Actually you couldn't go wrong by perusing the November 1977 issue of the
NG&SL Gazette Red Mountain on the Wye article. Although about the Silverton Railroad the techniques used there with sifted raw white plaster to partially cover the ground and structure rooves are very realistic. The module took Best in Show at Valley Forge NMRA convention.
If you can't locate a copy I could shoot the pages.
UpSideDownC
in New Zealand