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Monday, 18 April 2016

The Devensian Ice Limit in the Isles of Scilly

This is my carefully considered ice edge map for the Devension glaciation of the Isles of Scilly.  The archipelago is a fantastic "geomorphology laboratory" in which it is possible to pick up obvious signs of glacial deposition in one locality before moving to another locality a hundred metres away to find that signs of glaciation are quite absent. The ice edge was quite sharp, but I agree with other authors that it was quite short-lived, and that it might well have been linked to a surge.  The morainic ridges described by Hiemstra et al (2006) seem to me to be very convincing glacial constructional features, but they are not terminal moraines that mark the ice's greatest extent.  So they might mark retreat stillstands of the ice edge, or else short-lived readvances.

In the map above the yellow line shows the southern limit of the Hell Bay Gravel, used by some as a proxy for the ice edge.  But it's not an accurate line, and neither is the Hell Bay Gravel a distinctive deposit -- so the yellow line is nothing more than an approximate guide.

The blue line shows the suggested ice limit of Mitchell and Orme (1967), as adopted by Scourse and other authors more recently with relatively minor changes in the shallows around Northwethel, St Helen's and Tean Islands. Scourse (1998), in the GCR volume on the Quaternary of SW England, straightens out the blue line as it is shown above, but it is uncertain whether this adjustment is based on detailed observations.

My main criticisms of the blue and yellow lines as shown above are as follows:

1.  There are fresh glacial deposits outside them, for example on Bryher, St Mary's and St Agnes.

2.  They do not take proper account of topography, and of the tendency of glacier ice to flow round obstacles and into troughs or depressions.  Given the shape of the sea floor in the archipelago, and given that Devensian ice has clearly overtopped the higher moors of Bryher, Tresco and St Martin's, it is unrealistic to draw straight ice edges that include hills over 40m high and also the straits between the present day islands.  The straits and sounds must have contained lobes or tongues of ice.

My red line on the map is drawn to enclose all of the fresh till exposures I was able to identify during the course of a week in the Scillies in April 2016. The red dots show the locations of the main exposures / outcrops of till which I noticed on my travels.  Ice lobes are now suggested in the sound between Tresco and St Martin's, in the strait between St Martin's and Tean, in the narrow channel between Tresco and Bryher, and in the wide sound between Samson and St Agnes.

I suggest that Samson and the Eastern Isles lay outside the glaciation limit, but that Annet might have been over-ridden.  I have not visited any of these, so observations from others are welcome.  To the west of Bryher, Gweal must also have been over-ridden.

When did this last ice incursion occur?   Scourse and others suggest a date of c 20,000 - 18,000 years BP, and this seems entirely plausible.

Finally a word about ice movement directions.  Scourse and others have suggested that the ice that affected the Isles of Scilly was moving from NE towards SW.  I disagree with this.  In relatively unconstrained situations, ice always moves perpendicular to the ice edge.  So it is entirely sensible to propose that the Devensian ice of the Irish Sea Glacier moved into the archipelago broadly from NW towards SE, except in the southern islands where it might well have moved through the Northwest Passage into St Mary's Road from west to east.  That makes perfectly good glaciological sense, and also explains the distribution of glacial sediments.

This might seem to be a radical redrawing of the glacial map of the Scilly Isles, but I contented that it is much more accurate than earlier attempts and that it is supported on the ground.  Much to my own surprise, the impact of Devensian ice on this remote archipelago off the tip of Cornwall was much greater than has previously been supposed.


Ian Evans said...

Well done, it is clear that an ice advance over hilly topography will produce a crenelated margin. Your three southernmost new exposures are the most critical in extending the ice limit, and others working on this topic should focus on checking these.

BRIAN JOHN said...

Thanks Ian. Yes, agree that St Marys and especially St Agnes need to be looked at carefully -- we must encourage QRA members to go there on holiday and do some checking.......

And I.m quite convinced about an even larger glacialtion that covered the whole of the archipelago. As far as I can see, there is no part of the Scillies devoid of erratics in the lower head and raised beach.