122 //. F. Gleland — Formation of Natural Bridges. 



the formation of the bridge the stream, which now flows 

 under, then flowed upon the surface of what is now the arch 

 and probably plunged over a fall a short distance below the 

 present site of the bridge. While the stream was flowing over 

 this fall a portion of the water was percolating through a 

 joint plane or other crack up stream and discharging into the 

 stream under the fall, enlarging its passage by its solvent 

 power. In the course of time this passage became sufficiently 

 large to contain all of the water of the stream, and the bridge 

 resulted. It is not possible to say what the length of this 

 underground passage was. It must have been somewhat 

 longer than at present, but " whether one hundred feet or sev- 

 eral hundred feet" it is not possible to determine. 



The description of some wonderful natural bridges in Utah,* 

 in a recent paper, suggests an explanation similar to that 

 given above, except that, in the case of these bridges, the rock 

 is said to be a sandstone (pink or gray) instead of a limestone. 

 The most probable explanation is that, at one time, the river 

 flowed over a fall a short distance below the lowest bridge 

 and that, as the stream was cutting back, a portion of the 

 water was pouring through a fissure up the stream and reap- 

 pearing at the brink of the fall, dissolving out the cement of 

 the sandstone along its course. This underground passage was 

 gradually enlarged by the washing out of the unconsolidated 

 sand, resulting in a tunnel of sufficient size to hold the entire 

 volume of the stream. After this event the valley was eroded 

 to nearly its level. This process was repeated three times with 

 the formation of three bridges. When it is remembered that 

 one of these bridges spans a canyon 335' wide, that the lower side 

 of the arch is 357' above the stream and that the material of 

 which they are constructed is sandstone, it will be seen that 

 any explanation requiring a tunnel of great size extending for 

 a long distance is untenable. It is, however, unsafe to do 

 more than speculate upon the formation of these bridges, since 

 so little is known of the rock of which they are composed. 



In the Yellowstone National Park occurs a small natural 

 bridge of rhyolite. The bridge consists of two vertical slabs of 

 lithoidal rhyolite, parts of the contorted layers of lava flow, 

 which stand nearly vertical in this place. f They are slightly 

 curved and are separated by open crevices with roughened 

 scoriaceous walls. Of the two slabs forming the ledge the 

 eastern is two feet thick at its ends and thinner in the middle. 

 There is a space of two feet between it and the western slab, 

 which is four feet thick. " The span of the arch is about 30 

 feet and it rises about 10 feet, the top of the bridge being some 



*W. W. Dyar, Cent. Mag., vol. lxviii, 1904, pp. 505-511. 



f Geo!, of Yellowstone Nat. Park, U. S. G. S. Mon., vol. 32, pt. II, pp. 386-7. 



