Phil, Joe did not use terrains but rocks. Normally you don't have this problem with terrains as you can configurate the influence of slope easy and the texture follows.

In the above sample is a rock. In the first image it was just left as it is, in the second I've changed turbulence and rotation of the texture. I also have changed the Z-bumps to 0.3. Both images are rendered final. You can play more with it to get different or better effects.

:-)Guitta