1880.] xyjfj [Stevenson. 



Drainage and Erosion. 



Two forks of Powell river, Pigeon and Roaring, rise on the southern 

 side of Black mountain and unite at the head of Big Stone gap, by which 

 the resulting stream crosses the Stone Mountain fault. The South Fork of 

 Powell river rises at the foot of Powell mountain near Little Stone gap ; re- 

 ceives tributaries from the north side of that mountain as well as from the 

 valley between it and Wallen's ridge ; and, after crossing the fault 

 of Poor valley ridge, by a shallow gap, joins the river at about a 

 mile below Big Stone gap. Powell Mountain river flows between 

 Stone mountain and Poor Valley ridge, for say fifteen miles, to opposite 

 Pennington's gap, where it receives the North Fork. That stream rises on 

 the slope of Black mountain and flows across the Stone Mountain fault by 

 way of Pennington's gap. The main river, thus increased, flows soutli- 

 wardly for a little distance, and then crosses the fault of Poor Valley ridge. 



The North Fork of Clinch river rises in the valley between Wallen's 

 ridge and Powell mountain, and is separated by a low divide at its head 

 from Wildcat creek, a tributary to the South Fork of Powell river. It 

 flows along a monoclinal valley to the North Fork gap, which is sometimes 

 known as Slemp's gap, where it turns toward the soutli-east. Within three 

 or four miles, its course is again changed and the stream once more flows 

 south-westwardl3% following the foot of Powell mountain in a monoclinal 

 valley. 



Stock creek, another tributary to Clinch river, rises in Hunter valley, 

 flows along the synclinal for several miles, then turns and flows up 

 the dip to Clinch river. Clinch river itself rises far to the east of the 

 region examined by me, and flows for a long distance near the line of the 

 Stone Mountain fault, which it crosses and recrosses ; but, at several miles 

 east from the Little Stone gap, its course is changed, and the river for a 

 "while flows with the dip ; but the course is again changed, and the direc- 

 tion becomes very nearly that of the strike. 



It is sufficiently clear that the gaps through Stone mountain and Poor 

 Valley ridge do not owe their origin to any convulsion of nature. The 

 strike of the limestone and the conglomerate across Big Stone gap is per- 

 fectly true, and no evidence of a cross-fault or fracture is appai'ent there or 

 in the coal-field behind the mountain. The absence of fracture is even 

 more apparent in Pennington's gap through Stone mountain ; for this gap 

 is a tortuous passage-way with bold cliflTs of the vertical conglomerate beds 

 projecting from both sides. These cliffs afford ample evidence that erosion, 

 not convulsion, produced the gap. The same condition is seen in the gap 

 by which the South Fork of Powell crosses Wallen's ridge or the Poor 

 Valley ridge fault. Tlie North Fork gap in Powell mountain is as dis- 

 tinctly due to erosion as is the monoclinal valley along which the stream 

 flows above the gap, or the tunnel by which Stock creelc flows through a 

 hill near Clinch river. 



It may be stated positively that not one of the numerous gaps by which 



PROC. AMER. PHILOS. SOC. XIX. 107. N. PRINTED OCTOBER 27, 1880. 



