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Saturday, 25 August 2012

The ice sheet - ice cap - snowfield continuum


If you look back over many of the photos of Greenland published on this blog over the past two years or so, you will see over and again that there is a continuum from the glacier centre to the glacier periphery which can be described as ice sheet --> ice cap --> snowfield.  The boundaries between these three zones are sometimes quite clear (in the northern hemisphere, maybe in July and August) but for the rest of the year an intermittent blanket of snow will make boundaries and junctions very difficult to pick up.   You can get some clues as to what is going on.  For example, take a look at these two satellite images from NW Iceland:




The top one shows the plateau of Dranga and the bottom one shows the nearby plateau of Glama.  Approx the same area is covered in both photos.    They might look very similar, but note that in the top photo the edge of the snow-covered area is quite sharp, whereas in the bottom photo it is much more diffuse.  To some degree this is because the top photo is a summer photo and the bottom one shows a springtime situation -- but the snowline and snow edge is certainly much sharper on Dranga.  There is one further clue -- look carefully at the Dranga photo and you will see small projecting lobes running down into the radiating valleys.  These are little outlet glaciers -- not very healthy ones, but they do at least still exist.  In the case of Glama there is no ice cap at all -- there are no traces of glaciers flowing into the troughs -- everything white in the photo is seasonal snow which melts every summer and reforms again every winter.

Now then -- on the matter of Devon and Cornwall.  I am pretty convinced that during the Devensian there were little ice caps like the Drangajokull on Exmoor, Dartmoor, Bodmin Moor and the Mendips, and maybe on other uplands as well.  On some of the lower hills, and on Salisbury Plain, we will have had a situation analagous to that of the Glama Plateau -- a lot of permafrost, a lot of snow, but no actual glacier ice.

.... and on the coasts of Devon and Cornwall, and in the great inlet of the Somerset Levels?  Almost certainly an ice edge of the Irish Sea Glacier, looking something like this:


This is a "fingered" ice sheet edge in West Greenland.  Below is a more complex landscape, in Peary Land, North Greenland:


There are several types of "Ice Age" landscapes here.  To the left is the edge of the Greenland Ice Sheet, fingering into the low-lying areas as ice lobes.  There are several ice caps.  One of them (top left) is connected to the ice sheet, but probably has its own dynamics.  Just south of the centre of the photo is another small and almost circular ice cap -- maybe the Dartmoor Ice Cap looked rather like this.  The higher land beyond the ice edge -- especially in the northern part of the photo -- is covered with ice patches, small glaciers, perennial and seasonal snowfields.  Finally the lower lying areas are both ice free and snow-free in the summer -- but here there is very thick permafrost.

2 comments:

Constantinos Ragazas said...

Brian you write,

“In the case of Glama there is no ice cap at all … [just] seasonal snow which melts every summer and reforms again every winter. “


And also,

“ … on Salisbury Plain, we will have had a situation analagous to that of the Glama Plateau -- a lot of permafrost, a lot of snow, but no actual glacier ice.”

No actual glacier ice.
What about local frozen bodies of water? If the chalkland was not protected but was seasonally exposed how do you explain the lack of “chalkland [periglacial] patterns” in Salisbury Plain as these are found in abundance in Eastern England?

Kostas

BRIAN JOHN said...

There are lots of chalkland areas, Kostas, which do not have chalkland periglacial patterns on them -- and many of these are hilly areas and sloping surfaces. Forget about your locally frozen water bodies. We are not going there again......