@Colin, when you combined the two terrains, did you get a single surface? Or are they still overlapping faces and loose geometry (edges), as they may seem from the picture?
The OP asked for a solid group.
I used MeshWrapper Tool and it created a new single grid mesh for the entire surface (it can by composed by several groups/terrains/pieces). Then you’ll add the Skirt and make it a Solid Group (as I already show you how to do it).
