I think the fake rock is called "gunnite." I may have that spelled wrong. It is wet concrete sprayed or blown onto a metal mesh framework, which I think is often attached to a wooden frame. It lasts a pretty long time. The concrete is painted, too. As a teenage and young adult guy, I walked all over that stuff at the float trip, diving bell and flooded mine. It did easily support me, it did not give way at all. However, as mentioned above, after a while the weather gets into it. Cracks develop. Hey, concrete "rots" after awhile, and weakens. Remember there was a waterfall at Grandfather's Mansion, and a rocky outcropping outside the Springhouse restaurant entrance, and the Infinity Room at the tree house was made of the stuff, too. All gone, folks! That cave at the old float trip, now AP, lasted from 1969 until at least what, 2005? That's pretty good, but I know to cut costs the maintenance folks replaced the old stuff as it wore out. Too bad some of the old character of the park is lost when that stuff is torn out and replaced.