NATURAL BRIDGES BY STREAM EROSION 



315 



until at length an opening was made which permitted the water of the 

 stream to cut off its meander by passing under the. arch thus made. 



a. Roch hridge across Swifts Camp Creek, Campion, Kentucky. — The 

 so-called "Eock Bridge" near Campton^ Kentucky (plate 18, figure 1), 

 furnishes an excellent example of a bridge of this origin. This bridge 

 was visited by the writer in July, 1909, and is described here for the first 

 time. It is situated in the fioor of a narrow valley from 125 to 150 feet 

 deep, and is composed of a coarse red sandstone which is almost conglom- 

 eratic in some portions. The top of the bridge is between 15 and 20 feet 

 above the surface of the stream; the length of the span is 50 feet and the 

 width varies from 6 to 12 feet. The thickness of the arch is about 8 feet. 



A 



315 





^•Ai 



FrauRB 1. — Diagram indicating Origin of Rock Bridge 



It will be seen from the accompanying diagram (figure 1) that the aban- 

 doned meander is long for its width, and that the sharp curve which the 

 stream formerly made was such as to produce rapid cutting on the inside 

 of the curve. The abandoned meander is now filled with alluvium and 

 is cultivated. The cause of the meandering seems to have been the find- 

 ing of a more resistant stratum by the stream in its downward cutting. 

 It is possible, however, that a temporary baselevel was reached by a halt 

 in the elevation of the region, by temporary lowering of the land, or by 

 some temporary dam. 



h. South Dakota hridge. — This bridge (plate 18, figure 2), located 

 south of White Eiver below the mouth of Porcupine Creek, in the Bad 

 Lands of South Dakota, and formed of White Eiver beds, is described by 

 Barnett^ as follows: 



» Journal of Geology, vol. xvi, no. 1, January to February, 1908, pp. 93-95. 



