@chris a Chris, hadn't thought about using the spray foam for more craggy surfaces. So far I have used this method for "rolling" hills not steeper grades like you show. I'll have to try that. I presume the foam adheres to the batting pretty well. Nice videos.
@Mark Boyce Mark, the base paint you use will show through, so its important the color you pick will represent the area. I like an earth brown but would go with a darker gray green if it was supposed to be shale or rock. I found Walmart has cheap flat paint - if you can find someone to mix it up for you. Buy the gallon, the batting soaks it up like crazy. And I always find areas that I missed or the paint did not take well. After the ground cover is down it can take several days for everything to completely dry. When I used wood glue (buy the gallon) it took overnight to set properly and could use an extra day before painting. As Chris shows, get multiple types of ground cover, color and texture variations makes the scene believable. There are at least two types of cotton batting. I use the medium weight stuff. The light weight is far too thin IMO. Kind of funny, the base material will tell you where to put the various covers as it clings to certain areas and not to others.
In areas where I want rock to show through, after everything is dry I'll go back and cut out the section and glue (hot melt) in the plaster rock face I want. The only bad thing about hot melt is the glue strings it leaves behind and getting hot fingers when you stick them accidentally into the fresh hot hot-melt.
Lastly the spray foam is nasty stuff. I constantly get it on my hands regardless of how careful I try to be wear latex/vinyl gloves on both hands.