Two Natural Bridges of the Cumberland 

 Mountains 



BY WILBUR A. NELSON. 



There is probably no portion of Tennessee more beautiful or more in- 

 teresting to the traveler than the Cumberland Mountains a rugged belt 

 which stretches nearly north and south across the State, forming a natural 

 barrier between the middle and eastern portions of the State. 



Among the many things of interest to the geologist and general ob- 

 server are two natural bridges, which occur in this region. One, which 

 has been known for years, is about three miles southwest of Sewanee in 

 Franklin County, and is a point of interest to all visitors on the mountain. 

 During the summer of 1913, the writer observed another, located two 

 miles northwest of Dayton, on the eastern escarpment of Walden Ridge. 

 It is so surrounded by trees that unless its location were known one would 

 pass within a hundred yards of it and be ignorant of its existence. 



Mode of formation. The history of the two bridges is similar. The 

 Cumberland Mountains are composed of horizontal beds of alternating 

 sandstones and shales. The bridges occur in the thick sandstone, where 

 the latter forms bluffs along the face of the mountain. A marked vertical 

 jointing, parallel to the face of the escarpment, gives the abundant rain- 

 fall opportunity to disintegrate channels 50 to 100 feet back from the es- 

 carpment. These channels finally reach, at a lower level, a softer layer 

 of sandstone which generally occurs near the base of the bluff. The water 

 disintegrates the rock very easily and makes its way to the base of the bluff 

 through a joint. This at first a small joint, gradually enlarges until a 

 sufficient size has been reached, when the process is rapidly augmented 

 by the thin bedded sandstone failing down of its own weight, thus com- 

 pleting the formation of the bridge. Eventually the processes that form 

 the bridge destroy it by causing it to collapse and leave only a jagged gate, 

 through which the water flows. 



In addition to the water forming a channel along the joint plane of the 

 sandstone, there occurs at each of the two bridges a small spring, which 

 gushes out at the base of the solid sandstone wall, just back of the bridge 

 arch. While the surface water, which flows down through the vertical 

 joints is responsible for separating the arch from the sandstone that caps 



