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Wednesday, 5 January 2011

Last Glaciation Protective Snow or Ice Cover in Southern England

There is a lot in the literature about the manner in which certain areas beyond the edges of active glaciers or ice sheets can support thin, cold-based and largely static ice which does very little to the landscape apart from providing a "protective blanket" over a landscape previously affected by permafrost.

I came across this example from Scoresby Land - Jameson land in East Greenland, where there are a number of "landform" and depositional puzzles which have caused confusion among geomorphologists.  Lena Hakansson has tried to date erratic boulders and rock surfaces (using cosmogenic dating methods) and found that the distribution of ages could only be adequately explained by postulating intermittent exposure to radiation, with snow or ice cover shutting off this exposure at certain key times -- in particular coinciding with the peak of the last glacial episode.

So she has postulated the above scenario, with active glacier ice streaming along the broad fjord of Scoresbysund (right hand edge of the diagram) and with a more or less stagnant cover of snow and ice blanketing the landscape to the east.  This blanket, with ice up to 200m thick in places, is shown by the light blue area in the cross-profile, with frozen ground beneath (dark blue).  The interesting thing is that this blanket has capped or sealed older glacial and periglacial deposits, as well as scattered erratics, effectively preserving them from further erosion or disturbance for the duration of the snow / ice blanket.

This sort of scenario seems to fit southern England pretty well, for a period of 5,000 years or more near the peak of the Devensian glaciation.  I still think it's quite possible that the uplands of Dartmoor, Bodmin Moor and Exmoor might have supported thin covers of firn or glacier ice which coalesced with the Irish Sea Ice coming onto the coasts of Somerset, Devon and Cornwall from the north and west -- but further south and east what did the landscape actually look like?  At times, parts of the ground surface must have been snow free, allowing the formation of periglacial features and permitting rapid solifluxion on steep slopes, but  there must also have been episodes when the WHOLE landscape must have been snow-covered even during the summer months -- but with inadequate precipitation for the development of glaciers.  Snow and firn maybe -- but probably no glacier ice.

When I was in Antarctica I worked on a snow-covered (and ice free) peninsula called Byers Peninsula, on Livingston Island.  Not far away was the edge of the ice cap.  Only at the height of the summer season (for maybe a week or two) does the snow cover melt back sufficiently to see what's underneath it.  When we were there, early in the summer season, we plodded about in the snow and missed all sorts of details (and quite a complex landscape) which were somewhere down there, beneath our feet.......

8 comments:

Anonymous said...

Brian,

Your first post in the new year brings us even closer together! I am very pleased that you are coming around to my view that Salisbury Plain had a 'local ice' sheet cover possibly around the same time when Stonehenge was made. This helps explain the unexplainable!!!!

Constantinos Ragazas

Robert Langdon said...

Happy New Year Brian!

200m of permafrost Ice over Stonehenge is you 'worst' case scenario at the end of the Devonsian?

If so, thats equivalent to a thousand times the rainfall that fell on Australia and would the chalk base be last to thaw?

Do you still insist that it all fell into the sea?

Thank you for all your research - you have been an excellent source of information!

Robert Langdon

BRIAN JOHN said...

No no, Kostas -- there is a vast gulf between us! I am talking about conditions in Southern England at the peak of the Devensian cold phase, when the Irish Sea Glacier was expanding and then retreating. The time frame is between 26,000 yrs BP and 14,000 yrs BP. That's a total of 12,000 years -- generally very cold indeed, with assorted oscillations still imperfectly understood -- there were some ice advances and retreats within that time frame. This was the Palaeolithic -- very few human beings in the UK, with few signs of tribal organization. Even during the Mesolithic, after the retreat of the ice, there is no evidence of interest on the part of tribal groups in the moving or erection of standing stones.

All of the archaeological evidence -- which I have no reason to doubt -- puts the construction of Stonehenge into the period after 6,000 yrs BP.

So your "local ice sheet" is at quite the wrong time. As I have said before, there were cold episodes during the Older Dryas and Younger Dryas, ending about 10,500 yrs BP. Permafrost was certainly present at these times, and indeed there may have been an extensive seasonal snow cover in the Younger Dryas over Salisbury Plain, but we are still thousands of years adrift from the stone-using episides at Stonehenge.

BRIAN JOHN said...

Hi Robert -- Happy New Year! Permafrost ice is in the ground, not above it. Snow, firn and ice can accumulate on the ground surface if there is sufficient precipitation in the form of snow and if accumulation rates exceed ablation rates. I doubt that there was ever a blanket 200m thick in this area -- but maybe some reasonable thicknesses (20 30m?) in some of the deeper valleys, with much thinner cover over the ridges and interfluves.

In this sort of environment, when summer melting occurs, the meltwater finds its way down-river and eventually to the sea. But in very arid conditions a large amount of moisture can be sublimated or simply absorbed back into the atmosphere with no passage through a "liquid water" phase. I doubt that it was ever THAT arid on Salisbury Plain -- and indeed all of the signs of solifluxion seem to point to the presence of a mobile active layer with quite a bit of moisture. But not enough for total landscape inundation of the type you are proposing!

Constantinos Ragazas said...

Brian,

We agree that Salisbury Plain had a 'local ice' cover (possibly as thin as 30m, to satisfy Robert) as recently as 14,000 BP. Some articles I read even claim that this area was experiencing sever freezing temperatures as recently as 12,000 BP. There is no reason to believe that there was no ice cover during this period, if there was 'local ice' cover a bit earlier with freezing conditions.

In answering Robert you write,

“... all of the signs of solifluxion seem to point to the presence of a mobile active layer with quite a bit of moisture. But not enough for total landscape inundation of the type you are proposing!”

Such presence of solifluxion suggests the effects of a 'local ice sheet' melting gradually and more prominently along its edges, where the ice joints the hillsides. That such solifluxion is found, I believe you said, mainly on slopes seems to confirm this view of a slow melt-away of a 'local ice cover' of Salisbury Plain.

So where is this “...vast gulf between us!” ?

You write,

“All of the archaeological evidence -- which I have no reason to doubt -- puts the construction of Stonehenge into the period after 6,000 yrs BP. “

My understanding is that these dates are based on radioactive carbon dating of organic material found all over Salisbury Plain. And as you said in an earlier post last year (I have a very good memory when it comes to what interests me!) all these carbon dates are remarkably consistent to the same time period. If there are no organic materiel found in Salisbury Plain that date earlier than these dates, doesn't it raise the question that there were no organic material growing in Salisbury Plain earlier? And why would that be? Perhaps because Salisbury Plain was under the cover of ice prior to these dates!!!!

The date 6,000 BP would be only relevant if we assumed that the stones of Stonehenge were transported and erected from the 'ground up' by men. And that is what drives the current interpretation of all the scientific evidence and 'facts on the ground'. But if we consider that the stones of Stonehenge (and of all other such sites) could have been transported there along the surface of a 'local ice cover' by men and nature, and were 'dropped from above' rather than 'raised from the ground', than the date 6,000 BP is of no consequence. Such construction could have been done by men earlier than 6,000BP and with little social organization and no significant technical skills.

Look at the 'facts on the ground', Brian, and use some good old common sense. The evidence is already there at Stonehenge and at all other such sites to prove my 'smart ice theory' as I describe in my paper “The un-Henging of Stonehenge”. But the resistance to disbelieve is too strong for most people to believe what Stonehenge is telling us.

Constantinos Ragazas

BRIAN JOHN said...

Kostas, you are putting words into my mouth here. I do NOT agree that there was a local ice cover c 30m thick as recently as 14,000 BP. I said there might have been a patchy snow cover and that the snow banks might have been up to 30m thick in the deeper coombes and maybe on shady parts of scarp faces. I specifically said I do not think there would have been glacier ice present.....

The radiocarbon dates that are associated with the stone settings are pretty consistent -- suggesting work on these settings around 5,000 - 4,000 BP. There ARE earlier radiocarbon dates from the Stonehenge area -- associated with Mesolithic post holes. The earliest date I have seen is around 9,000 BP. I'm confident that many other C14 dates will be obtained in the future for the Late Glacial - Early Holocene period -- the problem is that the best places for getting organic sediments and C14 dates are peat bogs -- and there aren't many of those on pr near Salisbury Plain!

Solifluxion on steep slopes has nothing to do with the melting away of a local ice cover -- it does have something to do with the melting of permafrost in the soil and deeper layers of the chalk.

Robert Langdon said...

I must be going blind as the diagram to me shows the light blue above the dark blue frozen land. The dark blue blanket is 200m (according to Lena), but you estimate 30m could you explain the discrepancy?

RJL

BRIAN JOHN said...

You read her diagram much too literally -- -- it's just a diagrammatic representation of what she thinks was the scenario. Also the scale of Jameson Land is much greater --her transect is over a distance of 200 km or more, with much greater terrain differences too.

I don't understand why you should have a problem with my suggestion of snow banks or firm up to 30m thick in the deeper coombes of Salisbury Plain. Just common sense.