Note that you don't want to do the rendering with QUADS - those are only generally suitable for quadrilaterals which lie exactly in a plane, but for terrain they rarely will. If you can see both sides at once, then you'll likely get visual glitches, and it's quite likely you'll be able to if you look at the edge of the terrain from a suitable angle.
We shouldn't really be using QUADS for the tubes either (and there's a FIXME to that effect), but it's harder to get a view such that you see a glitch for those.
