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Friday, 22 December 2017

More fun and games on Lundy Island

On this map from Carr, Hiemstra and Owen (2017) the "established" Late Devensian ice limit is shown with a solid line. Established by whom?  It is inaccurate in Pembrokeshire, and the dashed line showing an ice edge close to the coasts of Devon and Cornwall seems to be a much better fit for the field evidence

Here is a new paper:

Landscape evolution of Lundy Island: challenging the proposed MIS 3 glaciation of SW Britain
Simon J. Carr, John F. Hiemstra, Geraint Owen of the Geologists’ Association 128 (2017) 722–741

A B S T R A C TLundy Island, in the Bristol Channel of south-west Britain, holds a pivotal place in understanding theextent and timing of Quaternary glaciations in southern Britain, in particular the timing, extent anddynamics of the Irish Sea Ice Stream during the Devensian glaciation. New geomorphologicalobservations and revised interpretations of geomorphological and cosmogenic exposure data lead tothe conclusion that Lundy was not covered by ice in the last (Devensian) glaciation. Geomorphological features are related to surface lowering by means of granite weathering under mainly periglacial and cool-temperate conditions. Previously reported cosmogenic ages are re-interpreted to reflect a dynamic equilibrium of cosmogenic nuclide production and surface lowering during a prolonged period of subaerial granite weathering. This re-evaluation of the geomorphology of Lundy Island challenges recently proposed interpretations of early glacial cover of Lundy (MIS 4-3) and for cold-based ice cover at the Last Glacial Maximum (MIS 2), and instead supports existing regional ice sheet reconstructions. This study demonstrates that a robust, coherent geomorphological framework is fundamentally important to support the validity of detailed geochronological and stratigraphic investigations

It’s always good to see an old-fashioned geomorphological row going on — conducted, as ever, in the politest possible terms. In this paper, Carr, Hiemstra and Owen go after Chris Rolfe and others who have suggested (rather uniquely, it has to be said) that the most dramatic recent glaciation of Lundy Island was during the Early Devensian, at a time when most workers consider the Bristol Channel basin to have been ice-free. We have considered the Rolfe thesis here:
and you can find other discussions on Lundy by typing "Rolfe Lundy" into the search box........

I have been somewhat sceptical about the "Rolfe theory", largely on the basis that it appears to be unsupported elsewhere. My own feeling was, on reading the Rolfe et al (2012) paper, that the cosmogenic dates / exposure ages on which it depends were all inaccurate for some reason or another. Maybe the cause of the false (?) readings was intermittent rock surface cover or rock surface breakdown by weathering……… 

Anyway, when I got at this PDF I was rather interested in what the new authors had to say. The abstract is above.

I was rather concerned at the outset by the authors’ assumption that there are accepted “regional reconstructions” and “determined ice margins” for other parts of the Celtic Sea / Bristol Channel arena. They also refer to "the established regional model of glaciation of the Southwest British Isles” as if it is soundly based and widely accepted. Well, I don’t think it is, and since the LGM maximum of the Irish Sea Glacier is in the wrong place in West Wales on Figure 1, it is very probably in the wrong places elsewhere as well.

The authors give a summary of the geology and regional context of Lundy, and when they consider the borehole and sedimentary evidence from the floor of the Bristol Channel. Strangely, they are disinclined to believe the data with respect to glacigenic sediments beneath sea-level, and suggest that the deposits referred to as “glacigenic” may not actually have a glacial origin because there are no known scouring or streamlining features in the vicinity. I don’t follow that line of reasoning at all, since some glaciated areas have streamlined features and others do not. Anyway, at an early stage in the paper it’s fairly obvious where the authors are heading.

In the “evidence” part of the paper, the authors seek to systematically dismantle the claims by Rolfe et a that there are streamlined granite surfaces and associated features, whaleback forms, grooves and meltwater channels, erratic scatters and perched boulders, tors, faceted clasts and diamictons, and periglacial materials. Then they reinterpret the cosmogenic dates as corrupt because of the gradual lowering of granite surfaces by granular disintegration and “grus” production.

These are the conclusions:
An evaluation of the geomorphological evidence previously reported by Mitchell (1968) and Rolfe et al. (2012, 2014) in support of Devensian glaciation of Lundy Island cannot be reconciled with any glaciologically-plausible or coherent glacial landsystems. Re-examination of the geomorphology of Lundy Island identifies a suite of related landforms that reflect a combination of subaerial processes of pneumatolysis, granite weathering and slope movement operating under cool-maritime temperate and periglacial conditions, with more recent impact of wildfires and human agency. Critically, this study finds no evidence for past glaciation, contrary to previous work. A revised model of Late Quaternary landscape evolution of Lundy Island dominated by granite weathering periodically enhanced by periglacial conditions offers a simple and realistic explanation of previously-published cosmogenic nuclide dates. This revised model removes the need to explain an additional MIS 3/4 glacial advance not recorded elsewhere in the southwestern sector of the BIIS, and conforms to the established regional glacial stratigraphy of the Irish Sea Ice Stream, as well as previously-reported periglacial development of tors on nearby Dartmoor.
So where does this leave us? Some of the evidence presented here is convincing, and the authors are right, I think, to question some of the assumptions of Rolfe et al about what happened on Lundy in the Early and Late Devensan. But just as Rolfe et al have “forced” the evidence into their hypothesis, and have found traces of glaciation everywhere, Carr et al have gone to the other extreme, and have forced all their evidence into a non-glacial hypothesis. In particular, they seek to argue that the so-called “glacial features” described by Rolfe et al "cannot be reconciled with any glaciologically-plausible or coherent glacial land systems” — I find that unsettling, since landforms and deposits in close juxtaposition do not need to be contemporaneous or even to belong to the same “glaciological plausible” episode. Just because granite surfaces degrade and break up to create scatters of grus and even broken granite cobbles, that does not mean that the surfaces are not glaciated; and just because assorted degradational processes are at work in grooves and bedrock channels, that does not mean that they have not also been affected by glacial meltwater. Just because there are tors nearby, that does not mean that nearby surfaces cannot have been streamlined; and I have seen far too many meltwater channels adjacent to streamlined surfaces to think that the two are mutually exclusive.

The jury is still out. I remain convinced (not having been to Lundy, and having to depend on published evidence) that the island has been affected by overriding ice during the Devensian, and that there are real problems of timing or dating. The reference to an Early Devensian glaciation is anomalous, since one would expect to see evidence of it in Pembrokeshire and elsewhere. And I remain convinced that this new paper by Carr et al does nothing to confirm the correctness of "the established regional model of glaciation of the Southwest British Isles”. I still think that strange lobe of ice pushing out into the Celtic Sea does not make sense.

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