More recently, a letter in The Orcadian newspaper held a clue to solve the mystery.
The letter writer, Morris Pottinger (who I understand once lived on Orkney, but now resides on the other side of the Pentland Firth in Caithness), was discussing a problem associated with the ferry port of Burwick in South Ronaldsay.
"I studied old pre-war tidal charts and new post-war ones, and was struck by the difference from pre-Churchill Barriers and afterwards in relation to the Lothar Rock outside Burwick.
Pre-war, the tides did indeed hasten on flood or ebb past the Lothar, but after 1945 the flow increased massively at that point on the south-east corner of South Ronaldsay.
The effect of this changed, and increased, flow on Caithness is that no longer does shell sand come in at John o' Groats, Duncansby or the other shell sand beaches we had.
A look at the east side of the Barrier between Burray and South Ronaldsay tells you where it now lodges. Not too far to find a reason.
The four channels now Barriered no longer allow ebb and flow to pass through, channelling all the tides either direction past the Lothar..."
In the above map (pre-barriers!), Burray can be seen top right, Burwick is on the south-west corner of South Ronaldsay and Duncansby Head is shown in the north-east corner of Caithness, on the Scottish mainland.