Hi Monte,
The landforms are all cardboard strips covered with Scuptamold (a plaster/papier-mache mix). This is then painted a neutral brown colour. Once that has dried I paint it again with matte medium and sprinkle dirt on it. Then I eye-dropper matte medium diluted about 1:4 with a mix of water and isopropyl alcohol.
The small scattered stones are real; the larger rock-forms are plaster (cast from Bragdon molds).
The eroding bank was more complicated. I wanted to show old river stone being uncovered, so the dirt is mixed with pebbly beach sand. The top-soil is brown felt which has had its edge frayed a bit and speckled with some grey spray-paint. Then grass is put on top of that.
Another quiet scene (I see one of my stumps needs a bit more dirt at the bottom):
Cheers,
Jeff.