354 UNIVERSITY OF VIRGINIA PUBLICATIONS 



stream capture, and has been widely quoted in text-books as a type exam- 

 ple of the process. 



GEOLOGIC INFLUENCES OF THE REGION FAVORING STREAM CAPTURE. 



The conditions chiefly responsible for the concentration of the large 

 number of antecedent streams into a single system are attributable to 

 structure and the unequal resistance of the rocks to erosion, especially 

 difference between the lithologic rock types found in the Blue Ridge 

 and the Great Valley. 



The sandstones, hmestones, and shales deposited during Paleozoic 

 times and forming the region were laid down practically in horizontal 

 position, and their eastern margin was for the lower members of the series 

 at least considerably east of the position now occupied by the Blue Ridge. 

 It is highly probable that even the lower members of the limestone group 

 extended east of the Blue Ridge as the Lower Cambrian silicious sedi- 

 ments evidently did. The uplift and folding follo^ving the Paleozoic 

 cycle of sedimentation in the Appalachian IMountains province resulted 

 in reversing the drainage of the region from a northwesterly to a south- 

 easterly direction. This marked the beginning of the consequent system 

 of drainage and the first cycle of erosion. 



The uppermost members of this great thickness of Paleozoic sediments 

 were of a silicious • character, and must have completely covered the 

 great limestone formation near the base of the column and the igneous 

 rocks of the Blue Ridge. By long erosion of these folds beds of different 

 lithologic character were brought to the surface which varied widely in 

 their ability to resist erosion. The ultimate result was th'e almost entire 

 removal of the sediments from the crest of the Blue Ridge exposing the 

 core of resistant igneous rocks and a great belt of limestone in the Valley 

 region, with Massanutten Mountain remaining as a monadnock capped 

 with resistant sandstone; and in the Alleghany Ridges province overlying 

 sandstones removed only in narrow belts to a sufficient depth to expose 

 the underlying limestone. 



The streams flowing across the region transverse to the axis of fold- 

 ing, subsequently found themselves working on beds of widely varying 

 degrees of resistance in different parts of their courses. East of the Alle- 

 ghany Ridges the chief barriers of resistant beds were the sandstones 

 of Massanutten Mountain, the cherty beds of the Shenandoah limestone 

 group, and the igneous rocks and Cambrian quartzite of the Blue Ridge. 

 These were the chief and controlling barriers in the path of the southeast 



