NABSQNO 12S-613821-3866501. Keystone Bridge might easily be considered an example of the petrified log variety of waterfall natural bridge except for the fact that it does not span a streambed! Indeed, the arch is on a ridge-top and might be viewed as a special variety of shelter natural arch. However, this classification does not really capture the unique attributes of this arch either. The best way to put this "odd ball" into the taxonomy is in the irregular category.
Keystone Bridge is located in Petrified Forest National Park east of Holbrook, Arizona. It is accessed via a very short trail off of the Blue Mesa loop road.
It is worth noting that fractures in the lintel, and even the slippage along one fracture, does not disqualify Keystone Bridge as a natural arch. These characteristics can be viewed as incidental to how the arch formed and are clearly not the result of rocks being piled up to form the arch.
Other Irregular Natural Arch examples: