That's what I was thinking too. Which (a) could make it a location outside of Narnia, (b) a location in Narnia we never see in canon, or (c) the hill that the Stone Table is on, though that seems less unlikely. (BUT, in the movie, they do show the Stone Table as being surrounded by standing stones, so the PTB could have had that in mind.) I like the non-Narnia location theory.

I am so intrigued by the World Ash Tree! Theory thrown at snacky up above.