In the Wending Woods in DA:O:A, there is an arrangement of stones which go back to some sort of Tevinter enchantment. When you first arrive, there's a stone missing, but I've found the missing stone, put it in place, and played around with activating the other stones that light up to create the lines of fire. There's obviously some sort of puzzle to solve here, but I can't figure out what I'm supposed to do.
Can someone give me a gentle, non-spoilery hint as to what I need to do to solve this puzzle? Alternately, is there an in-game object that I need to get that will point me in the right direction?