Washed surfaces and ice-transported boulders

 

 

Shoreline nwith a heavy concentration of erratic boulders on the coast of Granöören, near the eastern tip of Blidö, Stockholm Archipelago.  The coastline here is cut into a thick deposit of till, and the fines have been washed out by wave action.  When a "stillstand" occurs, with a rough equivalence of eustatic sea level rise and isostatic recovery rate, the concentration of boulders on the shoreline may be more pronounced.

I am intrigued that the geologists and geomorphologists who are "embedded" in the Stonehenge establishment still apparently believe that the big boulders dotted around the coasts of the Bristol Channel were transported by  floating sea ice and icebergs rather than by glacier ice.  This flies in the face of everything we know about glacial processes and about the Pleistocene history of the region, since nobody has yet demonstrated that the relative  positions of global sea level and the Bristol Channel coasts were close to those of today at a time when debris-laden icebergs could have been grounded between the tide marks.  On the contrary, on those occasions when ice-rafted debris might have been moved about in the channel, relative sea level must have been far below that of the present day, and the coastline must have been many miles away from its present position.   To argue that isostatic depression of the landmass caused the coastline to sink by an amount precisely equivalent to the eustatic sea level fall involves special pleading -- and there is no evidence to support it.

I am genuinely at a loss to understand what is to be gained by the continued promotion of the lRD (ice rafted debris) hypothesis, unless you want to fly in the face of the evidence and pretend (for rather obvious reasons) that the coasts of Devon and Cornwall were never glaciated.........

I am reminded of this rather silly argument every time I paddle the kayak around the coasts of the Stockholm Archipelago, which were once submerged beneath 100m or more of sea water.  The boulder-lined shorelines that we see everywhere are all the products of wave sapping of rock surfaces and exposed glacial sediments.  Wave action across a relatively narrow vertical range of a metre or two (there are no tides in the Baltic) removes all the finer materials -- clay, silt, sand and gravel -- and leaves behind the cobbles and the boulders.   If you tried to suggest to any Swedish geomorphologist that ice- rafting had anything at all to do with the presence of these big boulders, you would be laughed out of court.

So can we just have a bit of common sense here?