THE 



POPULAE SCIENCE 



MONTHLY 



MAY, 1911 



THE FORMATION OF NORTH AMERICAN NATURAL 



BRIDGES 



By Professor HERDMAN F. CLELAND 

 williams college 



ALTHOUGH there are more than fifty natural bridges of consid- 

 erable size in North America, comparatively few persons have 

 ever seen one, the reason being that, with the exception of the Virginia 

 bridge and the natural bridge in North Adams, Mass., most of them are 

 more or less inaccessible. 



A bridge, according to the usual definition, is a structure that per- 

 mits one to pass from one side of a depression to another, whether that 

 depression be a railroad cut, a street or a river. As used in this article 

 a natural bridge is a natural stone arch that spans a valley made hy 

 running water; a natural arch being a structure that does not span a 

 valley of erosion. 



Although a number of descriptive articles on natural bridges have 



Fig. 1. Block Diagram illustrating the Formation of a Natural Bridge in 

 Limestone by the Partial Caving in of the Roofs of Tunnels. 



VOL. LXXVIII. — 29. 



