Design the layout of your rocky terrain or mountaintop with a "skeleton" of stage platforms. Connect a series of platforms that vary in height to cover larger areas of the stage. Provide for a graduation of heights from stage level to the highest point desired. Connect these levels so actors can move from one level to another safely. Remember to plan for entrances and exits for the best "blocking" of movements, including fast "escapes." Cover visible sides with hardboard facias.
Position the platforms on the stage. If the mountaintop set will be a single unit that does move during the entire play, connect the platforms securely to the stage floor and to each other using angle irons, cleats or cross-bracing. If the mountain has to move, install casters on each platform base unit. Remember to engineer a means to anchor these rolling platforms into position for scene action and then release them.
Create rocks with various materials. Faux rocks normally used to cover home exteriors are very expensive. An affordable alternative is to make your own boulders and rocks using polystyrene (often sold under the brand name Styrofoam) cut into various rock shapes using saber saws or hack saw blades. You can stack and glue the foam in "sandwiches" to achieve more dimension, and then sculpt the edges of foam rocks with rasps or files. Styrofoam can be purchased in sheets from local home improvement stores. Rigid urethane foam, which is sturdier than polystyrene and more finely sculpted, can be ordered on line in block form. Both types of foam will readily accept stage paint.
Glue these foam rocks to the platform side facias using construction adhesive. Study pictures of rocky locations to achieve a realistic arrangement. Paint the rocks to suit. Remember that real rocks are never uniform in color. A fast way to achieve the look of smoother "sandstone" mountains is to drape old carpeting over the platform skeleton, tacking it down into interesting "alluvial folds." Glue on foam rock outcroppings here and there for realism. Paint the carpeting in rock colors using standard stage paint and brushes or a paint sprayer. Rocks can also be made out of draped chicken wire covered with paper mache. Harden these rocks further with coats of diluted white glue after painting.