The texture is filling the entire 0-1 UV space most likely and all your cube's UVs are inside said space so yes the whole cube will be affected. I'd just layout the cube UVs then paint/assemble the surface and rock layer texture in Photoshop/Gimp/Mudbox/Zbrush or whatever so they're sitting over the appropriate UVs. Displacement also breaks apart surfaces in areas where are are UV seams, there are remedies for that found on these boards, I remember being in a thread about it.