NATURAL BRIDGES BY WAVE ACTION 327 



eluded among natural bridges because the valley which it spans for a 

 portion of the year, at least, probably contains a small stream. The valley 

 can not, however, be said to be formed by stream erosion. 



C. Bridges initiated by Solution 



In regions in which the surface rock is thick and fairly soluble tunnels 

 as well as natural bridges may be formed, the only difference between a 

 tunnel and a bridge being the greater length of one than the other. Prob- 

 ably the greater number of the natural bridges of the world — though cer- 

 tainly not the most imposing — are to be included in this division. 



1. BY SEEPAGE 



Natural bridges may be formed by seepage through a joint or other 

 crack in the bed of a stream, usually approximately at right angles to the 

 course of the stream, thence along a bedding or other plane and discharg- 

 ing under a fall or rapid. 



a. The Virginia Natural Bridge. — This remarkable natural bridge 

 may be considered a type of bridges formed by seepage. In bridges of 

 this character the cavity which later produced the bridge was formed by 

 water percolating through a joint or fissure athwart the stream, thence 

 along a bedding plane and emptying under a fall or rapid of the stream. 

 The channel thus formed was gradually enlarged until all the water of 

 the stream was diverted from the stream bed below the point of ingress, 

 leaving a bridge. The height of the arch of the natural bridge above 

 the stream will naturally depend upon the amount of cutting subsequent 

 to the formation of the bridge, and to the weathering of the under side 

 of the arch. Bridges formed in this way can readily be distinguished 

 from those formed by the caving in of all but a small portion of the roof 

 of a cavern by the fact that the top of bridge of the former was plainly 

 at one time the bottom of the valley. 



"On the north side" the summit of the bridge is 236 feet above the water, 

 and this part of the arch has a thicliness of 44 feet and a span of from 45 to 

 60 feet. The western edge is about 10 feet higher and the eastern edge about 

 10 feet lower than the central point." 



h. The OJilahoma Natural Bridge. — A natural bridge of unusual 

 origin occurs 15 miles southeast of McAlester, Oklahoma.^® The lime- 

 stone, which is almost vertical in this place, permitted the water of the 



" C. D. Walcott : National Geographic Magazine, vol. 5, 1893, pp. 60, 61. 

 IS The information concerning this bridge was obtained from Prof. C. N. Gould in a 

 letter, January, 1910. 



