I found this small erratic of Carboniferous Limestone in the Irish Sea till at Gwbert, near Cardigan, today. It has two beautiful brachiopod fossils in it, and as befits a far-travelled stone its surface is covered in striations.
Where has it come from? The best bet is that it has come from somewhere in central Ireland, and limestones rich in braciopods seem to be especially common in the Burren area on the west coast. Here is an example:
However, from what we know of past iceflow directions, a source somewhere near Dublin seems more likely:
Geological map opf Ireland -- the light blue tint shows the area of Carboniferous Limestone outcrops.
If the erratic really did come from Ireland, there must have been much more mixing of ice streams and erratic material in the Cardigan Bay -- St Georges Channel area than has been previously recognized............
We also have to consider Anglesey and the North Wales coast as source areas, although the outcrops there are of very limited extent. This is from the GeoMon Global Geopark web site:
.............highly fossiliferous limestones outcrop all long the east coast southwards from Lligwy to Penmon [and along the coast of North Wales from the Great Orme to Prestatyn]. These rocks were laid down in cycles as the sea transgressed and retreated as many as 11 times over North Wales. On Anglesey, only four of these cycles, which consist of limestone followed by sandy beds and then mudstone were deposited. The rocks are rich in brachiopods and corals and often exhibit palaeokarstic surfaces and rarer sandstone pipes.
An Anglesey origin for the erratic is, on balance, most likely since this accords most closely with the ice flow pattern shown on the map above.
An interesting dilemma.........