My problem is this - I want a hf of a slanted hill/cliff with a different texture on top to the bare rock shown at the sides (the exposed cliff).

At the moment I am doing it by having two copies of the hf, one of them scaled slightly higher in the Y direction and set slightly back and left. This isn't very satisfactory for a variety of reasons (I can't get any turbulence in to the edge apart from anything else, which leads to an over-regular thickness of the two materials).

Is there a better way to do this?

Current code and image below.

height_field // white cliff
{
 tga
 "c1.tga"
 smooth
 scale <300, 75, 400>
 translate <-175,-5,-50>
  pigment {rgb<1,1,1>}
}

height_field // green cliff
{
 tga
 "c1.tga"
 smooth
 scale <300, 80, 400>
 translate <-176,-5,-49>
 pigment {rgb <0,1,0>}
}

// An infinite planar surface
// plane {<A, B, C>, D } where: A*x + B*y + C*z = D
plane
{
  y,  -1.0
  pigment{rgb<1,1,0>}
}