I'm not sure how realistic you want to get, but you can use Bryce 7, which, for now, is free at DAZ3D. You'd first need to make a black and white height map using an image editor like Photoshop or GIMP. Then you'd apply that to what's called a "landscape" and Bryce will duplicate the height map in 3D. Then you can apply colors and adjust the height and detail. You can see examples in my Planet Eben thread in the Finished Maps forum.