154 GEOLOGY, 



in this way. A similar bridge is now in process of development in 

 Two Medicine River in northwestern Montana (Fig. 145). Once in 

 existence, a natural bridge will slowly weather away. 



It is not to be understood that all natural bridges have had this 

 history. They are sometimes developed from underground caves 

 when parts of their roofs are destroyed, as well as in various other ways. 



Fig. 141. — Effect of columnar structure on weathering. Big Bad Lands, S. D. (Bar- 

 ton, U. S. Geol. Surv.) 



Folds. — The erosion of folded strata (anticHnes and synclines) leads 

 to the development of distinctive topographic features. So soon 

 as a fold begins to be lifted, it is, by reason of its position, subject to 

 more rapid erosion than its surroundings. For the same reason the 

 crest of the fold is hkely to be degraded more rapidly than its lower 

 slopes, and must suffer more degradation before it is brought to base- 

 level. Folds are usually composed of beds of unequal resistance, 

 and as the degradation of a fold proceeds, successive layers are worn 

 from the top, and the alternating hard and soft layers composing it 

 are exposed. So soon as this is accomplished, adjustment of the streams 



