@Madrood, thank you for this thread.
I have read the Guardian piece and note the explanation of games of O and X is the only one offered. And that without any evidence.
Given that the examples are all in or about large buildings of unreinforced masonry, there might be a more straight forward alternative explanation. And it is this:
A website (ThinkHazard.org) describes Galicia as having a low earthquake. "Low" is described as "a 2% chance of potentially-damaging earthquake shaking in your project area in the next 50 years. Based on this information, the impact of earthquake
should be considered in the project, in particular during design and construction."
While the constructors of large buildings in Galicia would not have had the benefit ThinkHazard.org, they may have had the benefit of "community memory". Or even the results of past damaging earthquakes in their vicinity. (A Richter 4.4 event was recorded near A Corruna in October 2021. Another website (
Earthquakes ) reports the following near Sarria in the last 90 days:
Richter 5 = 1; Richter 4 = 0; Richter 3 = 0; Richter 2 = 45 (of which one happened "17 hours ago")
So, the nicely rounded depressions in stones for a larger building maybe intended to while away an idle moment.
Or they may have been intended for a lick of mortar to help the unreinforced masonry stay in place.
Of course, more research is needed (anyone for tennis). And you pays your money and you take your choice.
Kia kaha tātou (you all take care and be strong)