The term "Iwakura" (rock-seat) refers to boulders or mountains in which a god is enshrined or alternatively--and in particular when termed an "Iwasaka" (border-rock)--to the boulders or mountains that form the borders either of shrines or of the Tokoyo (the spirit realm).
磐座（いわくら）神が鎮座する岩や山または、特に磐境としたときは、神域や常世との端境である岩や山をさす。