NORTH AMERICAN NATURAL BRIDGES 427 



Fig. 13. A Nati kai, Aimh neak Santa Ckuz, Oalifobxia, fobmed by Wave Action. 



be seen by a study of a map of the region. Another Kentucky bridge 

 formed by the lateral erosion of a tributary stream has produced a perfo- 

 ration (Figs. 11 and 12). The water flows through the opening into the 

 tributary stream when the river is high and from the tributary to the 

 river when the water in that is high. One at least of the Utah bridges 

 may have been formed in this way. 



On the coast of California is a natural arch (Fig. 13) made by the 

 beating of the waves against a clifl; of soft shale. The top of the arch 

 is so level that a team of horses can be driven across it in safety. It 

 was formed by the partial falling in of the roof of a sea cave. Open- 

 ings of a similar nature are not uncommon on rough and stormy coasts 

 but it is seldom that a structure so perfect as this is formed. 



It will be seen from the above that natural bridges are formed in 

 many ways ; that they are not confined to any particular kind of rock, 

 nor are they restricted to any particular region but are found alike in 

 deserts and fertile lands, in mountains and on plateaus. 



Natural bridges are short lived, geologically considered. The marble 

 natural bridge in North Adams, Mass., for example, was formed many 

 years after glacial times, but already a portion of the bridge has fallen 

 in. Since the great ice sheet is believed to have disappeared from 

 Massachusetts between 20,000 and 80,000 years ago and since this 

 bridge was not formed until long after it had vanished, it will be seen 

 that the life is not long as time goes. Nevertheless, short lived as they 

 are geologically, some of them probably were in existence when the hu- 

 man race was verv vounsr. 



