H. F. Cleland — Formation of Natural Bridges. 121 



the water containing carbon dioxide (C0 2 ) gradually increased 

 the size of the crack until it was still further enlarged by the 

 erosion of the stream. The stream was finally entirely diverted 

 from its former channel at B to its present course. The gorge 

 from the dam to the pre-glacial valley is a succession of broken 

 pot-holes varying in size up to 6 or 8 feet in diameter, showing 



Fig. 2.— The North Adams Natural Bridge as seen from the south.* 5 * For- 

 merly the bridge probably extended nearly to the foreground of the picture. 



that after the tunnel was made the gorge was largely excavated 

 in this way. The pre-glacial valley "in which the Hudson 

 Brook flows below the gorge is broad but to some extent 

 choked with glacial drift. 



The origin of the famous Natural Bridge of Lexington, Va.,* 

 as explained by Walcott, was similar to that of the Natural 

 Bridge of North Adams, Mass., but is on a larger scale. Before 



* National Geographic Magazine 1893, vol. v, p. 59. 



