NORTH AMERICAN NATURAL BRIDGES 



425 



Fig. 10. Block Diagram to show the Formation of a Natural Bridge by the 

 Perforation of an Entrenched Meander. (See Fig. 9.) 



Fig. 11. Block Diagram to show a Bridge formed by the Tunneling at a 

 Meander Bend of a Narrow Divide between the Main and Tributary Stream. 



deposit (travertine) but also the whole of the cultivated valley of about 

 25 acres. The bridge has a span of 140 feet, a height of more than 125 

 feet and a width of about 400 feet. In fact the bridge is so large that 

 the visitor is likely to walk over it, as the writer did, without knowing 

 he is on it. The top of the bridge is under irrigation and produces 

 crops of alfalfa. 



The formation of the bridge is simple and the process can some- 

 times be seen when moist, drifting snow forms a bridge across a small 

 valley. Several large springs that empty into the valley on the east con- 

 tain lime in solution which, upon evaporation or loss of carbonic acid 

 gas, is deposited. For many years this deposit has been accumulating 

 and with sufficient rapidity to force the stream (Pine Creek) to the 

 west side of the valley. In one place the deposition was rapid enough 



