You might try using a Painted Shader to paint in additional displacement there (or paint out the heightfield). For the former, you would just add a Displacement Shader with the Painted Shader as input; for the latter, you would add the Painted Shader as a mask for the Heightfield Shader. If you paint out the heightfield, you'll end up with a hole there which you would then want to fill with procedural data, or perhaps flat; you could do this by using the Painted Shader output as the Mask input for a Power Fractal, but inverted so the Power Fractal *only* displaces where the Heightfield is masked out. Then you would adjust Displacement Offset and Amplitude to get the effect you want. I'm not certain how well this will work, but may be worth experimenting with.
- Oshyan
