Been doing some MAJOR work in the Balancing rock area. Got all the rocks made and installed, and they all move!! I pulled back some of the paper mache and installed some K'nex gears all powered by one Lego motor. All of these gears, 18 of them, power seven columns of rocks, each with a piece of flexible plastic tubing in them. In this plastic tubing, a metal rod bent at different angles is driven by the gear and the different bends in the rod as it turns moves the rocks. The plastic tubing basically protects the inside of the foam from the metal rod and it keeps the foam rocks from spinning with the rod
Here's a video of the rocks in action:
They are a little shaky, but that is because they haven't be permanently installed, they will run a little smoother when they are installed permanently and the "ground" is put back in.