I am not an expert on cairns, don't get me wrong, but to me this looks like someone has taken part of the wall behind down, notice how the wall looks rather shorts of stones on the left hand side..and tried to build a cairn...it does not have the look of a cairn...and it is strange how all the stones look clean, both on the "cairn" and the wall... almost looks like it was built yesterday..i may be wrong...but i don't think or believe this is prehistoric...it may be a cairn...but it is a modern cairn.