362 UNIVERSITY OF VIRGINIA PUBLICATIONS 



doah plain in the Vallej' limestones, completed probabl}^ in -Lafaj^ette 

 time (see pi. VII). During the early part of the Shenandoah cycle the 

 rival drainage systems were Goose Creek and the Potomac (fig. 5). The 

 Potomac was the more able bocUed stream because it was likely the larger 

 of the two, and because the Blue Ridge at Harper's Ferry was, on account 

 of width, a less effective barrier than at Manassas Gap. During the latter 

 part of the Tertiary cycle the j^oung Shenandoah River, a tributary to 

 Potomac River, captured the headwaters of Beaver Dam Creek on the 

 northwest side of the Blue Ridge at Snicker's Gap and possiblj^ other 

 small streams in the northern part of the valley. During the Shenandoah 

 cycle it extended its course, headward along the foot of the Blue Ridge 

 and captured the entire Goose Creek system at Manassas Gap, diverting 

 its waters to the Potomac. This was the last important capture in the 

 development of the present Shenandoah system (see pi. I). 



The headwaters of Goose Creek were also intercepted by other trib- 

 utaries of Potomac River in the western part of the Valley and Alleghanj' 

 Ridges provinces. It is a matter of interest that while several streams 

 in the central part of the Shenandoah Vallej^ region head well back in 

 the Alleghany Ridges province and flow eastward across the ridges for 

 considerable distances to join the Shenandoah, similar antecedent streams 

 are neither fomid in the southern part of the Vallej' region in the vicinity 

 of the James River drainage, nor in the northern part in the vicinity of 

 the Potomac. 



Similar conquests have been accomplished by tributaries of the Poto- 

 mac in the Alleghany Ridges district. In manj^ instances a single drain- 

 age line has been intercepted a number of times before the present adjust- 

 ment of the drainage was accomplished. 



The Recent cycle. A third uplift of the region brought the Shenandoah 

 cj^cle of erosion to a close. During the latter part of this cycle there 

 was some deposition of material in the form of gravels and sand derived 

 from the neighboring mountains. This material is now found on well 

 preserved but small remnants of the Shenandoah plain and probably cor- 

 relates with the Lafayette. The recent cycle of erosion has dissected 

 the Shenandoah plain into an intaglio and the streams are now flowing 

 in U-shaped valleys with flood plains which vary in width depending on 

 local lithologic conditions. They may be as much as a mile ■ndde in 

 places, or they may be entirely absent. Temporarj^ base levels are locally 

 in process of development above barriers of resistant material in the 

 streams. 



