TERTIARY WORK IN PENNSYLVANIA. 573 



(1888) of the final report of the Geological Survey of that State. Here we 

 find attention called to the great work of the forces of denudation in fashion- 

 ing the present form of the land, and in illustration of this truth the gap cut 

 by the Delaware is mentioned. Now, it is not to be doubted that the open- 

 ing of this gap was a difficult piece of work, on account of the great resist- 

 ance of its rocks ; nor can it be questioned that the gap is a superb illustra- 

 tion of erosive action ; but in instancing it as an example of great volume of 

 erosion, and saying nothing of the vastly greater volume of erosion that has 

 been accomplished in the same period of time over the lowlands on either 

 side of the mountain in which the gap is cut, it is clearly implied that the 

 lowlands were already lowlands when the river began the work of cutting 

 the gap in the mountain. From this the reader must infer that the moun- 

 tain owes its height over the surrounding country to a local uplift, and this is 

 fundamentally wrong. The relief of the mountain over the surrounding 

 country is due entirely to its superior resistance. The surface of the rocks 

 that now occupy the lowlands was, at the close of the period of Tertiary uplift, 

 raised as high as the surface of the baseleveled mountain. The mountain is 

 still high simply because it has not worn down as the low lands have. The 

 story is the same whether we consider the Delaware, the Lehigh, the Schuyl- 

 kill, or the Susquehanna ; these being the only large streams that cut their 

 ways across the mountain. 



A matter of interest in the development of the Tertiary lowlands is the 

 abstraction of the headwaters of certain small streams that once crossed the 

 mountains, and their diversion into the larger rivers along the strike of some 

 weak stratum up-stream from the gaps, according to principles formulated 

 by Heim, Gilbert and Lowl. The incipient gaps are thus deserted and re- 

 main as notches, not traversed by streams, and know^n as "wind gaps." The 

 most famous of its class is between the Delaware and Lehigh gaps ; others 

 occur near the Schuylkill and Susquehanna and further southwestward. 



The continuity of the great valley across the line of the rivers is a matter 

 of particular interest. From Newburgh to Alabama there is a continuous 

 belt of lowland, bordered on the northwest by the wall of Medina sandstone 

 and on the southeast generally by the inner division of the crystalline belt. 

 Northeast of the Hudson its features are less regular, apparently because 

 here the thickness of the overlying shales that have been folded with the 

 limestones was so great and their position with respect to the Cretaceous 

 baselevel was so adjusted that they now form considerable hills or even 

 mountains, retaining in the Berkshire valley district even a full measure of 

 the height of the Cretaceous peneplain. But southwest of the Hudson the 

 limestone belt is more continuous, and it is very generally reduced close to 

 the Tertiary baselevel. Slate hills remain somewhat above it, but fail by 

 many hundred feet of retaining the height of the old peneplain. It is clear 



LXXXIV— Bum,. Geol. Soc. Am., Vol. 2, 1890. 



