NORTH AMERICAN NATURAL BRIDGES 



421 



There are probably many small streams that are spanned by bridges 

 of this sort, but few of them have been reported. Two such occur in 

 Vennont. 



In Kentucky are several arches with an unusual origin which 

 should perhaps be included under the term natural bridge. They were 

 formed in a plateau composed of horizontal sandstone and limestone by 

 the cutting back of the heads of two streams flowing in opposite direc- 

 tions in deep valleys. The streams continued to cut back until only a 

 narrow ridge or divide separated their basins. This divide was in time 

 perforated by the action of water, wind and frost until at length a fine 

 bridge resulted. One of these (Fig. 5) near the station of Nat- 

 ural Bridge on the Lexington and Eastern Railroad, is 32 feet high and 

 66 feet wide. There are three bridges, or arches, of this origin within a 

 radius of three or four miles. 



In narrow mountain valleys natural bridges are sometimes formed 

 by a large rock falling down the mountainside and wedging into the 

 valley. In Switzerland two bridges of this sort are actually in use by 

 pedestrians, but none has been reported in this country, though many 

 doubtless exist. An unusual bridge formed by gravit}' (Fig. 6) is one 

 consisting of a large slab which was separated from one side of a valley 

 and fell to the other side. When the crack was filled with debris a 

 usable bridge resulted. 



In the Yellowstone National Park a natural bridge (Fig. 7) com- 

 posed of a lava made up of vertical plates of compact and porous rock 

 spans Bridge Creek near Yellowstone Lake. The bridge, although only 

 forty feet high, is very interesting, both because of its rugged beauty and 

 of its unique origin. An examination shows that the bridge is made of 

 two vertical slabs of lava, one two feet and the other four feet thick, 

 separated by an opening two feet wide. The bridge was formed as fol- 



FiG. 4. Diagram showing the Cocrse or the Kicking Horse Riter unobr 

 THE Bridge. (See Fig. 3.) 



