It's a circular argument (but there's this one - http://www.themodernantiquarian.com/post/39772 ). It's a rock that supports the view that stones were intended to point out trackways and those walkers are on the Pennine Way. (This section is known to be at least Roman - paved in parts - and is still known as the Maiden Way).

I'm so superstitious about Orkney (and Wales) that I'm prepared to believe all of the rocks in those landscapes were probably stood up 'once upon a time'. There is no objective way of determining which is prehistoric and which not. It always comes down to an expert opinion or painstaking excavations that may, or may not, yield dateable remains.