There's also one right next to the highest mountain in Silverwood. There's an achievement for climbing the highest mountain so I won't say where it is to avoid spoiling it for people trying for that achievement.
I found it at level 14, and it gave me a one-handed weapon level 15 weapon that was significantly better than my current two handed weapon. While I was there, some Defiant player who was around level 30 showed up and got something very good, too. (I know what he got because it was broadcast as a shard first).
I told a co-worker about it, and he and his girlfriend took six character there. Five of them got very good items, and one got something that was kind of meh.
So that's at least two Ancient Cairns. I wonder how many more are out there?
Given that these seem to mostly give very good gear, and that it seems to be dynamically adjusted to your calling and your current level, I wonder if it would be best to avoid looting these until high level?