Large rock surfaces are usually cast in sections from Gunnite or fiberglass. Latex rubber molds are first made from actual rock outcrops. Using a chop gun for fiberglass or a cement pump for Gunnite, the molds are coated with the appropriate casting medium. A reinforcing framework of steel is affixed to the back sides of the cast rock sections. This work is all done off site. These sections are, in turn, fastened to a steel framework for the rock structure built on site. Typically, the finished walls are backed with urethane foam for added strength. Seams between the sections are patched and sculpted to create a perfect blend. Base coloration is added to the Gunnite or fiberglass gelcoat and the finished rock surface is "weathered" with penetrating pigments or epoxy. In some special situations the rocks may be cast in urethane (where weight is a consideration) or plaster/fiberglass.