Ramsey Island -- a major source of glacial erratics

 

This is a fabulous image of Ramsey Island, off the western tip of the St Davids Peninsula in Pembrokeshire. Click to enlarge.

I have posted on the geology of the island before:

https://brian-mountainman.blogspot.com/2016/01/ramsey-island-geology.html 

https://brian-mountainman.blogspot.com/2015/01/ramsey-island-as-major-source-of.html

Over the years, many geologists have assigned Pembrokeshire erratics to the various igneous outcrops on Ramsey -- and indeed in my posts about the Sleek Stone "giant erratic" near Broad Haven I have suggested that it might be a greenish or bluish quartz porphyry (rhyolitic) from the SW part of the island.   Thousands of tonnes of this material might have been entrained by overriding ice and transported south-eastwards across Ramsey Sound to the mainland.  

The darker coloured "giant erratic" on Ramsey Sound may have come from further north on the island;  one of the boulders in the Storrie Collection (from Pencoed in Glamorgan) is assigned by Bevins and Donnelly to one of the volcanic debris flow outcrops of Ramsey; and it may be that some of the fine-grained lava erratics from St Bride's Haven are also from Ramsey.  Somebody needs to do a systematic study.

Might the Limeslade erratic also have come from Ramsey?  I have some doubts about that, since the initial analyses suggest it is a metamorphosed coarse dolerite with a greenish colour, and I don't think such a rock type exists on Ramsey Island.  Watch this space.......

So if, as seems highly likely, large numbers of erratics have been entrained from Ramsey Island, it's intriguing that there are no obvious plucking or quarrying sites on the lee (or downglacier) flank.  The boulders -- sometimes weighing hundreds of tonnes -- have apparently been incorporated into the ice on the western and south-western hill masses, perhaps with later moulding removing all traces of the "extraction locations."  

This suggests to me that the assumption (which we often see in the literature) of moulding on the upglacier flanks of obstacles, and plucking (and erratic entrainment) on the downglacier flanks, is not very reliable.  The island does not have an obvious roche moutonnee shape.  I have of course also suggested that bluestone erratic entrainment on Preseli happened preferentially on the northern -- or up-glacier -- side of Mynydd Preseli, and I have discussed this at some length with various glacial geomorphology colleagues.  Again, watch this space......