Dune: Many thanks for your help, I have looked into it, you have put the compute normal before the fake stones ? It seems to work but only, as Cyphyr suggest, with procedural terrain. It is as Terragen don't recognize the second heightfield or the interaction between the two. I just don't understand at all.
The only solution I have, is to restrain the maximum altitude, I doesn't much affect the scene, and it's close to the result I would obtain.