In geological time frames, the erosion of the Bryce Canyon goes very fast. And you can actually see this at many places, such as on this spot where you can see that the roots of this old tree are hanging over the cliff. This is of course not the original relative position of the tree vs the rim, when it was a youngster. The rim erodes in a westward direction at a rate of 1 feet per 65 years on average. So just imagine where the rim was 300 years ago.