This 80 page PDF document (over 60 pictures) discusses the creation of outdoor garden railroad layout scenery using a papier-mâché like methodology that utilizes burlap strips soaked in a type of cement called “plastic cement” over a ridged framework (either pressure treated lumber or rebar, covered with chicken wire or hardware cloth). This technique is simple, the materials are cheap, and you can work on it over long periods of time (as you have time available).
1stclass.mylargescale.com/tj-lee/Trains%20-%20Building%20Scenery%20-%20Cliffs%20Rev%2001-01-08.pdf