More on glacial erosion by ice sheets

I have just made a new visit to the outer islands of the Stockholm Archipelago in Sweden, where many detailed observations were made more than 40 years ago prior to the publication of the text called Glaciers and Landscape.  Once again, I have been mightily impressed by the vast range of glacial erosional features in an area that lay right at the heart of the Scandinavian ice sheet every time it expanded and moved southwards towards Denmark, Germany, Poland and the other southern Baltic lands.

The archipelago is basically an area of ancient shield rocks including granites and gneisses and metamorphosed rocks that were originally shales, sandstones, greywackes, lavas and ashes -- over 3,000 milliion years ago.  The complexity of these rocks is striking, and many of them have had such complicated histories of deformation and alteration that they are difficult to classify.  At any rate, they are extremely beautiful, with colours ranging from grey and blue to green, black and brown, with some rocks bright red or in shades of pink.  Some rocks, predominantly made of quartz fragments, are white or buff-coloured.  There has been such a long history of erosion here that the landscape is essentially one of gently undulating plateau surfaces not much above present sea-level, with deep "tunnel valleys" incised into these surfaces and coinciding for the most part with faults or other lines of weakness.

There is an incredible micro-morphology of glacial erosional features which can be seen around the coasts on thousands of small islands to the north and south of Stockholm.  Most of these features date from the Devensian glacial episode, and they are remarkably fresh.  They are easy to examine because the whole of this area was deeply depressed isostatically by the weight of overlying glacier ice, and was submerged to a depth of about 150m following ice wastage.  Since that time (about 9,000 years ago) the land has been rising as a result of isostatic recovery, which has gradually slowed to the current rate of about 5 mm per year.  The islands are still "growing" -- at a rate which humans can appreciate over the span of a single lifetime.  On the higher parts of the islands vegetation has had time to establish itself and to create thin soils, but in many coastal areas virtually all soft sediments from till, lake and sea floor deposits have been washed away during the process of emergence from the sea, leaving behind clean rock surfaces with occasional erratic boulders and raised beaches of cobbles and rounded boulders.  The ice here has moved from the north towards the south, with minor deviations which can usually be attributed to the "control" exerted by the shape of the rock surface.  On the clean rock surfaces one finds wonderful examples of virtually every kind of glacial erosional feature:  roches moutonnees (large and small), striations and gouged grooves, chatter-marks, crescentic cracks and gouges with the ends sometimes pointing up-glacier and sometimes down-glacier, and sinuous P-forms which appear to have been caused by water at the base of the glacier or by heavily saturated sediments moving under pressure like toothpaste being squeezed out of a tube.

Below are some of my recent photos, which can be looked at in conjunction with my posts on glacial erosion and entrainment. The photos are all from Rodloga Storskar, in the outer part of the Stockholm Archipelago.

Chatter-marks on a smoothed and abraded pinkish granite rock surface.  These transverse fractures are almost at right angles to the direction of striations, and are probably caused by massive forces on the glacier bed, at a time of high basal friction when traction forces were applied to the rock surface -- literally causing it to tear.  This is a early phase in rock breakup.  The ice has moved from bottom R to top L.

Striations, deep parallel gouges, tensional cracks and striations, all on a rock surface that has been moulded and polished.  These features probably relate to several different ice movement directions -- most probably all during the Devensian.

Here the ice has moved from top L towards bottom R --notice the fine polishing and abrasion on the smooth rock surface.  Notice too that there is something that looks like a graze or a flesh wound in the centre of the photo.  The colour is slightly more pinkish than on the smooth area.  Here a process of tearing and gouging has just commenced -- look at the fractures and roughened surface.  This process has also affected the area towards the top R of the photo.  This is the first phase in the formation of a roche moutonnee, with different processes affecting the up-glacier and down-glacier faces.  Pressure differences have something to do with it, but what we see here is clearly not the result of a single large tool being pressed on to the glacier bed -- if tools have been involved, there must have been many of them........