I remember making 'tracks' for Monster Truck Madness and they used height maps for the terrain (256x256). Then, they also used what I think they called (if I remember correctly) blocks, which were cubes that you could stretch to any height, making them rectangular (How do you say rectangle in 3 dimensions?). I think these were just seperate objects that were included with the height map and aligned perfectly with the vertices of the map to allow you to create vertical surfaces and surfaces overhead. Mostly in the game, the blocks were used to create things like bridges, since they had a top and a bottom (you could adjust the bottom as well) so you could drive on them or under them. Or they were used to make straight edges (man-made cliff so to speak), since the best you could get with the height map was a near vertical edge.