Deleting Landscape portion

This is my first time creating landscapes. Once it is fully created and I got all the heights how I want, is there a way to just delete/cut out a portion of it. That way it just drops off and I completely get rid of a portion of the map I would never intend to use. I was also thinking perhaps I would like to seperate that portion and be able to make it two seperate pieces so I can create a large crack in the ground that causes you to fall to your abyss.

NOTE: I would not want to shrink the map, just get rid ofcertain portions of the map.

I like to keep things simple and not have to try to remember that I purposely left a space blank and that it wasn’t an accident. Also, I wouldn’t want anything to load past here anyways so I was thinking it would save game resources.