How can I close this hole in the terrain?

Hello, what is the best way to close the hole between these surfaces, to be able to join them and then convert them into a 3D terrain?

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I would _DupEdge on those two inner surfaces and your hole.
Then _UntrimHole (effectively removing the hole for now).
Now use the edges that you duplicated to _Trim away the portion of the ground surface under the inner surfaces.