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Wednesday 27 February 2013

Dartmoor Glaciation disputed

Just in case there is anybody out there who thinks that archaeology is the only discipline within which assorted tribes slug it out on their battles for supremacy, rest assured that the same thing goes on within geomorphology too.  In a series of posts in June last year I reported on the new paper by Prof David Evans and others in which they presented the evidence of glaciation on Dartmoor:

I found this evidence very persuasive, especially since it was backed up by modelling work and had a very strong internal consistency about it.  Furthermore, the guys responsible for that paper know their glaciers pretty well, having worked all over the world.

Now a senior geomorphologist named Allan Straw has thrown a spanner into the works, with a short paper entitled "Dartmoor Glaciation -- Fact of Fiction?" and published in Quaternary Newsletter, Vol 129, Feb 2013, pp 46-51.  He runs through a number of lines of evidence, defending in each case the old and traditional view that almost everything on Dartmoor can be explained by reference to a very long history of periglacial action combined with slope processes. 

What should one make of the new spat?  Well, it's all innocent fun -- another case of bright young things coming up with a radical new theory and a senior academic defending the status quo, to which he has no doubt held a lifelong allegiance.  That having been said, I don't find Allan Straw's defence of the ancient tradition very convincing -- it presents no new evidence, and simply seems to be a demonstration of scepticism,  suggesting that certain named features MIGHT just be periglacial after all, and not glacial.....

It will be interesting to see how David Evans and his colleagues respond to this criticism, as they undoubtedly will.....


Anonymous said...

I suppose in the terms of this blog the interesting point would whether stones might have been moved over long distances.

As a rank outsider in geomorphology the exact distinction between glacial and peri-glacial is a grey zone. One year it might be glacial, a year later peri-glacial. Over one hundred years it might be a bit of both.

Dartmoor/Somerset was close to the borderline - a bit of both. Brian, please help me understand why this intellectual debate is important to our quest?

Anonymous said...

T'was ever thus!


BRIAN JOHN said...

Anon -- there is plenty on this site which describes the different processes that operate in glacial and periglacial environments. It's crucial to out understanding of how and when the stones got to Salisbury Plain to know how many glaciations affected the area, and how extensive they were. More in the pipeline -- watch this space!

Constantinos Ragazas said...


Most impressive photo. Is this rock free standing or embedded? And if embedded as a bedrock outcrop, can it be so explained as resulting from glacier advance?

Also, the upper sections show more wear and at shorter intervals than the bottom ¾ of the rock. What glacier or periglacial processes can account for that? Easy to see how water could have!


BRIAN JOHN said...

This is one of the Dartmoor tors -- as in all tors this is a BEDROCK outcrop. Tors are residuals -- ie bedrock outcrops left behind when (for one reason or another) the more easily eroded material around them has been removed. Glacial or periglacial processes can help in removing the surrounding rock debris -- but in hot climates that is achieved by other processes like weathering, gravity and wind winnowing. On the Breton coast water action might have something to do with it, but not here on the summits of a broad upland area.

The fracture patterns are highly variable in granita areas -- but they might be tighter towards the top of these tors because of pressure release processes.