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Monday, 26 June 2017

Roches moutonnees in the Stockholm Archipelago

 Two images of moulded and streamlined topography on Pre-Cambrian Basement rocks in the Stockholm Archipelago. Location -- west side of Rödlöga Storskär.  In both photos the ice has moved from right to left.  Note the streamlined ridges and gullies.  In the lower photo we see the stoss side of a large roche moutonnee -- apart from a few plucked faces on miniature roches moutonnees, all the surfaces are smoothed and rounded off by powerful abrasion.  The ice here has been under compression, probably melting on its bed.


I have done many posts on this topic before -- and will probably do more in the future, if fate decrees that I have more visits to the extraordinary and beautiful islands of the outer archipelago.  I have just spent three days out there. and these notes and pictures are the result.

Wherever you look on the washed rock surfaces there are asymmetrical forms.  We would call many of them roches moutonnees, and they occur on many different scales.  Some of them are substantial hill masses, over 20 m high, and others are measurable in centimetres.  They should not be confused with whaleback forms or rock drumlins, which tend to have abrasion / polishing features on all flanks;  in contrast,  roches moutonnees always have gentle up-glacier (stoss side)  slopes that are polished and steep down-glacier (lee side) slopes that are fractured and steep.  Different processes are at play -- and the key to the form is the ability of ice to exert immense pressure and to "quarry" vast chunks of bedrock which are then entrained and carried away.  This is how erratic slabs, boulders and pillars are formed........

In the archipelago the ice moved consistently from north to south, with minor variations dictated by local topography.  There was erosion on a considerable scale during the Devensian glaciation, with the ice of the Scandinavian ice sheet again grinding down old surfaces affected by ice many times before. Parts of the eroded surface are covered by till and fluvioglacial materials, but because the surface was covered by water after being covered by ice, it has been well washed, with fines carried away into deeper water -- leaving behind a classic erosional landscape that can be examined in minute detail.

Herewith some more photos:

 A mini-roche moutonnee, just a few cm high.  The watch shows the direction of ice movement.  Smoothed and abraded face on the right, and plucked (fractured) face on the left, shown up by the lighter colour on the rock.

 This roche moutonnee is a bit bigger.  Here the direction of ice movement is again obvious.  The stoss side is heavily striated, and the fractures on the lee side are quite complex. There have probably been several phases of block removal.
 
 Another small roche moutonnee.  Here it looks as if there has been just one phase of block removal -- the fractured face is much simpler.

 There is a more complex fracture pattern on this lee side, and the shape of the roche moutonnee is quite irregular.  Note the big crescentic gouge on the abraded surface in the background

8 comments:

TonyH said...

How do you compare any formations in Preseli? Any similarities?

BRIAN JOHN said...

Yes -- I think Carningli displays some roche moutonnee chatacteristics (ice-smoothed slabs to the north, plucked face and debris on the south face.) Foel Drygarn also looks like a very big roche moutonnee, and as for Carn Meini, it is a classic. I have published various pics showing those wonderful glaciated pavements on the hill summit and the south-facing plucked face. You can stand on the top, looking down at the cliff below, and admire the places from which large blocks of spotted dolerite have been carried away by the ice.......

TonyH said...

.....or, of course, to the more highly imaginative/ obsessed in the archaeology fraternity, admire the places from which Stone Age Man left his orthostats ready for transportation in whichever direction and by whichever human transport mode had been decided for their delivery over at the edge of Salisbury Plain......

BRIAN JOHN said...

Ah yes -- quarries here, there, and wherever you want to see them.....

TonyH said...

As even Paul MacCartney, late of The Quarrymen (before he went on to other things), put down in music and verse:-

Listen to Here, There and Everywhere" coupled with "Wanderlust" - both very Carn Meini & Preseli, folks.

https://www.youtube.com/watch?v=HuphFPEqJqw

Beautiful and engaging - just like 'dem Human Transport Claims - but do not be seduced....

TonyH said...

THE DEFECTIVE/REJECTED BLUESTONE? THE PICNIC TABLE?

"Captain says there'll be a bust,
This One's not to be"

P MacCARTNEY

BRIAN JOHN said...

I have always been intrigued by the tendency of the human transport brigade to see abandoned / rejected / unsatisfactory / broken / sunken / lost bluestones all over the place. The literature is full of them. As you say, the picnic table at Rhosyfelin is one of them; the long pillar-shaped stone at Carn Meini is another; the broken stone at Carn Meini is another; and people have gone hunting for them on the bed of Milford Haven. If only I had had the chance to take some of these people on a field trip to Iceland or Greenland, to show them what happens when a big glacier covers a landscape and then retreats..........

Many of the problems of this world arise from the fact that people do not know enough about natural processes. There should be compulsory geomorphology from the age of three, that's what I say!

TonyH said...

Absolutely agree about people not knowing, or finding out about, natural environmental processes.

As James Taylor says in one of his most beautiful songs, "Gaia", [on his album, Hourglass]:-

Ron run run said the automobileand we ran
Run for your life take to your heels
Foolish school of fish on wheels....

....We thought we ought to walk awhile
So we left that town in a single file
Up and up and up mile after mile after mile

We reached the tree line and I dropped my pack
Sat down on my haunches and I looked back down
OVER THE MOUNTAIN
Helpless and speechless and breathless

Someone's got to stop us now
Save us from us Gaia
No one's gonna stop us now