The Rivers and Yalleys of Pennsyl/vmiia. 199 



12. Tertiary elevation and denudation. — This broad lowland 

 IS a lowland no longer. It has been raised over the greater part 

 of its area into a highland, with an elevation of from oue to three 

 thousand feet, sloping gently eastward and descending under the 

 Atlantic level near the present margin of the Cretaceous forma- 

 tion. The elevation seems to have taken place early in Tertiary 

 time, and will be referred to as of that date. Opportunity was 

 then given for the revival of the previously exhausted forces of 

 denudation, and as a consequence we now see the formerly even 

 siirface of the plain greatly roughened by the incision of deep 

 valleys and the opening of broad lowlands on its softer rocks. 

 Only the harder rocks retain indications of the even surface 

 which once stretched continuously across the whole area. The 

 best indication of the average altitude at which the mass stood 

 through the greater part of post-Cretaceous time is to be found 

 on the weak shales of the Newark formation in New Jersey and 

 Pennsylvania, and on the weak Cambrian limestones of the great 

 Kittatinny valley ; for both of these areas have been actually 

 almost baselevelled again in the Tertiary cycle. They will be 

 referred to as the Tertiary baselevel lowlands ; and the valleys 

 corresponding to them, cut in the harder rocks, as well as the 

 rolling lowlands between the ridges of the central district of 

 Pennsylvania will be regarded as of the same date. Whatever 

 variations of level occurred in this cycle of development do not 

 seem to have left marks of importance on the inland surface, 

 though they may have had greater significance near the coast. 



13. Later changes of level. — Again at the close of Tertiary 

 time, there was an elevation of moderate amount, and to this niay 

 be refei-red the trenches that are so distinctly cut across the Ter- 

 tiary baselevel lowland by the larger rivers, as well as the lateral 



Fig. 4. 



shallower channels of the smaller streams. This will be called 

 the Quaternary cycle ; and for the present no further mention of 

 the oscillations known to have occurred in this division of time 

 need be considered ; the reader may find careful discussion of 

 16 



