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Saturday, 9 April 2016

Glaciation on the Isles of Scilly



 Striated stones from glacial deposits on the north coast of St Martin's (credit:  Peter Knight)

Here we are on the Isles of Scilly, and already I have found glacial deposits outside the ice margin as shown on the maps of previous field workers.  More of that anon.  In the meantime, this is a nice summary from Peter Knight.

From Peter Knight's web site:
http://www.petergknight.com/scilly/glaciers.html

If you know Scilly at all, you will be familiar with the gravelly orange cliffs of "ram" or "head" that overlie the bedrock all around the islands' coasts. This is NOT glacial sediment, so don't get excited. Actually it is still quite exciting, as it is what we call a "periglacial" deposit, created by mass movements (soil creep and landslides) when the ground was going through periods of thawing and freezing during the ice age when glaciers were nearby but not actually covering the islands. The glaciers only seem to have covered a tiny little bit of the north of Scilly, and that is where you can find the glacial sediments: the "till" that contains the erratics. The glacial sediment generally has more clay and silt in it compared with the dominantly sandy gravel of the ram or head around the rest of the islands, but both the till and the ram have lots of stones and boulders in, so it is not always easy at all to tell the difference! In section of cliff on St Martins, there is a layer of till sandwiched between layers of ram above and below it. What geologists call the "type site" for the Scilly till, in other words the location where they had a good look at it and were able to describe and define it properly, is Bread & Cheese Cove on the north east side of St Martins. In fact, this location features in a guide to the UK's most important Geological Sites relating to the history of the last 2 million years or so that was published in 2015 (see reading list below). Over the years a number of professional (and amateur) glacial geologists have explored the northern coastlines of Scilly looking for glacial deposits, and it seems that you might strike lucky anywhere along that thin strip of coastline across the northern perimeter of Scilly from the eastern end of St Martins around to the northern coasts of Tresco and possibly even Bryher. It has been suggested that the White Island bar might have been pushed up as a ridge of rocks by the bulldozer effect of the front of the glacier, and I think (although I have not seen this published anywhere) that the ridge, a few metres high and a few hundred metres long, that runs parallel to the coast around the north west corner of St Martins overlooking White Island looks very much like a glacial moraine (a ridge of debris deposited or pushed up by the ice). There are is also a layer of gravels on Tresco (called the Tregarthen gravels!) that are supposed to have been deposited in streams running off the glacier, and there is a line of boulders at Shipman Head on Bryher that is supposed to be a glacial moraine: a ridge of debris pushup up by the edge of the ice. Some people have argued that the top of Round Island shows signs of glacial erosion, indicating that the glacier actually ran over the top of it.


Key references on the glaciation of the Isles of Scilly:

• Scourse, J.D. (1998) The Quaternary History of the Isles of Scilly (Chapter 8 in "Quaternary of South-West England", edited by S. Campbell et al.) Joint Nature Conservation Committee; Springer Science.
 • McCarroll, D. and others (2010) Exposure-age constraints on the extent, timing and rate of retreat of the last Irish Sea ice stream. Quaternary Science Reviews, Volume 29, Pages 1844-1852.
 • Hiemstra, J.F. and others (2006) New evidence for a grounded Irish Sea glaciation of the Isles of Scilly, UK. Quaternary Science Reviews, Volume 25, Pages 299-309.

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