342 CYCLES OF DENUDATION 



The present height of these peaks is due to subsequent reelevation. 

 This plain is called the Kittatinny peneplain, because the ridge 

 of that name in Pennsylvania and New Jersey is one of the rem- 

 nants of it. To the observer who can overlook the billowy ridges 

 of the present range their even sky line is very striking, and these 

 ridges are all composed of the hardest rocks, which all rise to 

 nearly the same level. To reproduce the plain it would be 

 necessary to fill the valleys between the Blue Ridge on the east 

 and the plateau on the west up to the level attained by the hard 

 ridges, and this would give a gently arched surface, sloping very 

 gradually to the Mississippi valley and the Atlantic. On this 

 peneplain were already established the great streams which flow 

 to the ocean, such as the Susquehanna and the Potomac. 



Next the peneplain was raised very gradually to a height of 

 1400 feet in Virginia, diminishing in both directions from this 

 point, and the denuding forces once more attacked and dissected 

 the plain, the larger streams holding their transverse courses and 

 sawing through the hard strata, which were left standing as ridges 

 by the cutting of the longitudinal valleys along the more destructi- 

 ble beds. Denudation had cut down the softer beds to one gen- 

 eral level, called the Shenandoah peneplain, the period of rest 

 being long enough to bring all the areas of soft and soluble beds 

 to this level, but not to materially lower the ridges of the more 

 resistant strata. 



"The swelling of the Appalachian dome began again. It rose 

 200 feet in New Jersey, 600 feet in Pennsylvania, 1700 feet in 

 southern Virginia, and thence southward sloped to the Gulf of 

 Mexico. ... In consequence of the renewed elevation, the 

 streams were revived. Once more falling swiftly, they have 

 sawed, and are sawing, their channels down, and are preparing 

 for the development of a future base-level." (Willis.) 



This example is sufficient to show the value of the physio- 

 graphic method to the geologist in supplementing the knowledge 

 derived from the study of sedimentation. 



