248 National Oeographic Magazine. 



of the Juniata breached the eastern slope of the Nittany-Bedf ord 

 range and pushed the divide westward, they at last gained pos- 

 session of the Siluro-Devonian monocline on its western slope ; 

 but beyond this it has not been possible for them yet to go. As 

 the streams cut down deeper and encountered the Medina anti- 

 cline near the core of the ridge, they sawed a passage through it ; 

 the Cambrian beds were discovei-ed below and a valley was 

 opened on them as the Medina cover wore away. The most 

 important point about this is that we find in it an adequate 

 explanation of the opposite location of water-gaps in pairs, such 

 ais characterize the branches of the Juniata below Tyrone and 

 again below Bedford. This opposite location has been held to 

 indicate an antecedent origin of the river that passes through the 

 gaps, while gaps formed by self-developed streams are not 

 thought to present such correspondence (Hilber). Yet this 

 special case of paired gaps in the opposite walls of a breached 

 anticline is manifestly a direct sequence of the development of 

 the Juniata headwaters. The settling down of the main Juniata 

 on Jack's mountain anticline below Huntingdon is another case 

 of the same kind, in which the relatively low anticlinal crest is as 

 yet not widely breached ; the gaps below Bedford stand apart, as 

 the crest is there higher, and hence wider opened ; and the gaps 

 below Tyrone are separated by some ten or twelve miles. 



When the headwater streams captured the drainage of the 

 Siluro-Devonian monocline on the western side of the ancient 

 dividing anticline, they developed subsequent rectangular branches 

 o-rowing like a well-trained grape vine. Most of this valley has 

 been acquired by the west branch of the Susquehanna, probably 

 because it traversed the Medina beds less often than the Juniata. 

 For the same reason, it may be, the West Branch has captured a 

 considerable area of plateau drainage that must have once 

 belonged to the Ohio, while the Juniata has none of it ; but if so, 

 the capture must have been before the Tertiary cycle, for since 

 that time the ability of the West Branch and of the Juniata as 

 regards such capture appears about alike. On the other hand, 

 Castleman's river, a branch of the Monongahela, still retains the. 

 drainage of a small bit of the Siluro-Devonian monocline, at the 

 southern border of the State, where the Juniata headwaters had 

 the least opportunity to capture it ; but the change here is 

 probably only retarded, not prevented entirely ; the Juniata will 

 some day push the divide even here back to the Alleghany Front, 

 the frontal bluff of the plateau. 



