A Mystery Photograph

I am slowly making headway on my photobook of my trip to Scotland.  This photo was taken across from Reef Beach on Lewis.  I am trying to find out what the mysterious form on the hill is.  I've tried the usually reliable Professor Google but to no avail.  I found only one photo that mentioned a "mysterious sword on the hill" taken by another visitor.  I too think it looks like a giant sword.  I am hoping, maybe, some Scottish blippers can tell me what it is otherwise it will remain a mystery which isn't a bad thing, really.  Gift#253: a little mystery in one's life.