DRAINAGE CHANGES OP THE SHENANDOAH 357 



suppose that the Shenandoah Hmestones, which are immediately above 

 the quartzites, were also involved in this anticline, and extended farther 

 eastward. Remnants of limestone found in the Piedmont Plateau are 

 the probable correlatives in age of the lower members of the Shenandoah 

 group of limestones which compose the Great Valley. Assuming this 

 correlation to be correct the limestone was evidently removed from the 

 Blue Ridge province during the Kittatinny cycle of erosion. During this 

 period of removal of the less resistant limestone, the Blue Ridge was a 

 weak belt instead of a barrier as today. If this assumption of the east- 

 ward extension of the limestone is correct, its eastern limits extended 

 some distance east of the present location of the Blue Ridge. „ If this 

 limestone belt east of the Blue Ridge really existed the peculiar change 

 in the course of James River below Lynchburg from a southeasterly to 

 a northeasterly direction and later back to a southeasterly course, is 

 easil3^ explained by resort to capture by a strike stream working on this 

 limestone belt west of a barrier of resistant rock. Like changes in direc- 

 tion in the same relative position also occur both in the case of Roanoke 

 and Rappahannock rivers. The evidence of limestone remnants found 

 in the Piedmont province and the changes in direction of these streams 

 at the same relative positions in their courses, seems to add special weight 

 to this hj'pothesis. 



The second cycle [Tertiary i^f) plain). Uplift of the Kittatinny plain 

 rejuvenated the streams and a second cycle of erosion was inaugurated. 

 After the removal of whatever formations covered the limestones of the 

 Valley, a broad belt of soluble rocks was exposed west of the Blue Ridge 

 which had now become a barrier by offering to erosion the basaltic and 

 granitic igneous rocks of its core, and the resistant Cambrian quartzites 

 which flank it on the northwest side, and in places occupy its crest. The 

 erosion of overlying material resulted in superimposing the streams on 

 limestones in the Valley region and hard resistant rocks in the Blue Ridge; 

 Massanutten Mountain remaining as a barrier by failure of erosion to 

 remove the thickness of hard quartzite which occupies its crest. 



During this cycle of erosion most of the stream captures which occurred 

 in the process of drainage adjustment took place. Only one important 

 capture remained for the succeeding cycle. The first step in the inter- 

 esting series of drainage changes which occurred in the early part of this 

 cycle was the capture of all small streams flowing through shallow gaps 

 in the then low Blue Ridge by tributaries of the major streams. The 

 distribution of the large and small wind gaps in the Blue Ridge argues 

 strongl}' against capture of all these streams by the young Shenandoah, 



