DRAINAGE CHANGES OF THE SHENANDO-AH 353 



Rockfish Gap near Waynesboro is the second in size in the Blue Ridge 

 delimiting the Shenandoah Valley region. It seems quite probable that 

 the stream which developed it has been intercepted at a number of points 

 on the Valley side and all evidence of its existence obhterated, except 

 probably that part of Middle River northeast of Staunton. 



The relative distribution of the large and small wind gaps in the Blue 

 Ridge is significant not only of ancient drainage lines but of the order in 

 which they were abandoned by their streams. These wind gaps furnish 

 the only evidence of drainage conditions of the region during the various 

 stages of its existence. Figure 2 shows the distribution of wind gaps in 

 the Blue Ridge opposite the Shenandoah Valley between Harpers Ferry 

 and- Front Royal, which is more or less typical of the northern Blue Ridge 

 in Virginia. 



Fig. 2. Longitudin-al Profile of the Blue Ridge Mountains from Harpers 

 Ferry to Front Royal, showing Position of Gaps. 



After considering these peculiar and significant relations of the Shen- 

 andoah system to the physiographic features of the region it becomes 

 strikingly evident that the present drainage lines conform only in part 

 to what they were at the beginning of the first cycle of erosion. This 

 original drainage, reconstructed on the basis of the evidence at hand, is 

 shown in figure 3. The streams are of. the consequent type, folloTvdng 

 courses with their directions determined by the original slope of the region 

 at the close of the Paleozoic era. Only by starting with a condition of 

 this sort can the present drainage and physiography of the region be 

 satisfactorily explained. That the Shenandoah system is the result of 

 the concentration of a large number of small streams which held their 

 courses across the Blue Ridge has long been recognized. The well known 

 case of the capture of Beaver Dam Creek by the Young Shenandoah 

 opposite Snicker's Gap* has become one of the best known instances of 



* Willis, Bailey, The Northern Appalachians, National Geographic Monographs, 

 vol. i, pp. 169-202. 



