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Wednesday, 17 July 2013

Roche moutonnees - Stockholm Archipelago

 Ice movement directions in the Weichselian (Devensian) glacial stage, c 20,000 years ago, in Southern Sweden.  The following photos all come from within the area marked by the red circle.  With minor variations, the ice movement in the Stocholm Archipelago has  been within a few degrees of north - south.  If you are lost in a fog, without a compass, all you need to do is go ashore and look for the striations.......

 Typical small roche moutonnee on a small island off Blido, in the outer archipelago.  The smoothed (up-glacier) face is on the north side, and the rough or plucked face is on the south side.  The latter is much steeper, as we can see -- because this is the face from which blocks of rock have been dragged away.

 Another typical small roche moutonnee, on Salskaret, off Blido.  Again we can see the contrast between the northern streamlined and smoothed face, and the steeper plucked face on the south side.


This is the most beautiful small roche moutonnee I have ever seen, on the west side of Rodloga Storskar.  The roche moutonnee form is almost perfect, and to the south of what is left of the "parent rock" there are three detached blocks -- and maybe more, under the sea surface.  here we can see two of the blocks, which have not been moved very far by the overriding ice.

Here we can see all 3 of the detached blocks -- the first one is about 20m away from the parent rock.

Towards the end of the last glacial episode, the original roche moutonnee had been reduced to such a small size that the remnant was about to be broken up by the inexorable compression on the up-glacier side and the tension exerted on the rock surface on the down-glacier side.  The big crack we can see in the photo was about to be exploited, and another big block was about to be dragged from the flank of the 'parent" feature -- but then the glaciation came to an end, the ice stopped moving, and the erosional process was never completed.......




2 comments:

Myris of Alexandria. said...

I believe Ixer and Bevins are about to submit a paper entitled "Chips off the old block:The Stonehenge Debitage Dilemma.
There must be a cold nip in the air.
Very lovely, very persuasive photographs. Now were Rhosyfelin to be orientated correctly.
I must banish these naughty thoughts.
Excellent.
A too warm Myris,not a drop of eau de Nile to be had.

BRIAN JOHN said...

We can only take the Rhosyfelin analogy so far -- the ridge sticking out into the valley s certainly not a roche moutonnee. But what we see everywhere in the Stockholm Archipelago is the manner in which processes operate on the base of an ice sheet which is flowing across an extensive area. It's called areal scouring -- very different from the processes that operate more intensively in a concentrated linear situation -- ie in a glacial trough. Beneath a moving ice sheet stream like the Irish Sea Glacier there will have been smoothing and polishing, and plucking, in hundreds or thousands of different situations, with the details determined by topography and glacier dynamics.