In order to turn spazerMANIAc56's solution into something useful, I scanned the map:
The red circles are to ease the search for the small red crosses. If I missed any, please let me know and I'll add them.
Edit: After going for the achievement myself now (yes, I know), it appears as though not every cross points to a stone. Either that, or I am too stupid to find them (which is a very real possibility). That the in game map is 3D doesn't really help pinpoint the location, but overall the map proved useful.