First of all I painted the entire canvas with washes of Indian yellow (by the way - did you know this pigment was first produced by feeding cattle mango leaves and collecting their urine?) and burnt sienna , which gave a me a glowing surface to work with. The trick would be to keep layers transparent so that I didn't lose this. After sketching in the landforms with ultramarine blue I began to find the forms of the ranges , using a purply grey on the furthest to make them recede. i worked forward through the painting, slowly trying to build transparent layers without dulling that initial glow. I feel pretty happy with the end result.
PS. It's odd, but once I'm painting I never think about working from "top to bottom" , it's always "far to near" , which goes to show ,I guess, that I really am back there in spirit painting on the side of the road !