1. Chose a suitable and realistic place on your layout where an avalanche of a certain kind would run across the tracks;
2. Tear off some plastic wrap, the Saran kind, and lay it in place. Inside the margins of the Saran Wrap, so that you can grasp the uncovered edges later, either fashion a form with cardboard, like you would to pour a basement. The idea is to contain what you're going to put there next;
3. Use sculptamold, or some terrain goop, to create a base shell that will conform to the contours under the Saran Wrap, and also to the rails (this last bit is important because the rails will act as retainers due to their running inside slots created during the sculptamold/good pour);
4. With the pour hardened, now pour over some talus, chunks of grit, or whatever, some light filtered sand, in successive layers. Add some slightly diluted Mod Podge Matte (NOT GLOSS) medium from Wally's craft section. The idea is to make a solid block to the extent possible, but certainly to bond the top pours to the goop put down first. When that cures, you have your avalance;
5. For winter avalanches, with more snow that anything, just liberally cover the same 'avalanche' with W/S 'snow.