422 



THE POPULAR SCIENCE MONTHLY 



Fig. 5. A Natural Bridge near the Station, Natural Bridge, Kentucky. 



lows : at one time the stream flowed over a fall which is now represented 

 by the top of the bridge ; in the course of time the freezing of the water 

 between the thin vertical plates of which the slabs are composed at the 

 foot of the fall forced them apart, making it easy for the water from 

 the fall to wear away the rock at its foot and to excavate a cave. This 

 cavity was gradually extended up stream until a porous layer was en- 

 countered through which the water of the stream poured into the cavity, 

 thus forming a bridge of the first of the two slabs. The same process 

 was continued with the undercutting of the second slab. In this way a 

 natural bridge was formed. It is hardly probable that another struc- 

 ture made in the same way is in existence. 



It does not seem possible at first thought that a natural bridge more 

 than 125 feet high could be formed by the deposit of lime from water 

 (travertine), but such a bridge (Fig. 3) occurs near the little Mormon 



Pig. 6. Block Diagram to show a Bridge formed by the Headward Cutting of 



Two Streams in a Plateau Region. 



