560 



W. M. DAVIS — DATES OF TOPOGRAPHIC FORMS. 



plain. Going to the summit of one of the furthest of these ridges, that of 

 Bald Eagle mountain, back of Bellefonte, in the center of the state, the ob- 

 server may see toward the southeast a series of even-topped ridges, all of 

 Medina sandstone, all in synclinal or anticlinal foldings, and all truncated 

 in the same systematic manner. It must not be inferred that they accord 

 with geometric exactness, but only Avith geographic exactness, such that 

 if the intervening lowlands were filled up to the crest-lines a very even pene- 

 plain w^ould be formed. Looking northwestward from the same ridge, the 

 plateau of western Pennsylvania rises beyond a valley of soft Silurian and 

 Devonian rocks. Still the same general elevation is preserved, but the type 

 of form is altered in accordance with the nearly level position of the strata. 

 The upland of the plateau is a gently hilly country, noAV deeply dissected by 

 valleys of the post-Cretaceous cycles of development. The upland is not a 



Figure 3 — Cretaceous Peneplain in New Jersey and Pennsylvania. 



constructional plateau, that is, its surface does not consist of a single stratum 

 of rock or formation of rock ; it is an eroded surface, a peneplain on which 

 successive formations crop out, one shingled on the next as we cross the 

 country. In this denuded upland, a new^ set of valleys is now cut in conse- 

 quence of its elevation above its former lowland attitude. At Emporium, 

 for example, a short walk from the town up to a hill-top near by, a spur of 

 the plateau, gives a fine view of the surrounding gently rolling upland, as 

 well as of the deep valleys that the roads and railroads follow' from town to 

 town. When it is recognized that the upland itself has sufl^ered a consider- 

 able erosion and yet has a generally even surface, and that deep valleys are 

 sunk beneath it, we are forced to that it once as a whole stood lower than 

 now ; that while lower it was reduced to a rolling peneplain ; that since then 

 it has been raised to its present height, and that during and since this eleva- 

 tion the present deep valleys have been sunk below its old surface. How 



