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Saturday, 1 August 2015

Quarrying and entrainment beneath an advancing glacier


While looking at some material relating to the Wisconsin Glaciation of North America, I came across this interesting diagram in the Illinois State Geological Survey web site.

It's interesting because it portrays pretty well the exact scenario that I envisage for the quarrying and entrainment of blocks and other debris from the area around Craig Rhosyfelin in North Pembrokeshire.  What we have in this idealised scenario is an old valley transverse to the direction of advancing ice flow,  with plucking and entrainment along shear planes within the ice.  This enables blocks and other debris to be transported in an englacial situation.

Nothing new under the sun......

7 comments:

TonyH said...

Natural Geomorphological events, including glacial processes, need to be understood properly by all Archaeological teams, even if these include participating Geologists, so that interpretation of landscapes takes account of these.

Not everything in the landscape is man - made!!

Not every "quarrying" activity involves humans!!

chris johnson said...

Very interesting Brian.

It would be good if these processes are taught to everybody in schools. It helps me appreciate the world around me and emphasises the ephemeral nature of much we consider to be rock solid certainties.

BRIAN JOHN said...

Well, there's a nice text-book I can point you towards, Chris! "Glaciers and Landscape" -- sold incredibly well for more than 20 years, which is a very long life for a university text book. You might pick one up cheaply somewhere......

Evergreen said...

Periglacial striping at Avebury?

https://mobile.twitter.com/mgprolix/status/628197464110202881

TonyH said...

Even popular American musician James Taylor has had a decent go at drawing attention to the Forces of Nature in his recent 2015 song, Montana:-

I'm not smart enough for this life We been living
A little too slow for the pace of the game
It's not I'm ungrateful for all We been given
But nevertheless just the same
I wish to my soul I was back in Montana
High on my mountain and deep in the snow
Up in my cabin, over the valley
Under the blankets with you
Over the ocean from here

Who can imagine the scale of the forces
That pushed this old mountain range up in the sky?
Tectonic creation, erosion, mutation
Something to pleasure God's eye........

From "Before This World" CD, 2015

BRIAN JOHN said...

That phrase just seems to be thrown out because it is flavour of the month. those marks look like solutional rills to me -- always assuming that they run down the maximum gradient of the chalk surface...... The darker areas might be due to enhanced solutional activity (and maybe staining by humic acid) associated with tree roots.

TonyH said...

www.stonehengeandaveburywhs.org/avebury-between-the-monuments-exca

This should work. Otherwise, put MARK GILLINGS LEICESTER UNIVERSITY BETWEEN THE MONUMENTS AVEBURY into your trusty Search Engine and simply press the button for this, amongst others.

Dr Mark Gillings was on the Clatford, near Marlborough, dig with MPP & Josh Pollard, that I joined in with 2 or 3 years ago. Mark has worked in the Avebury/ Marlborough Downs landscape with Josh since the 1990's, and is NOT closely linked with MPP.

His term - time academic email is:-

mg41@le.ac.uk

I note that Mike Allen is involved in the Western Avenue Avebury excavations. He is one of the two "environmentalists" on MPP's Stonehenge Riverside Project. That was where "periglacial stripes" became Flavour of the Month/ Year/ Decade. Think there is some photographic sign of them on the above (or similar) report from the current 3 -year dig on the W Avenue at Avebury. The ground, to my knowledge, is fairly flat.