Here's my attempt at a very similar idea about four years ago. I did basically what pogmahone suggested to you. Two terrains, one turned upside down (turn 180 degrees on the x or z axis) and added lots of spikes in the lower terrain and lots of streching and squeezing until I got proportions I liked. I cheated by placing a disk between the two terrains to avoid the problem of matching the two profiles and blending the different material along the seam. One other thing. I placed the camera inside a glass sphere to get the flat ground plane to curve along the horizon line. There was a tutorial on this but I forget where and by who - it was four years ago and memory is the second thing to go... (Don't laught guys, everyone did bubble and glass sphere pictures when they first got Bryce)