NATURAL BRIDGE and NATURAL ARCH. A natural bridge has been defined as a "natural stone arch that spans a valley of erosion. A natural arch is a similar structure which does not span a (true) erosion valley." These forms are produced in numerous ways. Most of our larger natural bridges are the re sult of the cutting through of the necks of deeply-entrenched meanders (q.v.), so that the river cuts off an oxbow, just as on a typical modern flood plain, except that the meander is in a deep gorge or valley and cuts through the narrow neck of rock underneath the sur face. (See MEArtnras). This is the origin of the famous Pont d'Arc in France. The group of natural bridges in Southern Utah are also of this type. The most noted of these are the Augusta, Edwin, Caroline and Rainbow bridges. The Augusta Bridge has a height of 265 feet, a span of 320 feet, a thickness of arch of 83 feet and a width of roadway of 35 feet.
In limestone regions, solution along joints may form long tunnels, or, if only part of the tunnel roof remains, natural bridges. Some
times in case of waterfalls, part of the water, before reaching the brink of the fall, may work down through a joint in the bed and come out below the fall. If this passageway enlarges sufficiently to accommodate all the water of the stream, the former fall may be abandoned, leaving its old crest line as a natural bridge. The famous natural bridge of Virginia is per haps of this or similar origin. It is 236 feet high, has a span of about 50 feet and a thick ness of arch of about 40 feet. Other natural bridges are formed by various other means, as by wind erosion undercutting ridges in deserts, wave erosion along shore lines, the falling in of parts of roofs of caves, leaving part of the roof as a bridge and many other minor causes. Consult Cleland, 'North Amer ican Natural Bridges' (in Bulletin of the Geo logical Society of America, Vol. XXI, 1910, pp. 313-338).