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Saturday, 25 August 2012

Where are the chalkland patterns on Salisbury Plain?

We have talked about this before.  New development.  This is an interesting map (made originally by Sparks and West, from many sources) showing the main occurrences of periglacial features in Great Britain. It's pretty rough, and many occurrences of single ice wedges (for example) are missing -- but you get the general idea.  Again, the SE quadrant of the country, outside the Devensian glacial limit, is where we see the greatest concentrations of periglacial features associated with permafrost and surface movements in the active layer during the summer months.  But there are quite a few features in Scotland, well within the Devensian limit, and also in Devon and Cornwall -- an area now subject to much scrutiny following the publication of the recent paper on the Dartmoor Ice Cap.  One problem is that these features are difficult to date.  When they occur close to the ground surface -- as in Pembrokeshire -- fossil ice wedges can reasonably be assigned to the Younger Dryas, since we know that it was very cold indeed, with permafrost widespread.  We can probably assume that most of the periglacial features of Pembrokeshire, South Wales, Devon and Cornwall, are of approximately the same age.  This still allows for the possibility of quite a heavy cover of glacial ice, firn and snowfields in SW England during the peak of the Devensian, around 20,000 years ago.

One question which has never -- to the best of my knowledge -- been satisfactorily answered is this:  "Why are there prominent chalkland patterns on the chalklands of eastern England and Kent, but not on the chalklands of Wiltshire and Southern England?"  It may be that this has something to do with the nature of the chalk itself, or with topography, but that explanation is not entirely satisfactory -- so we have to wonder whether there might have been a more extensive and thicker snow blanket in Wiltshire, for example, and a colder and more arid climate further east, permitting the development of much thicker permafrost and the formation of more prominent periglacial features in the landscape.

1 comment:

Constantinos Ragazas said...


I remember a similar discussion we had in your blog more than a year ago and have been patiently awaiting when you return to this very important question!

At that time, I suggested the reason why there are no similar periglacial features at Salisbury Plain is because of a local ice cover of the area. With the recent paper by Stephan Harrison and others on the Dartmoor Ice Cap, the possibility of a Salisbury Ice Cover increases. The Avenue periglacial stripes (and in other locations more exposed to the weather -- like meltwater stream bottoms and retaining basins) add to the evidence for my hypothesis.

The accumulating evidence, Brian, in pointing in my direction …

Have you checked the reference I make to you and your blog in my essay “The Metaphysics of Physics” ?

Just in case, and for others interested also, I am including it again below: