328 H. r. CLELAND NORTH AMERICAN NATURAL BRIDGES : 



stream to flow down along a bedding plane, thence through a crack or 

 joint from which it issued beneath a fall which existed at the contact of 

 the limestone and a shale. The arch is said to be about 10 feet high 

 above the stream level and probably 25 feet across. 



c. The Massachusetts Natural Bridge. — The marble natural bridge at 

 North Adams, Massachusetts, furnishes another illustration. With the 

 possible exception of the Virginia Natural Bridge, that in North Adams 

 (plate 25) is one of the most interesting and unique in the eastern half 

 of the continent. Hawthorne in his American Note Booh speaks of its 

 beauty and of the pleasure he often took in visiting it. It spans Hudson 

 Brook, a small stream emptying into Beaver Creek, a tributary of the 

 Hoosac Eiver. 



A short distance above and below the bridge the brook flows through 

 a gorge of white marble 30 to 60 feet deep and from 5 to 40 feet wide, 

 the average width above the bridge being from 5 to 10 feet and below 

 from 10 to 30 feet. The gorge is cut in a coarsely crystalline marble of 

 Cambro-Ordovician age which belongs to the Stockbridge formation. 



The top of the bridge is 44 feet above the water of the stream, the 

 arch itself being about 8 feet thick in the middle. The span of the 

 bridge is less than 10 feet long and the width, at present, is 25 feet. 

 Hitchcock^^ states that when he studied the region (before 1839) there 

 were two natural bridges, "though the upper [ ?] one is much broken." 

 Only one bridge is in existence at present, but the second one, described 

 by Hitchcock, appears to have been below, not above, the present one. 



"On examining the course 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."^* 



The bridge was formed as follows: When the stream flowed into 

 the gorge through the ancient channel, it plunged over a fall into a 

 preglacial 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 the 

 water containing carbon dioxide (CO2) gradually increased the size of 

 the crack until it was still further enlarged by the erosion of the stream. 

 The stream was Anally entirely diverted from its former channel to its 



19 Edward Hitchcock : Geology of Massachusetts, vol. 1, 1841, pp. 287-288. 



20 H, F. Cleland : Formation of natural bridges, American Journal of Science, vol. xx, 

 August, 1905, pp. 120, 121. 



E. O. Hovey : Celebrated American caverns, pp. 14 and 206. 



