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Saturday 7 November 2015

Ice-contact environments

Since you asked -- or maybe would prefer not to know -- here are a couple of photos that might help you to understand the contents of our new Rhosyfelin paper, when it appears.  What we are proposing (Dyfed Elis-Gruffydd, John Downes and me) is that the deeper deposits at Rhosyfelin are partly rockfall deposits (see previous post) and partly a mixture of meltout till and fluvioglacial gravels.  We envisage an environment very similar to this one, on the edge of the Russell Glacier in western Greenland.  (You see similar associations of sediments in all glaciated regions.)

The essential features here are (a) dirty ice, well laden with sediments close to the ice edge; and (b) fluvioglacial sands and gravels laid down by shifting meltwater streams.  There is quite strong relief here, but when the glacier has melted away, and when the ice has also melted out from within the ice-cored moraines, all that is left is an undulating surface of meltout till mixed with fluvioglacial materials.  There will be some lodgement tills, but most of the deposits will have the characteristics of "flow tills" -- which have literally flowed or slipped down an ice surface and accumulated against the base of the slope.  As I have said many times before, this is the sort of environment in which "anything can happen, and usually does...."

When the ice has all melted out, you may get something looking like this landscape of hummocky moraine:

This is near Bruarjokull in Iceland.  Sometimes, close to existing glacier snouts, there may still be some ancient buried ice deep down beneath the surface.  In other cases, you will get even less surface expression, and a subdued landscape of mixed till and fluvioglacial materials:

Above: scattered morainic debris on James Ross Island, Antarctica


Myris of Alexandria said...

These sediments must be a nightmare to work on both literally and intellectually.
Is brash ice particularly rude I wonder why it is called brash, I kept noticing it in Acts of God, sequel,sequel, think Antarctica,more perfidious yanks there.
Just finished reading the last 6 Graham Hurley Faraday and Winter novels, bereft I have done them. Splendid writing, solid but racy and much brashness.

BRIAN JOHN said...

No idea why brash ice is so called....... and where did hoodoos come from?

yes, these ice-contact sediments are a nightmare. Much discussion just now in geomorphological circles about whether the coarse glacial / fluvioglacial sediments on the south coast of Gower are in primary or secondary position..... the ones exposed in Langland Bay are particularly difficult.

Geo Cur said...

Up here brash is the detritus left after forestry operations .Closer to German breche =break than anything gaelic .Couldn't it be much the same for brash ice , i.e. bits (broken))?

BRIAN JOHN said...

Thanks Geo -- that sounds exactly right to me. Brash ice consists of all sorts of broken fragments -- some bits from icebergs, some bits from sea ice etc. Larger fragments that have fallen off icebergs are called bergy bits -- they are very dangerous, because they sit low in the water, often have smooth tops, and can do serious damage if you hit them at speed....

Myris of Alexandria said...

Bergy bits, sounds slightly worse than a late night Kentish Town kebab.
The etymology sounds plausible but not convinced unless early artic explorers were German.
Wonder if the origin is from a language north Of Leubeck?
Christmass for me is the coming of Emmanuel and marzipan. Buy in bulk, the sweets not saviour(ies).
Just refound spectacular radiometric dates, more soonish.