subsequent erosion and caving had greatly decreased the length of the underground channel and produced the short canyon above the present entrance portal. owing to the thinness of the roof, the first part of the tunnel caved rapidly, whereas the erosion and caving of the lower part was much slower. a condition now has been reached where the entrance and exit portals will be eroded with about equal speed and at the same time the tunnel will be greatly enlarged in diameter by down cutting and by caving of the roof. under these conditions the bridge eventually should assume the arch structure characteristic of famous natural bridges in other parts of the world.