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Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Tuesday, 9 July 2013

A very erratic erratic on a washed surface

One of the nice things about coming to Sweden every summer is that I get to wander about in the Stockholm Archipelago, which is like an open text-book on glacial erosional processes.  Some of the landforms and the detailed "micro-morphology" left by overriding ice are quite extraordinary -- and in part their survival after c 15,000 years of exposure, following the retreat of the Devensian ice, is down to the fact that this whole area has been subjected to considerable isostatic recovery.  The process is still going on.  This means that the rock surface shown in this photo was submerged beneath the sea  not that long ago, and that it has been "washed" by coastal processes including wave action which have removed most of the silt, clay and gravel from any till left behind when the ice retreated.  So we get these remarkably clean and smooth rock surfaces, with erratics scattered all over the place, just waiting to be photographed........

Here the bedrock is a metamorphosed Precambrian pink granite, and the erratic boulder is made of a grey gneiss which will have come from somewhere to the north.

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