Thanks guys. I've been experimenting with a double set of strata (the shader) at various angles and hard layer heights and such. But that takes so much time that I haven't found the perfect solution yet. Small scales of hard layer depth (2) and spacing (9) seem to work quite nicely for very small shaly areas.
Anyway, here's an experiment with also some shadow throwing clouds (only secondary) and mist (low soft cloud layer), but I missed my goal. Back to the rocks, and I might indeed do some voronoi, Ryan.