Natural Arch and Natural Bridge, two terms that are generally used interchangeably, but are sometimes considered to have slightly different meanings. Both are rock formations carved into arching forms by such erosional forces as wind and running water. The chief distinction is that natural bridges are more likely to span streams and valleys.
Natural bridges and arches occur in many parts of the United States. Those in the southwest, particularly southeastern Utah, are probably the most outstanding. More than 80 natural arches of red sandstone form the chief attraction of Arches National Park, near Moab, Utah. West of Blanding, Utah, is Natural Bridges National Monument, site of three great natural bridges. Rainbow Bridge, in one of the remotest parts of southeastern Utah, is the world's largest known naturally arched formation—290 feet (88 m) high and 2 78 feet (85 m) wide. It is a national monument. Probably the most notable natural bridge in the eastern United States is Natural Bridge, near Lexington, Virginia.
A special type of natural arch is the sea, or marine, arch. It is formed by the cutting action of waves on a headland or island. Percé Rock, on Gaspé Peninsula in Quebec, Canada, is a classic example of a sea arch.
