I used plaster cloth that comes in a roll about 8 inches wide. First crumple up newspaper to desired contours and secure with masking tape. Cut off pieces of plaster cloth, dip in a tub of water for a couple of seconds, and lay over the newsprint. It dries to plaster hardness but is much easier to apply and less messy than real plaster. I then sprayed the plaster with appropriate brown, gray and green enamel paint and covered with various "soft" scenery made for train layouts (ground up foam). Looks fairly realistic I think, but plaster is plaster, even when covered with grass, and cars taking an off-road excursion usually bear the consequences. If I had to do it again, I might use foam. I remember seeing an article years ago by Weird Jack Stinson (maybe on Old Weird Herald site?) that explained a variation of the sculpted foam technique described above. I think he used expanding insulating foam from a squeeze gun (like carpenters use to fill gaps around windows and doors). Squirt it on, shape it a bit, let it dry, paint it, and voila -- car coddling soft rocks!
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