Welcome to the wonderful world of mountains, valleys, canyons and bridges. I have used plaster of paris and towels, WS plaster cloth over newspapers, and found both difficult and messy. Others get great results.
I have become a fan of Styrofoam and have carved up about 20 sheets of the stuff. It makes a mess, but it vacs up nice. I have done a few threads on terraforming to show what I do. There are some foam guys a lot better than I.
WS makes a white foam, that I find too expensive and I have not tried it. I get foam scrap from construction sites. I have used all four colors, mixed together. I love carving the stuff. I use a steak knife. If you are interested in this method, there are several of us who will be glad to share and learn as we go.
Hminky has a great tutorial on using ceiling tile for limestone. I have tried that and it works.