60 C. D. Walcott—The Natural Bridge. 



met with in passing up from the river are highly inclined lime- 

 stones and shales of middle or upper Cambrian age. These 

 are succeeded by the massive Knox dolomites, which are nearly 

 vertical or inclined slightly westAvard. A few hundred feet 

 beloAV Natural bridge the westward dip decreases very rapidly, 

 and at the bridge the beds are nearly horizontal, while a short 

 distance above they are rising westward and dipping eastward 

 toward the bridge at an angle of 5° to 10°. This increases to 

 20° to 25° higher up the stream. 



A diagramatic section of the rocks cut through in the can- 

 yon of Cedar creek gives the outline shown in figure 1. The 

 bridge is at A, Lace falls at B, and James river at C. No attempt 

 Is made to show the depth of the canyon or gorge through which 

 Cedar creek flows. 



It is not supposed that the present Cedar creek began to 

 wear its channel across the edges of the upturned beds from 

 B to C when the present topographic features were established ; 

 on- the contrary, it began its work long before, under conditions 



Figure 1. — Attitude of Strata at Natural Bridge. 



and in rocks that have since disappeared in the general erosion 

 of the surrounding country. The course of the stream was 

 determined by circumstaiices connected with the life history of 

 James river. When the latter obtained a new lease of active life 

 and lowered its channel through the Blue ridge, Cedar creek 

 began to cut down itS bed in the peneplain and to prepare the 

 way for the possibility of the existence of an arch over its chan- 

 nel. 



The general mode of formation has long been described for 

 this and other natural rock bridges. In this case in detail it 

 is considered to be as follows : Cedar creek was engaged for a 

 considerable period in excavating the gorge from the James river 

 to a point not far below the present site of the bridge, where a fall 

 appears to have existed, the summit of which was not far if at 

 all below the present level of the top of the bridge. About this 

 time the water found a subterranean passage in the limestone fur- 

 ther up the stream than the present site of the bridge, and through 

 this it flowed and discharged beneath the brink of the falls. 



