4i8 



THE POPULAR SCIENCE MONTHLY 



appeared in recent years, the explanation of their origin has, for tlie 

 most part, been omitted or has been unsatisfactorily given. In this 

 paper an effort is made to show how the more important North Ameri- 

 can natural bridges were formed.^ 



In the older geologies and geographies we were taught that all nat- 

 ural bridges were formed in one and the same way. According to this 

 time-honored theory natural bridges resulted from the partial caving in 



of the roof of an underground tun- 

 nel or cavern, the portion of tlie 

 roof left spanning the chasm be- 

 ing a natural bridge. That nat- 

 ural bridges must occasionally be 

 formed in this way is evident. 

 For example, in Edmonson County, 

 Kentucky, where the Mammoth 

 Cave is situated, it is estimattd 

 that there are 100,000 miles of 

 underground passages. In the 

 course of time these passages will 

 be widened and the rocks above 

 them will be worn down by surface 

 erosion until, at length, the roofs 

 will almost completely disappear, 

 leaving portions standing here and 

 there as natural bridges. What is 

 happening in Kentucky now has 

 heen going on for counties* ages in 

 linie?tore regions in other paits of 

 t!ie world with the possible forma- 

 tion (Fig. 1) and later destruction 

 of natural bridges. It is a rather 

 curious fact, however, that although many small natural bridges have 

 this history, as, for example, a number of bridges in Florida, Iowa, Mis- 

 souri and other states, yet, as far as known at present, none of the 

 world's great natural bridges has this origin. 



The Virginia natural bridge may be taken as a type of natural 

 bridge formed by solution aided by cracks (joints). This can best be 

 explained by a theoretical case. Let us suppose that a short distance — 

 100 or 200 feet — above the brink of Niagara Falls the water of the 

 river should find a crack athwart its course in the limestone bod of 

 the river and that the water seeping througli this crack should flow 

 along the top of a lower layer, and reappear underneath the fall as a 



'For a more complete discussion see "North American Natural Bridges, 

 with a Discussion of their Origin," Bulletin of the Geographical Society, Vol. 

 21, pp. 313-338, July, 1910. 



Fic. 



TiiK Xonxii Adams, Mass., 

 Natihai. HHiiMiK. 



