You don't necessarily need to connect the clipfile as a child to a surface-layer Bob.
You can also connect it to the direct input on the far left. The only thing you lose is control over restrictions and coverage.
A way to avoid this is first to still connect the clipfile as a child to a seperate surface layer.
Secondly, uncheck colour for that layer and connect it's output to the input of the shader which generates the surface for your stones.
Another way would be to do the second step differently. You can also connect the shader-chain, which generates the surfacing of the stones, to the input port of the surface-layer (from step one).
It depends a bit on how you have set up your stones.
Cheers,
Martin
