SOME SUBORDINATE RIDGES OF PENNSYLVANIA 125 



the so-called Harrisburg peneplain level of 700 to 1,000 feet shown 

 in many even-topped ridges and intermontane floor areas, a series 

 of. ridges averaging 1,200 feet in height, which he correlates with 

 the Weverton, Maryland, peneplain,^ and ''remnants of inter- 

 mediate erosion plains" from 1,550 to 1,700 feet in height. 



Facts such as the foregoing give rise to doubts and queries 

 which are of more than local significance. To admit the inadequacy 

 of the Tertiary erosion to reduce completely some subordinate 

 ridges to the valley-plain level seems to be overlooking the question 

 of their origin as much as does the affirmation that the height of 

 subordinate ridges represents a lowering and modification of an 

 original, higher peneplain surface. As the geologist follows for 

 miles two closely parallel ridges of even crests but of different 

 heights, he is likely to think, as the writer does, that the origin of 

 the two is the same, and that any explanation of the method of 

 development of the one should be applicable to the other also. If 

 the one represents a portion of the dissected and mutilated surface 

 of an older peneplain, likewise, by the same reasoning, the other 

 must represent something; but by no stretch of the imagination 

 can some of the lower ridges of Pennsylvania be considered peneplain 

 remnants. 



Many years ago Hayes^ expressed the belief that a lower ridge 

 with even crest can be produced by reduction from a higher pene- 

 plain, as "corrasion is practically absent and only the forces of 

 erosion are in play. Hence if the ridge was originally level it might 

 remain so indefinitely or until it had been reduced far below the 

 former surface of the baselevel plain." 



Dr. T. C. Hopkins,^ whose experience in central Pennsylvania 

 entitles him to express an opinion, explains the development of 

 these low ridges — the terraces of "terraced mountains"'' — by 



' x\rthur Keith, "Geology of the Catoctin Belt," U.S. Geol. Surv., Fourteenth Ann. 

 Rept., 1892-93, Part 2, p. 388. Also W. B. Clark and E. B. Mathews, "The Physical 

 Features of Maryland," Md. Geol. Surv., VI (1906), 87, 88. 



^ C. W. Hayes, "Physiography of the Chattanooga District, in Tennessee, 

 Georgia, and Alabama," U.S. Geol. Surv., Nineteenth Ann. Rept., 1897-98, Part 2, 

 p. 27. 



3 Recent personal communication to the writer. 



''T. C. Hopkins, Elements of Physical Geography (1908), pp. 338-39. 



