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Monday, 23 May 2016

More on the glaciation of the Isles of Scilly

 The Little Popplestones morainic ridge on Bryher -- one of the glacial landforms which suggest where the Devensian ice margin was located either at the time of maximum extent or slightly later.

Many thanks to Prof Danny McCarroll for sending a PDF of his new paper. It's a very interesting one, making many valid points. I'll do another post on its implications. For the moment, there's an interesting section devoted to the Isles of Scilly. This is what Danny says:

I know of only one place in the British Isles where it is possible to stand in the landscape and see clearly where the limit of glaciation lies, and that is the Isles of Scilly. The ice limit is clear because the islands are formed entirely of granite and have only been glaciated once, so that south of the ice limit erratics are completely absent. The Island of St Marys, for example, has no erratics and on the southern coast of St Martin’s, less than 3 km to the north they are also completely absent. Walking the few hundred metres northward over the moorland of St Martin’s, however, reveals a very gradual transition marked only by the appearance of a few scattered pebbles of mixed lithology. The same transition occurs on the adjacent island of Tresco. On the northern shores of both islands erratic pebbles are abundant and on St Martin’s, at Bread and Cheese Cove, there is even a small remnant of glacitectonized sediment (Hiemstra et al. 2006). The available dating evidence suggests that the ice limit on Scilly was produced during the last glacial cycle (Scourse 1991; McCarroll et al. 2010; Chiverrell et al. 2013). The important point here is that the position of the ice margin on Scilly, irrespective of age, is revealed only by a light dusting of erratics; there is no clear geomorphological evidence. If Scilly had been glaciated more than once, and erratic pebbles were widespread, I doubt that the present limits would even be visible.
Trimline Trauma: The Wider Implications of a Paradigm Shift in Recognising and Interpreting Glacial Limits
Danny McCarroll
Scottish Geographical Journal, 2016
Published 27 Feb 2016
http://dx.doi.org/10.1080/14702541.2016.1157203


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As I have pointed out to Danny in a private message, there are a number of important inaccuracies in that paragraph.  My points of criticism are as follows.

I know of only one place in the British Isles where it is possible to stand in the landscape and see clearly where the limit of glaciation lies, and that is the Isles of Scilly.  You can't see it all that clearly --you only pick it up through careful fieldwork and mapping of erratic distributions and sediments.  As I have said before, James Scourse and others claim to be able to see a difference between the glaciated tors and the unglaciated ones -- but that is actually not at all easy in the field.  In any case, when we do pick up a line it does not mark "the limit of glaciation"  -- it simply marks the greatest extent of Devensian ice.


The ice limit is clear because the islands are formed entirely of granite and have only been glaciated once, so that south of the ice limit erratics are completely absent.   This is wrong.  The islands have been glaciated at least twice.  Erratics are not completely absent outside the Devensian limit.  There are masses of them, when you look for them in the sediments exposed along the cliffline.....

The Island of St Marys, for example, has no erratics.......... That is not correct.  There are plenty of erratics on the island -- as described in previous posts on this blog.

.........and on the southern coast of St Martin’s, less than 3 km to the north they are also completely absent. I suspect this is an unwise statement! I didn't have a chance to explore the south coast in detail, but there are bound to be erratics there, as there are everywhere else. 

Walking the few hundred metres northward over the moorland of St Martin’s, however, reveals a very gradual transition marked only by the appearance of a few scattered pebbles of mixed lithology. The same transition occurs on the adjacent island of Tresco. On the northern shores of both islands erratic pebbles are abundant and on St Martin’s, at Bread and Cheese Cove, there is even a small remnant of glacitectonized sediment (Hiemstra et al. 2006). The available dating evidence suggests that the ice limit on Scilly was produced during the last glacial cycle (Scourse 1991; McCarroll et al. 2010; Chiverrell et al. 2013). The important point here is that the position of the ice margin on Scilly, irrespective of age, is revealed only by a light dusting of erratics; there is no clear geomorphological evidence. I'll go along with most of that.  The light dusting of erratics seen on the ground surface within the Devensian limit is quite marked on Tresco, St Martin's and Bryher.  But there are glacial landforms in the Scillies, as I have indicated here:

http://brian-mountainman.blogspot.co.uk/2016/04/glacial-landforms-on-isles-of-scilly.html 

The old moraines at the northern tip of St Martin's are pretty convincing, as described by Hiemstra et al in 2006.  And there are at least two moraine ridges on Bryher too.  I think they mark retreat stages / readvances, not the outermost ice positions.  But there certainly is geomorphological evidence which needs to be interpreted.    

If Scilly had been glaciated more than once, and erratic pebbles were widespread, I doubt that the present limits would even be visible.  Well, it's clear that the islands HAVE been glaciated more than once, and that erratic pebbles are widespread.  Nonetheless, it is still clear that there is a Devensian limit that can be picked up, largely because of the associated glacial landforms and because all of the coherent glacial sediments observed seem to relate to this most recent glacial phase.

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