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Saturday, 26 March 2011

The Devensian ice margin in West Wales

In Jansson and Glasser's study of the Welsh Ice Cap (2008) the authors come up with a suggested LGM limit which is remarkably similar to mine, as shown above.  There are minor differences of detail, and the ice limit inland is difficult to fix with any certainty because it now looks as if the ice edge was not stationary at this margin for any length of time.  There is no terminal moraine that can be examined in the field -- just a "feather edge" of younger glacial deposits overlapping older glacial deposits.

As I have noted before, the Devensian ice did impinge on the South Pembrokeshire coast, south of Milford Haven.  However, it is uncertain whether this ice was thick enough to impound a glacial lake within the area we now know as the Milford Haven waterway.

6 comments:

Constantinos Ragazas said...

Brian,

There is something that puzzles me about your map above in this post. You have an area that you designate 'ice free area?' that is long and relatively narrow surrounded by the Irish Sea Glacier Ice and the Welsh Glacier Ice. And this is mainly over unobstructed open sea. Being that ice is so fluid and under pressure and movement will fill in any such gaps, how could this 'Italian boot' like area in your map be free of ice? Could it just be that over such area (water for the most part) there cannot be found any evidence for there being ice?

Too often in our scientific research we are limited by the evidence that our theories and instruments are meant to uncover. The truth lies in that which we currently don't recognize -- though always in front of us!

Curious,

Kostas

BRIAN JOHN said...

Hi Kostas -- please forget about the sea ..... it wasn't there at the time! Around 20,000 years ago all of the area shown as ice free would have been barren tundra, much of it probably snow covered.

Constantinos Ragazas said...

Brian,

As a geomorphologist, are you saying that barren tundra have the power to keep glaciers at bay and away? Or is this another example of drawing conclusions from missing evidence? Like dismissing glacier ice from Salisbury Plain because the evidence we are looking for is not the evidence we find?

Kostas

BRIAN JOHN said...

Nothing magic about tundra -- it just happens to be the "almost glacial" environment, so is most frequently found beyond an ice edge. A glacier skids to a halt when it runs out of energy. There is lots of evidence of Devensian glaciation inside the line, and very little evidence outside it, which is best interpreted as being inherited from a very old glaciation (Anglian?).

Constantinos Ragazas said...

Brian,
Thanks for clarifying this tundra mystery. I can see that if there is no 'new ice' available to form on a landscape then the glacier will not advance beyond such point. How far typically from a glacier edge do such “almost glacial” environments exist?

Kostas

BRIAN JOHN said...

I suggest you check out "tundra" on Wikipedia. the tundra zone, which we can also refer to as periglacial or (sometimes) the permafrost zone can be many hundreds of miles wide. It grades into a sort of scrub woodland and then into taiga. In upland areas the tundra may be very restricted in extent, with coniferous forest in close proximity on the lower mountain slopes and in the valleys..