168 GEOLOGY 



falls and the new would then be bridged by rock (bf" and /'", Fig. 

 135). The natural bridge near Lexington, Va., (Fig. 136), almost 

 200 feet above the stream which flows beneath it ; is believed to 

 have been developed in this way; but it is not to be understood 

 that all natural bridges have had this history. 



Folds. The erosion of folded strata (anticlines and synclines) 

 leads to the development of distinctive topographic features. So 



Fig. 136. The Natural Bridge of Virginia. (U. S. Geol. Surv.) 



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 likely 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 alter- 

 nating hard and soft layers composing it are exposed. When this 

 is accomplished, adjustment of the streams is likely to begin, and 



