I made an image in PS, used a gradient, experimented till I got where I wanted, then reversed the process for the surrounding terrain. Took the images into the terrain editor. These are 25% size of the actual images I used, but I am sure you get the drift. Mind you I played around for ages before I got it almost right.

The greatest part of wisdom is learning to develop  the ineffable genius of extracting the "neither here nor there" out of any situation...."