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Thursday, 6 October 2011

Deep Freeeze in the Mendips

On doing some more investigating (with thanks to the works of Derek Ford and Hugh French) it appears that the Nahanni Karst analogy is not a bad one if we are seeking to understand the landscape of the Mendips.  It appears that the Nahanni region WAS glaciated, more than 300,000 years ago, but has remained ice-free since then.  There appear to be no erratics on the karstland surfaces, and only very thin soils -- have these traces of glaciation (for there must have been some) simply been dissolved away?

The following may be of interest:


The top diagram shows Derek Ford's interpretation of how the Nahanni karst might have evolved -- with very complex relationships between inherited landforms (eg glacial meltwater drainage channels) and intermittently frozen water either in free situations or incorporated into limestone bedrock.  The idea that permafrost conditions simply prevent karst processes from operating must be wrong -- things are much more complex.

The bottom text extract is from High French's excellent book on periglacial landscapes.  Plenty to think about here, if we seek to answer the question: "Have the Mendips ever been glaciated?"

** Hugh M French 2006 "The Periglacial Environment."

We may also be able to learn something from this:

The largest icefield in the Rockies, COLUMBIA ICEFIELD, is mostly drained by sinkholes surviving in the limestone and dolostone beneath it. The waters flow in great caves through Mount Castleguard, to emerge as spectacular springs in the valley of Castleguard River (a headwater of the North Saskatchewan River). It is the world's pre-eminent example of modern subglacial karst.

2 comments:

Constantinos Ragazas said...

Brian,

Quoting from the quote in your post,

“The largest icefield in the Rockies, COLUMBIA ICEFIELD, is mostly drained by sinkholes”

For a beautiful photo of an ice sinkhole (perfectly circular) check my article, “The un-Henging of Stonehenge”. This shows how Nature can form circular holes on an ice sheet – a key hypothesis in my Stonehenge theory.

Kostas

BRIAN JOHN said...

You are talking about moulins, which are often circular because of the rotation or swirling of the water that descends into them from surface meltwater streams. You need large quantities of meltwater to create this effect.