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Tuesday, 19 February 2013

Dating the Dartmoor Tors



This is an interesting paper, just published. I haven't had a chance to read the full thing yet, but it appears from the abstract that the cosmogenic dates for the Dartmoor tor surfaces are remarkably young, clustering in the period 36,000 - 50,000 years BP.  In other words, the rock surfaces were first exposed during the long period of periglacial conditions before the build-up of the short-lived Dartmoor Ice Cap which we have discussed earlier.

I still have a healthy scepticism for cosmogenic dates, since there are many things that can influence them and lead to false ages, but I'll reserve judgment until I have looked over the paper properly.

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

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. 

6 comments:

Constantinos Ragazas said...

Brian,

What shields stones from air exposure? I can only think of water, ice and soil. If these tors are found to be so young, isn't it more likely an ice cover shielded them from exposure? Can we rule out sea water at that altitude? And if the tors were buried in soil the question is what eroded so much soil away? Back to water and ice once again. So back to ice!

Nice arrangement of stones in the photo you posted! I've seen similar in the MOMA in New York City. Happen to know the artist?

Kostas

BRIAN JOHN said...

Kostas -- seasonal or perennial snow is far more important than ice in shielding a rock surface in a periglacial climate. Other things are vegetation -- grasses, shrubby plants or overshading trees, if the climate is right. In this sort of environment, peat is also a factor. Forget the sea water and the lakes....

Constantinos Ragazas said...

Brian,

I agree. Unlikely sea water to have been the cover up!

But something about cosmogenic dating. What happens when the exposed surface flakes off over time due to weathering?

Kostas

BRIAN JOHN said...

That is exactly the point, Kostas. If the surface is continually crumbling of flaking during the time of cosmogenic bombardment, then you will get a falsely young date -- or a date that simply dates the "micro-surface" of the rock, but not the tor itself. Now the authors may have considered this -- I will reserve judgment until I have read the full paper.

Constantinos Ragazas said...

Brian,

Science can get too clever by any measure. Better to stick to what 'makes sense'. And bluestone human transport does not make sense!

Kostas

BRIAN JOHN said...

Cosmogenic dating is pretty good, but I am cautious about it. Preferably several different methods should be used in parallel -- and no amount of technical wizardry can be a substitute for good geomorphology. The latter may be missing in this case -- we shall see....