THE BOOK
Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Saturday, 24 September 2011

The Times Atlas and the Greenland ice edge

Many of us will have followed one of this week's big stories -- about the Times Atlas misrepresenting the extent of ice wastage around the edges of the Greenland ice sheet.  Part of the reason for their misunderstanding seems to be the misapprehension that ice sheets have nice simple edges that move forwards during the waxing phases of glaciations and then move back when a glaciation comes to an end.  So the simple idea is that you can just draw a line on a map, and that's sorted.......

But this is not what happens, except where you have a gently undulating plain under the ice, as in parts of SW Greenland today.  Elsewhere, as the ice thins, the edges of the ice sheet tend to break up into scores or hundreds of smaller ice sheet fragments or ice caps, on plateau surfaces and mountains, while the valleys between them are deglaciated and become green.  Troughs that were under the ice are also exposed -- to become fjords near the coastline.  Look at these pics from Google Earth:

 The top image is of Nordvestfjord near the coast of east Greenland; I defy anybody to draw a line on a map showing where the ice sheet edge is today -- plateau ice caps, glaciated uplands, and outlet glaciers and valley glaciers everywhere.  Many of these ice masses are in a relatively healthy state -- we cannot just assume that they are wasting away at a catastrophic rate.

The lower pic gives a very different impression.  Here in SW Greenland the ice edge is retreating across a deeply scoured lowland, and as the ice thins the edge does indeed retreat in the classical sense. 

So -- generalisations are bound to lead to trouble.......

This pattern of ice sheet growth and ice sheet breakup is very close to what Henry Patten and others are finding in their models for the last (Devensian) glaciation.  If you run Henry's model of the expansion and contraction of the Welsh ice cap you will see a sort of flickering effect, with little ice caps (including one on Preseli) coming and going in step with climatic oscillations, eventually being "gobbled up" by the main ice cap, and then becoming independent and isolated again as the glacial episode draws to a close. 

And the relevance for the South-west of England and Stonehenge?  Well, I am increasingly convinced that there WERE small independent snowfields (and maybe actual glacier ice, frozen to the bed) in the upland areas including the Mendips, Exmoor, Dartmoor and Bodmin.  As the ice thickened in the Bristol Channel and as the Irish Sea Glacier started to move in, these independent areas of ice became incorporated, or coalesced together, to create a very complex area on the "ice sheet periphery" -- with some areas of flowing or moving ice and other areas where the ice may have been stagnant, playing a protective rather than erosional role.  We need to get this picture into the heads of all those who take part in this debate, because too many people see the Irish Sea Glacier as something which had a sharp and definable edge to it.  Visions of vertical ice cliffs trundling in from the west......   I myself plead guilty to portraying the ice front as such, in an attempt to simplify and explain.  Maybe it's time to make things more complicated and more realistic.....

8 comments:

Constantinos Ragazas said...

Brian you write,

“Well, I am increasingly convinced that there WERE small independent snowfields (and maybe actual glacier ice, frozen to the bed) in the upland areas including the Mendips, Exmoor, Dartmoor and Bodmin.  As the ice thickened in the Bristol Channel and as the Irish Sea Glacier started to move in, these independent areas of ice became incorporated, or coalesced together, to create a very complex area on the "ice sheet periphery" -- with some areas of flowing or moving ice and other areas where the ice may have been stagnant, playing a protective rather than erosional role.”


Thank you!

Kostas

BRIAN JOHN said...

I was expecting a post from you, Kostas! But don't get too excited -- I still don't believe your "sheets of ice" theory any more than I did when I first encountered it. My suggested mechanism for the growth of the Irish sea ice sheet is very different from yours.

The Stonehenge Enigma said...

Brian

I'm confused!

Is this debate about the Devensian glaciation in an attempt to provide evidence that your bluestones were shipped in the recent past - as we know that chlorine-36 dating shows the stones were exposed about 14,000BP i.e. when the ice caps had melted?

Or do you still believe these was placed by the Anglian Ice age 400,000 years ago?

RJL

BRIAN JOHN said...

Wouldn't want you to be confused, Robert. I'm just trying to understand how this ice sheet and its peripheral ice caps and snowfields might have behaved -- it's immaterial which glaciation we are talking about, since glaciers obeyed the same rules 450,000 years ago as they did 20,000 years ago. So the Devensian -- if we can get to sort it out properly -- will give us a guide to what happened in the Anglian.

Constantinos Ragazas said...

Brian,

I do not claim to know how the ice sheets that covered southern UK formed! I can only make some plausible suggestions. All that matters to me, however, is that they existed! And post by post, inch by inch, you are coming closer to this view too! I couldn't be more excited!

Kostas

BRIAN JOHN said...

Careful, Kostas. High blood pressure can be very damaging to the health......

Scissor Sister said...

Cue The Pointer Sisters lyrics for
"I'm So Excited!"

Constantinos Ragazas said...

Scissor Sister,

Pleased about the excitement! And I thought I was the only one …

Brian,

I think with my head and not my heart. I am as cool as a cucumber!

Not to worry!

But thanks for worrying!

Kostas