Just wondering, how deep is the water in these locations?
The drop from the mainland to the Ireland end of the Causeway (which formed after the end of the Cretaceous) appears to be pretty steep (those who have seen it and are in this thread can better tell us) and the Causeway doesn’t go all that far above the water. It then plunges and (if a faint old memory serves) curves more or less, and gets lost under perhaps millions of years of sedimentation. I couldn’t find anything about the identification of the formation having been identified anywhere at depth. Suffice to say, the deepest part of that strait is probably in the area of 1000+ feet.
> The old name for the Giant’s Causeway was clachanafomhaire roughly translated as ‘the stepping stones for the Fomorians’.
http://www.doeni.gov.uk/niea/protected_areas_home/aonb/aonb_causeway/aonb_causeway_giants.htm