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Thursday, 21 March 2013

The dating of the Dartmoor Tors

 Greater Dartmoor, showing the locations used for the sampling of the granite tors for cosmogenic dating purposes.  In the north of the marked area, the whitish overlay shows the presumed extent of the Devensian ice cap.

The proposed extent of the Dartmoor ice Cap in the late Devensian. Seven samples were taken for cosmogenic dating from within this area assumed to have been covered by ice.

Many thanks to Yanni Gunnel for sending this article, which I have mentioned earlier on this blog. 

"The granite tors of Dartmoor, Southwest England: rapid and recent emergence
revealed by Late Pleistocene cosmogenic apparent exposure ages."
Yanni Gunnell, David Jarman, Régis Braucher, Marc Calvet, Magali Delmas, Laetitia Leanni,
Didier Bourlès, Maurice Arnold, Georges Aumaître, Karim Keddaouche.  in: Quaternary Science Reviews 61 (2013) pp 62-76

This is not something I know a lot about, but the article is well presented and carefully considered, and I find it convincing.  Here is the Abstract:

Dartmoor, in SW England, is a classic periglaciated granite upland adorned with a population of over 150 tors. The origin of the tors has been controversial, but their emergence by differentiation after stripping of regolith during Pleistocene cold phases is accepted. However, their actual age has been unknown, with possible scenarios ranging from preservation since the early Middle Pleistocene to relatively short-lived landforms in a maritime climate with high denudation rates. The latter is now
supported by 32 cosmogenic surface exposure dates from 28 tors across the whole upland. The distribution of apparent 10Be ages peaks strongly in the Middle Devensian (36-50 ka), which with corrections for weathering and limited ice shielding could be interpreted as Early Devensian. These ages are much younger than those found for three glacially unmodified Cairngorms tors, and somewhat younger even than glacially modified Cairngorms tors. The dates show little spatial variation. Although an ice cap has now been modelled in the heart of northern Dartmoor, tors here are of median age, suggesting that ice cover sufficient to shield tors from incoming radiation was of short duration. The few younger tor ages support the idea of continuing landform instability across the landscape, with weathering flakes redeveloping soon after inferred loss of top pillows by gelifraction or gravitational toppling. The few older tor ages have no systematic explanation, and may indicate inheritance from an earlier cycle of bedrock near exposure.

Since most tors are modest in height (typically 2-5 m), volumetrically insignificant, and often
in advanced stages of disintegration, the general impression is that they are evanescent features, which emerge and quickly disappear during every Pleistocene climatic downturn. Tor populations may thus flicker across the landscape rather randomly over the Quaternary. The remarkably consistent age of the present tor population could be associated with a stripping event at the start of the Devensian, but fuller analysis must await closer controls on tor denudation rates in different climatic phases, and on ice cover extent and duration. These results only date extant tor surfaces, not the landscape, but as the best available erosion pins they have evident value in exploring theories of the evolution of Dartmoor during the Quaternary.

Much of the paper is concerned with sampling procedures and correction factors -- but the results are interesting.  Of the 32 samples taken, the oldest was 117,000 yrs BP and the majority were clustered around 40,000 - 45,000 yrs BP -- in the Early Devensian.  The youngest date was around 9,000 yrs BP.   At face value, this means that the rock surfaces (if not the tors themselves) are surprisingly young -- much younger than the surfaces of tors in the Cairngorms, for example.  This means that the landscape is surprisingly dynamic, evolving quite rapidly under the influence of effective weathering processes. the authors do not think that the results contribute much to the debate on whether or not Dartmoor had its own ice cap.  as they say, such an ice cap would have been so short lived that it will not have skewed the cosmogenic readings very much at all.  Indeed, it may well be that while the ice cap was present, the rest of the ground surface on Dartmoor will also have been protected -- by perennial and seasonal snowfields.

What we still do not know is the extent to which the results are skewed as a consequence of granite weathering and erosion.  Are we looking here at artifices, or meaningful dates?  In an attempt to resolve this issue, the authors estimate or calculate rates of ground surface lowering or denudation.  The figures suggest that amid the dartmoor tors the ground surface is being lowered at rates between 76 mm per millennium and 6 mm per millennium.  Mostly the figures suggest between 10 mm and 25 mm per millennium.

All in all, an interesting contribution to our knowledge of what happened in the Ice age in SW Britain..... and I particularly like the idea of the "flickering" tors -- coming and going over a very long period of time depending on the upturns and downturns of the climate.

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