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Wednesday, 9 June 2010

More on the English Channel ice foot



The Celtic Sea "glacier model" made by Dr Alun Hubbard and colleagues

I am more and more convinced that there is so much erratic material scattered along the coasts of the South-West Peninsula and the English Channel that we have to think very seriously about the role played by floating ice and the ice foot in the evolution of the coastline. Many years ago I wrote (with my colleague David Sugden) a hefty paper called "Coastal Geomorphology of High Latitudes" (Progress in Geography, Vol 7, pp 54-132.) In it, we included a pretty comprehensive piece about the ice foot, and touched on other matters including iceberg drift and sediment and erratic transport by floating ice. Recommended reading, though I say so myself.......

OK -- for these coastal processes typical of high latitudes to operate in Southern England, we need a very severe glacial / periglacial climate, floating glacier ice (ie a calving ice edge) somewhere appropriate for iceberg and ice-floe drift under the influence of winds and currents, and also sufficient isostatic depression to bring the shoreline down to the approx level of the sea surface of the time. We must be talking here about the peak of a glacial episode, with sea-level (Wolstonian? Anglian?) around 140m below the present. A simple calculation tells us that we need approx 400m of ice over the adjacent land surface, to achieve the necessary loading, or maybe even thicker ice a little distance to the north, and then a proportional or elastic depression of areas beyond the ice edge. The map above gives us just about the right scenario.

WITHOUT ICE FILLING THE CELTIC SEA AND THE BRISTOL CHANNEL WE JUST CANNOT GET THE RIGHT CONDITIONS TO EXPLAIN THE MASS OF FAR-TRAVELLED ERRATICS THAT WE FIND AROUND THE COASTS OF DEVON, CORNWALL, AND THE ENGLISH CHANNEL. Following deglaciation, isostatic uplift and eustatic sea-level rise have brought sea and land up to their present positions, although there may have been various transgressions and regressions during this process of recovery, since we cannot expect everything to have been in phase.

So let's take the map as a working model -- it will need modification, but it gives us the following:

1. A mechanism for transporting "bluestones" from the west into the environs of Somerset and the Wiltshire Downs.

2. A floating ice edge to the west of the Scilly Isles, providing ideal conditions for the calving of icebergs and brash ice into deep water.

3. A source for erratics and other debris to be carried by floating ice broadly eastwards into the English Channel, driven by prevailing winds and tidal and other currents.

4. An explanation for the erratics and till deposits (and ice edge meltwater channels) on the west-facing coasts of Devon and Cornwall.

5.Conditions perfect for the operation of "ice foot processes" along the shorelines of the Channel Coasts.

Hey presto. All sorted.

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