Dorothy and the Wizard in Oz by L. Frank Baum

For a considerable distance the way led straight upward in a gentle incline, and the wanderers made such good progress that they grew hopeful and eager, thinking they might see sunshine at any minute. But at length they came unexpectedly upon a huge rock that shut off the passage and blocked them from proceeding a single step farther.

This rock was separate from the rest of the mountain and was in motion, turning slowly around and around as if upon a pivot. When first they came to it there was a solid wall before them; but presently it revolved until there was exposed a wide, smooth path across it to the other side. This appeared so unexpectedly that they were unprepared to take advantage of it at first, and allowed the rocky wall to swing around again before they had decided to pass over. But they knew now that there was a means of escape and so waited patiently until the path appeared for the second time.