An Ailsa Craig erratic boulder (much sampled by geologists) on the beach at White Park Bay on the north coast of Northern Ireland. This is just the tip of the boulder -- it's the biggest Ailsa Craig erratic I have seen.
The pic is from Peter Wilson's interesting video on glaciation and glacial features of the Irish Mountains:
https://www.youtube.com/watch?v=voQYLd5svmc
Peter mentions that Ailsa Craig erratics are found all the way along the eastern coast of Ireland and as far to the south-east as Cork Harbour. That's 350 miles from the source in the Firth of Clyde. Perfect spot provenancing? Well, not quite, since we don't know how extensive the original micro-granite intrusion might have been before thye onset of glaciation and the recent (Holocene) rise of relative sea-level.
The bedrock in White Park Bay is Upper Cretaceous chalk or "white limestone". As a matter of interest, one of the larger erratics found at Kenn, not far from Bristol, is made of "white limestone" and is assumed to have come from Northern Ireland -- transported by the Irish Sea Ice Stream.
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