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Monday, 23 January 2012

Ice over All

Some members of our faithful blogging community have difficulties with the fact that ice caps and ice sheets develop ice surface profiles that are sometimes quite independent of the shape of the ground underneath.  So we effectively have a submerged landscape beneath a largely featureless ice expanse such as we see today in the centre of Greenland.

This is beautifully illustrated in the latest reconstructions of the Devensian Welsh Ice Cap by Hubbard et al (2011).  Above there are two cross profiles -- the top one for North Wales and the lower one for Mid Wales.  Note that there were a few small nunataks in North Wales (including Snowdon and Glyder Fawr) and that the ice surface was at about 1000m  In Mid Wales the whole of the landscape was submerged, and the summit of the ice dome was at about 1200m.

In general, ice velocities are greatest where the ice is thickest -- and the ice flows in ice streams, often in deep river valleys or troughs.  Where the ice is very thin -- near the mountain summits -- the ice temperatures will be low, and the ice will be "cold based" -- with very little or no erosion and with even quite delicate features like tors effectively protected.

Don't assume that this reconstruction is correct in every detail -- discussions are ongoing.  But the evidence does seem to be converging, and with every year that passes, the models and the "ground truthing data" get closer together......

7 comments:

Henry Patton said...

The north Wales transect probably underestimates ice thickness - the modelling does not take into account any Irish Sea ice buttressing along the northern coastline, so it is possible that all summits here were ice covered.

BRIAN JOHN said...

Thanks Henry -- yes,the model will differ from reality in that maybe ALL of the flanks of the ice cap will have had Irish Sea Ice to contend with -- and a more sophisticated model will no doubt build this in. But I fully accept that you use a relatively simple model to start with and then build in other complicating parameters. one by one... softly softly catchee monkey.

Anonymous said...

Brian

The model does not take into account isostatic transformation of the land surface including the mountains?

'Shorelines of North America' (2009) has shown that this degree of ice coverage compressed the land by as much as 700 metres - although the profile maybe similar, if not somewhat less severe and squatter.

This changes the dynamics of the ice flow and possible rock movement, making your theory impossible, as it would have to travel 'up hill' without leaving a trace or collecting on high ground that lays in front of Stonehenge, which would have not suffered the same transformation.

James Clark

BRIAN JOHN said...

James

If there is one thing I know about, it's shorelines and isostatic depression. You have got it all wrong -- please have a look at some of the other posts on this blog -- put in "isostatic adjustment" or some such thing into the search box.

Isostatic effects beneath and beyond ice edges are now well known, following many years of study on all continents. Remember too that sea-water also has an isostatic effect -- there is a lot of weight in a column of 120m of sea water!

BRIAN JOHN said...

James -- forgot to mention -- the rule is 3:1 for ice -- ie 300m of ice on the land surface will result in 100m of crustal depression. The crust normally responds by flexing rather than by faulting, and depression extends well beyond the ice edge. Then you have "forebulge" effects too. Gets a bit complicated...

Anonymous said...

Brian

The point is how did your rocks flow 'uphill' to a place without 'isostatic adjustment' (Stonehenge)?

Are you reinventing the rules of ice flow as well as isostasy?

James

BRIAN JOHN said...

James

I am reinventing neither. Glaciers move rocks uphill and downhill all the time. Bedrock topography has relatively little to do with it. The direction of ice flow is determined above all else by the surface gradient of the ice. So long as there was a continuous ice surface gradient from Pembrokeshire to Stonehenge there is nothing in principle (or in the laws of physics) to prevent the glacier from transporting stones all the way to the site of the monument.