you now need to apply fancy displacements. Be careful that the patch size in your compute terrain node is very small, like 0.01, otherwise (for some reason that escapes me), the displacments will look funny.
So next step:
- check patch size
- i.e. add a surface layer, minimum altitude at where your arch begins, and add a powerfractal with displacements to it. Be careful about the feature size and the displacement amplitude. It's better to use the measure tool to measure the rough dimensions of your arch before applying displacements.
Regards,
Frank