The cheat you want to use is "modifyNeighborhoodTerrain on", with no quotations, obviously. But in order to place a lot over your newly created gorge, you'll need to go to C:\Program Files\EA GAMES\The Sims 2\TSData\Sys\Neighborhood.ini, open that file, scroll down near the bottom, find the line that says "MaxLotSlope=20.0" and change the "20.0" to 80.0, or any other number less than 90. Do this for every expansion you have, ie, C:\Program Files\EA GAMES\The Sims 2 University (or Nightlife, Pets, Seasons, Apartment Life)\TSData\Sys\Neighborhood.ini.
That cheat only came out with Free Time. If you don't have that expansion, you'll have to get a premade neighborhood terrain with a built-in gorge.
Oh, and back up your Neighborhood.ini file just in case you screw it up, or it turns out I'm talking completely out of my arse.