I tried low cloud following terrain, and masked by the horses (painted shader). But I broke a render like these off after nearly 5 hours, as it wasn't even halfway; just took me too long. These only took, what, 12 minutes or so.
Never mind the title, they always start differently. Daniil's erosion shader took care of the debris areas, works like a dream!
EDIT: and another one with some v3 clouds, some imaginary early amphibs and (too red) saltwort. But the erosion was what this started with.