UPPER SUSQUEHANNA VALLEY 291 



Susquehanna flows has an altitude at this point of about 600 feet. It is 

 bordered on the west by a low ridge of Pottsville sandstone which rises 

 to an altitude of 1,500 or 1,600 feet; behind this, and stretching to the 

 northwest for a distance of 10 miles, is a belt of Lower Carboniferous and 

 Devonian shales which have a remarkably regular surface altitude of 

 from 1,200 to 1,300 feet. It is true that the region is extremely hilly 

 and that many knobs rise above the general level, but when it is con- 

 sidered that the region lies at a considerable distance from the sea it is 

 surprising that it should be reduced to so uniform a level. This hilly 

 region is bounded on the northwest by the Pocono plateau, which rises 

 abruptly to an altitude of from 2,200 to 2,400 feet. The difi'erentiation 

 of these features is extremely sharp, and it seems impossible to explain 

 the intermediate plateau on any other supposition than that it once 

 stood near baselevel and was subject to uninterrupted erosion through 

 a long period of time. The adjacent regions show traces of the same 

 surface, but at no other place is it so well developed as in this particular 

 locality. 



The Juniata valley has not been completely mapped, and therefore it 

 is impossible to trace continuously the peneplain in that direction ; but 

 recently some excellent maps have been made of its headwaters, which 

 show features similar to those just described. In the great Nittany 

 valley at the head of the stream, the limestones have been reduced to a 

 fairly even surface at an altitude of from 1,200 to 1,400 feet. The Devo- 

 nian shales in the same region have sufl'ered similar reduction, so that 

 it seems probable that the Harrisburg peneplain was developed in the 

 vicinity of Hollidaysburg and Altoona. 



The Somerville plain of Professor Davis seems to show at intervals 

 along the Susquehanna river, rising from an altitude of 400 feet in the 

 vicinity of Harrisburg to possibly 700 feet at Pittston. Above this point 

 it does not appear in the harder rocks of the Allegheny plateau. Simi- 

 iarly up the Juniata river it seems possible to trace it to above Holli- 

 daysburg at an altitude of about 900 feet. Thus it appears that this 

 region suffered an amount of deformation similar to that of the Potomac 

 valley in middle Tertiary time, and that since the development of the 

 Somerville plain it has suffered deformation along an axis presumabl}^ 

 corresponding with or west of the Allegheny front. 



From the above review of the evidence in eastern Pennsylvania it seems 

 to the writer that the Harrisburg peneplain must be recognized as the 

 most important topographic feature of Tertiary time, and that it repre- 

 sents an extremely long period of undisturbed erosion. The Somerville 

 plain, which is so well exposed on the weak rocks in the eastern part of 



