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