In praise of Ailsa Craig

Thee discussion about erratic transport on the western coast of Great Britain -- in the context of the "Altar Stone debate" -- reminds me of the importance of Ailsa Craig, a lonely rock in the Firth of Clyde.

The microgranite that makes up the bulk of the rock is most famous because it is the "rock of choice" for the manufacture of curling stones, because of its hardness, its resistance to percussion fracture, and its appearance.   It's a white rock with minute bluish specks; one of my old teachers, Francis Synge, taught me to recognize it back in days of yore when we wandered about together on the beaches of the Irish Sea coasts.  It was uncanny -- he seemed to be able to recognize small Ailsa Craig pebbles from twenty or thirty yards away!

Ailsa Craig microgranite pebbles (?) collected on the Pembrokeshire beaches. Most are well rounded because they have been washed out of Irish Sea till and incorporated into modern storm beach environments.  I'm somewhat uncertain about a few of these stones, but you get the general idea........

Harrison et al describe the variations in colour and texture of the microgranite found in different parts of the island.

 The Ailsa Craig microgranite -- one of the photomicrographs from Harrison et al (1987).  There are considerable variations in particle size, but the rock is nonetheless very recognizable in hand specimens.

HARRISON, R. K., STONE, P., CAMERON, I. B., ELLIOT, R. W. and HARDING, R. R. 1987
Geology, petrology and geochemistry of Ailsa Craig, Ayrshire. Rep. Br. Geol. Surv., Vol. 16, No.9, 29 pp.

Quote:  The tough microgranite  survived well during transport as glacial erratics and, being a distinctive rock type, can readily be observed in till deposits; Figure 1 shows the localities from which erratics of Ailsa Craig microgranite have been collected (summarised by Charlesworth, 1957). The first discovery was made in the Isle of Man (Kendall, 1891)and subsequently a wide distribution of erratics was recorded from Pembrokeshire Jehu, 1904) to Donegal (Corkey, 1937). The pattern illustrates the flow directions of the two interfering ice sheets which, at various times, have crossed the island. The dominant influence was ice originating in the Scottish Highlands and flowing south and south-west. However, at times ice emanating from the Southern Uplands and flowing north-west has carried erratics into Kintyre and the north of Ireland (Figure 1). Doubtless many of the erratics came to their final resting place by extremely devious routes.

https://nora.nerc.ac.uk/id/eprint/505002/1/Ailsa%20Craig.pdf

The "fan" of Ailsa Craig erratics around the coasts of the Irish Sea, Cardigan Bay and 

St George's Channel (Harrison et al).  Other occurrences lie outside this line -- for example on the Cardigan Bay coast at least as far east as Cardigan.

If you look at the reconstructions of ice flow across the British Isles, it is generally agreed that the major ice shed (ie the highest part of the ice sheet) lay to the north of Ailsa Craig, in the Glasgow - Aran - Mull of Kintyre area.  It was this dome -- which moved back and forth -- which supplied the bulk of ice flowing broadly northwards, westwards and southwards.  Because the Ailsa Craig erratics are all quite small (I have never seen one larger than a human fist) I would argue that they have been comminuted during a very long history of ice entrainment, transport, dumping, fresh entrainment and so forth during multiple glacial episodes.  I agree with Harrison et al that the stones have ended up at their "final resting places" after multiple changes of transport direction.  So the length of the arrows on the map is no guide at all to the distance of travel.  I would not be at all surprised to find Ailsa Craig erratics on any or all of the beaches on the Bristol Channel coasts.

As far as I know, no Ailsa Craig erratics have been found in the Hebrides or in any till or beach locations to the north of the island. That supports the idea that the ice shed or accumulation zone of the Irish Sea Ice Stream was more or less fixed within a limited geographical area. That means it is unlikely (but not impossible) that glacial erratics would have been carried southwards from Northern Scotland or from the Hebrides and then incorporated into the "erratic load" of the Irish Sea Ice Stream.

The Irish Sea Ice Stream / Ice Piedmont as it might have been during the 

Devensian Glaciation.  Other contemporaneous ice caps, ice streams and glacier front positions not shown.   Ailsa Craig is just south of the Ice Shed label on the map.

As for the other Scottish erratics on the shores of the Bristol Channel, it is generally assumed that they might have come from the igneous and sedimentary outcrops of the Midland Valley and Galloway.