Every arch in the park is as unique as a fingerprint, telling its own personal story of rock, water, time and change. When discussing them scientifically, however, it's helpful to group them into categories by their shape or apparent mechanism of formation.
Types of Arches
Cliff Wall Arch
This is the most obvious kind of arch: standing alone, independent of other rock walls or fins, proudly spanning an easily visible light opening. A few arches of this type have been called "windows" for the scenic views they frame.
This type of arch forms when a pothole (small depression) on top of a rock mass merges with an alcove on a rock face. The light opening is often smooth and rounded at the top, casting light down into a room-shaped opening below. Measuring these arches requires the skills of an experienced climber.
A natural bridge is distinguished from other types of arches because of its location astride a stream or stream channel. Water need not be flowing year-round; the massive namesake features at Natural Bridges National Monument often cast their shade onto a dry creek bed below.
There are only a handful of known natural bridges at Arches, all of which -- such as Walk Through Bridge in the Fiery Furnace -- require a bit of trekking to get to.
There are many, many more rock holes in the park than just the official 2,000+ named arches. Most of these are simply too small; spans must have a light opening of at least three feet in one direction to count as an official arch. Gaps formed by rock falls (the space between two boulders at rest, for example) or vertical cracks or joints in bedding planes also do not count as legitimate arches.