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Monday, 19 January 2015

Ramsey Island as a major source of erratics?




I have been struck, in my recent reading, by the number of times that Ramsey Island gets a mention as a possible erratic source.  Most recently I was looking at the paper by Bevins and Donnelly about the Storrie collection of erratics that were gathered up from the Pencoed area of Glamorgan.  They describe a number of sub-rounded fragments made of "mud-rich volcanic debris-flow" material and assign Ramsey Island as a possible source.  They also describe other rock types (gabbro and rhyolitic tuff) from the adjacent St David's Head area.  They also describe three "far-travelled boulders" from the Cowbridge area which are ignimbrites very similar to those found on Ramsey.

Add to this Sid Howells's instinct that quite a few of the erratic pebbles collected on Flat Holm are from Ramsey (still to be confirmed by detailed analyses)...........

And add to this the speculation as to Ramsey origins for some of the more unusual erratic pebbles and boulders collected by the Geological Survey field researchers in the period 1904-21.

Griffiths (1940) identified two erratic types in south Pembrokeshire that he traced to Ramsey Island.

Finally there are the suggestions -- in my mind at least -- that some of the far-travelled erratics on Lundy Island and in the Baggy Point - Croyde - Saunton area might also have come from Ramsey.  Much more thin-section work is needed!

Should we be surprised if Ramsey Island turns out to be rather an important source for far-travelled erratic material?  Not a bit of it.  If Irish Sea Ice was coming in from St George's Channel -- ie from the north-west --  at a time when there was no sea water in the vicinity at the time, ice would have been rising up from a bedrock floor beneath -100m OD to override the western cliffs of the island which are over 100m high.  So ice would have been flowing UPHILL on quite a steep gradient, from NW towards SE, over a distance of not much more than 20 km, and rising 200m (c 660 ft) in the process.  Perfect conditions for compressive ice flow, and perfect conditions for the entrainment of erratics.

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