NATURAL BRIDGES BY SOLUTION 



333 



division in that '^a. crevice or open joint penetrating the entire thickness 

 of the roof has undoubtedly been one of the chief factors determining 

 the tnnneL'^ "Large slabs of rock have fallen from the under side of 

 the bridge along the crevice." "The opening 

 underneath the bridge is 200 feet long, 20 

 feet high, and 50 feet wide." ^®. It appears 

 from the above that the bridge owes its ori- 

 gin to a longitudinal crack through whicli 

 the water seeped, enlarging a tunnel beneath 

 the surface. 



e. Bridges in western OJcIahoma. — Certain 

 bridges in the Gypsum Hills of western Okla- 

 homa may belong to this division. It is pos- 

 sible, however, that they should be included 

 imder C, 2. No descriptions of these bridges 

 are available. 



SUMMARY OF DIVISION C 



Figure 10. — Diagram to sJiow 

 hoic a natural Bridge may he 

 formed iy Gravity 



A boulder fallen from the moun- 

 tain side has lodged in a nar- 

 row ravine 



It will be seen from the above that bridges 

 initiated by solution are of three or four types: (1) Those formed by 

 seepage along joints and bedding planes, thence emptying under a fall or 

 rapid; (2) those formed in a region of thick limestone or gypsum by sub- 

 aerial erosion, with the accompanying enlargement of the caverns and the 

 sinks, and (3) by the local remains of comparatively superficial tunnels. 



D. Bridges formed by Gravity 



1. BY A STONE WEDGED IN A NARROW CHASM 



In mountainous regions, where erosion is rapid, the deep, narrow val- 

 leys which abound may be bridged over by a large mass of rock which is 

 carried from the mountain side into the valley (figure 10). Several ex- 

 amples of this sort are discussed by Friih. Bridges of this t}^e, in daily 

 use by foot passengers, span the Tamina Valley, in Switzerland.^" Such 

 l)ridges are probably not uncommon in the narrow mountain gorges of 

 this country; but as they are unusually small, it is difficult to learn of 

 their location. The illustration (plate 26, figure 2) from the Big Horn 

 Mountains shows a rather poor example from Wyoming. 



2«E. R. Buckley : Geology of Miller County, Missouri, Bureau of Geology and Mines, 

 vol. 1, series 2, 1903, p. 13. 



" J. Friih : Naturhriicken. p. 358 ; also Friih. Xachtrag zu Naturbriicken, 1907. Tafel 1. 

 St. Gallien. 



I 



