I've been creating geologic block diagrams using 3D solids. I'm not sure this explanation is clear, but here goes:

I'd like to take a polygon mesh that represents topography as a solid object down to an XY plane (at -2500 feet, for example). This I can use to trim other objects that might represent a rock layer, or a fault zone, and show how they would intersect the surface.

I've created a 3D solid using closed contours that were extruded, stacked, and joined. But the countour interval is 40 feet and that creates a stair-step surface that does not have a smooth look, especially when combined with an air photo draped upon a polymesh surface.

Furthermore, the geologic features themselves could be created using combinations of mesh to solid objects.

I've tried various approaches. I can explode a mesh and get 3D faces, which in turn can be extruded and joined, with variable results. Some faces extrude up, others down. I've combined two objects created this way, but using the opposite extrude distance, and then joining them. Again with variable success.