202 National Geographic Magazine. 



cut into the plateau during the Tertiary cycle are narrow here, 

 because the rocks are mostly hard. The steep slopes of the canon- 

 like valley of the Lehigh and the even crests of the ridges mani- 

 festly belong to difEerent cycles of development. Figs. 6 and 7 

 are gaps cut in Black Log and Shade mountain, by a small 

 upper branch stream of the Juniata in southeastern Huntingdon 

 county. The stream traverses a breached anticlinal of Medina 

 sandstone, of which these mountains are the lateral members. A 

 long narrow valley is opened on the axial Trenton limestone 

 between the two. The gaps are not opposite to each other, and 

 therefore in looking through either gap from the outer country 

 the even crest of the further ridge is seen beyond the axial valley. 

 The gap in Black Log mountain, fig. 6, is located on a small frac- 

 ture, but in this respect it is unlike most of its fellows.* The 

 striking similarity of the two views illustrates the uniformity that 

 so strongly characterizes the Medina ridges of the central district. 

 Fig. 8 is in good part an ideal view, based on sketches on the 



■^"';;i!!Hv!^;:'''^^^^^^ 



Fm. 8. 



upper Susquehanna, and designed to present a typical illustration 

 of the more significant features of the region. It shows the even 

 crest-lines of a high Medina or Pocono ridge in the background, 

 retaining the form given to it in the Cretaceous cycle ; the even 

 lowlands in the foreground, opened on the weaker Siluro-Devo- 

 nian rocks in the Tertiary cycle ; and the uneven ridges in the mid- 

 dle distance marking the Oriskany and Chemung beds of inter- 

 mediate hardness that have lost the Cretaceous level and yet have 

 not been reduced to the Tertiary lowland. The Susquehanna 

 flows distinctly below the lowland plain, and the small side 

 streams run in narrow trenches of late Tertiary and Quaternary 

 date. 



If this interpretation is accepted, and the Permian mountains 

 are seen to have been once greatly reduced and at a later time 

 worn out, while the ridges of to-day are merely the relief left by 



* Second Geol. Surv. Pa., Report T3, 19. 



