can you elaborate a little closer what you are intending to do, right now i have about a dozen of questions, and i am not sure if thats helpful if i keep asking all of them.
Are you refering to drape command?
in case you did not create this image, if you have such curves on the bottom like the orange one, then you use Extrude with option solid and pull it up to the upper edge of the terrain plate as indicated on my image. after that you use boolean difference. during boolean command first choose the surfaces you want to cut out then use that previously extruded block as a cutting object, the commands will tell you what to select.
I have a feeling the topic starter wants to create a smooth surface touching the (presumably) topographic contour lines. In that case, he could use
DupEdge to copy the edges, trim or join them to the required width of the new surface, and
Loft these curves to create the surface. If this gives undesirable results, the curves could be smoothed by rebuilding those with
It is a weird landscape though, I have never seen contour lines undercutting each other like that.
p.s. if Matt9 struggles in describing his problem in the English language, maybe he should do it in his native language, and most likely someone on the forum can translate it (we seem to have many nationalities visiting here ).