How can I stop fake stones picking up the colour and displacement of the underlying ground layer?
I don't know how but I've sorted it displacement earlier in the chain was causing the problem, I hope.
Stones can have their own unique textures.
If you are using a distribution shader or surface layer (as you should), under Slope restraints, it's the Slope key, select terrain normal.
Check this out - http://www.planetside.co.uk/forums/index.php?topic=4443.0
I found it buried in File Sharing.
Cheers all