It's quite a pretty one isn't it.
I love the way that the diagram (taken from a photograph) has a copyright notice on it. What a load of bollocks!
How did you know the plank was 5ft long? :-)
Width of the link avenues. Just a guess, but a better one than "it was an effect of the spiritual power lines - a sort of overload caused by the tension of the "twin" solstice."