Thank you, Jo
The software looks complicated, but once you grasp what all the different types of node do its not so difficult to make a texture from scratch like that
The hardest part wasn't the textures. It was forcing the non-orthographic camera to take a more or less parallel perspective shot, which entailed backing it away from the terrain by about 10 miles (to scale) and vastly increasing the focal length from the normal 35mm to about 500mm. This in turn meant the entire atmosphere got in the way, and all I could see was a hazy white-out. So I had to create a new atmosphere that had no decay, fog, or mist, but which retained enough scattering to fill the shaded areas with sufficient reflected and refracted light, or they would just have been black.
The state-of-the art default photometric Vue atmosphere is designed to behave as realistically as possible, but it really gets in the way if you want to do something a bit odd like this