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Thursday, 5 November 2015

Windscoop



Here is another of my posts that has nothing to do with Stonehenge, but something to do with glacial geomorphology.......

I found this attractive photo of a windscoop.  Don't even know which country it's from...... but it's a nice photo, which illustrates perfectly the scale of a typical windscoop and its essential features.  The person gives the scale.

There are several processes at work here.  Normally windscoops are prominent on the flanks of nunataks or mountain peaks sticking up through a glacier or ice sheet surface, like this:

 One requirement is a dark surface, which leads to heat absorption on the rock and scree, and that leads to warmer air in circulation around it, which leads to enhanced ice-melt and snow-melt, which leads to a windscoop being formed and maintained.  If the ice is itself dirty (as in the lower photo), that too will enhance the rate of melting.  One requirement, if the windscoop is to survive, is a modest or slow rate of ice movement in the glacier.

If there is a high accumulation rate, as in the upper photo, you may see no glacier ice in the windscoop, but all surfaces may be covered in fresh snow accumulations.

The main process that keeps the windscoop open is, as the name implies, the wind itself.  Nunataks create airflow turbulence, and where there is turbulence there are eddies which can be strong enough to lift loose snow out of the trough or trench, and keep it looking like a moat around a castle....

4 comments:

TonyH said...

Do nunataks always take on a certain physical formation once the glacial processes have been completed? Do we have examples of nunataks in Preseli, elsewhere in South or Mid - Wales, Dartmoor, or even Exmoor etc?

BRIAN JOHN said...

I have put up several posts on nunataks -- do a search on the blog. Often it;s very hard to tell whether a summit or isolated hill has stood above an ice surface. I have speculated on that as a possibility for Carningli, near Newport,on the basis that there is till on the lower slopes but not on the summit, and on the basis that there are ice-smoothed slabs on the lower slopes and flanks of the hill as well. Sometimes you get a distinct "trim-line" or even a morainic ridge which shows you the altitude reached by the ice -- then you may have scree and frost-shattered clitter above the trim-line, and till below it. As for some of those hill summits in Somerset -- and maybe even the Mendips -- this becomes an interesting problem. Short answer with ref to Cley Hill -- a strange outlier of chalk with a capping of Tertiary sediments -- is that yes, it could well have been a nunatak close to an ice edge, although it is only about 80m high. But it's certainly prominent, and the standard explanation that it was somehow isolated by an ancient sea does not seem very sensible to me. I have passed it many times, and pondered upon it -- next time go go to see the family in Nunney, I must make an effort to examine it properly!

TonyH said...

Let me know when you intend making a visit to The National Trust's Cley Hill, Brian, and we may be able to meet up in the car park below before climbing up it. It's relatively close to me.

BRIAN JOHN said...

OK Tony -- will do.....