This is. or rather this was a popular chiselled mark. What is it and why was it made?
(Not very clear here, but looks sort of like this...
_
/|\
A surveyor's mark.
http://en.wikipedia.org/wiki/Benchmark_%28surveying%29
Well done Robert. 10/10... I'm hoping you had the answer before checking the pic name...?
(Used to mark height above sea level...these days we use other means...sad me thinks.)