H. F. CLELAND NORTH AMERICAN NATURAL BRIDGES 



above and below the bridge, some of which are very deep. The water 

 flows under the ledge (plate 20, figure 2) between "1" and "2." At "3" 

 (upstream) the stream flows over a fall into a basin called the '^death 

 pot," and reappears at "4." 



3. EROSION ASSISTED BY FROST ACTION 



a. Yellowstone Natural Bridge. — The origin of the natural bridge 

 across Bridge Creek, Yellowstone National Park, is probably unique, 

 since it is unlikely that the conditions that made it possible have occurred 

 elsewhere. 



I 



Figure 6. — Diagram of the natural Bridge across Kicking Horse River 

 Indicates that the bridge was formed by pot-hole action 



The bridge (plate 21, figure 1), which consists of two vertical layers of 

 lithoidal rhyolite, stands 40 feet above the stream bed and has a length 

 of span of about 30 feet. The layers of which the bridge is composed 

 are slightly curved and are separated by open crevices with roughened 

 scoriaceous walls (plate 21, figure 2). Of the two slabs forming the 

 l)ridge the eastern is 2 feet thick at its ends and thinner in the middle; 

 this is separated from the western slab, which is 4 feet in thickness by a 

 space of about 2 feet. 



