I use plaster soaked paper towels over a corrugated cardboard grid. I prefer to add to an area rather than cut away the box. The rod is my oil pipeline and the cardboard is the future road. The hole is for a rockface. I am adding a pond in the center.
I hadn't decided about how I was doing the pipeline. I glued a styrene tube onto the terrain with hot glue. This would permit me to model the terrain around the pipe buried for the roadway. I would slide the pipeline through the tube when I added it permanently.
I cut the cardboard to represent the road. I added the rockface in the hole banged in the plaster.
I made a hump on the hill by putting newspaper into a plastic bag. The bag was taped to the hill with masking tape.
The road embankment was formed with masking tape. The rock embankment made from ceiling tile had been completed except for final painting. A cutout for the pond had been made in the plywood.
I use regular paper towels and floor leveler.
I make small strips with the paper towels. I use a disposable paint tray to mix the plaster. Cleanup of the tray just requires to flex the tray and the dried plaster falls out.
I mix the plaster into water until it is the consistency of heavy cream. I apply the soaked strips to the terrain. This forms my "bedrock" basic terrain shape.
I apply a second "soil" coat of Structolite. This is a slow setting gypsum plaster containing perlite particles. I mix it in a plastic tub and apply it with a plastic putty knife.
The Structolite should be stiff enough to stay in place when placed on a embankment. Cover all the white plaster "bedrock".
The Structolite is formed into the final terrain configuration. It is slow setting and allows the terrain to be configured at leisure.
I made the one field as though it had been eroded after the earth was taken for the road fill. The basic road surface is added.
I added erosion at the base of the hill in the road.
I model the 1870's and most roads are dirt with ruts. I take a stiff paint brush and draw it up the road. This places a basic texture to the road surface. Later the road will be detailed.
The Structolite will set overnight. It will turn a dark gray. The Structolite should be applied at least an eighth inch thick. If it is too thin a layer it will dry and not set. It will be a light gray in color. Spots around the edges that dried will be taken addressed with the paint coating.
After the Structolite has set I paint it with my latex paint base color.