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Although movement by glaciers seems an attractive idea for the Altar Stone it is a non-starter. Glacial flow is off the Cairngorm/Grampian highlands north across Caithness and Orkney - Peach et al mapped this 100 years ago or more and in the last few years BRITICE have showed the same. This means that the erratics from Orkney would have been transported out into the North Atlantic, and there is no obvious mechanism for them to have been picked up by southward-flowing ice.
Dr Brian John’s comments are clear and we concur with them. He says....
Next, I have no problems with what the authors say about ice flow directions and the likelihood of glacial erratic transport. It is indeed very difficult to conceive of ice flow conditions that might have facilitated the glacial transport of a large sandstone erratic from the ORS Orcadian Basin all the way to Salisbury Plain, via either a west coast or east coast route. As far as we know, the ice flow in the last (Devensian) Glaciation was from SE towards NW across Orkney, as described earlier on this blog.
brian-mountainman.blogspot.com/search?q=+Orkney+ice+flow
Source: Adrian Hall et al, 2016
I am afraid not even a little way south.
Something is not right. This message is just to keep things from messing up down the road