NATURAL BRIDGES BY STREAM EROSION 317 



c. Bridge at Attica, Indiana. — A natural bridge over which a roadway 

 passes, said to be about 30 feet in height from the stream bed to the top 



of the bridge, spans Bear Creek near Attica, Indiana. From the very 

 imperfect description of the bridge, which it has been possible to secure 

 l)y correspondence, it appears that this bridge was formed, as in the case 

 of the Campton bridge, by the stream tunneling through the isthmus of 

 the incised meander. 



d. Pont d*Arc, France. — A discussion of natural bridges formed by the 

 perforation of the isthmus of incised meanders would be incomplete with- 

 out a mention of the famous Pont d^Arc across the Ardeche Eiver, in 

 east central France.^ 



This, the most imposing natural bridge in Europe, has a height of 

 about 60 meters and a width of 65 meters and is composed of a hori- 

 zontally bedded limestone. The size of the cavity is due both to the 

 deepening of the valley and, after the hole was made, to the crumbling 

 of the interior surface of the arch by the agencies of the weather. 



Fruh includes this bridge in his division B, I, "Limited to easily solu- 

 ble rocks (carbonates and sulphates)." The fact that bridges of sand- 

 stone are formed by the perforation of the neck of an incised meander, 

 shows that mechanical erosion and not solution, as Friih supposes, is the 

 important agent in the formation of bridges of this type. Solution is 

 probably a negligible factor. A glance at the map (figure 2) of the 

 region in the vicinity of Pont d'Arc shows the origin to be as described 

 above. 



e. Bridges of Southeastern Utah. — Probably the largest and most 

 impressive natural bridges in the world are those which have been de- 

 scribed from San Juan County, Utah. These bridges may be reached 

 either from the north, by leaving the Eio Grande and Western Eailroad 

 at Thompson Springs, thence by stage to Moab and Bluff ; or, by leaving 

 the Denver and Eio" Grande Eailroad on the east, going by stage to Cortez 

 and Bluff. The three best known bridges are about 65 miles northwest 

 of the latter village. 



The courses of the streams in this region, as described by H. L. A. 

 Culmer (the artist who painted the pictures illustrated in this article, 

 plate 19, figures 1 and 2), are very suggestive : 



"The windings of Grand Culch [for example] are such that in one place we 

 find ourselves separated by a wall only 60 feet in thickness from the spot we 

 left some time before, although we have since traveled nearly a mile." 



' J. Frvih : Tiber Naturbriicken und verwandte Formen ect. Jahrbuch der Naturwissen 

 schaftllchen Gesellschaft (St. Gallen, 1905), p. 368. 



Reclus : The earth and Its InhabUants, vol. II, Europe, pp. 96-97. 

 Martel : Les Cervennes, p. 291. 



