The Scilly Nunatak

Satellite image of the Isles of Scilly.  Maybe at the time of the Last Glacial Maximum (LGM) the scene did not look so different?  Just replace the blue with white........ 

Enough of intellectual contortions.  For the last few years all of the geomorphologists (including me!) who have been mulling over the significance of the Late Devensian ice limit on the north coast of the Isles of Scilly have been trying to work out how an ISIS  terminus (or lateral ice edge) at this position could be squared with an ice edge far to the south, on the continental shelf edge.  Most people have shrugged their shoulders and tried to explain that on the eastern flank of the ice stream the ice really did stop here; and some have become so wedded to this idea that they have become really rather angry that I should have had the temerity to say that there was evidence of active ice further south, on the coasts of St Agnes and St Mary's islands.

My map of proposed ice extent, published in Quaternary Newsletter, caused considerable anger in some quarters..........

Well, in the last few days I have been reading Ed Lockhart's excellent PhD thesis, and have once again been involved in intellectual convolutions as I tried to work out whether his evidence really does support his own conclusions about the "eastern ice edge".........  

This morning, while having my morning shower, I realised that the only way out of this impasse is to propose that the Scilly island group was a single nunatak at the time of the LGM, completely surrounded by flowing glacier ice; and that the "eastern ice edge" was some distance to the south and east.  If there were any moraines associated with this limit, they are on the sea bed.

I propose that the ice edge on the northern islands, on the up-glacier nunatak flank, was above present sea-level; as suggested by Scourse, Hiemstra and others, active ice affected the islands up to an altitude of c 40m.  Further south, on St Mary's and St Agnes, the "glaciation limit" was around present sea-level.  And on the SE or down-glacier flank it was below present sea-level, explaining why there appear to be no fresh glacial deposits on the SE-facing coasts.

The map below updates the map published with my QN article (2018) by adding an ice edge along the eastern and south-eastern margins of the archipelago.

A reconstruction of what the Scilly nunatak might have looked like.  The areas coloured green and blue on the bathymetric chart are the shallows -- probably, at the LGM, these areas were covered by extensive snowfields, while the island summits were exposed and would have been visible as rocky hill masses during the summer season.

Here are some "nunatak analogies" -- many illustrations show very spectacular nunataks with steep cliffs and rocky pinnacles -- but most of them are in reality rather boring in appearance......

I have been taking a look at some of the ice sheet fringes (on Google Earth) to find analogies, and this one hit me between the eyes.  It's an image from East Greenland, not far from the ice sheet edge.  These are not jagged mountain summits projecting through the ice, but a series of broad and gentle hill summits which must have been overridden by the ice sheet in the past but which are now in the ablation zone -- the ice is not thick enough or active enough to overtop the summits.   The scale is very similar to that of the Scilly Nunatak -- the up-glacier nunatak edge is between 6 km and 8 km long.  Along this edge the ice is blocked, and we can see that a sinuous frontal moraine has been formed -- just as happened in the Scilly Isles in the Late Devensian.  The unimpeded ice to left and right of the nunatak continues out towards the coast.  There are no traces of moraines above the ice surface on the nunatak flanks, or on the down-glacier edge, which will be at a lower altitude than the up-glacier edge.  This is a rather spectacular analogy.........

It's interesting that some of the models by the BRITICE group and others have also shown the Scilly archipelago as a nunatak, surrounded by streaming ice.  Another factor that feeds into the discussion is the "accepted wisdom" that the ISIS terminated at the northern coast of the archipelago, meaning that sea-bed studies of sediments and bedforms have been largely ignored to the south and east of the islands, with effort (quite understandably) concentrated further to the west and south-west, in the centre of the perceived ice stream.

So if evidence is missing, it is because hardly anybody has tried to collect it......

I believe that there is nothing in the literature re sea floor sediments and bedforms to contradict this "nunatak" hypothesis.  As ever, comments and additional information are welcome.