A rock

Is it a modern sculpture of a human head in the Easter Island style? Or perhaps an ancient head crafted by the Pictish people?

No, I am afraid that it is rather more prosaic. It is a large angular rock set into the top of a WW2 concrete anti-tank block. It's purpose was to tear off the tracks of any Nazi tank that attempted to climb over it. Thankfully it was never put to the test.