314 H. F. CLELAND NORTH AMERICAN NATURAL BRIDGES 



Page 



2. By the combined action of subaerial and subterranean erosion 329 



3. By the partial caving in of superficial tunnels 331 



a. Florida bridges 331 



6. New York bridges 332 



c. Iowa bridges 332 



d. Missouri Natural Bridge 332 



e. Bridges in western Oklahoma 333 



Summary of division C > 333 



D. Bridges formed by gravity 333 



1. By a stone wedged in a narrow chasm 333 



2. By a rock separated from one bank and leaning against the oppo- 



site bank 334 



E. Bridges formed by deposition < 334 



1. By deposition of travertine 334 



2. By snow and avalanches 336 



Conclusions 336 



Catalog of North American natural bridges 337 



Introduction, with Definition 



There are few topographical features that excite greater interest than 

 natural bridges. This is due, no doubt, to some extent to their rarity 

 and to some extent to the questions which arise as to the force or forces 

 which have been at work to produce such structures. Although rare, the 

 total number on the North American continent is quite large. In this 

 paper, which does not include a description of all the natural bridges of 

 North America, thirty-eight are mentioned. 



The terms "natural bridge" and "natural arch" have been so often 

 used as synonyms, both in common parlance and in scientific literature, 

 that it will be necessary to define the terms. In the restricted sense in 

 which the term "natural bridge" is used in this paper, a natural bridge 

 is a natural stone arch that spans a valley of erosion. A natural arch is 

 a similar structure which, however, does not span an erosion valley. It 

 will readily be seen from the above definitions that many of the archea 

 which are formed by wind-blown sand, frost, solution, and wave action 

 are not strictly natural bridges. 



The classification used in this paper is one based entirely on origin, 

 since all others that have been considered lead to more or less confusion. 



A. Natural Bridges initiated by Stream Erosion 



1. BT THE PERFORATION OF THE NECK OF AN INCISED MEANDER 



Some of the most imposing of the world's natural bridges were formed 

 by the undercutting on the inside of the curve of an intrenched meander 



