

Kostas

BRIAN JOHN said...

Kostas -- round mounts and dimpled depressions? From where I'm sitting (inn the cottage in Sweden!) I don't recognize any round mounts -- Salisbury plain is an undulating chalk downland with a few ridges and broad rises and a few lower areas coinciding with broad river valleys -- some dry and some carrying permanent streams. Because this is chalk, there are many solution hollows in this surface, but generally they occur in the lower areas where water can collect and where solution can be concentrated. The other things you describe sound to me like prehistoric earthworks of various sorts -- with man rather than natural processes responsible. if you want to see some real dimpling on a hill summit take a look (on Google Earth)at Foel Drigarn at the eastern end of the Preseli Hills -- the summit is covered with circular pits that look like small bomb craters but which are actually the remnants of Iron Age hut circles.

Constantinos Ragazas said...

Brian,

Thanks for the reference. Tried a fly-by of Foel Drigarn with Google Earth but could not locate the site. I did get some poor pictures of the site, however, using google search.

Interesting! But these are not the features I saw and tried to describe to you. What I saw in these poor pictures of Foel Drigarn are three small round mounts in a straight line with many 'pot holes' marking them all around.

I wish I had taken photos of the round mounts I saw while visiting Stonehenge last summer, but I didn't! But if you look at Silbury Hill you will see similar such features:

1) Flat round top with a depressed centered (not quite a 'dimple' in the case of Silbury Hill)

2) Circular segmented ditch at the perimeter of the distinct top portion of the Hill

3) Straight 'avenue' along the side of the Hill going from the top all the way to the bottom.

These features Brian are very different from Foel Drigarn but are more similar to the picture from Sweden in your post.

Enjoy the rest of your rest at Sweden!

Kostas

BRIAN JOHN said...

Thanks for the good wishes, Kostas.

Silbury Hill is one of the classic ancient man-made features -- or are you suggesting that it too is a natural phenomenon, created by some process unknown to science?

Constantinos Ragazas said...

Brian,

I am not suggesting anything! But I am questioning EVERYTHING!

If these features that I listed in my previous post are man-made, what are the reasons for them? Certainly an “avenue” that runs straight up a steep hill does not make sense! A more winding spiraling pathway to the top is more 'humane'!

Kostas

Anonymous said...

Dear Constantinos
My patron saint is St Constantine XI Palaeologos Porphyrogenitus.
The day the world stopped.
Sadly the dimple on the top of Salisbury Hill is due to the collapse of 19th 20th cent. excvations in the core of the mound. Nothing natural at all totally anthropogenic
GCU In two minds aka iconoclast.

BRIAN JOHN said...

Agree with Anon on this -- the history of excavations an Silbury Hill is well known and well recorded in a copious literature. Kostas, I don't want to spend any more time on Silbury Hill, discussing whether it is man made or not. It is a man-made feature -- the evidence for that is overwhelming.

My previous mentions of Silbury Hill have concentrated on the occurrence, or otherwise, of bluestone fragments in the material incorporated into the hill mass, or scattered on the surface. There was the old rumour that Atkinson had found bluestone fragments there, but had failed to report the finds since they did not fit with his preferred chronology. Still not sure what the truth of the matter is on that score.....

BRIAN JOHN said...

Agree with Anon on this -- the history of excavations an Silbury Hill is well known and well recorded in a copious literature. Kostas, I don't want to spend any more time on Silbury Hill, discussing whether it is man made or not. It is a man-made feature -- the evidence for that is overwhelming.

My previous mentions of Silbury Hill have concentrated on the occurrence, or otherwise, of bluestone fragments in the material incorporated into the hill mass, or scattered on the surface. There was the old rumour that Atkinson had found bluestone fragments there, but had failed to report the finds since they did not fit with his preferred chronology. Still not sure what the truth of the matter is on that score.....

Constantinos Ragazas said...

Thanks Brian, Anon for your comments …

I wont belabor the point, but I do remain skeptical as many things about these earthworks features just don't make sense to me.

Very interesting Brian that it was rumored Atckinson found “bluestone fragments in the material incorporated into the hill mass, or scattered on the surface”. What was the source and history of that rumor? Any further investigation made on this? Why or why not?

Can you maybe post on that subject? It may be very relevant to our theory!

Kostas

BRIAN JOHN said...

Kostas -- it's all on the site already. Just type "Silbury Hill" into the search box and all will be revealed.....

I wasted a lot of time discussing foreign fragments at Silbury Hill -- arising from a copy of a copy of a copy of an ancient typescript that I got from somewhere, which had a jumble of aggregated data on it. I got that seriously wrong, and had to apologize to my long-suffering fellow bloggers for the mistake!

Constantinos Ragazas said...

Brian,

Used the 'search feature' in your blog to find posts and comments on bluestone fragments. Two points to make.

Your blog site is fast becoming a suppository of much information and a great public forum on Stonehenge. It is truly impressive how much there is!

Second, this confusion and controversy about bluestone fragments between Stonehenge and South Wales can be easily resolved by some deep bore holes extracted at places like Silsbury Hill and other round mounts.

But my sense is that more of such bluestone fragments will be found along the sea coast and not so many scattered inland!

All this of course is critically significant to your glacier transport theory. Could this then be the reason why no field research is done to resolve this question?

Kostas

BRIAN JOHN said...

Thanks for your kind remarks, Kostas. there have been almost 40,000 hits on the site now -- which means, I suppose, that there are quite a few people out there who are actually THINKING about Stonehenge instead of simply accepting the standard story as publicised by EH and assorted academics.

Regarding boreholes, there are of course many borehole records published, as well as many detailed records of digs. I'm not sure that these records will show up bluestone fragments -- I wouldn't put any money on it. What they are much more likely to show (in places like the Somerset Levels) is a sequence of Quaternary deposits that may -- in some places -- include glacial deposits like till and fluvioglacial sands and gravels. These won't be preferentially arranged round the coastline -- they could occur anywhere within the limits of the glaciated area.

Constantinos Ragazas said...

Brian,

Coincidentally I was reading an article in Wikipedia on Silbury Hill and also the latest post in Robert Langdon's blog about Mesolithic clearing of forests for the development of agriculture. Two interesting facts that have emerged from these:

1) Robert quotes a University of Manchester study which estimates the Mesolithic UK population to be 2,500 people.

2) The Wikipedia article quotes Prof. Atckinson's estimates that it took 500 men some 15 years to build Silbury Hill.

A population of some 2,500 people would have not many more able bodied men than 500. Are we to believe the entire able bodied population of all UK spend some 15 years just building Silbury Hill? And what about all the other (thousands) of prehistoric monuments and earthworks? Didn't prehistoric people have to make a living?

It doesn't make sense to me Brian! The population size and man-hours needed to built all these monuments just does not add up!

But I wont go any further, since I know how you feel about this …

Kostas

BRIAN JOHN said...

we are a bit off-topic here! This is supposed to be about ice sheets and glacial erosion.

As you know, I don't accept Robert's dating of Stonehenge to the Mesolithic. It's a standard technique in pseudo-science to take a "fact" that may or not be agreed by the academic community, to demonstrate that the "fact" is wrong, to go on to say that the established academic view is therefore up the creek, and to then put in your own crazy theory instead. You then flag up your theory as the greatest scientific breakthrough of the millennium. Hum hum. Fine fun and games, but not science.

Constantinos Ragazas said...

Brian,

I am not interested in pseudoscience! I don't believe in pseudoscience. Nothing that I have considered is pseudoscience. So why you accuse me of pseudoscience?

I am not Robert! Everything I have proposed is 'falsifiable'. I have even suggested experiments to 'falsify' my own claims.

Brian, intellectual courage and integrity demand that we question EVERYTHING!

Please post my comment on C14 dating. I have serious questions. Perhaps your readers may provide helpful answers.

Kostas

BRIAN JOHN said...

Take it easy, Kostas! My comment wasn't directed at you at all -- as you will see if you read it carefully.....

Constantinos Ragazas said...

Brian,

In fairness to Robert, he does provide the reference to the University of Manchester study that shows the Mesolithic population of the UK was 2,500 people. [The Mesolithic mammal funa of Great Britain, S Maroo and DW Yalden, 2000 Mammal Society, Mammal Review 30,243-248]

Kostas

BRIAN JOHN said...

I don't have any particular axe to grind on this one -- so I'm not bothered how big the population was. but the Mesolithic lasted a very long time -- 5,000 years or more, depending on where one draws the boundaries. the population at the beginning of the period will have been very much smaller than the population at the end. And I'm not sure how wise it is to depend upon an article which is 11 years old for one's definitive statement on this.... I have sen other figures suggesting that towards the end of the Mesolithic the UK population was around 100,000.

Constantinos Ragazas said...

Your point is well taken, Brian.

There is a larger picture here, however, that should be considered: the relationship between the size of population and the size of public works.

If we consider the number of man-hours required to do all the thousands of prehistoric 'public projects' and the population size that the land could have supported during the Mesolithic/Neolithic, we would have a quantifiable approach to this age old question of who built these sites.

I don't know the answer to this question, Brian. But I think it is worth knowing. And if the math does not add up, then perhaps we will be more open minded to consider other explanations consistent with the scientific data that is now accumulating.

Kostas

BRIAN JOHN said...

This is a perfectly valid question. As you know, I have always argued that Stonehenge was never finished, and that the builders (over successive generations) could never quite decide what they wanted or how to complete the work. A case of high aspirations but limited resources. It's all in my book and in the YouTube video called "Stonehenge Unhinged." It would fit this scenario perfectly well to say that they ran out of stones in the local area and had inadequate resources to travel a long way to fetch them. It would also be perfectly reasonable to suggest that the tribal groups on Salisbury Plain simply did not have enough people to complete the work......