two ways I've seen - cut and layer stacks of
pink or blue insulation foam, or use the
expanding foam stuff used for insulation, just have to sand it to how you want it.
Tbh the former is probably better for cliffs, the latter for more "hills". Or you could do a mix of both types of course, depending on your layout. Both should be fairly light weight but durable. And paintable. (**Be very careful cutting or sanding either of these!** try to do it somewhere easy to cleanup, wear a mask, a good one, as you really do not want to be breathing these particles!)
The other way would be with the more traditional wire frame, plaster or paper mache method. Would be even lighter since it'd be hollow but also not necessarily as durable, or able to take a lot of heavy weight. I'm not sure on this method as it's not one I've done before.
You can look up all kinds of videos on youtube from Terrain tutor and others - especially model railroaders who do this a lot - to get more ideas or more explicit instructions.
Good luck

It looks like it'd be a lot of fun to make!