II. F. Cleland — Formation of Natural Bridges. 119 



Art. XV. — The Formation of Natural Bridges; by 

 Herdman F. Cleland. 



Until recently the text-books of Geology and Physical 

 Geography have given the idea, whether intentionally or not, 

 that natural bridges are universally formed by the partial caving 

 in of a long cavern, the bridge being that portion of the roof 

 strong enough to span the cavity.* The belief seems to be 

 prevalent that these cavities extended for long distances, a 

 condition comparable to that which would exist if the greater 

 part of the roof of Mammoth Cave should fall in, leaving a 

 small portion as a bridge. This theory is simple and logical 

 and is one which immediately appeals to the reader, but, as will 

 be seen from the examples cited in this paper, not only is it 

 not of universal application but it must be exceptional rather 

 than otherwise. The writer was led to this study by an 

 examination of the natural bridge near North Adams, Mass. 

 which has long been considered to be a typical example and 

 proof of the formation from caverns. 



The North Adams Natural Bridge spans Hudson Brook and 

 has been an object of more than local interest for many years 

 both because of its natural beauty and because of the rarity of 

 these objects. Hudson Brook is a small stream emptying into 

 Beaver Creek, a tributary of the Hoosick River. From the 

 dam (shown in the sketch tig. 1) to the pre-glacial valley the 

 brook flows through a gorge 30 to 60 feet deep and from 5 to 

 40 feet wide, the average width above the bridge being from 

 1 to 10 feet and below from 10 to 30 feet. This gorge is cut 

 in a coarsely crystalline marble which, because of its color and 

 texture, presents a striking appearance. The rock is Cambro- 

 Silurian and belongs to the Stockbridge formation. 



The top of the natural bridge is 44 feet above the water of 

 the stream and the bridge itself is about 8 feet thick. The 

 span of the bridge is less than 10 feet long and the width at 

 present 25 feet, but at one time it probably extended a short 

 distance farther south where it is now fallen in. It is 

 extremely difficult to take a good photograph of the bridge 

 because, as will be seen from the sketch, the stream turns 

 sharply both above and below. Because of this condition it 

 was found necessary to make a drawing, in order to give a cor- 

 rect idea of its appearance. 



Prof. E. Hitchcock described the North Adams Natural 

 Bridge and published a rough drawing of it in 1841. f Con- 

 cerning this drawing he says, "I thought it better that a sketch 



* Chamberlain and Salisbury, Geol., vol. i, pp. 145-147. 



f Geology of Mass., by Edward Hitchcock, vol. i, 1841, pp. 287-288. 



