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Saturday, 23 July 2016

Erratic content of Isles of Scilly glacial deposits



 Modern beach deposit, Gugh Island.  Many of these pebbles have come from old deposits exposed in the nearby cliffs.  There are sandstone and shale erratics in this suite of cobbles -- in spite of the fact that Gugh Island is supposed not to have been glaciated.

Grateful thanks to James Scourse for publishing this information in his 1991 paper:
Scourse, J.D.  (1991) Late Pleistocene Stratigraphy and Palaeobotany of the Isles of Scilly. Phil Trans Roy Soc B, December 1991, Volume: 334 Issue: 1271.
A long time ago, but geological information does not date!  The hand sample identifications were made by JR Hawkes of the BGS Petrology Unit.

Below we reproduce info from the appendices of the paper, relating to the Scilly Till examined at Bread and Cheese Cove on St Martin's Island and to assorted samples collected from the related Hell Bay Gravel.


Above:  features of the Scilly Till



Pebbles identified in hand samples -- Nos 1-56, Hell Bay Gravel

The great range of erratic materials is interesting.  Particularly intriguing are the Lower Palaeozoic sandstones and the red / pink / purple / greenish sandstones and marls  which Dr Hawkes speculates as possibly coming from Devonian outcrops in Pembrokeshire, Devon or Ireland.  Brightly coloured Devonian sandstones are quite widespread, but brightly coloured Cambrian sandstones do not outcrop so frequently, and the most obvious source would be the southern coast of the St David's Peninsula around Porth Clais and Caerfai.  How easy is it to differentiate these from the Devonian sandstones?  As far as I know, nobody subsequently has attempted to work out whether these sandstones have come from Cambrian or Devonian outcrops -- there's an interesting project for somebody.  The samples are apparently still held at the School of Ocean Sciences at Bangor University.

The heavy metal suite from the till is also interesting -- perhaps we could get a view from Myris on that?

Additional information:  Dr Hawkes has also found a 10-tonne erratic block of olivine basalt on Great Crebawethan, one of the western rocks not far from the Bishop Rock Lighthouse, and more than 4 km west of St Agnes. This is reported in Scourse (1991).  Who knows what other large erratics are still to be discovered?

3 comments:

MoA said...

I am not certain what can be said about the heavy mineral suite as it is very broad.
Distinguishing between the Cambrian and Devonian red beds should be simple- an/?the authority on the Caerfai red beds is no less than Dr Peter Turner of I and T Altar Stone 'fame' he is also an authority on red beds generally -I think one of the standard red bed tomes is his. (He has recently rejoined the pet rock boys in a series of papers on the Lower Palaeozoic Sst debitage)
The Caerfai red sst -very small outcrops- were the subject of some granny- instigated Mills and Boon archy nonsense regarding Stonehenge bracers being made from them, alongside golden-eyed Irish sailors and Wicklow panned riches.

The trouble with macroscopical descriptions is that they are just that. As has been said here before fine to medium-grained siliciclastics are bloody difficult.
Trouble is they have to be seen and probably sectioned before any Swag ing (Scientifc wild-arse guess.

But a very interesting post.
M

BRIAN JOHN said...

I wonder if Peter would like to have a look at those pebbles? They are apparently all up in Bangor....... just a matter of asking Jim Scourse for a look.......

BRIAN JOHN said...

That 10-tonne lump of olivine basalt -- am I right in thinking that is most likely to be from Northern Ireland or western Scotland? As far as I am aware, there are no rocks of this type in West Wales, Devon and Cornwall -- so we must be looking at really long-distance glacial transport here.......