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Sunday, 4 June 2017

Washed moraines on the west coast of Sweden

 Washed moraine surfaces on the west side of the island of Rörö, in the archipelago to the north of Göteborg, Sweden.  The expanse of pebbles and boulders seen in the lower photo is still washed by extreme storm waves in the winter, and because of the very exposed position of this coast a vast quantity of flotsam and jetsam is thrown up every year.   Children have had considerable fun here, creating a structure which is part den and part work of art......

It's amazing what you find when you go for a walk.  We arrived in Sweden a couple of days ago for the annual hols, and our good friends Anita and Bengt suggested a trip to the little island of Rörö, which could be reached after a short ferry trip.  There were lots of interesting pebbles there, so they said.  Well, I can never resist looking at interesting pebbles, so we packed up a picnic and off we went.  

What an amazing landscape!  When we reached the western side of the island we found ourselves in a strange wilderness of gravel, pebbles and boulders of all sizes, in places quite difficult to walk over, interspersed with patches of scrubby west coast vegetation, rock outcrops,  and a few pools of water.  This "klappersten" terrain (meaning pebbly stone terrain) is up to 500m west-east, and stretches along the coast for maybe  1.5 km.  In places there are clear ridges of storm beach material, and in some of the inlets there are clear strandlines, as can be seen in the Bing image below.  The ice-moulded terrain of the greater part of the island (quite typical of the west coast archipelago) is shown grey in the image, and the pebbly terrain to the west shows up as white.  The ice moved across this area roughly from NE towards SW.

Most of the strandlines are less than 20m above present sea-level.  I reckon that big storm waves sometimes affect the surface up to a height of c 10m and maybe 50m inland from the shore.

I thought at first that this whole area was made up of a large suite of strandlines or raised beaches, but I quickly noticed that the degree of stone and boulder rounding was not sufficient to justify that interpretation, and the only possible alternative explanation is that this must be a vast expanse of morainic debris laid down during one of the big retreat stages of the Scandinavian Ice Sheet after 20,000 years BP.  Sea level must have been higher than it is today because of isostatic loading, but on a rising coast the morainic expanse was not affected by wave action for very long -- the result being a washing out of finer material and a survival of most of the sub-angular and faceted stone shapes typical of glacial moraine.

So -- this is a classic washed moraine landscape.  It's probably featured in all the Swedish glacial geomorphology text books -- but I have not had time to check!

Just before putting this post together, I checked out the maps of Scandinavian Ice Sheet retreat stages, and sure enough I see that the outer part of the archipelago in this area reveals fragments of the "Halland retreat moraine complex" dated at around 16,800 - 16,200 years BP.  So there we are then-  my observations slot very nicely into the established chronology.

On this map the Halland coastal moraines are shown, on the outermost islands of the western archipelago.

Source:  "Investigating the last deglaciation of the Scandinavian Ice Sheet in southwest Sweden with 10Be exposure dating", Journal of Quaternary Science 27(2):211-220 · February 2012, by Nicolaj Krog Larsen, Henriette Linge, Lena Håkansson, Derek Fabel

1 comment:

TonyH said...

I hope you are obtaining copies of the Pembrokeshire Herald over there in Sweden. You assured us you would keep us posted on any further sightings of the Feral Man of Pembrokeshire (previous Post).

Or is he now over in southern Sweden? That would be a curious coincidence. Not a bit of publicity for a new book, perhaps?