336 H. F. CLELAND NORTH AMERICAN NATURAL BRIDGES 



neath the arch (plate 28, figure 1) of the bridge are several caves of con- 

 siderable extent, from the roof of which hang stalactites and from the floor 

 of which stalagmites arise. These caves are reached from below by lad- 

 ders which have been erected by Mr. Goodfellow, the owner of the bridge. 



The extent of the terrace above, and including the bridge, is about 25 

 acres, and is covered with a good soil which is irrigated from the springs 

 and produces abundant crops of fruit and alfalfa. A small portion of 

 the north end of the cultivated tract is apparently not underlaid with 

 travertine, but is formed by sediment carried in by a small stream. This 

 may, however, also be underlaid by travertine. Travertine terraces, such 

 as that of the natural bridge, also occur in Fossil Creek and Beaver Dam 

 Creek, on the road between Pine and Camp Verde and Flagstaff. No 

 natural bridge, however, has been built in either of these valleys. 



The natural bridge of Papigno,^® east of Terne, is, according to Keller, 

 of this origin. Another bridge near Hierapolis, in Asiatic Turkey; an- 

 other at Mesquitin, between Tunis and Constantino, in Africa, but which 

 is now said to have fallen, are said to be of this origin. ^^ Two travertine 

 bridges are also reported from Chile which span a gorge of the Eio Maipo. 



2. BY SNOW AND AVALANCHES 



Snow and avalanche bridges should be mentioned here, since they are 

 strictly natural bridges. 



Conclusions 



It will be seen from an analysis of the above — 



(1) That natural bridges occur in glaciated as well as unglaciated re- 

 gions, and that the former, at least, have required a short time, geologic- 

 ally speaking, for their formation. 



(2) That they must necessarily be short-lived structures, the life de- 

 pending upon several factors — the character and composition of the rock, 

 the climatic conditions, etcetera. The North Adams bridge, for example, 

 is a postglacial structure which was evidently formed during the later half 

 of postglacial time, judging from the width of the gorge above and below 

 the bridge. 1 



(3) That they are formed of various materials — limestone, sandstone, 

 marble, gypsum, conglomerate, marl, clay, shale, schist, and lava. 



(4) That they fall into at least five well marked divisions, some of 

 which have several subdivisions. 



2» Pet. Mitt, 1881, pp. 329-330, quoted by Frtih, p. 362, 

 39 Giissfelt, as quoted by Friih. Ibid., p. 363. 



