My starting state was open framework with the bridge already in place.
I used some plastic 1/2" 'hardware cloth' (bought at Home Depot) to create the sides of the valley. It is stapled to the wooden frame.
In places where the mesh could not be strongly anchored, I supported it from below with pieces of Styrofoam.
Some areas were easier to create with layers of Styrofoam, cut with a hot wire cutter and glued in place.
Since things were about to get messy I covered all the track and the bridge.
I then spread on a layer of plaster. I use the bucket of dry-wall plaster available in home repair stores such as Home Depot or Lowe's. It is premixed and you get 5 gallons (19 liters) for about $13.
I put on a few layers. The first layer tends to show the ripples of the plastic mesh supporting the cotton sheeting. When that was dry, I added another layer and used a tiny trowel to obtain a smooth surface where there will be grass. On areas that will be cliff face I applied additional plaster in horizontal bands and used the edge of the trowel to make lots of horizontal 'ledges'. I did not worry about making things angular yet.
Be aware that when this plaster dries it will shrink, especially in thicker areas. The shrinkage results in cracks. I leave the big cracks in places where there will be rocky outcrops and I fill in the cracks in other areas. This step may have to be repeated over a few days as all the plaster dries.
Once the plaster was fully dried, I took a chisel and jabbed at all the ledges I had made to break pieces off and make them angular.
At this stage the shape of the valley and cliffs is done but it looks like a winter snowscape.
I painted all the areas that were to be rock face, with black paint. It is important to get the paint into all the holes and cracks. I had to view the cliffs from all angles and ensure that there were no white holes visible. Areas that were to be vegetated, I painted green. I used latex paint that I had, but one can also use tempura or acrylic paint.
Once the black paint was dry, I went over it with brown paint. This time however, I did not attempt to fill every hole and crack, leaving the black paint visible in such places.
The next step was to add vegetation. I spread Elmer's School glue (white glue) on the green painted areas and spread on Woodland Scenics light green course turf.
The cliffs also need to get vegetation on every surface where plants can take root. That means almost every little horizontal ledge. To get glue onto these ledges I used a 1-1/2" paintbrush dipped in a 6:4 diluted glue mixture, and I wiped the glue onto the ledges in a downward motion, with the handle of the brush lower than the bristles. This ensures you don't get glue on the entire cliff face. I then sprinkled on various turf material.
Once the foundation vegetation was in place I added some bushes, etc.
My next step was to add the waterfalls and river to the valley, which is document here.