and the travertine strong enough to arch over the creek and buttress itself on the opposite bank, thus forming a natural bridge. The deposition does not seem to be going on as actively now as formerly, but in one place the lime is being deposited so rapidly that hats, shoes* and other objects left in the spray are coated over with a thick layer of lime in a few months. Underneath the bridge are caves of considerable size adorned with stalactites and stalagmites.
The greatest natural bridges of the United States, and of the world, are found in southeastern Utah. These were formed in a manner so simple that the explanation may, at first, seem inadequate. The streams which they now span have great bends and formerly had greater ones. As they deepened their beds they kept cutting away on the inside of these curves. In some cases the streams probably cut through the necks of these meanders without the formation of bridges, but in four instances the stream perforated the neck of the bend, forming natural bridges. These Utah natural bridges are enormous, varying in height from 108 to 308 feet, and in length of span from 186 to 275 feet. The bridges are made of red sandstone and occur in a high plateau in which the streams have cut canyons hundreds of feet deep.
An interesting bridge of a similar origin spans Swifts Camp Creek (Figs. 9 and 10) in the mountains of eastern Kentucky. Although the top of the bridge is but 15 or 20 feet above the surface of the stream and the length of the span is only 50 feet, yet it illustrates as well as the great Utah natural bridges the manner in which the stream has worked to accomplish this result. The greatest natural bridge in Europe, the Pont D'Arc across the Ardêche has a similar origin, as can readily
formed as shown in Fig. 11.
