Make a terrain height you want:




Add Walls:
- walls must be placed at the jagged edge.




Select one of these tools: (red or blue)
- brush with smaller arrows (red) will lowers the edge very slowly.
- brush with bigger arrows (blue) will instantly lower the edge to a lowest terrain point (green).




Set brush size (must be x1):
- other brush sizes does not seem to work.




Use selected tool at jagged edge to lower the rock:




Result: