Canyon Strata in Cycles

I’m trying to create a realistic rock strata material for a model of a canyon I have, but I am running into trouble with creating a material that only maps to the rock faces on the sides of the canyon. Does anyone know of a technique to achieve a good rock strata effect for cycles that is only applied to canyon walls?

The UVs are currently set up from a top-down camera projection.


The effect I am looking for:

Very simple test:

It uses the Geometry > Position value to drive a color ramp and is therefore completely independent from any texture mapping. If you use an image texture you could e. g. overlay the color ramp over the texture in order to have a color variation. Might be a bit finicky to set up, though…