Here's the simplest test terrain I can imagine. I had to zip it up to attach it to this post. As you go around the island in cm eye, watch the base of the cliffs pull back from the shoreline. In a plane, you'll be going slower and it's less noticeable.
I was testing a number of things. One of them was what would happen if I drove a jeep off the top of the cliff. Usually every thing goes okay, but if you hit the side of a lower slope just right, you fall through the terrain. That's another something to watch out for in building cliffs. For now, I'll make sure the terrain is flat at the bottom if I expect to drive a GV to the top.