120 77. F. Clelancl — Formation of Natural Bridges. 



should be taken by one not at all accustomed to drawing, than 

 that no memento be left of this interesting place," (there was 

 danger at that time that the bridge might be destroyed by the 

 quarry-men.) Hovey* in his " Celebrated American Caverns" 

 describes this bridge but gives the locality as Adams, Mass. 



"^FF Dim. 



The explanation of the for- 

 mation of the North Adams 

 Natural Bridge, as given by 

 Hitchcock and accepted by 

 Hovey, is that it is the section 

 of the roof of a cavern, the 

 ends of which have falleirin. 

 In illustration of this point, 

 Hovey states that, "the com- 

 bination of cave, chasm and 

 natural bridge, on Hudson 

 Brook, Mass. is even a better 

 example (than that of the 

 Natural Bridge in Virginia) 

 of the same thing," i. e., 

 "that what are now open 

 canons were once caves, the 

 arch being merely a remnant 

 of an ancient cave roof." 



On examining the conrse of 

 the stream and the rock in the 

 vicinity of the North Adams 

 Natural Bridge one is struck 

 with the width of the joints, 

 and the fact that the stream 

 has, for a portion of its course, 

 followed the joint planes. In 

 the upper part of the accom- 

 panying sketch (lig. 1) the 

 relation of the stream to the joint planes is indicated by the 

 dotted lines A-A. The channel through which the stream 

 flowed previous to the formation of the bridge is also well 

 marked a few feet to the west at B. A pot-hole situated near 

 the edge of the gorge at B is further evidence of the former 

 position of the brook. 



The bridge was probably formed as follows: When the 

 stream flowed into the gorge through the ancient channel, it 

 plunged over a fall into the pre-glacial valley. Some of the 

 water in the joint plane nearest the present bridge seeped 

 through an approximately horizontal crack a short distance 

 under the present arch of the bridge. The solvent power of 



* "' Celebrated American Caverns," H. C. Hovey, pp. 14 and 206. 



42- 



Fig. 1. — Sketch map of Hudson 

 Brook, Mass. , showing the position of 

 the natural bridge, the joint planes 

 A-A, and the pre-glacial valley. 



