I struggled with it too. Distant terrains are 'easy' to texture, very close-by landscapes are 'easy' if you add enough foliage, but large mid-distance terrains that are not just rocky, but consist of grasses, especially with herbs and such are really hard to do convincingly.
Try a lambert shader after the foliage shaders, that'll give you a bit more light in the dark blobs. Especially if you have two or more fake stone shaders mimicking low foliage you easily get black shadowed areas.
Perhaps PT would do that better, btw.