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Wednesday, 25 May 2011

The Dalsetter Erratic

 The Dalsetter Erratic (about 1m across) near Boddam, near the southern tip of the 
main island of Shetland

Those who expect to see erratic trails or erratic fans between source point and dumping point conveniently forget that there are well-known examples (from all glaciated areas)  of erratics that are so extremely erratic that they are almost anomalous.  One such is the Dalsetter Erratic on Shetland -- a large boulder made of a rock called "tonsbergite" which has come from the Oslo district in Norway.  Luckily it has a very distinctive colour and texture -- so identification was made a long time ago, and did not have to depend upon modern geochemistry techniques etc.

So how did this erratic boulder get from A to B?  It has to be the result of glacial transport, at a time when ice from the Scandinavian Ice Sheet was so extensive that it crossed the Norwegian Trench and invaded the Orkney and Shetland island groups.  There is no great doubt about that, since there is other evidence that can be adduced -- but in spite of a thorough search across 7 km of terrain near this erratic, no other erratic of similar type has ever been found.  In fact (please correct me if I'm wrong!) no other erratic from Norway has been found on the Shetlands.......

So because the Dalsetter Erratic is "anomalous" do we have to discount ice action and hypothesise that it was actually carried by human beings?  Well, that has been proposed, on the grounds that the erratic might have been carried ashore as ships ballast by the Vikings -- but there is no way that they would carry boulders of this size as ballast.  In any case, there is quite convincing evidence that the erratic was dug out of a local deposit of glacial till -- and that seems to put an end to the discussion.

So where is the erratic train, if there is one?  Simple answer -- under the sea.  Not so different from the Preseli / Stonehenge scenario, when you think about it.

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