They're all on two (maybe three) of the map squares. Most are on the one Broken Blade, with the rest being in the forest, but my memory is fuzzy as to whether there were anywhere else. If there were, it was a square right next to one of those two. I killed all 23 relatively quickly, and I think you'll know when you got them all because you don't get the petrified wood drop until you kill the last one (assuming you know what I mean by the petrified wood drop... it's another quest so I won't go further than that to avoid any further spoilers).