Topographic controls on glaciation in the Isles of Scilly

This map shows the Scilly Islands Archipelago as it was around 5,000 years ago, at the transition between the Neolithic and the Bronze Age.  The solid green area shows the land area and the lime green area is the intertidal zone.   People could walk with ease between all of the islands except for St Agnes

Fed up with Waun Mawn today. Time for a change of topic.  Been thinking about this:  if a surging glacier with a low surface gradient, at a time of low sea-level, approached the Isles of Scilly from the NW, what would be the shape of the ice front?  (This is more or less what happened, maybe around 27,000 years ago, so the question is not just theoretical...........)

Glaciological controls would be hugely important, of course, but  the other thing that would be of vast importance would be the shape and nature of the bed over which the ice was flowing. Sea-level was low at the time, and the coast was far away.  So the buoyancy of ice was not a factor here.  We can see from the map where the topographic obstacles to ice flow are, and we can assume that the ice would have preferentially flowed across the uncoloured areas on the map, other things being equal.  But they were of course not equal, and because the ice was flowing broadly from the NW towards the SE the contours on the ice surface would have been running approx NE to SW.  So you would expect the NW coasts of the islands to be more heavily inundated than the SE coasts of St Agnes, Gugh and St Mary's.  You can pick up more details of the low-level terrain on this marine chart.

The lower of these two maps is easier to read, but for some reason the western edge has been chopped off it.  

Because the ice edge in the Devensian was more than 250km away, out towards the edge of the Celtic Sea shelf, there must have been a massive glacier to the west and very close to the archipelago.  At one time it reached the island coastlines, and because ice always flows perpendicular to the ice edge, the ice (as indicated above) must have flowed broadly from NW towards SE.

So here's a question:  would the ice only have affected the extreme northern coasts of the archipelago?   No way -- it must have flowed into St Mary's Sound and it must have affected the coasts of St Agnes and St Mary's.  It's almost certain that it would also have affected the islands of Annet and Samson -- so they need to be examined carefully for glacial traces.

As I have shown in previous posts, there are indeed glacial traces (in the form of thin patches of till, maybe redistributed by slope and ice melt processes) on the north and west coasts of St Agnes, and at Cape Morval on the island of St Mary's.

                             

On the above map the red line shows the Devensian ice limit as agreed by lots of other field workers, striking the north coasts and located well offshore further south.  If the ice had behaved like this it would have been a major aberration -- so it is much more in line with glacier behaviour to envisage a crenulated glacier edge, with tongues of ice pressing into all of the low-lying areas in the north and maybe leaving some of the higher parts of the islands ice-free.  Ice must have flowed into St Mary's Sound broadly from the west.

Lo and behold, that is exactly what the field evidence shows -- see the distribution of glacigenic deposits.  Maybe over-cautiously, I have left Samson outside the glaciated area -- must go back and check!  Annet must, I am sure, have been glaciated. And it is not beyond the bounds of possibility that the  north coasts of the Eastern Isles were also affected by ice.  

Anyway, here is the map with the black line showing my proposed ice edge somewhere around 27,000 years ago.  Theory and ground truthing coincide, as indeed they should, if the theory is sound.......